MINISTERUL EDUCAŢIEI NAȚIONALE

UNIVERSITATEA TRANSILVANIA DIN BRAŞOV BRAŞOV, EROILOR NR. 29, 500036, TEL. 0040-268-413000, FAX

0040-268-410525

Școala Doctorală Interdisciplinară

Facultatea de Ingineria Lemnului

Ing. LIU Xin You(刘新有)

PhD Thesis / Teză de doctorat

ABSTRACT/ REZUMAT

Contributions to the study of ageing phenomena of wooden substrate and

traditional materials for transparent finishes – a comparative approach for

Europe and China with applicability in furniture conservation/ restoration

Contribuţii la studiul fenomenelor de îmbătrânire a suportului lemnos şi

materialelor tradiţionale de finisare transparentă - o abordare comparativă pentru

materiale reprezentative din Europa şi China, cu aplicabilitate în conservarea/

restaurarea mobilei

Domeniul de doctorat: Inginerie Forestieră

Doctoral field: Forest Engineering

Conducător ştiinţific

Prof.dr.chim. Maria Cristina TIMAR

BRAȘOV, 2017

2

MINISTERUL EDUCAŢIEI NAȚIONALE

UNIVERSITATEA TRANSILVANIA DIN BRAŞOV BRAŞOV, B-DUL EROILOR NR. 29, 500036, TEL. 0040-268-413000, FAX 0040-268-410525

RECTORAT

D-lui (D-nei) .......................................................................................

COMPONENŢA

Comisiei de doctorat

Numită prin ordinul Rectorului Universităţii Transilvania din Braşov

Nr. 8593 din 14.06.2017

PREŞEDINTE: Prof.dr.ing. Mihaela CÂMPEAN

Universitatea Transilvania din Brașov

CONDUCĂTOR ŞTIINŢIFIC: Prof.dr.chim. Maria Cristina TIMAR

Universitatea Transilvania din Brașov

REFERENŢI: Prof.dr.ing. Song Lin YI

Beijing Forestry University, China

Prof.dr.ing. Ion SANDU

Universitatea A. I. Cuza, Iași

Prof.dr.ing. Marina CIONCA

Universitatea Transilvania din Brașov

Data, ora şi locul susţinerii publice a tezei de doctorat: 27 iulie 2017, ora 11.00, Institutul

Confucius – Universitatea Transilvania din Brașov, Clădire Rectorat, parter, B-dul Eroilor 29,

Brașov.

Eventualele aprecieri sau observaţii asupra conţinutului lucrării vă rugăm să le

transmiteţi în timp util, pe adresa liu.xinyou@unitbv.ro

Totodată vă invităm să luaţi parte la şedinţa publică de susţinere a tezei de doctorat.

Vă mulţumim.

3

CUPRINS

Pg. teza Pg. rezumat

Lista de abrevieri 34 7

Introducere 35 8

1. Stadiul actual al cercetarilor și progresele în domeniul tezei 37 9

1.1 Mobilierul tradițional din Europa și China - patrimoniu cultural 37 9

1.2 Conceptul modern de conservare a patrimoniului cultural 51 9

1.3 Îmbătrânirea materialelor 55 10

1.4 Concluzii generale ale studiului de literatură 78 14

2. Obiectivele de cercetare și faze 79 14

2.1 Scopul tezei 79 14

2.2 Prezentarea obiectivelor 80 14

3. Metodologia de cercetare 83 15

3.1 Conceptul metodologic 83 15

3.2 Materiale 84 16

3.4 Teste de îmbătrânire 97 17

3.5 Metode și echipamente de investigare 104 18

3.6 Metode pentru analiza materialelor de finisare și calității suprafețelor finisate 112 19

4. Caracterizarea speciilor de lemn și a materialelor de finisare tradiționale selectate 121 19

4.1 Specii de lemn 121 19

4.2 Materiale de finisare 137 21

5. Tehnologii de finisare si caracterizarea suprafețelor 156 26

5.2 Aspecte macroscopice 156 26

5.4 Rugozitate 160 26

6. Comportamentul la îmbătrânire a speciilor lemnoase studiate 168 27

6.1 Îmbătrânire indusă de temperatură (A1) 168 27

6.4 Îmbătrânire indusă de UV 182 29

6.5 Îmbătrânire naturală simulată în condiții de interior 190 33

4

6.6 Influența factorilor de îmbătrânire - aspecte specifice legate de îmbătrânirea lemnului 196 34

6.7 Concluzii privind îmbătrânirea lemnului 204 37

7. Comportamentul la îmbătrânire a materialelor de finisare studiate 207 39

7.2 Efectele îmbătrânirii asupra structurii chimice a peliculelor de finisare 211 39

7.3 Aspecte specifice privind îmbătrânirea materialelor de finisare 229 49

7.4 Concluzii 230 48

8. Comportamentul la îmbătrânire a suprafețelor de lemn finisate 231 50

8.7 Influența factorilor de îmbătrânire și a speciilor de lemn asupra comportamentului de

îmbătrânire a suprafețelor finisate

269 51

8.8 Concluzii privind îmbătrânirea suprafețelor de lemn finisate 276 55

9. Concluzii generale, contribuții originale și cercetări viitoare 278 57

9.1 Concluzii generale 278 57

9.2 Contribuții originale 282 60

9.3 Direcții viitoare de cercetare 283 61

BIBLIOGRAFIE 284 61

Scurt Rezumat (Engleză/ Română) 66

CV 67

5

CONTENT

Pg. thesis Pg. abstract

List of abbreviations 34 7

Introduction 35 8

1. Literature survey - present knowledge and developments in the thesis’ field 37 9

1.1 Traditional European and Chinese furniture as cultural heritage 37 9

1.2 The modern concept of cultural heritage conservation 51 9

1.3 Materials ageing 55 10

1.4 Overall conclusions of literature survey 78 14

2. Research objectives and phases 79 14

2.1 Aim of the thesis 79 14

2.2 Outline of Objectives 80 14

3. Research methodology 83 15

3.1 Methodological concept 83 15

3.2 Materials 84 16

3.4 Ageing procedures 97 17

3.5 Investigation methods and equipments 104 18

3.6 Analyses methods for finishing materials and the quality of finished surfaces 112 19

4. Characterisation of the selected wood species and traditional finishing materials e 121 19

4.1 Wood species 121 19

4.2 Finishing materials 137 21

5. Finishing technologies and characterisation of the finished wood surfaces 156 26

5.2 .Macroscopic aspects 156 26

5.4 Roughness 160 26

6. Ageing behaviour of the studied of wood species 165 27

6.1 Temperature induced aging (A1) 168 27

6.4 UV induced ageing 168 29

6.5 Natural ageing in indoors conditions 182 33

6

6.6 Influence of the ageing factors - specific aspects of wood ageing 190 34

6.7 Conclusions on wood ageing 196 37

7. Ageing behaviour of the studied finishing materials 204 39

7.2 Effects of ageing on the chemical structure of the coating films 207 39

7.3 Specific aspects of ageing of finishing materials 211 49

7.4 Conclusions 229 48

8. Ageing behaviour of the studied finished wood surfaces 230 50

8.7 Influence of ageing factors and wood species on the ageing behaviour of the

finished wood surfaces

231 51

8.8 Conclusions on the ageing of wood finished surfaces 269 55

9. Conclusions, original contributions and direction of future research 276 57

9.1 General conclusions 278 57

9.2 Original contributions 278 60

9.3.Further research directions 282 601

References 283 61

Summary (English/ Romanian) 284 66

CV 65

7

List of abbreviations

No. Abbreviations Complete names Remarks

1 R1,2,3 Research report 1,2,3 /Raport cercetare Original

2 Ex1,2,3 Exams 1,2,3 /Examene în stagiul de doctorat Original

3 O1,2... Objectives 1,2… /Obiective Original

4 F European ash / Frasin European (Fraxinus excelsior) Original

5 N European walnut / Nuc European (Juglans regia) Original

6 P Sycamore maple / Paltin de munte (Acer pseudoplatanus) Original

7 S Chinese ash / Frasin chinezesc (Fraxinus mandshurica) Original

8 H Chinese walnut / Nuc chinezesc (Juglans mandshurica

Maxim)

Original

9 T Paulownia / Paulonia (Paulownia elongata), Original

10 BW Beeswax / Ceară de albine Original

11 LO Linseed oil / Uei de in Original

12 SL Shellac / Șelac Original

13 CW Chinese wax / Ceară chinezească Original

14 TO Tung oil / Ulei de Tung Original

15 CL Chinese lacquer / Lac chinezesc Original

16 MC, U Moisure content / Conținut de umiditate Common use

17 D-A1,2,3,4 Diagram of ageing tests A1,2,3,4 / Diagrame teste de

îmbătrânire A1, 2, 3, 4

Original

18 RH Relative humidity / Umiditate atmosferică relativă Common use

19 T Temperature / Temperatură Common use

20 UV Ultra Violet radiation / Radiații ultraviolete Common use

21 Z Zone / Zonă Original

22 TLM Transmitted Light Microscopy / Microscopie optică prin

transmisie

Common use

23 RLM Reflected Light Microscopy / Microscopie optică prin

reflexie

Common use

24 BF Bright Field / Câmp luminos Common use

25 DF Dark Field / Câmp întunecat Common use

26 TL Transmitted Light / Lumină transmisă Common use

27 SC Solid content (%) / Conținut de substanță solidă (%); Common use

28 ρl Density în liquid phase / Densitate în stare lichidă Common use

29 Rvk The reduced valley depth / Parametru rugozitate Common use

30 aT, Temperature ageing sensitivity slope factor / Factor pantă

de sensibilitate la îmbătrânire indusă de radiații UV

Original

31 aUV UV ageing sensitivity slope factor / Factor pantă de

sensibilitate la îmbătrânire indusă termic

Original

32 AAI Acceleration index- Apparent / Indice aparent de accelerare Original

33 EAI Acceleration index-Effective / Indice efectiv de accelerare Original

34 GASI Global ageing sensitivity index / Indice global de

sensibilitate la îmbătrânire

Original

35 R-E Relative E-colour difference ratio / Raport relativ al

diferențelor de culoare

Original

8

Introduction

The patrimonial value of historic furniture as a representative part of world cultural

heritage is generally acknowledged, as well as the necessity of a scientific approach for its

efficient conservation.

This would be not possible without a deep knowledge of the component materials or

without understanding the ageing phenomena naturally occurring in time and affecting the

properties of those materials. Both wood substrate and traditional finishing materials do

undergo specific ageing phenomena affecting their properties. Colour changes, blurring or

crackling of transparent coating films and loss of adherence are common aspects for many

historic furniture objects. They are related to ageing phenomena, which up to a certain extend

contribute to the specific increased aesthetics of old wooden artefacts, known as patina.

However, the progress of the physical and chemical processes defining ageing, accelerated by

the environmental factors, can result in undesirable change of properties up to degradation.

The importance of understanding and knowing the ageing behaviour, respectively the

ageing resistance of materials for conservation has been clearly stated (e.g. Feller 1994,

Baglioni and Giorgi 2006, Maxwell 2005). However, the available data on the ageing

behaviour of different wood species, finishing materials and finished wooden surfaces is still

very limited compared to the diversity of materials employed in historic furniture, especially

when considering different cultures.

This PhD thesis started with Literature survey (Ch.1), comprising a short overview of

furniture history in Europe and China, theory of ageing and a summary of present knowledge

and developments in the field of thesis. The research objectives and phases (Ch.2) were based

on the present situation and the aim of the thesis. An original methodological concept was

elaborated and applied (Ch.3). Relevant materials were selected for experiments and

characterised (Ch.4). These were: three European wood species [European ash (Fraxinus

excelsior), European walnut (Juglans regia), Sycamore maple (Acer pseudoplatanus)] and

three Chinese wood species [Chinese ash (Fraxinus mandshurica), Chinese walnut (Juglans

mandshurica Maxim), Paulownia (Paulownia elongata)], alongside three types of traditional

finishing material from Europe (beeswax, linseed oil and shellac) and also three

corresponding ones from China (Chinese insect wax, Tung oil and Chinese lacquer). More

than 2000 samples of wood (unfinished and finished surfaces) alongside finishing materials

films on glass lamellas were prepared (Ch.5) and tested in 5 ageing tests, employing the main

ageing factors. Ageing phenomena were monitored by direct observations, microscopy,

measuring the colour changes and FTIR investigations (Ch.6, 7, 8). The results represent a

contribution to the present knowledge and available research based data on the ageing of

wooden substrate, traditional finishing materials and finished surfaces, from a comparative

perspective for Europe and China. This is well reflected in the conclusions of the thesis and

directions of further work (C.9). This data have direct applicability in the field of scientific

furniture conservation as relevant comprehensive data bases.

The thesis itself is structured on 295 pages, including 9 chapters, 62 tables, 167 figures

and 187 references. The annexes of the thesis (392 pages) represent important data-bases for

the subject under study. Images cards in electronic format are included.

9

1. Literature survey - present knowledge and developments in the thesis’

field

1.1 Traditional European and Chinese furniture as cultural heritage

Furniture is a generic term defining objects intended to support/assist various human

activities, such as seating, sleeping, eating, working, resting etc. (Jim 2007). However, from

ancient times to present days furniture cumulates functionality and aesthetic /artistic valences,

being a relevant expression of human culture and civilization in continuous development.

In the long history of furniture, several and very diverse factors, from geographical and

economical to social and cultural, as summarised in Fig. 0-1, influenced development and the

in-time evolution of furniture. From simple items, strictly linked to a necessity, to complex

objects, cumulating multi-functionality, interesting structure and technical solutions alongside

exquisite decoration, historical furniture from different periods and regions, reflect

development of a society, its knowledge and technical achievements and economical power,

as well as symbolic values and religious belief, myths, etc. Therefore, historic furniture

represents worldwide an important part of cultural heritage and identity.

Fig. 0-1 Diagram of historical furniture evolution under the influence of different factors (Liu-original).

The patrimonial value of old furniture / (wooden) artefacts results not only from their age,

material value and history, but also from the valuable information on the cultural and

technical level of the community creating and employing those objects.

1.2 The modern concept of cultural heritage conservation

The European Confederation of Conservator-Restorers' Organisations (E.C.C.O.),

defines ―objects to which society attributes a particular artistic, historic, documentary,

aesthetic, scientific, spiritual or religious value‖ as "cultural property"; which constitute a

―material and cultural heritage to be passed on to forthcoming generations‖.

10

These components of the conservation-restoration concept are summarized in the scheme

from Fig. 0-2. The scheme also highlights that in all the action levels science and scientific

investigation should be employed. Any conservation-restoration action is based on respecting

the 10 basic principles of conservation-restoration, out of which the necessity of scientific

investigation for diagnosis is an important one.

Ageing and degradation are phenomena that occur over time, causing modifications of

both the aspect and the properties of materials. The actual effects of ageing and degradation

processes are characteristic to each material, as a function of its nature, composition and

structure. Similarly, the ageing resistance/sensitivity of a material is influenced by its nature,

structure and composition, making it to react in a specific manner to the ageing and

degradation factors in a given environment.

Fig. 0-2 A scheme of components of the conservation-restoration concept

1.3 Materials ageing

Ageing of materials may be defined as a slow, gradual and irreversible alteration of the

chemical or physical structure of a material with the passage of time. Usually, this alteration

Repairing defects

Completion of missing parts, Reintegration

RESTORATION

Storage / Handling

Adequate environmental

conditions

PREVENTIVE CONSERVATION

(INDIRECT)

Treatments

Stopping degradation

StabilisationConsolidation

REMEDIAL CONSERVATION

(DIRECT)

Scientific investigation

research

Scientific investigation

research

Scientific investigation

research

11

has a detrimental effect on the material properties, leading to a gradual loss of the designed

function and ultimate failure or unacceptable loss of efficiency (adapted after David 2009).

Though the ageing of materials or products implies changes to their original state, it does

not necessarily just mean deterioration or degradation. Ageing can also cause, in some cases,

formation of new substances and stabilisation, which might be a desirable effect (Smidt et al

2012).

Ageing is always related to chemical changes in the materials composition and structure,

which are actually reflected in changes of the physical and mechanical properties. The main

ageing factors (agents) are: light /UV radiation, temperature and oxygen, which cause specific

chemical degradation processes, such as: photolysis (photo-degradation), thermolysis

(thermo-degradation) and oxidation. Oxidation is always part of aerobic ageing processes and

this can be promoted by light or temperature in photo-oxidation and thermo-oxidation

reactions. This means that in real ageing processes more factors may be involved and the

chemical changes and related mechanisms are complex.

Furthermore, chemical processes determining ageing may be influenced by other

environmental factors, such as the relative atmospheric humidity, promotting hydrolytic

reactions. In outdoors conditions, where direct contact with water is possible, washing out of

the initial products resulted by photolysis or thermolysis may occur. Moreover, fluctuations of

relative humidity and/or temperature can cause other physical changes /structural changes in

the material (e.g. fissures, cracks), which may accelerate ageing and degradation.

Consequently, the presented ageing factors can not only affect the chemical composition of a

material, including wood, but also its physical and mechanical properties (Kránitz et. al. 2016).

An original principle scheme defining ageing and the main ageing factors and influencing

factors is proposed in Fig. 0-3. This is further developed in Fig. 0-4, where the principle

mechanisms involved, including the cause- effect link between chemical changes and

properties changes are reflected.

12

Fig. 0-3 A schematic theoretical representation of the chemical and physical effects of different individual ageing

factors and influencing factors on a material (original)

Fig. 0-4 A schematic theoretical representation of the complex phenomenon of materials ageing under the combined

action of different ageing factors and influencing factors, highlighting the cause-effect relation between the structural

chemistry changes and physical properties changes (original)

Ageing and degradation can not be easily differentiated when the same causing factors

are involved and the effects caused are considered detrimental (unwanted decrease of

properties). Actually, there is a limit of acceptance of a certain property affected, called

minimum acceptance level or threshold, after which the change of materials properties are

considered degradation and this will lead to end of service life (see Fig. 0-4). This means that

the resistance to ageing of a material is related to both its chemical stability and physical

durability under stress in use (Feller 1994, Smidt et al 2012, David 2009)

Thus on a time evolving axis, light /UV radiation, oxygen and temperature, assisted by

relative humidity / frequently variable in an environment with also variation of temperature,

will cause progressive ageing, which may evolve to degradation until an end of service limit.

In other words, ageing might be considered as an evolving, continuous phase, associated to

still acceptable changes of properties, before degradation.

13

With this respect it is important to highlight that actually the preventive conservation of

artefacts, based on the control of the environmental factors (temperature and its variation,

relative humidity and admissible variations, quality and level of illumination, quality of air),

which should be monitored and maintained in imposed limits, must be the principal concern

of conservators and restorers in order to delay accelerated ageing and to prevent degradation.

It is also important to understand that the conservation conditions should be adapted to

the type of artefacts and that scientific research is needed to better understand ageing and

degradation mechanisms in order to help setting up appropriate conservation norms.

Variability of ageing properties within a group of materials (e.g. among different wood

species, among different types of finishing materials) and their presence together, combined

in the same artefact, complicates the problem and make useful more research.

Considering the preventive conservation norms, a differentiation of ageing and

degradation might be considered: ageing is a normally occurring phenomenon, happening

slowly in time, even when respecting the conservation norms, whilst degradation is an

ultimate change of the materials properties occurring (accelerated) when the conservation

norms are not respected. In this context it is worth mentioning that more degradation factors,

such as the biological ones, will become active in high humidity conditions, exceeding the

conservation norms. Thus the term of degradation cumulates not only advanced properties

changes, exceeding an acceptable limit, caused by the ageing factors, but also other specific

degradation phenomena, caused by other factors than light, oxygen and temperature.

Ageing of wood species and traditional finishing materials, with relevance to cultural

heritage conservation is an important and actual research topic. The statistics data included

in Tab. 0-1-Tab. 0-3, referring to the number of research articles studied for this thesis is

relevant.

Tab. 0-1 Statistics on literature on ageing (based on 100 references in the thesis)

Ageing materials

Publication years

Wood ageing Traditional finishing materials ageing

Number Percent % Number Percent %

2011-2017 31 46 19 58

2001-2010 29 43 12 36

1991-1999 5 8 2 4

Before 1990 2 3 0 0

Total 67 100 33 100

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They also highlight the actual focus on this research subject and which areas/ aspects need

more investigations.

Tab. 0-2 Statistics on literature on wood ageing (based on67 references in the thesis)

Ageing factors

Monitoring methods

TC

TC RH (%) Light induced

UV/VIS

Colour changes 6 1 1 19

FTIR investigation 5 14

Mechanical properties 5 1 2 5

Chemical modification 8 1 4

Tab. 0-3 Statistics on literature on traditional finishing materials ageing (based on 33 references in the thesis)

Ageing factors

Finishing materials

TC

TC RH (%) Light induced

UV/VIS

Others

Waxes 3 - - 2 3(identification)

oils 4 1 8 1(identification)

Resin 3 - 1 8 2(identification)

Others 1 3 -

1.4 Overall conclusions of literature survey

The literature survey focused on the fundamental aspects of the thesis and based on it the

followings were possible:

Selection of relevant wood species and finishing materials for the experimental

research;

Development and application of relevant ageing tests associated with adequate

investigation methods

Synthesis of literature data on the specific ageing mechanisms of wood and

traditional finishing materials in relation to the ageing factor, as necessary input

basis for understanding and interpretation of experimental data in this research.

2. Research objectives and phases

2.1. Aim of the thesis

This thesis aimed at studying the ageing phenomena of some relevant wood species and

traditional finishing materials for historic furniture, by conducting appropriate accelerated

artificial ageing tests, which should simulate as relevant as possible the natural ageing

phenomena occurring over long periods of time, in order to get useful data for scientific

conservation. The whole concept is based on a comparative approach selecting 3 wood

species and 3 finishing materials characteristic for Europe and another set of 3 wood species

and 3 finishing materials characteristic for China.

15

2.2. Outline of Objectives

In addressing this general aim several objectives were formulated and followed during

the whole PhD programme, as detailed in Tab. 0-1.

Tab. 0-1 The objectives of this PhD thesis

O1 Comparative study on the traditional European and Chinese furniture:

O2 Understanding the complexity of materials ageing phenomena and

summary of current knowledge / research data in the field of thesis

O3 Selection and characterisation of relevant wood species and traditional

finishing materials from Europe and China

O4 Finishing of wood samples (traditional materials and application techniques)

O5 Developing and conducting appropriate accelerated artificial ageing tests

on the selected wooden species, finishing materials and finished surfaces

O6 Monitoring the effects of ageing phenomena by adequate investigation

methods

O7 Corroboration of experimental data and theory to reveal specific aspects of

the ageing for the investigated materials.

O8 Evaluation of the influence of artificial ageing conditions /factors and

materials characteristics /properties: comparative behaviour of wood

species and finishing materials.

O9 Building data-bases and formulation of research based conclusions with

practical application in the field of scientific conservation of furniture/wood

Legend Theoretic – literature survey

with original contributions

Experimental research

3. Research methodology

3.1. Methodological concept

The methodological concept is integrated in the general concept of the thesis

organisation (blue square area in Fig. 0-1). This included selection of relevant materials and

adequate investigation methods for their characterisation, before and after several ageing tests

in order to reveal and monitor the ageing effects. The methodological concept was originally

developed based on a thorough literature survey and specific research needs in order to meet

the thesis objectives.

16

Fig. 0-1 The methodological concept in the general scheme of the thesis

3.2. Materials

Based on the literature survey and also considering availability of wood species and the

concept of a comparative approach, the six selected wood species alongside their research

codes in this thesis are presented in Tab. 0-1. The dried material of all six wood species was

17

mechanically processed into test samples following the steps presented in

Fig. 0-2:

Tab. 0-1 The six wood species selected for the experimental research

Europe Romanian

language China

Chinese language

Pin yin

European ash (F)

Fraxinus excelsior Frasin

Chinese ash (S)

Fraxinus mandshurica

Shui Qu Liu

(水曲柳)

European walnut (N)

Juglans regia Nuc

Chinese walnut (H)

Juglans mandshurica Maxim

He Tao Qiu

（核桃楸）

Sycamore maple (P)

Acer pseudoplatanus Paltin

Paulownia (T)

Paulownia elongata

Tong Mu

（桐木）

Fig. 0-2 The mechanical processing of test wooden samples

Six types of natural, traditional finishing materials were selected, namely: Beeswax,

Chinese wax, Linseed oil, Tung oil, Shellac and Chinese lacquer. They represent three classes

of traditional finishing materials: waxes, oils and natural resins /lacquers, each with one

material as representative for Europe and a second one as representative for China. These

finishing materials with their research codes are presented in Tab. 0-2

Tab. 0-2 Finishing materials selected for research

18

Class of finishing

materials

Materials selected for research

EUROPE CHINA

Product’s name Code Product’s name Code Chinese symbol

Waxes Beeswax BW Chinese wax CW 虫蜡

Drying oils Linseed oil LO Tung oil TO 桐油

Resins/lacquers Shellac SL Chinese lacquer CL 生漆

The Chinese symbols of these materials are related to their origin as resulting from the

explanatory pictures in Fig. 0-3 Explanation of the Chinese symbols for the three traditional

Chinese finishing materials Fig. 0-3.

Chinese wax Tung oil Chinese lacquer

Fig. 0-3 Explanation of the Chinese symbols for the three traditional Chinese finishing materials

3.4 Ageing procedures

Four accelerated artificial ageing tests (A1-A4) under the action of the main ageing

factors, namely UV radiation, temperature and extreme variation of atmospheric humidity

(RH) or temperature were conceived and run in the Feutron FKS programmable climatic

chamber.

Tab. 0-3 Codes and conditions of the experimental ageing tests

Code Ageing agent Conditions Time/cycle

(step)

No. of

cycle (step) Diagram

Total

time

A1 Temperature 100oC /RH 55% 72h 4 D-A1 288h

A2 Cold check

(variation of T)

T: 50oC:1h;

-15 oC/1h

20 oC:1h

RH 55%

3h 10 D-A2 30h

A3

Relative

humidity

(variation of RH)

T=20oC,

RH: 25% for24h

85% for24h

48h 5 D-A3 240h

A4 Radiation UV UVA –UVB

T=40 oC, RH 55%

4*6 h UV

3 D-A4

72h

(UV)

A5 Natural ageing

(simulation)

Indoor conditions,

exposure glass

filtered sunlight

1 month 6 (12) -

6 (12)

months

19

These tests are based on previous research experience in this field (Tuduce 2012) and

literature data (Borrega and Karenlampi 2008; Chen et al 2012; Chen et al 2014; David 2009;

Matsuo et al 2009; Matsuo et al 2010; Matsuo et al 2011 and Tolvaj and Faix 1995).

Supplementary, a simulated natural ageing test in indoors conditions, under the action of

sunlight filtered by glass window (A5) was set and run for 6 months (this test is being

continued). The five types of tests, coded as A1, A2, A3, A4, A5 and their characteristic

conditions are presented inTab. 0-3.

3.5. Investigation methods and equipments

In Tab. 0-4 are summarised the investigation methods employed in this research for

characterisation of wood materials, finishing materials and finished surfaces as well as for

revealing and monitoring the ageing phenomena.

Tab. 0-4 Investigation methods of wood materials, finishing materials and finished surfaces

Methods Wood material Finishing materials Finished surfaces

Macroscopic

- direct visual assessment

- photo/scan

Yes Yes Yes

Optical microscopy TLM / RLM TLM/(BF/DF) RLM

Color measurements

CIE-Lab system

Yes No Yes

FTIR investigation Yes Yes Yes

Note: TLM: transmitted light microscopy

RLM: reflected light microscopy

BF: bright field; DF: dark field

3.6. Analyses methods for finishing materials and the quality of finished

surfaces

Alongside of above investigation methods, some analyses methods for finishing

materials and the quality of finished surfaces were also employed.

Solids content: can evaluate the material quality and also is necessary to calculate

the thickness of each finishing layer if the application rate Csp [g/m2] is known.

Roughness measurements were performed in order to evaluate and compare the

quality of the surfaces before (prepared by sanding) and after finishing in the six

variants (European ash and sycamore maple).

Cross-cut test is a method for assessing the resistance of films of finishing materials

to separation from substrates (international standard ISO 2409:2007). The finished

surfaces selected for this test were the six wood species finished in six variants with

the six different finishing materials considered in this research.

The resistance of finished surfaces to a selection of cold liquids was assessed in this

research based on the method described in EN 12720 (Furniture-Assessment of

surface resistance to cold liquids). All the variants of finished wood surfaces (6

wood species 6 types of finishing materials) investigated in this research were

20

tested with respect of their resistance to cold liquids before and after accelerated

ageing tests (A1-A4).

4. Characterisation of the selected wood species and traditional finishing

materials

4.1. Wood species

From the images produced at various magnifications, the largest magnification 200X

contains more detailed information, but the area included has less anatomical details, while

the smallest magnification 40x gives more information about the pores distribution within and

between annual rings, as well as distribution of parenchyma. Therefore, examination at all

these magnifications is useful. The microscopic images of transversal section 100X for the

investigated species: European ash (Fraxinus excelsior), Chinese ash (Fraxinus mandshurica),

European walnut (Juglans regia), Chinese walnut (Juglans mandshurica Maxim), and

sycamore maple (Acer pseudoplatanus), Paulownia (Paulownia elongata) are presented in the

figures Fig. 0-1 (4.3-4-8 in the thesis, on the three sections).

a b c

d e f

Fig. 0-1 Micrographs of six wood species on transversal section 100x: a. European ash (Fraxinus excelsior), b. Chinese

ash (Fraxinus mandshurica), c. European walnut (Juglans regia), d. Chinese walnut (Juglans mandshurica Maxim), e.

sycamore maple (Acer pseudoplatanus), f. Paulownia (Paulownia elongata)

The six recorded wood species all belong to hardwoods, their FTIR spectra being very

similar. All FTIR spectra present strong absorption bands of O-H groups at around 3320-3340

cm-1

, of C-H asymmetric, symmetric stretching at around 2910-2930 cm-1

and of C–O

stretching in cellulose and hemicelluloses at around 1030 cm-1

(frequently employed as

internal reference for spectra normalisation), as main common features of wood samples.

Furthermore, a series of peaks are present in the fingerprint region 1800-600cm-1

(Fig. 0-2).

They can be assigned to the different structural parts of the principal chemical components of

wood (cellulose, hemicelluloses and lignin).

21

Fig. 0-2 The FTIR –ATR spectra of the six wood species in fingerprint range (1800-600cm-1)

In the interpretation of wood FTIR spectra are of great importance the peaks assignable

to the main chemical components as follows:

lignin at around 1506 cm-1

, 1600 cm-1

, 1453-1456 cm-1

, 1324 and 1240 cm-1

;

hollocellulose (cellulose and hemicelluloses) at around 1370 cm-1

and 1155 cm-1

;

hemicelluloses at around 1730 cm-1

(unconjugated carbonyl groups present mostly

as acetyl groups in hemicelluloses, mostly pentosanes as xylan)

cellulose at around 900 cm-1

.

Moreover, several other peaks have a contribution of more components. The peak at

around 1640 cm-1

assigned to conjugated or aromatic ketones can be associated to some

structural features in the structure of lignin, such as carbonyl or unsaturated C=C groups in

the position of the propane side chain of lignin (Chen et al. 2014). These can be reactive

sites in ageing processes. This is absent in the FTIR spectra of two kinds of walnuts, while

the absorption (shoulder) at 1150 cm-1

is absent for Chinese ash.

4.2. Finishing materials

The macroscopic aspect of the finishing products employed in this research can be

observed in the pictures in Fig. 0-3. Beeswax is a homogenous solid with light yellow colour.

Chinese wax is white solid, with a specific structure, including opaque amorphous material

and crystalline, translucent, high reflective, granules. Linseed oil and Tung oil are

homogenous liquids with light yellow colour and characteristic smell, the colour of linseed oil

is a little darker than of Tung oil. Shellac flakes are high-glossy, translucent with

orange-brownish colour. Chinese lacquer is a viscous whitish (milky to cream colour) liquid

with sour smell, which colour turns quickly to light brown when exposed to open air.

K:\FTIR A0\F-M-A1-0 blc-sm-MEDIU-N.0 Frasin Instrument type and / or accessory K:\FTIR A0\H-M-A1-0 blc-sm-MEDIU-N.0 Nuc Ch A1-0 Instrument type and / or accessory K:\FTIR A0\N-M-A1-0 blc-sm-MEDIU-N.0 Nuc Instrument type and / or accessory K:\FTIR A0\P-M-A1-0 blc-sm-MEDIU-N.0 Paltin Instrument type and / or accessory K:\FTIR A0\S-M-A1-0 blc-sm-mediu-N.0 Frasin China Instrument type and / or accessory K:\FTIR A0\T-M-A1-0 blc-sm-mediu-N.0 Pawlonia Instrument type and / or accessory

2014/5/12 星期一2014/6/17 星期二2014/5/12 星期一2014/5/12 星期一2014/5/12 星期一2014/5/8 星期四

Paulownia1730.0

9

1643.0

4

1595.2

9

1504.7

3

1455.6

1

1420.8

2

1368.9

8

1324.0

6

1235.4

2

1027.0

0

899.7

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Fig. 0-3 The macroscopic aspect of the six finishing materials: a. beeswax, b. Chinese wax, c. linseed oil, d. Tung

oil , e. shellac flakes, f. Chinese lacquer.

The microscopic investigation of the films casted on microscopic glass lamellas revealed

characteristic differentiating features, as can be observed in Fig. 0-4

The microscopic structure of beeswax film is quite regular with a microcrystalline

aspect. Chinese wax forms a high reflective film with a specific less regular pattern with

wavy areas alternating with geometrical/rhomboidal areas of 100 - 200 µm. The

microstructure of the drying oils films was observable only under dark field examination. For

linseed oil microscopic structure is quite regular with fine granular aspect. A characteristic

different microstructure, with wavy aspect, could be distinguished also for the Tung oil film.

Also formation of some cracks in the film is visible in this case. For the shellac film is

observed a very smooth and compact microscopic structure, with some small craters with

diameters of about 20µm. For the Chinese lacquer film was observed a quite regular

microscopic structure with a certain roughness, in which can be observed dark brown insular

areas (mostly of around 20 - 40 µm) on a more uniform whitish film with micro-crystalline

structure. This is in accordance to studies of Ta-kayuki Honda et al. (2008) and explainable by

the complex composition of this material.

a b c

d e f

Fig. 0-4 The microscopic aspect of films of selected finishing materials at 40X magnification: a. beeswax, b.

linseed oil, c. shellac, d. Chinese wax, e. Tung oil, f. Chinese lacquer

Infrared spectroscopy, especially the modern FTIR – ATR technique, represent a

well-recognised investigation tool for characterising the chemical structure of organic

compounds. In Fig. 0-5, Fig. 0-5 and Fig. 0-7 are presented the FTIR–ATR spectra

a b

c d

e f

40 X 80 X

a b

c d

40 X 80 X

a b

c d

a b

c d

40 X 80 X

a b

c d

40 X 80 X

a b

c d

a b

c d

23

experimentally recorded for the six investigated finishing materials (Fig. 0-5 for waxes, Fig.

0-5 for oils and Fig. 0-7 for natural resin).

Fig. 0-5 Comparative FTIR spectra of investigated waxes

(whole range 4000-400 cm-1)

Fig. 0-6 Comparative FTIR spectra of investigated oils:

whole range 4000-400 cm-1

Fig. 0-7 Comparative FTIR spectra of investigated natural

resins: whole range 4000-400 cm-1

Fig. 0-8 Comparative FTIR spectra of investigated oils

samples: a)Tung oil hardened film, b) Tung oil liquid

form, c) linseed oil hardened film, d) linseed oil liquid

form (from Liu et al 2015c)

As shown in these figures, the FTIR spectra of these six finishing materials have some

common peaks associated to common chemical features/moieties related to their chemical

composition/structure, such as: 3300-3400 cm-1

due to the hydroxyl –OH groups;

2910-2930 and 2850 cm-1

as a double peak attributed to C-H stretching of aliphatic methylene

–CH2- and methyl -CH3 , 1710-1730 cm-1

due to unconjugated carbonyls C=O stretching in

carboxylic acids –COOH, esters –COOR or free aldehydes/ketones groups overlapping with

C=O stretching of unsaturated esters. This band is located to lower wavenumbers for

free acids and at higher wavenumbers for esters. The absorption bands at 1244 and 1153 cm-1

represent the O-H bending and C-O stretching of aliphatic carboxyl and hydroxyl groups

overlapping with C-O stretch of –COOR. The C-O stretching of ether/acetal linkages

appears at 1030- 1070 cm-1

. The absorption bands at 936 and around 730 cm-1

may be

attributed to the rocking vibration of the C-CH3 superimposed with C-H wag of the

disubstituted trans olefins (Derry 2012, Farag and Leopold 2009, Stenberg 2004, Izzo 2010,

Schonemann 2011, Guillen and Cabo 2003, Pereda 2010, Huang 2013, Baeten et al 2010,

Beltran et al 2015, Kim and Eom 2015, Ma et al 2015, Duce et al 2015 and Regert et al 2015).

FTIR spectra not only reflect characteristic chemical features of the finishing materials

allowing in-between comparisons, but also highlight the chemical structure changes

E:\Pt Andrei de la Miha\FTIR PT TEZA\A1-nomolizat\Lamele-N\L-BW-A0 -blc-sm-M-N.0 Lamele Bruker ATR

E:\Pt Andrei de la Miha\FTIR PT TEZA\A1-nomolizat\Lamele-N\L-CW A0 blc-sm-M-N.0 Lamele Bruker ATR

2015/6/10 星期三

2015/6/10 星期三

2912.6

2

2846.0

9

1730.2

81696.2

4

1463.3

01424.0

2

1299.4

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1215.0

91169.8

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Chinese wax

Beeswax

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2015/6/10 星期三

2015/6/10 星期三

3428.7

0

2923.7

6

2855.5

9

1719.7

5

1589.5

8

1455.9

9

1379.4

5

1254.2

8

1164.1

01118.3

31067.2

9

971.8

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741.1

8705.8

4

Tung oil

Linseed oil

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2015/6/10 星期三

2015/6/10 星期三

3362.7

7

2923.5

8

2854.2

0

1730.4

81696.5

71621.0

8

1454.5

0

1371.9

5

1267.3

01238.9

51153.3

9

1071.8

5

1005.3

9

935.5

7

721.6

4

Shellac

Chinese lacquer

500100015002000250030003500

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E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\LO -pe lamela MEDIE blc-sm-N.0 LO -pe lamela Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\LO picatura MEDIE blc-sm-N .0 Ulei de in Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\TO -pe lamela MEDIE blc-sm-N.1 TO -pe lamela Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\TO picatura MEDIE blc-sm-N.0 Ulei de tung Instrument type and / or accessory

2014/2/7 星期五

2014/9/3 星期三

2014/2/7 星期五

2014/9/3 星期三

3518.6

1

3008.7

8

2926.8

6

2855.7

0

1740.0

3

1589.4

7

1455.5

3

1378.6

9

1254.5

1

1163.6

01125.6

71069.0

91069.0

1978.7

2

780.5

3740.0

9705.6

6

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associated to the chemical processes of curing, as it is the case for the drying oils and the

Chinese lacquer.

Spectra in

Fig. 0-8 (from Liu et al 2015c) are a good illustration. The main absorption bands in the

two drying oils spectra are those for hydroxyl (~ 3400 cm-1

), methylene (2922 and 2853 cm-1

),

carbonyl (1740 cm-1

) and ν(C–O) in C–O–C in esters (~1255, 1171 and 1125 cm-1

). The main

differences between Tung oil and linseed oil are the well defined /increased absorptions at

around 1600 cm-1 and 740 cm-1 assigned to double bonds, more numerous in the case of

Tung oil with an increase amount of unsaturated fatty acids, as well as a higher absorptions at

1254 cm-1

and the better resolved peaks at 1170 and 1070 cm-1

, assigned to C-O-C bonds.

Comparing the spectra of the two oils in liquid form with those of cured films, some

changes are visible. For Tung oil is observed a slight increase of hydroxyl related band at

3400 cm-1

, following oxidative processes (Meiorin 2013). At the same time, the bands at 1456

cm-1

, attributed to bending vibration of the aliphatic methyl and methylene groups, and 978

cm-1

attributed to ω(CH) wagging in isolated trans CH=CH, are slightly increasing denoting

cis-trans isomerisation. The shoulder at about 2956 cm-1

of the methylene band 2925 cm-1

,

assigned to the C-H stretching in methyl groups disappeared after curing, suggesting

cross-linking. Similar alterations of the bands 3400 and 2925 cm-1

occurred for linseed oil.

The small peak at around 3009 cm-1 due to ν (C-H) in unconjugated cis C=C-H disappeared,

while bending vibration at 1456 cm-1

visibly decreased. A strong increase of carbonyl band at

1740 cm-1

and of the peak at 1254 cm-1

, assigned to C-O-C oxiranes, demonstrate the

importance of oxidative processes in the chemical mechanism of curing. Appearance of a

small absorption 975 cm−1

gives the indication of the formation of isolated trans-configured

double bonds (Schonemann et al 2011). These changes are in good accordance to the

chemistry of these oils and their curing mechanism, presented earlier in this chapter.

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\LO -pe lamela MEDIE blc-sm-N.0 LO -pe lamela Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\LO picatura MEDIE blc-sm-N .0 Ulei de in Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\TO -pe lamela MEDIE blc-sm-N.1 TO -pe lamela Instrument type and / or accessory

E:\Pt Andrei de la Miha\ANDREI PRELUCRAT\FINISHING MATERIALE\TO picatura MEDIE blc-sm-N.0 Ulei de tung Instrument type and / or accessory

2014/2/7 星期五

2014/9/3 星期三

2014/2/7 星期五

2014/9/3 星期三

3518.6

1

3008.7

8

2926.8

6

2855.7

0

1740.0

3

1589.4

7

1455.5

3

1378.6

9

1254.5

1

1163.6

01125.6

71069.0

91069.0

1978.7

2

780.5

3740.0

9705.6

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Fig. 0-9 FTIR –ATR spectra of Chinese lacquer: raw urushi (black), Kurome urushi (blue) and hardened film (red) – from

Liu et al 2014 b

The FTIR spectra in Fig. 0-9 reflect the chemical structural features of raw Chinese

lacquer and changes occurring during Kuromisation and curing. The main absorption bands in

the raw urushi lacquer spectra for hydroxyl (~ 3400 cm-1

and ~ 1360 cm-1

); methylene (2922

and 2853 cm-1

) and the aromatic ring (~1600, 1455 and 920 cm-1

) denote a phenolic structure

with a long aliphatic side chain. Presence of aliphatic double bands C=C is highlighted by

absorptions at 1623 and 720 cm-1

, while carbonyl groups determine the small absorption band

at ~ 1740 cm-1

. Absorptions bands at 1280, 732 and 698 cm-1

were assigned by Honda et al

(2008) to catechol in monomer form. Kuromisation and curing is associated to FTIR

detectable structural changes: decrease of hydroxyl related bands (3400 and 1360 cm-1

)

occurring in parallel with an increase of carbonyl band (1740 cm-1

), following laccase induced

oxidation process. Furthermore, C-C and C-O coupling reactions occur. The vibrations

characteristic to aromatic C-H stretching and bending (3010 and 920 cm-1

) and C= C (1623,

720 cm-1

) are slightly decreasing. A drastic decrease of the absorption band at 1270 cm-1

,

which nearly disappears for the cured film, suggests formation of a polymerised catechol

structure, in accordance to literature (Honda et al 2008).

As discussed above these six selected finishing materials are very important for

European and Chinese traditional furniture. Their properties and ageing study is important for

furniture conservation and restoration. In Tab. 0-1, there are summarised some comparative

aspects for European finishing materials and Chinese materials

E:\FTIR -ANDREI\Liu 05.11.2014 CL\MEDIU\CLsolid -nou -MEDIU-N.0 Lac chinezesc Bruker ATR

E:\FTIR -ANDREI\Liu 05.11.2014 CL\MEDIU\lichid CL -nou MEDIU-N.0 Lac chinezesc Bruker ATR

E:\FTIR -ANDREI\Liu 05.11.2014 CL\MEDIU\lichid CL 40 -nou MEDIU-N.0 Lac chinezesc Bruker ATR

11/5/2014

11/5/2014

11/5/2014

3362.5

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3007.9

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2922.3

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2853.4

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1741.7

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1622.9

91600.9

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1454.8

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1356.9

4

1270.9

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1156.0

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1076.3

41018.9

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771.9

5719.4

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Kurome lacquer liquid

Raw lacquer liquid

Raw lacquer solid

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Tab. 0-1 Comparative aspects of selected finishing materials

Materials comparative aspects

European materials Chinese materials

Waxes Beeswax – produced by honey bees of the

genus Apis

Mixture of long chain aliphatic

compounds, mainly esters

The main component of beeswax is the

ester of palmitic acid (C16) with the

triacontanyl alcohol (C30), corresponding

to the formula

CH3-(CH2)14-CO-O-(CH2)29-CH3

- Film forming –physical process.

Chinese wax – produced by insects

named Ceroplastes ceriferus

Mixture of long chain aliphatic

compounds, mainly esters

Chinese wax consists mainly in esters

of long chain alcohols and acids

(C26-C30) mostly C26, C28, which result

by hydrolysis in approximate equal

proportions

- Film forming –physical process

Drying oil Linseed oil – obtained by pressing from

the seeds of Linum usitatissimum, linseed

(also called flax)

Composition: triglycerides of

polyunsaturated fatty acids with

unconjugated double bonds;

Cis configuration of the double bonds

- Film forming – chemical oxidative

process

Tung oil – obtained by pressing from

the nut of the Tung tree (Vernicia

fordii)

Composition: triglycerides of

polyunsaturated fatty acids whose

double bonds are partly or fully

conjugated;

Trans configuration of the most of the

double bonds

- Film forming – chemical oxidative

process

Natural

resin/lacquer

Shellac – secretion of insects Kerria lacca;

- collected in solid state; further

processing

Composition: (poly) ester of an aliphatic

hydroxy-acid with a sesquiterpene acid

- The main aliphatic polyhydroxy acid:

9, 10, 16-trihydroxy palmitic acid (aleuritic

acid):

HOCH2-(CH2)5-CH(OH)-CH(OH)-(CH2)7-COO

H.

- A major acid with sesquiterpene

structure is jalaric acid, a monocarboxylic

acid with one aldehyde and two hydroxy

groups, along with a smaller proportions

of other related sesquiterpene

hydroxy-acids

- Film forming –physical process

Chinese lacquer – sap of Rhus

verniciflua;

- collected as liquid - raw urushi is a

viscous, milky white colloidal system of

water in oil emulsion; further

processing

Composition:

- the oil phase is of aromatic nature,

represented by a mixture of

pyrocatechols (orto-di-phenols)

substituted in position 3 with long

saturated and unsaturated (dienes,

trienes) aliphatic chains (C15, C17),

known under the name of urushiol

(60-65%)

Film forming –bio - chemical oxidative

process – enzymes involved

27

5. Finishing technologies and characterisation of the finished wood surfaces

5.2. Macroscopic aspects

A general view of the finished samples in comparison with the unfinished controls, for

six wood species and six types of finishing materials is presented in the Image Card (IC_I-1).

For exemplification, Fig. 0-1 presents comparatively the macroscopic aspect for unfinished

and finished samples (6 finishes) of Paulownia.

Fig. 0-1 Macroscopic aspects of Paulownia: a. unfinished, b. beeswax finished, c.linseed oil finished, d. shellac finished,

e. Chinese wax finished, f. Tung oil finished, g. Chinese lacquer finished

Finishing generally enhances the beauty of wood and highlights the wood texture.

Furthermore, a saturation of natural colour or a colour change, depending on the colour and

refractive index of the finishing material is obtained. For the finishes in discussion it could be

easily observed that waxes tend to keep the natural aspect of wood almost unchanged, only a

barely visible yellowing trend being observed for beeswax. Oils finishes nicely highlight

wood structure and a more vivid, slightly darker and shifted to yellow colour appears, more

obvious for linseed oil compared to Tung oil. Shellac is a reddish-brown resin, so that shellac

finishing brings about a correspondent colour change (darkening and shift to red and yellow).

Moreover, shellac polishing is a filled-grain finish giving a nice glossy surface. The samples

finished with Chinese lacquer undergo the most obvious colour change to a nice dark red

colour and glossy surfaces (filled grain finish).

The colour evolution of the other wood species is presented in the Image card IC-I-1.

Obviously that the effect of finishing is more or less evident as colour change, depending on

the initial natural colour of the respective wood species. The nice contrasts between the

studied species is differently influenced by the finishes applied, enriched or diminished, while

saturated colours could be obtained by finishing with some products, especially oils. Looking

at this image card it could be better understood the esthetical impact of finished surfaces and

the importance of studying and understanding their in time alteration due to ageing.

5.4. Roughness

Roughness measurements were performed only on two series of samples made of wood

species with different anatomical structure (see chapter 4.1 in thesis): European ash and

a

b

T+BW

c

T+LO

d

T+SL

e

T+CW

f

T+TO

g

T+CL

28

sycamore maple. European ash is a typical ring porous hardwood with large pores up to

400-500 m in earlywood and about 100 m in latewood, while sycamore maple is a diffuse

porous hardwood with small vessels up to about 80-100 m in the earlywood.

Fig. 0-2 Comparative roughness profiles for unfinished and finished samples of sycamore maple

Each of these series contained wood samples before finishing and wood samples after

finishing in the six variants. All the test pieces were initially similarly prepared for finishing

by successive sandings.

The results obtained consisted in roughness profiles (as illustrated in Fig. 0-2 for

sycamore maple in all the six finishing variants.

6. Ageing behaviour of the studied of wood species

6.1. Temperature induced aging (A1)

A general view and first impression on the effect temperature induced ageing on the

aspect of the wood samples of the two ash wood species result from the pictures in Fig. 0-1.

The pictures Fig. 0-1 in show that two ash wood species samples suffered following

thermal induced ageing a colour change, generally visually perceived as darkening. However,

this colour change was different among the wood species. With the increase of ageing time,

this colour change increased.

To illustrate the effects of temperature induced ageing on the surface chemistry features

of the wood samples, comparative spectra of the six wood species before and after different

periods of temperature induced ageing are presented in the Annexes A-Fig.2-7. Due to

Type of samples Registered roughness profiles

Unfinished

Beeswax finished

Chinese wax finished

Linseed oil finished

Tung oil finished

Shellac finished

Chinese lacquer

finished

29

obvious similarities of these spectra for the six wood species,

Fig. 0-2 presents for exemplification the comparative FTIR spectra of European walnut

before ageing and after different periods of temperature induced ageing.

A0 A1-72h-T A1-144h-T A1-216h-T A1-288h-T

European

ash

Chinese

ash

Fig. 0-1 In-time evolution of aspect and colour of unfinished ash wood samples during temperature induced ageing

As reflected by the comparative spectra of control and aged samples of European walnut

(

Fig. 0-2), temperature ageing induced only slight chemical changes in the structure of

wood components. An obvious change is reduction of free hydroxyl groups (3400 cm-1

band)

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and more very small changes, better observed in the enlarged fingerprint region (1700-600

cm-1

). These were: a trend of initial decrease and then re-increase of the unconjugated

carbonyl band (1730 cm-1

), a slight decrease of the aromatic skeletal vibration of lignin at

1600 cm-1

and an apparent increase of lignin absorption band at 1506 cm-1

, while the

absorption at 1370 cm-1

, assigned to cellulose and hemicelluloses, seems to be little affected.

Fig. 0-2 Comparative FTIR spectra of European walnut before ageing and after different time temperature induced

ageing in the fingerprint region 1800–600 cm-1

These minute changes result better from the calculated ratios of some relevant absorption

bands, as illustrated in Fig. 0-3 for the ratios A1730/A1370 and A1506/A1370, comparatively

for all the six wood species.

a

b

Fig. 0-3 Two selected FTIR relative ratios for six wood species following temperature ageing for different periods of time:

a. A1730/A1370, b. A1505/A1370

A first obvious result is the different behaviour of the studied species suggesting their

different resistance to temperature ageing, also denoted by the different measured and

observed colour changes. Interpretation of these changes has been not a simple task, as

different temperature induced chemical processes occur, as detailed in a previous publication

resulted from the research for this thesis (Liu et al 2016 b). Temperature induces first

degradation of hemicelluloses starting with deacetylation and further hydrolytic

depolymerisation (decrease of the 1730 cm-1

band for unconjugated carbonyl groups

contained mostly as acetyl groups in hemicelluloses- xylan). Further in the process, thermo-

oxidative reactions occur (increase of the 1730 cm-1

band due to the formation of new

carbonyl and carboxyl groups) as well as some re-organisation and condensation of lignin

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structure, which in combination with the decrease of hemicelluloses content result in an

apparent increase of lignin. Due to these complicated and concurrent reactions the ratios

A1730/A1370 and A1506/A1370 in Fig. 0-3 had a wavy evolution during temperature

ageing. Moreover, the differences in the chemical composition of the studied species (for

instance content of hemicelluloses, ratio hemicelluloses/lignin, and presence of extractives of

various types) determined their different shapes for the species under study. It is worth noting

that for Paulownia it was registered the highest increase of the ratio A1730/A1370 and the

highest colour change. The role of thermo-oxidative processes in the formation of carbonyl

containing chromophores has to be considered as a possible explanation.

6.4. UV induced ageing

The evolution of the general aspect of the two ash wood species during the ageing test

under the action of UV radiation (A4) is obvious from the pictures in Fig. 0-4. It is generally

acknowledged that UV radiation represents a powerful ageing factor inducing significant

colour and chemical changes on the surface and subsurface of wood. Thus these results were

expected.

The FTIR spectra highlighted changes in the chemical structure of wood following UV

exposure, visible especially in the fingerprint region (1800-600 cm-1

), as illustrated for

European walnut in

Fig. 0-5. The other spectra on the whole range 4000-600 cm-1

are presented in the

Annexes of chapter 6 (A-Fig 16 to A-Fig. 21).

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A0 A4-24h-UV A4-48h-UV A4-72h-UV A4-72h-40C

European

ash

Chinese

ash

Fig. 0-4 In time evolution of aspect and colour of unfinished ash wood samples during UV induced ageing (A4)

The most obvious changes, in good accordance with literature (e.g. Chang et al 2002,

Pandey 2005, Pandey and Vuorinen 2008, Persze and Tolvaj 2012, Timar et al 2016) are the

decrease of the lignin associated bands at around 1506 cm-1

, 1600 cm-1

, 1455 cm-1

, occurring

in parallel with the increase of the bands for unconjugated carbonyl groups (at around

1725-1730 cm-1

) and conjugated and aromatic carbonyls (at around 1640 cm-1

). Carbonyls are

present in the structure of chromophores (colour generating) groups resulted by oxidation.

These UV induced chemical changes occur especially in the structure of lignin (the main UV

absorber among the principal chemical wood components), while cellulose and

hemicelluloses remain less affected, as suggested by the unaffected absorption band at around

1370 cm-1

.

Fig. 0-5 Comparative FTIR spectra of European walnut before ageing and after different times of UV induced ageing in

the fingerprint region 1800–600 cm-1

The evolution of the relative values of the characteristic FTIR ratios A1730/A1370

(unconjugated carbonyls/ hollo-cellulose) and A1506/A1370 (lignin/ hollo cellulose) during

the A4 ageing process, comparatively for the six wood species, is revealed in Fig. 0-6.

These diagrams illustrate the differences in the behaviour of the six wood species under

study, more obvious in the case of the FTIR ratio A1730/A1370 than A1506/A1370. For

instance, in the case of European and Chinese walnuts there are notable differences in the

ratio A1730/A1370, higher for the European walnut suggesting more advanced oxidative

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reactions leading to carbonyl containing chromophores, whilst the decrease of lignin was

almost similar for the two wood species. Consistent with this observation, colour changes for

European walnut were slightly higher than those measured for Chinese walnut.

a b

Fig. 0-6 Evolution of two selected FTIR relative ratios for the six wood species following UV induced ageing for different

periods of time: a. A1730/A1370, b. A1505/A1370

The measured colour data, as differences compared the control values for unaged

samples, were plotted against the calculated FTIR ratios A1730/A1370 and A1506/A1370 in

Fig. 0-7 and Fig. 0-8. Though these plots in Fig. 0-7 and Fig. 0-8 show a quite high dispersion,

some trends can be identified. Thus, the increase of global colour difference E and

yellowness b* in Fig. 0-7 occur in parallel with the increase of the relative FTIR ratio

A1730/A1370 (non-conjugated carbonyl groups/ hollo cellulose), suggesting a direct

correlation. On the other hand, colour difference E and yellowness b* in Fig. 0-8 increase

as the relative FTIR ratio A1506/A1370 decrease from the reference value 1 to nearly 0,

indicating advanced degradation of lignin and a direct correlation of colour change with lignin

degradation (see Tab. 0-1).

Fig. 0-7 UV induced ageing of wood (A4): Correlations between colour changes (expressed by the variation of colour

coordinates, b*, and global colour difference E) and modifications in wood surface chemistry, expressed by variation

of the relative FTIR ratio A1730/A1370

34

Fig. 0-8 UV induced ageing of wood (A4): Correlations between colour changes (expressed by the variation of colour

coordinates b* and global colour difference E) and modifications in wood surface chemistry, (expressed by variation

of the relative FTIR ratios A1505/A1370

These findings make sense as UV radiation first produces degradation of lignin and

carbonyl containing chromophores result by further oxidative processes (Fengel and Wegener

1984, Pandey 1995, Chang et al 2002, Pandey 2005, Pandey and Vuorinen 2008, Colom et al

2013, Tolvaj et al 2013, Tolvaj et al 2014a and Tolvaj et al 2014b, Timar et al 2016). It is also

known that photdegradation of lignin by UV radiation results mostly in yellowing as

chromatic change (e.g. Fengel and Wegener 1984, Muller et al 2003).

However, correlating colour and FTIR data is not straightforward, as an important

contribution of the extractives in the original colour and colour changes are not revealed by

FTIR. Furthermore, the global colour difference E results from the changes in chromaticity

(redness, yellowness coordinates) and lightness, for which the wood components may be

differently involved (Tolvaj et al 2013). With this respect controversy exists in the literature.

Mathematical linear correlations between E and the FTIR ratios A1505/A1370 and

A1730/A1370 were proposed by Pandey (2005), Rosu et al (2010), whilst non-linear

regressions considering all the colour parameters were proposed by Agresti et al (2013).

Tab. 0-1 Mathematical correlation between colour changes and the relative FTIR ratios A1506/A1370 and A1730/A1370

during UV induced ageing for two ash wood species

This research originally presents this analysis considering separately all the colour

parameters and highlights the particular influence of chemical changes on the different colour

parameters in relation to the mechanism involved. Moreover, the different behaviour of the

different wood species is demonstrated by our experimental data. The poorest correlation

confidence (R20.66) obtained for European ash may indicate a higher contribution of the

extractives to the measured colour changes.

Type of correlation Wood species Linear trend line Equation R-squared value

b* with

A1505/A1370

European ash y = -5.8509x + 6.8085 R² = 0.666

Chinese ash y = -8.2691x + 8.9014 R² = 0.9116

E with

A1505/A1370

European ash y = -6.9677x + 8.3281 R² = 0.6992

Chinese ash y = -8.4064x + 9.0189 R² = 0.914

E with

A1730/A1370

European ash y = 7.8294x - 7.6198 R² = 0.8959

Chinese ash y = 7.268x - 6.3767 R² = 0.789

35

6.5. Natural ageing in indoors conditions

The evolution of the general aspect of the ash wood species during the simulated indoors

natural ageing test (A5), during 6 months of exposure, results from Fig. 0-9.

A0 A5-1m-N A5-2m-N A5-3m-N A5-4m-N A5-5m-N A5-6m-N

European

ash

Chinese

ash

Fig. 0-9 In time evolution of aspect and colour of unfinished ash wood samples during indoor simulated natural ageing

Some colour changes as intensity and shade modifications could be observed, but clearly

less advanced than in the case of the accelerated UV ageing test (A4). Of course that both the

type of light (percentage of UV radiation) and its intensity in the two tests explain this

expectable results. Actually, running in parallel accelerated (A4) and natural ageing (A5) tests

and comparing the corresponding results is essential in estimating an acceleration factor and

judging the relevance of the accelerated test A4 in understanding the behaviour of materials

under normal exposure to light in indoors conditions. Such an approach and results were

previously published by the author for paulownia (Paulownia elongata) wood (Liu et al 2016

b).

The chemical changes induced by exposure to natural light in indoors conditions result

from the graphs in Fig. 0-10.

Similarly to the previous test, the most important chemical changes were degradation of

lignin (1506 cm-1

and 1600 cm-1

band) and oxidative processes leading to an increase of the

band at 1730 cm-1

assigned to unconjugated carbonyls. The corresponding FTIR spectra for

the 6 wood species are in the Annexes (A-Fig 25-30)

a b

Fig. 0-10 Evolution of selected FTIR relative ratios for the six wood species during indoors simulated natural ageing: a.

A1730/A1370, b. A1505/A1370

36

6.6. Influence of the ageing factors - specific aspects of wood ageing

The data presented in this chapter clearly show that the different ageing factors involved

in the accelerated ageing tests (A1-A4) and the natural simulated ageing in indoors conditions

(A5) demonstrate a different aggressively against wood. For instance, the comparative colour

changes, expressed by the global E values at the end of the testing period, following the 5

ageing tests, measured for European walnut are presented in

Fig. 0-11.

Fig. 0-11 E-colour difference at end of different ageing tests for European walnut

Based on these E values and the corresponding chemical changes previously discussed,

it is clear that the most aggressive factors are the UV light / natural light and the temperature.

Specific sensitivity of wood species to temperature and UV

In order to better highlight and compare the relative sensitivity of each wood species to

temperature and UV radiation and also to compare the resulting colour changes with those

occurring in indoors simulated natural ageing, the plots cumulated in Tab. 0-2 were proposed.

They show in-time evolution of colour changes (E) on a logarithmic time scale (log t), to

accommodate both short accelerated in boratory tests with the longer natural simulated ageing

test.

Linear regression lines were inserted to express the in-time evolution (on logaritmic time

scale) of colour changes due to temperature and UV, as functions of the following form: E=

37

alogt + b. The correlation parameters R2 (0.76-0.99) were encouraging for this approach.

Accordingly, the "a" values, which represent the slopes of these linear regressions in the A1

and A4 ageing tests, might be associated to the sensitivity of the respective wood species to

the ageing factors involved in those tests: temperature in A1 (aT) and UV in A4 (aUV). Higher

"a" slope values (aT, aUV) means higher sensitivity to the respective ageing factors, namely

more rapid and advanced colour change. Moreover, the comparison of the aT and aUV values

for a given wood species would indicate which one of the two ageing factors would cause

more advanced colour changes for the respective wood species.

These "a" slope values from the equations of the linear regressions are presented in the

first column of the Tab. 0-2, alongside the experimental data points and the linear regression

lines and associated equations. Intuitive symbols and colours were employed for both the data

points in the graphs and the corresponding "a" slope values: aT slope factor (in red) for

temperature induced ageing in the test A1 and aUV slope factor (in blue) for UV induced

ageing in the test A4.

Based on the aT slope factor, the studied wood species could be ranked in the following

order of increasing temperature sensitivity: Chinese ash (aT = 1.95) < Chinese walnut (aT =

2.48) < European walnut (aT = 2.90) < Sycamore maple (aT = 5.09) < European ash (aT = 5.35)

< Paulownia (aT = 7.16).

Based on the aUV slope factor, the studied wood species could be ranked in the following

order of increasing UV sensitivity: Chinese walnut (aUV= 3.31) < European ash (aUV = 3.87) <

European walnut (aUV = 4.22) ~ Chinese ash (aUV = 5.35) < Paulownia (aUV = 4.95) <

Sycamore maple (aUV = 8.27).

Tab. 0-2 Comparative evolution of colour change in the A1, A4 and A5 tests (on logarithmic time scale), linear

regression lines E = f (log t) = alog t + b and slope factors

38

ASSF-T-UV

(aT + aUV)

Regression lines E = alog t + b

Mathematical equations

ASSF-T-UV

(aT + aUV)

Regression lines E = alog t + b

Mathematical equations

9.22

(5.35 +

3.87)

6.21

(1.95 +

4.26)

7.12

(2.90 +

4.22)

5.79

(2.48 +

3.31)

13.36

(5.09 +

8.27)

12.11

(7.16 +

4.95)

Finally, a combined ageing sensitivity slope factor (ASSF-T-UV) was defined and

calculated as the sum of the two slope values. This might be an indicator of global ageing

sensitivity of the respective wood species in real conditions, where both temperature and UV

induced ageing processes could occur.

Accelerated ageing vs. natural ageing – acceleration factors

The relation accelerated /natural ageing is better highlighted in the combined graphs in

Fig. 0-13, where the in-time evolution of colour coordinates and colour difference (E) were

mirrored against the vertical axis crossing two opposite horizontal time axes (with different

scale units) at the 0 common point. The time x axis on the left refers to UV accelerated ageing

(A4 - with time units of 24h UV/40C) or temperature accelerated ageing (A1 – with time

units of 72 h/100C). The x axis on the right refers to natural ageing under the effect of

natural light filtered by window glass, each unit representing 1 month, meaning in average 30

days of exposure, cumulating a total of 720 h (day and night).

39

A4 VS. A5 A1 VS.A5

Fig. 0-12 Comparative in-time evolution of colour changes (L*, a*, b* and global colour difference E) for accelerated

ageing under UV radiation (A4- top) and temperature ageing (A1-bottom) vs. natural simulated ageing (A5), for

Paulownia wood – original, partially included in Liu et al 2016b

These graphs, in good accordance to the chemical results previously presented, show that

simulated natural ageing (A5) under indoor conditions relate better to those resulting from the

UV induced ageing (A4). In both these cases the total colour difference (E) results mainly

from yellowness change (superposition of E sand b* curves), this being actually related to

lignin degradation and formation of specific chromophores. On the other hand, in temperature

induced ageing (A1) the total colour difference (E) results mainly from the decrease of

lightness (negative L* values), changes in chromaticity being less important.

Fig. 0-13 Comparative in-time evolution of colour changes (E- colour difference) of the six selected wood species

during accelerated UV induced ageing (A4) and indoors simulated natural ageing test (A5)

Also the particular behaviour of the wood species (here Paulownia and European walnut)

is revealed by these original plots. Accordingly, to better highlight these particularities, Fig.

0-13 presents comparatively the in-time evolution of colour difference (E) for all the six

wood species considered within this research, in UV induced ageing (A4) vs. natural

simulated indoors ageing (A5).

40

Fig. 0-14 Variation of the average daylight time during the natural simulated ageing at the test location in Brasov

Based on these graphs acceleration indexes were calculated considering the respective

times in natural simulated ageing (T5) and UV accelerated ageing (T4) leading to a similar

global colour difference E, as presented in Tab. 0-3.

Tab. 0-3 Calculation of acceleration indexes for A4 vs. A5

Wood species E

UV ageing

time T4

[unit]-

Natural ageing

time T5

[unit]

Acceleration

index- Apparent

AAI

Acceleration

index-Effective

EAI

European ash

2 0.51 1.17 68.82 40.82

4 1.15 3.44 89.74 53.22

6 2.30 4.50 58.70 34.81

Chinese ash

2 0.45 0.93 62.00 36.77

4 0.90 2.56 85.33 50.61

6 2.02 4.00 59.41 35.23

Note

Acceleration index – Apparent AIA=T530days24h/T424h –considers the total

duration of exposure (day and night)

Acceleration index – Effective AIE=T5427h/T424h – considers only effective

exposure to sunlight; the average sunlight duration /month was calculated from

real daylight data for the corresponding time of exposure (months/year) (Fig. 0-14)

6.7. Conclusions on wood ageing

The following conclusions can be formulated based on this original and comprehensive

research on the ageing of the selected six wood species under the action of different ageing

factors:

1. The aggressively of the studied ageing factors were different: generally UV light and high

- temperature were the most active, followed by natural light.

2. Extreme variation of temperature and relative humidity could not be considered as

effective ageing factors, but more likely conditions that could assist the previously

mentioned ageing factors.

3. The ageing effects could be observed at macroscopic level, mostly as colour changes,

while specific chemical changes were revealed by FTIR investigation. Chemical changes

highlighted different ageing pathways depending on the ageing factors involved and

particularities related to the chemical composition of the wood species. Combined

analysis of colour changes and chemical changes revealed some correlation trends.

41

4. The sensitivity of the wood species to the most aggressive ageing factors was also

different. Comparing high temperature and UV as aggresivitity directly related to the

wood species under study, these can be divided in two groups: (i) species more sensitive

to UV than temperature: Chinese ash, Chinese walnut, European walnut, Sycamore maple

and (ii) species more sensitive to temperature than to UV: European ash, Paulownia. This

classification is based on the respective aT and aUV slope factors of the linear regression

lines showing in-time evolution of colour differences in accelerated temperature and UV

induced ageing tests.

5. A combined ageing sensitivity slope factor under the action of temperature and UV,

ASSF-T-UV, was defined as a possible way of expressing the relative sensitivity of the

wood species under the cumulated action of temperature and UV.

6. Comparing accelerated UV ageing with natural simulated indoors ageing (conditions

which are relevant for interior furniture and other old wood artefacts important for cultural

heritage) apparent and effective acceleration indexes could be calculated.

7. The calculated acceleration indexes varied during the ageing process, suggesting an initial

induction phase followed by acceleration (EAI around 50x) and then stabilisation (EAI

around 35x). The actual values of these indexes varied among the wood species, the

lowest acceleration being registered for sycamore maple, which suffered quickly

accentuated colour changes in both the accelerated and natural tests.

8. A model of approximating the global ageing sensitivity of the wood species under study,

by a dedicated index GASI, calculated as the sum of the colour changes E suffered in the

5 tests, was proposed (Tab. 0-4). The proposed GASI values can offer a global view on

the relative ageing sensitivity of the different wood species under similar ageing / storage/

indoors exposure conditions.

Tab. 0-4 Global ageing sensitivity index (GASI) of the six wood species (sum of E values under the action of different

ageing factors in A1-A5 tests) in comparison with the ageing sensitivity slope factor (ASSF-T-UV) in temperature and UV

induced accelerated ageing tests

Ageing factor

Ageing test

European

ash (F)

Chinese

ash (S)

European

walnut (N)

Chinese

walnut (H)

Sycamore

maple (P)

Paulownia

(T)

T, A1 14.81 5.83 7.72 6.77 14.12 18.01

Variation of T, A2 3.77 2.63 4.56 5.18 2.98 1.14

Variation of RH, A3 2.14 2.59 1.77 0.80 1.42 3.30

UV light, A4 8.74 9.16 9.49 7.02 18.18 10.69

Natural light, A5 3.78 5.04 4.74 4.50 14.82 9.40

GASI 33.23 25.25 28.27 24.27 51.53 42.54

ASSF-T-UV 9.22 6.21 7.12 5.79 13.36 12.11

Besides the ageing sensitivity slope factor (ASSF-T-UV), the GASI index could have

practical importance, as in real and long life-times of old artefacts the different ageing factors

may be involved in a combined synergetic action. Further research should consider the

validation and relevance of the new proposed ageing sensitivity indexes ASSF-T-UV and

GASI. Both these values indicate the following decreasing order of global ageing sensitivity

42

of the wood species: P (Sycamore maple) >T (paulownia) > F (European ash) > N European

walnut >S (Chinese ash) >H (Chinese walnut).

7. Ageing behaviour of the studied finishing materials

7.2. Effects of ageing on the chemical structure of the coating films

Temperature induced ageing (A1)

To illustrate the effects of temperature induced ageing on the chemistry of waxes, the

comparative spectra before and after 72, 144, 216 and 288 hours of ageing are presented in

Fig. 0-1 (for beeswax) and Fig. 0-2 (for Chinese wax).

Analysing the comparative spectra of control and aged beeswax samples in Fig. 0-1,

some minor changes are visible only in the area of carbonyls absorption around 1700 -1750

cm-1

. The slight increase of the area of the unconjugated carbonyl groups (1730 cm-1

) is

associated to the differentiation of a shoulder at around 1700 cm-1

, which might be assigned to

free carboxylic groups. It could be assumed that the free carboxyl group in the beeswax

increased from the hydrolytic thermal degradation of the initial ester groups during

temperature induced ageing (Kim et al 2015). Some oxidative effects, involving perhaps the

resulting long chain alcohols and their transformation to aldehydes, caused the increase of the

1730 cm-1

absorption.

Fig. 0-1 Comparative FTIR spectra of Beeswax on lamella before ageing and after different time temperature induced

ageing in the fingerprint region 1800–600 cm-1

In Fig. 0-2 spectra of Chinese wax after temperature ageing 72, 144, 216 and 288 hours

apparently show radical changes, so that one could even say these spectra were not for

Chinese wax. This might be actually the true, due to melting, flowing and sublimation

phenomena, leading to the disappearance of the most of the product from the glass lamella, as

also shown by microscopy (see previous section). Thus this spectra cannot be considered as

relevant for temperature aged Chinese wax, but should be assigned to some impurities (from

the wax or from the surrounding atmosphere), as detected by direct observation and

microscopy.

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Fig. 0-2 Comparative FTIR spectra of Chinese wax on lamella before ageing and after different time temperature induced

ageing on the range 3500-500 cm-1

Fig. 0-3 and

Fig. 0-4 present the comparative spectra before and after 72, 144, 216 and 288 hours of

temperature induced ageing for linseed oil and Tung oil.

These spectra show that some slight changes occurred after temperature ageing. An

increase and broadening of the absorption band at ~3400cm-1

, attributed to the hydroxyl

stretching vibration, was observed for the two types of oils and is related to oxidative

degradation processes and formation of free acids. The absorption band at 1730-1720 cm-1

,

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attributed to unconjugated carbonyls also seems to increase or become broader, due to the

formation of free fatty acids and other carbonyl compounds. Furthermore, the formation of

free carboxylic groups following thermal ageing of Tung oil results from the appearance of a

distinct small peak at around 1700 cm-1

. Also, for Tung oil it can be noted a more distinct

small band at 975 cm−1

, assigned to wagging vibration of isolated trans-configured double

bonds, which may suggest isomerisation and transposition of the 3 conjugated double bonds.

In contrast, a characteristic ageing pattern for linseed oil is the increase of the C-O stretching

vibration bands at around 1125 cm-1

, following oxidative processes.

Fig. 0-3 Comparative FTIR spectra of Linseed oil film (on glass lamella) before ageing and after different periods of

temperature induced ageing

Fig. 0-4 Comparative FTIR spectra of Tung oil film (on glass lamella) before ageing and after different time temperature

induced ageing

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a

b

Fig. 0-5 Variation of selected FTIR relative ratios following temperature ageing for different periods of time for drying

oils: a. Linseed oil, b. Tung oil

Fig. 0-5 presents the evolution of some calculated FTIR relative ratios, considering

absorption bands capable of illustrating the occurring chemical changes induced by

temperature. Oxidative processes leading to higher ratio of hydroxyl groups reported to

aliphatic chains (A3400/A2925-2855) is the most obvious one for both oils. Actually,

temperature induced ageing is somehow a continued curing process (Stenberg 2004, Izzo

2010, Schonneman 2011, Sammzadeh et al 2011, Liu et al 2015c).

The FTIR spectra in Fig. 0-6 illustrate temperature induced chemical changes in the

structure of shellac.

The shift of the absorption bands around 1700 cm-1

(unconjugated carbonyls), 1150 cm-1

(C-O in esters), and 1003 cm-1

(C-O stretching in ether/acetal linkages), all towards higher

wave numbers suggest transformation of free carboxylic groups to ester groups, as well as

possible etherification. This is sustained by the decrease of the absorption band of free

hydroxyls at around 3400cm-1

. Cross-linking in shellac structure by esterification or

etherification as result of ageing or storage has been reported previously in literature (Sartar

and Shrivastava 2000 a, Derry 2012, Poli et al 2014) and controversy still exists related to the

main chemical process involved.

Fig. 0-6 Comparative FTIR spectra of Shellac on lamella before ageing and after different time temperature induced

ageing: on the range 3500-500 cm-1

(top); in the fingerprint region 1800–600 cm-1(bottom)

The fact that chemical changes occur in the structure of shellac by thermal ageing results

also from the variation of the relative ratios of different absorption bands, presented in Fig.

0-8a, though a clear trend is difficult to be observed from the wavy curves. However,

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oxidative processes are linked to the formation of more hydroxyl groups at longer exposure

times, not to increase of carbonyls.

The FTIR spectra of thermally aged Chinese lacquer (Fig. 0-7) indicate an increase and

shift of the carbonyl absorption band at 1730cm-1

towards lower wave numbers suggesting

both oxidative effects and formation of more free carboxylic groups. Moreover, the broad less

distinct absorption band at around 1620 cm-1

, assignable to aromatic and ehenoid structures,

with a possible contribution of conjugated carbonyls and aromatic ketones (around 1640 cm-1

),

is increasing as area. The wide unresolved band between ~970-1070 cm-1

, overlapping 3

bands, becomes evidently higher and a distinctive absorption peak at ~1030 cm-1

is observed.

As absorptions in this area are related to C-O ether/ acetal linkages, this change may suggest

oxidative cross-linking effects.

The variation of some relative FTIR ratios during temperature induced ageing, presented

in Fig. 0-8b, seems more uniformous and clearly highlights oxidative reactions leading to

more carbonyl groups

Fig. 0-7 Comparative FTIR spectra of Chinese lacquer on lamella before ageing and after different time temperature

induced ageing: on the range 3500-500 cm-1

(top); in the fingerprint region 1800–600 cm-1(bottom)

a

b

Fig. 0-8 Variation of selected FTIR relative ratios following temperature ageing for different periods of time for selected

finishing materials: a. Shellac, b. Chinese lacquer.

UV induced ageing (A4)

Waxes seem to be very stable to the action of UV radiation, a well-known powerful

ageing factor. No directly detectable changes were observed for Chinese wax and only very

minor changes were observed in the fingerprint region for beeswax

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The detectable small changes in the FTIR spectra of beeswax were: shifting of the

absorption peaks at around 1168 cm-1

and 1465 cm-1

, assigned respectively to C-C linkages

and C-H bending vibration in waxes (Baeten et al 2010), to lower wave numbers and a slight

increase of carbonyl absorption at around 1730 cm-1

, occurring in parallel with appearance of

a shoulder at around 1700 cm-1

(assignable free carboxylic groups). These may indicate some

thermal splitting of esters groups and oxidative effects.

The small chemical changes occurring following UV ageing of beeswax are better

highlighted from the variation of the relative FTIR ratios in Fig. 0-9a. If these are compared

with those for Chinese wax (Fig. 0-9b) an opposite behaviour could be noted. This should be

correlated with the differences in their chemical composition, presented in Chapter 4/ section

4.2.1. However, more research is needed to clarify the undergoing chemical processes.

a

b

Fig. 0-9 Variation of selected FTIR relative ratios following UV induced ageing for different periods of time for waxes

films (on glass lamella): a. Beeswax, b. Chinese wax

The FTIR spectra Fig. 0-10 demonstrate a high UV resistance of the drying oil films.

Only very limited changes are barely detectable in the –OH absorption region (3400 cm-1

) and

C=C absorptions (around 970 and 740 cm-1

). These are better highlighted in Fig. 0-11 by the

calculated relative FTIR ratios.

a

b

Fig. 0-10 Comparative FTIR spectra of oil films (on glass lamella) before and after different periods of UV induced ageing:

a. Linseed oil, b. Tung oil, on the range 3500-500 cm-1

The evolution of the relative FTIR ratios for aged linseed oil suggest limited UV induced

photo-oxidation leading to more hydroxyl and carbonyl groups, the latter being formed very

likely by oxidation at the C=C double bonds. In the case of linseed oil, formation of hydroxyl

groups seems more advanced, while the existing C=C double bonds seem to be involved more

in cis-trans isomerisation than splitting by oxidation.

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a

b

Fig. 0-11 Variation of selected FTIR relative ratios following UV induced ageing for different periods of time for selected

finishing materials: a. Linseed oil, b. Tung oil

The FTIR spectra in Fig. 0-12a, for control and UV aged shellac films, demonstrate a

quite high UV resistance of this material, at least for the testing period of 72 h in this research.

An increase of carbonyl absorption (around 1730 cm-1

), without shifting, and a trend of

decrease and broadening of hydroxyl absorption (about 3400 cm-1

), suggest limited UV

induced photo-oxidation and possibly some cross-linking by esterification.

In contrast, more advanced chemical changes were induced by UV ageing on the

Chinese lacquer film (Fig. 0-12b). In this case the significant increase of the unconjugated

carbonyls absorption (1730 cm-1), alongside the decrease of -OH absorption indicate

extensive UV induced oxidation. The band at 1620 cm-1

(conjugated carbonyls, aromatic

ketones) is maintained by ageing, though the ratio between the two (A1730/A1640) is

changing radically. The limited UV resistance of Chinese lacquer has been previously

reported (Hong et al 2000, Xia et al 2011). If an apparent contradiction with the amazingly

good conservation of ancient Chinese lacquerware pieces seems to occur, then the fact that

when manufacturing those objects pigments (including iron based ones) were employed in

preparing the lacquer and the substrate should be considered. The property of pigments to

absorb UV radiation and to protect the films and substrates from the degradative action of UV

radiations is well acknowledged (Wei et al 2011, Hong et al 2000).

a b

Fig. 0-12 Comparative FTIR spectra of: a-Shellac and b- Chinese lacquer films (on glass lamella), before and after

different durations of UV induced ageing, in the range 3500-500 cm-1

The UV induced chemical changes for shellac and Chinese lacquer films, as well as the

significantly higher sensitivity of Chinese lacquer compared to shellac, result better from the

relative FTIR ratios plotted in Fig. 0-13.

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a

b

Fig. 0-13 Variation of selected FTIR relative ratios following UV induced ageing for different periods of time for selected

finishing materials: a. Shellac, b. Chinese lacquer

Indoors simulated natural ageing (A5)

Chemical changes induced by natural light filtered by window glass were very similar

with those occurring in the UV accelerated ageing test. After 6 months natural ageing,

changes comparable with those induced by 72 h of UV exposure (from high power sources)

were generally obtained, which demonstrate the efficiency of the UV tests. Therefore, the

comparative spectra for the waxes and oils films before and after indoors simulated ageing are

presented in the Annexes (A-Fig 67-70).

The variation of the relative FTIR ratios for the two types of waxes are presented in Fig.

0-14, showing evolving trends very similar to those in Fig. 0-9 (for UV ageing). However, it

seems very interesting to point out that oxidation of beeswax seems more accentuated in

natural ageing than UV ageing (relative A1730/ A2925 after 6 months natural ageing was

about 1.6, higher than the value of about 1.25 after 72 h UV ageing).

a

b

Fig. 0-14 Variation of selected FTIR relative ratios following indoor simulated natural ageing for different periods of time

for selected waxes: a. Beeswax, b. Chinese wax

The variation of the relative FTIR ratios for the two types of oils are presented in Fig.

0-15, showing evolving trends very similar to those in Fig. 0-9 and Fig. 0-11 (for UV ageing).

Oxidative effects leading to more hydroxyl and carbonyl groups occur. Again, in the case of

linseed oil the remaining double bonds C=C seems to be more affected by ageing than in the

case of Tung oil. It is worth remembering that isolated double bonds, mostly cis are in the

structure of linseed oil, while mostly, trans conjugated double bonds are present in the

structure of Tung oil.

50

a b

Fig. 0-15 Variation of selected FTIR relative ratios following indoor simulated natural ageing for different periods of time

for oil films (on glass lamella): a. Linseed oil, b. Tung oil

Fig. 0-16 Comparative FTIR spectra of Shellac on lamella before ageing and after different time indoor simulated natural

ageing: on the range 3500-500 cm-1

(top); in the fingerprint region 1800–600 cm-1(bottom)

The FTIR spectra of shellac in the fingerprint region

(

Fig. 0-16) clearly indicates oxidation with formation of more carbonyl acidic groups, as

the absorption at about 1700 cm-1

is increasing but is not shifting to higher wave numbers.

The absorptions at 1239 cm-1

(OH), 1150 cm-1

(C-O) and 1001 cm-1

(C-O from acids, esters)

are also increasing indicating oxidative phenomena. Curiously, the absorption for C=C at 720

cm-1

is increasing; a possible hyphothesis could consider the radicalic mechanism of

photo-oxidation, leading to formation of double bonds by H substraction in the termination

phase of the radicalic process.

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Fig. 0-17 Comparative FTIR spectra of Chinese lacquer on lamella before ageing and after different time indoor

simulated natural ageing: on the range 3500-500 cm-1

(top); in the fingerprint region 1800–600 cm-1(bottom)

The FTIR spectra of Chinese lacquer in

Fig. 0-17 are very similar with those obtained following UV induced ageing. A very

significant increase of the carbonyl absorption at 1730 cm-1

is registered alongside a decrease

and widening of the hydroxyls absorption and a decrease of -CH2- absorptions at 2925 and

1454 cm-1

. These changes may be associated to breaking of urushiol side chains and

formation of carbonyl containing structures. Moreover, the C=C double bonds are involved in

oxidative processes. These results are in good accordance with findings of Hong et al (2000),

following UV ageing of Chinese lacquer.

a

b

Fig. 0-18 Variation of selected FTIR relative ratios following indoor simulated natural ageing for different periods of time

for selected natural resin: a. Shellac, b. Chinese lacquer

The above described chemical changes induced by natural light in the structure of shellac

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and Chinese lacquer films are highlighted by the relative FTIR ratios in Fig. 0-18. The very

intensive oxidation of Chinese lacquer is illustrated by the relative FTIR ratio A1722/A2925

(carbonyl / methylen) reaching the value of 11 after 6 months of natural ageing. In

comparison, the similar value for aged shellac was only about 2.4.

The comparative effects of the different ageing tests on variation of selected FTIR

relative ratios for six finishing materials

The data presented in this chapter clearly show that the different ageing factors involved

in the accelerated ageing tests (A1-A4) and the natural simulated ageing in indoors conditions

(A5) demonstrate a different aggressive against the studied finishing materials. For instance,

the comparative FTIR relative ratios at the end of the testing period, following the A1, A4 and

A5 tests, are presented in Fig. 0-19-Fig. 0-21.

Fig. 0-19 The comparative effects of the different ageing tests on variation of selected FTIR relative ratios for waxes

Due to the fact that both beeswax and Chinese wax suffered melting and sublimation

during high temperature induced ageing, the spectra after 288 hours temperature aging were

not considered relevant. Fig. 0-19 shows comparative effects only for UV induced ageing (A4)

and Natural ageing (A5). UV radiation and natural light caused only minor and quite similar

chemical changes for both waxes (relative ratios close to 1.00). The evolution of the relative

ratios for Beeswax and Chinese following ageing was in opposite direction.

Fig. 0-20 shows the comparative effects of the different ageing tests on the variation of

selected FTIR relative ratios for linseed oil and Tung oil. The spectra of both oils at end of

different ageing (A1, A4 and A5) illustrated increasing of carbonyl absorption (around 1730

cm-1

) and hydroxyl absorption (arund 3400 cm-1

), due to oxidative processes. Compared with

UV ageing and natural ageing, temperature seems to be the most aggressive ageing factor for

both oils. This is consistent with the macroscopic aspect changes, presented in Error!

Reference source not found..

53

a b

Fig. 0-20 The comparative effects of the different ageing tests on variation of selected FTIR relative ratios for oils: a.

Linseed oil, b. Tung oil

a b

Fig. 0-21 The comparative effects of the different ageing tests on variation of selected FTIR relative ratios for natural

resin: a. Shellac, b. Chinese lacquer

Fig. 0-21a shows that the comparative effects of the different ageing tests on variation of

selected FTIR relative ratios for shellac:

After UV ageing 72h only minor changes occurred, the results were similar to those

recently reported by Sartar and Shrivastava (2000 a and b) (no significant chemical

changes at exposure times shorter than 75h UV induced ageing);

The FTIR relative ratios of 288 hours temperature aged shellac suggest

transformation of free carboxylic groups to ester groups during temperature ageing,

as well as possible etherification.

The FTIR relative ratios of 6 months natural aged shellac indicated oxidation with

formation of more carbonyl/ carboxyl acidic groups and other chemical changes.

Fig. 0-21b shows that the comparative effects of the different ageing tests on variation of

selected FTIR relative ratios for Chinese lacquer:

Some significant chemical changes occurred at end of 6 month natural ageing,

breaking of urushiol side chains and formation of carbonyl containing structures.

The effects of UV ageing on the FTIR ratios were similar with these of natural

ageing, but less obvious.

54

Temperature seems less aggressive ageing factors compared with UV, these FTIR

ratios illustrated formation of more free hydroxylic groups and oxidative

cross-linking effects.

7.3 Specific aspects of ageing of finishing materials

Some specific aspects of the mechanisms involved in the ageing of the finishing

materials under the action of the different ageing factors resulted from the experimental

research in this thesis, as summarised in Tab. 0-1.

55

Tab. 0-1 Summary mechanisms on types of produces

Class Type A1 A4, A5 Conclusions

Common Particular Common Particular

Waxes

BW

More or less

advanced

volatilisatio

n

- Thermal hydrolytic

degradation,

- Oxidation of

resulting alcohols to

aldehydes -

-Oxidative effects,

more advanced in

VIS light than UV

light

-Beeswax: thermal

degradation, ester

hydrolysis, some oxidative

effects, very stable to UV

ageing.

- Chinese wax: more

research is needed to clarify

the undergoing chemical

processes

CW

- Disappearance of

volatile components,

- No concluding

results

- Opposite

variation of FTIR

ratios

Oils

LO formation of

free fatty

acids

(3400,

1730)

- C-O increase

- oxidative effects formation of

free fatty

acids

(3400,

1730)

- During thermal ageing and

photo degradation both oils

suffered formation of free

fatty acids.

Linseed oil is more

sensitive during

temperature ageing

TO

C=C transposition and

cistrans

isomerization (975

740)

cistrans

Natural

resins

SL -

- Esterification and

eherification

cross-linking,

- oxidative

cross-linking effects

-

-limited UV

induced

photo-oxidation,

-some

cross-linking by

esterification

Etherification /

esterification occurred

during thermal ageing,

photo-degradation induced

formation of more carbonyl

groups

CL -

- Formation of more

free hydroxylic

groups

-oxidative

cross-linking effects

-

- breaking of

urushiol side

chains

- formation of

more carbonyl

- oxidative effects

Breaking of urushiol side

chains and formation of

carbonyl containing

structures

7.4. Conclusions

Ageing induced specific macroscopic aspect and/ or micro-structural changes of the

coating films. Some of them could be related to the physical phenomena associated to the

different ageing tests and the properties of the solid finishing layers, including melting/

sublimation/ vaporisation induced by temperature for the thermoplastic materials, such as

waxes.

Ageing generally induced more or less advanced chemistry changes. The actual

structural changes depended on both the ageing factor and the finishing material, meaning

class/type of material (wax, oil, resin) and the particular chemical features of each material,

respectively the mechanism of film formation.

56

UV radiation and high temperature were the most aggressive ageing factors in the

accelerated artificial tests. UV radiation generally caused photo-oxidative processes, while

temperature induced either cleavage of some chemical bonds (degradation /breaking of

structure) or formation of new chemical bonds (more advanced cross-linking).

Natural light caused pretty similar changes with those resulting from UV exposure, but

evidently at a much slower rate.

UV radiation and natural light caused only minor and quite similar chemical changes for

both waxes.

8. Ageing behaviour of the studied finished wood surfaces

8.7. Influence of ageing factors and wood species on the ageing behaviour of

the finished wood surfaces

Ageing factors relative aggressively vs. finishing materials and wood species

influence

The data presented in this chapter clearly show that the different ageing factors involved

in the accelerated ageing tests (A1-A4) and the natural simulated ageing in indoors conditions

(A5) demonstrate a different aggressively against the finished wood surfaces. The data also

show that the sensitivity of the six tested finishing materials to these factors is different. For

instance, the comparative colour changes, expressed by the global E values at the end of the

testing period, following the 5 ageing tests, measured for the six types of finished surfaces, on

the six wood species, are presented in Fig. 0-1 (waxes), Fig. 0-2 (oils) and Fig. 0-3 (natural

resins).

Fig. 0-1 E-colour difference at end of different ageing tests for the wood samples of different wood species finished

with beeswax(left) and Chinese wax(right)

57

Fig. 0-2 E-colour difference at end of different ageing tests for the wood samples of different wood species finished

with linseed oil (left) and Chinese wax (right)

Fig. 0-3 E-colour difference at end of different ageing tests for the wood samples of different wood species finished

with shellac (left) and Chinese lacquer (right)

Based on these E values and the corresponding chemical changes previously discussed,

some concluding remarks can be formulated as follows:

Generally the most aggressive factors for the finished surfaces were temperature,

UV light and natural light. In contrast, minimum effects were produced by the

extreme variation of humidity or the cold-check. An exception related to the

temperature effect /aggressively, was the Chinese lacquer, but its resistance to

temperature cannot be assumed just by colour changes, due to its original very dark

colour.

The relative ageing sensitivity of the finishing materials on study, expressed as

resulted colour change, depending on the ageing factor:

A1 (TC): linseed oil >> beeswax > Tung oil > shellac Chinese wax>> Chinese

lacquer

A2 (TC): linseed oil > beeswax >shellac Chinese wax Tung oil>> Chinese

lacquer

A3 (RH): linseed oil >> shellac > Tung oil > beeswax Chinese wax Chinese

lacquer

A4 (UV): Chinese wax > beeswax > linseed oil Chinese lacquer > Tung oil > shellac

58

A5 (VIS/UV): Chinese lacquer>>linseed oil Chinese wax beeswax Tung oil

shellac

The wood substrate (wood species) influenced the overall behaviour of the finished

samples: for the same type of finish the colour changes determined by each of the

ageing agents varied among the species under study.

Corroboration of the above findings demonstrate that the ageing behaviour of the

finished wood samples is specifically influenced by both the coating film and the

wood substrate, the measured effect being a combination of the ageing effects on the

finishing material and the substrate.

The relative ageing sensitivity (to the most aggressive factors T, UV, VIS) of the

unfinished and finished surfaces considering both the substrate and the coating film

is summarised in Tab. 0-1, where ordering was based on the recorded E values

(highest E values to smallest ones)

Tab. 0-1 The relative ageing sensitivity of the finished surfaces in different ageing tests, as a function of wood species

and the finishing materials

Ageing tests

Surfaces

Relative sensitivity to ageing

based on the E values registered at the end of the test

A1 (288h) A4 (72h) A5 (6months)

Un-finished T>F>P>N>H>S P>T>N>S>F>H P>T>S>N>H>F

Beeswax T>P>F>S>H>N P>T>N>S>F>H P>N>T>F>S>H

Chinese wax P>T>F>H>N>S P>S>H>F>T>N P>N>F>T>S>H

Linseed oil P>T>F>S>H>N P>F>H>T>S>N N>H>P>T>S>F

Tung oil P>T>F>S>H>N P>T>S>H>F>N N>H>P>T>S>F

Shellac P>T>F>H>S>N P>N>F>S>T>H H>N>T>P>S>F

Chinese lacquer P>N>T>S>H>F S>F>P>H>N>T P>F>H>T>S>N

Note: The codes of wood species are:

F- European ash; N- European walnut; P- Sycamore maple

S- Chinese ash; H- Chinese walnut; T- Paulownia-

In order to estimate the influence of the finishing material itself in this assembly

finishing material/wood substrate, the "relative E-colour difference ratio" (R-E): equal

to the ratio E finished samples /E un-finished samples, at end of the different ageing

tests were calculated. Based on this calculation it is assumed that values smaller than 1

would indicate that the finishing material played a protective role (smaller colour

differences induced by ageing in comparison to unfinished samples). In contrast, values

higher than 1 would indicate that the finishing material brings further colour changes to

those due to the substrate. In this case the measured colour differences cumulate the

contribution of the substrate itself (suffered by the respective unfinished wood species

during the same ageing test) and of that of the coating film.

These relative colour differences ratios are presented for Paulownia and European

walnut samples in the column graphs in Fig. 0-4. As the two wood species demonstrated

themselves different ageing sensitivity (see GASI and ASSF-T-UV) in chapter 6, with

paulownia generally more sensitive than European walnut, the aspect of these graphs is

different.

59

Fig. 0-4 Relative E-colour difference at the end of different ageing tests for Paulownia samples (left) and European

walnut samples (right)

For the highly ageing sensitive Paulownia wood, almost all the finishing materials

actioned as protective against ageing, so that the finished samples changed colour less than

the bare wood. Exceptions are linseed oil in the case of temperature ageing and Chinese

lacquer in the case of natural light ageing. This correlates well with the high sensitivity of

these materials to the respective ageing factors (see chapter 7).

In the case of European walnut, with a medium ageing sensitivity, the finishing materials

applied also actioned as protective against ageing, excepting indoors simulated natural ageing.

For this type of test all the finished samples, excepting those coated with shellac, changed

colour most than the unfinished ones. So, what it is observed is mostly the ageing of the

coating film itself.

Accelerated ageing vs. natural ageing – acceleration factors

In order to compare and correlate UV accelerated ageing with indoors simulated natural

ageing under the action of natural light filtered by glass the combined plots presented in Fig.

0-5 for Paulownia was proposed. They are similar to those presented in Chapter 6 for

un-finished wood and allow the calculation of the acceleration indexes AAI-apparent

acceleration index and EAI – effective acceleration index. The calculation principle is the

comparison of the testing times leading to similar ageing effects, in this case similar colour

difference. The calculated values are presented in Tab. 0-2, respectively.

Fig. 0-5 Comparative evolution of the E-colour difference during the accelerated UV test A4 (left x –axis) and simulated

indoors natural ageing (right x axis) for Paulownia samples finished in the 6 variants.

It is worth noticing that the acceleration indexes when comparing the same tests (A4/A5)

were different between unfinished and finished samples and also influenced by the finishing

and the wood species.

60

For the Paulownia samples the AAI and EAI values were only slightly different for the

finished samples compared to unfinished wood, excepting the samples finished with Chinese

lacquer (for which significantly lower values were calculated). This means that the UV ageing

test was quite effective in accelerate simulating natural ageing of both unfinished samples and

samples finished with waxes, oils and shellac (to some extent). Actually the colour changes

under UV light and natural light evolved similarly for the Paulownia samples unfinished and

finished (excepting Chinese lacquer).

The situation was different for other species. Differences result from the different ageing

sensitivity of the considered wood species in relation to the ageing sensitivity of the finishing

layers. This means that data from accelerated UV ageing of finished samples should be

considered with care when intended to predict colour changes for indoors furniture and data,

as acceleration indexes, determined for one species or one type of finishing material can’t be

extended to others.

Tab. 0-2 The calculated acceleration indexes (A4 test vs. A5 test) Paulownia wood samples

Wood surface E UV ageing time

T4

Natural ageing time

T5

Acceleration

index- Apparent

Acceleration

index-Effective

[unit] [unit] AAI EAI

Un-finished 2 0.37 1.00 81.08 48.09

4 0.74 1.84 74.59 44.24

6 1.45 2.80 57.93 34.36

Beeswax finished 2 0.36 0.96 80.00 47.45

4 0.72 1.99 82.92 49.18

6 1.08 2.76 76.67 45.47

Chinese wax

finished

2 0.43 1.02 71.16 42.21

4 0.86 2.32 80.93 48.00

6 1.91 3.78 59.37 35.22

Linseed oil

finished

2 0.38 0.67 52.89 31.37

4 0.74 1.56 63.24 37.51

6 2.06 2.89 42.09 24.96

Tung oil finished 2 0.58 1.85 95.69 56.76

3 0.87 2.64 91.03 54.00

6 2.23 3.94 53.00 31.44

Shellac finished 1 0.42 0.52 37.14 22.03

2 0.83 0.85 30.72 18.22

4 3.00 4.80 48.00 28.47

Chinese lacquer

finished

2 0.78 0.28 10.77 6.39

4 2.59 0.56 6.49 3.85

6 2.80 0.84 9.00 5.34

8.8. Conclusions on the ageing of wood finished surfaces

All the data in this chapter clearly highlights the complexity of this topic as phenomena

and mechanisms involved, influencing factors, necessity of multiple aspects investigation,

61

corroboration and data interpretation. Certainly that more investigation methods/ analytical

techniques and further experiments / extended testing times would be necessary, but on the

here presented results some useful conclusions, can be formulated and sustained:

High temperature, UV light from high power sources and natural light, even

filtered by the window glass, are important ageing factors affecting the aspect and

the surface chemical structure of both unfinished and finished wood surfaces in

indoors applications:

Extreme variation of temperature and relative humidity caused only minor

changes compared to the above factors, but they might be favourising ageing

conditions and/or causes of other physical changes affecting the wood artefacts in

inadequate adverse conservation conditions.

Ageing of finished surfaces was assessed by colour changes (L, a*, b, E)

and FTIR spectra, which were processed to highlight structural changes as

variation of the ratios of significant absorption bands. Correlation of these

investigations allowed a better understanding /interpretation of the results.

The actual ageing effect is the result of a complex interaction of both the

substrate and the finishing material with the ageing factor, being therefore

strongly influenced by the wood species, the type of finishing material and the

ageing conditions.

The Relative E-colour difference ratio (R-E) was defined and calculated, as

the ratio E finished samples /E un-finished samples, in order to highlight the

effect of the coating film ageing to the global colour change of the finished

samples.

Depending on the ageing sensitivity of the wood substrate (expressed by GASI

and ASSF-T-UV) and the finishing materials, R-E values smaller or higher than

1.00 were obtained, indicating either colour stability induced by coating or an

increased colour change during ageing due to the coating contribution.

Ageing affected in a specific way the quality of the finished surfaces, expressed

by adherence to the substrate and resistance to cold liquids.

Adherence of the oils was little affected by ageing, while UV ageing affected

adherence of the shellac film. The adherence of the Chinese lacquer was affected

mostly by temperature ageing.

Improved resistance to acetone was determined for the surfaces finished with oils

following UV ageing due to further UV induced cross-linking. Improved

resistance to alcohol and acetone was determined for the shellac finished surfaces

after temperature induced ageing, due to further cross-linking by esterification

processes.

Comparison of colour change data from accelerated UV ageing and natural

simulated indoors ageing under the action of natural light filtered by window

glass, allowed calculation of the acceleration indexes AAI and EAI. They show

the advantages and limits of accelerated tests in simulating real exposure

conditions.

62

The analysis of these values, different between unfinished and finished samples,

between wood species finished with the same material and different between

samples finished with different finishing material (same wood species) strongly

suggests that data resulting from accelerated tests should be considered with care

when extrapolated to real conditions.

Acceleration indexes are useful in predicting real behaviour only if they result

from tests on similar wood substrate and type of finishing material.

They demonstrate the utility of the comparative approach in this thesis, which

contributes with a significant amount of data on the ageing behaviour of six wood

species, each of them finished in six variants and exposed in 5 different ageing

tests.

These conclusions are important for both the field of scientific conservation of wood

artefacts and the wood technology sector.

9. Conclusions, original contributions and direction of future research

9.1. General conclusions

The main aim of this thesis was studying the ageing phenomena of six relevant wood

species and six types of traditional finishing materials for historic furniture, by conducting

appropriate accelerated artificial ageing tests, to simulate the natural ageing phenomena

occurring over long periods of time, in order to get useful data for scientific conservation.

Approaching this aim, theoretical and experimental research, with specific objectives

(detailed in chapter 2) was conducted.

As a general statement it could be appreciated that the aim and the specific objectives

were properly addressed and resolved within this thesis.

Theoretical research allowed, by analysis and corroboration of literature data and deep

studies into the mechanisms of ageing, the following important aspects, further applied in the

practical research:

A better understanding of the value of furniture as cultural heritage, with a long history

and diverse representation in Europe and China, of the common and differentiating

features, alongside the cultural interferences.

Selection of relevant wood species and finishing materials for the experimental

research.

A better understanding of significance of ageing of materials, as terminology and

complex phenomenon and of the principles of accelerated ageing testing methods.

Development of an original methodological concept of the project, including relevant

ageing tests, associated with adequate investigation methods.

A better understanding and interpretation of the original experimental data.

Experimental research fully covered the specific experimental research objectives,

allowing the following general conclusions:

A number of 1800 wood test pieces from the selected species were finished by

traditional techniques with selected finishing materials; alongside, 200 pieces of glass

lamella were also coated with these finishing materials.

63

Four (4) original artificial ageing tests and one (1) simulated natural ageing test

involving the main ageing factors: UV radiation, sunlight filtered by window glass,

temperature, extreme variation of temperature or relative humidity were accomplished

on wood, finishing materials and finished surfaces. The effects of ageing phenomena

were assessed by colour measurements, visual assessment, microscopic investigation

and FTIR investigation of surface chemistry changes.

Both visual assessment and colour measurements in the CIE-Lab system of wood

unfinished and finished revealed colour differences after ageing. These are documented

in the images cards: I-Effects of ageing on the general aspect and colour of the six wood

species, as a function of the type of finish, affecting contrast between species; II-In-time

evolution of ageing; III-Comparative affects of the ageing factors.

The ageing factors tested induced colour changes and chemical structure modifications

of different intensity, revealed by FT-IR spectra and their further advanced analysis by

calculation of ratios of relevant absorption peaks.

Temperature (A1), UV radiation (A4) and simulated natural ageing (A5), acted as

effective ageing factors, determining specific chemical changes and the most evident

colour changes, reflected by maximum E values, considering a certain wood species

and type of wood surface (finished and unfinished).

Extreme variation of temperature (cold check) and extreme variation of relative

humidity caused significantly smaller colour changes, while chemical changes were

nearly absent, demonstrating their secondary role in ageing, mostly as influencing

factors, not real ageing factors.

Wood species on study were affected differently by ageing factors involved; the ageing

resistance of the finishing materials on study was also different and depended on the

ageing factors.

Ageing of finished surfaces resulted as a complex interaction of both the substrate and

the finishing material, being therefore strongly influenced by the wood species, the type

of finishing material and the ageing conditions.

R-E values, were proposed to evident ate the contribution of the coating film,

indicating either colour stability induced by coating (R-E,1) or an increased colour

change during ageing due to the coating contribution (R-E>1).

Ageing affected in a specific way the quality of the finished surfaces, expressed by

adherence to the substrate and resistance to cold liquids; reduction of shellac solubility

in alcohol due to ageing was demonstrated;

Comparison of colour change data from accelerated UV ageing and natural simulated

indoors ageing under the action of natural light filtered by window glass, allowed

calculation of the acceleration indexes AAI and EAI. They show the advantages and

limits of accelerated tests in simulating real exposure conditions.

The analysis of these values, different between unfinished and finished samples,

between wood species finished with the same material and different between samples

finished with different finishing material (same wood species) strongly suggests that

data resulting from accelerated tests should be considered with care when extrapolated

to real conditions.

64

Acceleration indexes are useful in predicting real behaviour only if they result from

tests on similar wood substrate and type of finishing material.

The experimental results allowed also more specific conclusions on the particular ageing

resistance of the materials under study:

Wood ageing

The global ageing sensitivity index (GASI) and ageing sensitivity slope factor

(ASSF-T-UV), originally proposed in this thesis, indicate the following decreasing

order of global ageing sensitivity of the wood species: P (Sycamore maple) >T

(Paulownia) > F (European ash) > N European walnut >S (Chinese ash) >H (Chinese

walnut).

The sensitivity order to each individual ageing factor was slightly different as the

wood species differentiated themselves in terms of sensitivity to the most aggressive

ageing factors, based on the aUV and aT ageing sensitivity slope factors: (i) Sycamore

maple, Chinese ash, European walnut, Chinese walnut are more sensitive to UV than

temperature; (ii) Paulownia and European ash are more sensitive to temperature than

UV radiation.

The studied wood species could be ranked in the following order of increasing UV

sensitivity: Chinese walnut (aUV= 3.31) < European ash (aUV = 3.87) < European

walnut (aUV = 4.22) ~ Chinese ash (aUV = 5.35) < Paulownia (aUV = 4.95) < Sycamore

maple (aUV = 8.27).

The studied wood species could be ranked in the following order of increasing

temperature sensitivity: Chinese ash (aT = 1.95) < Chinese walnut (aT = 2.48) <

European walnut (aT = 2.90) < Sycamore maple (aT = 5.09) < European ash (aT = 5.35)

< Paulownia (aT = 7.16).

FTIR spectra and calculated ratios indicated high temperature induced hemicelluloses

degradation and UV light and natural induced lignin degradation.

Comparing accelerated UV ageing with natural simulated indoors ageing apparent and

effective acceleration indexes could be calculated (around 35x).

Finishing materials ageing

Ageing induced specific macroscopic aspect and/ or micro-structural changes of the

coating films:

Waxes: melting/ sublimation/ vaporisation induced by temperature for the

thermoplastic materials

Oils: yellowing by thermal oxidation

Shellac: pulver-like surface by photo-degradation

Chinese lacquer film lost elasticity during high temperature ageing.

Ageing generally induced more or less advanced chemistry changes. The actual

structural changes depended on both the ageing factor and the finishing material,

meaning class/type of material (wax, oil, resin) and the particular chemical features of

each material, respectively the mechanism of film formation.

65

Ageing of finished wood surfaces

High temperature, UV light from high power sources and natural light, even filtered by

the window glass, are important ageing factors, affecting the aspect and the surface

chemical structure of finished wood surfaces in indoors applications.

Ageing behaviour of the finished wood surfaces results from the contribution of both

the substrate and the coating film, so that depending on the ageing sensitivity of the

wood substrate and the ageing sensitivity of the finishing material, the colour of the

finished wood surfaces might be more or less affected by ageing compared to

unfinished wood.

Surface chemistry changes of wood finished surfaces are mostly related to the finishing

material itself, though some contribution of the substrate might be evident for some

products /ageing factors (e.g. thermal ageing of waxes) and should be considered in data

interpretation.

The quality of the finished surfaces in terms of resistance to cold liquids (water,

solvents) is affected by ageing in a specific way, depending on the finishing product and

the ageing factor. An increase of resistance to solvents might be considered as quality

for a new finish but a defect for conservation, when related to reversibility.

The quality of the coating film in terms of adherence to the substrate might be affected

negatively by ageing, depending on both the finishing material and the wood substrate.

9.2. Original contributions

The original contributions of the author are found in all phases of the research, the most

important being summarised as follows:

1. A brief analysis of historic furniture in Europe and China, highlighting specific

periods, characteristics and interferences.

2. A thorough, extensive study of literature and original synthesis of data highlighting

the main theoretical aspects of ageing of materials with specific aspects for wood and

traditional finishing materials.

3. The methodological concept of the project, integrating in an interconnectivity

manner the objectives and research phases, alongside appropriate testing and modern

investigation methods.

4. Characterisation and comparison between the European and Chinese traditional

finishing materials from three classes: waxes, oils and natural resins. These are

important not only for the conservation of cultural heritage, but also for development

of novel eco- materials starting from natural resources.

5. Characterisation (microscopic structure, chemical composition and ageing behaviour)

and comparison of similar wood species from Europe and China (Fraxinus excelsior

vs. Fraxinus mandshurica, Juglans regia vs. Fraxinus mandshurica) and

characterisation of Paulownia wood (Paulownia elongata), a wood species less

known but becoming important in Romania.

66

6. Determining the quality of the wood surfaces finished with traditional finishing

materials and a comprehensive study on their ageing behaviour.

7. Proposal of original indicators for the comparison of the ageing properties of the six

wood species under study, involving three main ageing factors and two

environmental parameters.

8. Highlighting the particular contribution of the wood substrate and finishing material

to the ageing behaviour of finished surfaces.

9. Two comprehensive data bases referring to the colour and chemical changes caused

by ageing for the six wood species, six finishing materials and the corresponding

finished surfaces.

10. The images cards illustrating colour changes induced by five different ageing

tests to the six wood species unfinished and finished in six variants with natural

traditional products.

9.3. Further research directions

Based on the experimental results and conclusions, the following aspects should be

considered in further research:

Temperature induced accelerated ageing of waxes at a lower temperature (for example

50C) with a longer period of testing;

Extending the testing period for UV ageing resistance;

Continuing the started simulated natural ageing test on unfinished wood, finishing

materials and finished surfaces on long term;

Conducting similar experimental research on selected softwood species from Europe

and China and extending the number of hardwood species tested;

Conducting similar experimental research on pigmented traditional finishing materials

(opaque finishes), especially for Chinese lacquer, shellac and oils in order to highlight

the influence of pigments on the ageing resistance of the respective coating films and

finished surfaces;

Building a relevant collection of old, naturally aged samples (wood, finished surfaces)

and their investigation in order to compare the characteristics of naturally aged

samples and artificially aged samples;

Implementing the useful results in the practice of furniture conservation and

restoration.

67

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Summary

The PhD-Thesis entitled „Contributions to the study of ageing phenomena of wooden substrate

and traditional materials for transparent finishes – a comparative approach for Europe and China

with applicability in furniture conservation/ restoration” deals with a complex, interdisciplinary and

actual research topic, leading to important, original scientific results and practical outcomes.

The research focused on the comparative ageing behaviour of six wood species from Europe and

China (European ash (Fraxinus excelsior), European walnut (Juglans regia), Sycamore maple (Acer

pseudoplatanus), Chinese ash (Fraxinus mandshurica), Chinese walnut (Juglans mandshurica Maxim)

and Paulownia (Paulownia elongata), six traditional materials for transparent finishes (beeswax,

linseed oil, shellac, Chinese wax, Tung oil and Chinese lacquer) and the resulting finished surfaces in

4 types of accelerated artificial ageing tests and one simulated natural ageing test in indoor conditions.

Microscopy, colour measurement and FTIR investigation were employed. The results represent a

significant contribution to the development of knowledge in the field of wood/ materials ageing.

Original contributions include proposal and definition of some specific indexes to characterise the

ageing sensitivity of wood/various materials.

The thesis itself is structured on 295 pages, including 9 chapters, 62 tables, 167 figures and 187

references. The annexes of the thesis (392 pages) represent important data-bases for the subject under

study. Images cards in electronic format are included.

Rezumat

Teza de doctorat intitulată "Contribuţii la studiul fenomenelor de îmbătrânire a suportului

lemnos şi materialelor tradiţionale de finisare transparentă - o abordare comparativă pentru

materiale reprezentative din Europa şi China, cu aplicabilitate în conservarea/restaurarea mobilei"

vizează o tematică de cercetare complexă, interdisciplinară, de mare actualitate, ce a condus la

rezultate științifice originale importante, cu aplicabilitate practică.

Cercetările s-au axat pe studiul comparativ privind comportarea la îmbătrânire a șase specii de

lemn din Europa și China: frasin european (Fraxinus excelsior), nuc european (Juglans regia), paltin

de câmp (Acer pseudoplatanus), frasin chinezesc (Fraxinus mandshurica), nuc chinezesc (Juglans

mandshurica Maxim), paulownia (Paulownia elongata) și a șase materiale tradiționale pentru finisaje

transparente (ceară de albine, ulei de in, șelac, ceară chinezească, ulei Tung, lac chinezesc), precum și

a suprafețelor finisate rezultate, în patru tipuri de teste de îmbătrânire artificială accelerateă și un test

de îmbătrânire naturală simulată în condiții de interior. Metodele de investigare folosite au fost:

microscopia, măsurarea instrumentală a culorii și spectrometria FTIR. Rezultatele reprezintă o

contribuție semnificativă la dezvoltarea cunoștințelor în domeniul îmbătrânirii lemnului /materialelor.

Contribuțiile originale includ propunerea și definirea unor indici specifici pentru caracterizarea

sensibilității la îmbătrânire a lemnului /materialelor.

Teza în sine este structurată pe 295 pagini, care cuprind: 9 capitole, 62 tabele, 167 figuri și 187

referințe. Teza este completată de anexe (392 pagini), ce reprezintă baze de date importante pentru

subiectul tezei. Teza include cartoteci de imagini (format electronic).

72

Curriculum vitae

Europass

Informaţii personale

Nume / Prenume LIU XINYOU

Adresă(e) Str. Bld 15 noimberie Nr.35-37, Brasov

Telefon(oane) 0040-0752086088

E-mail(uri) liu.xinyou@unitbv.ro

Naţionalitate(-tăţi) Chineză

Data naşterii 12.04.1982

Sex Masculin

Experienţa profesională

01.2016--prezent Manager de productie la Woodlands SRL din Hongkong, în Croația

10.2012-06.2017 Doctorand, activități de cercetare în domeniul de îmbătrânire a

materialelor

03.2012—06.2015 Profesor (voluntar) de limbă chineză, în cadrul Institutului

Confucius din Brașov

Perioada

Funcţia sau postul ocupat

Activităţi şi responsabilităţi principale

Numele şi adresa angajatorului

Tipul activităţii sau sectorul de activitate

Educaţie şi formare

Perioada

Calificarea / diploma obţinută

Disciplinele principale studiate /

competenţe profesionale dobândite

Numele şi tipul instituţiei de învăţământ

/ furnizorului de formare

Nivelul în clasificarea naţională sau

internaţională

10.2012-prezent studii doctorale postuniversitare- Facultatea de Ingineria

Lemnului, Universitatea Transilvania din Braşov

10.2010–07.2012 masterat – Structuri avansate din lemn și tehnologii inovative,

Facultatea de Ingineria Lemnului, Universitatea Transilvania din Braşov

12.2009–06.2010 Student în anul pregătitor de limbă română Universitatea

Babeş-Bolyai din Cluj-Napoca.

09.2005-07.2009 Stundent – Știința si Ingineria Lemnului, Universitatea de

Silvicultură din Beijing China.

Aptitudini şi competenţe

personale

Limba(i) maternă(e)

Limba(i) străină(e) cunoscută(e)

Autoevaluare

Competenţe şi abilităţi sociale

Competenţe şi aptitudini

organizatorice

Competenţe şi aptitudini tehnice

Competenţe şi aptitudini de utilizare a

calculatorului

Chineză

Română, Engleză

Limbi straine Ințelegere Vorbire Scriere

Română bine bine bine

Engleză bine bine bine

Sociabil, organizatoric, fire deschisă, amabil.

Computer Auto CAD, Autodesk 3dsMax, Photoshop, Word, Excel, Power Point.

Reparaţie si întreţinere a utilajelor.

73

Curriculum vitae

Europass

Personal information

Surname/name LIU XINYOU

Adrese Str. Bld 15 noimberie Nr.35-37, Brasov

Telephon 0040-0752086088

E-mail liu.xinyou@unitbv.ro

Nationality Chinese

Birthday 12.04.1982

Gender male

Work experience

01.2016--present Production Manager for Woodlands ltd from Hongkong, in Croatia

10.2012-06.2017 PhD studentresearch activity in the fields of materials ageing,

Transilvania University of Brasov

03.2012—06.2015 Chinese teacher ( volunteer) for Confucius Institute of

Brasov, Transilvania University of Brasov

Period

Occupation or position held

Main activities and responsibilities

Name and address of employer

Type of business or sector

Education and Training

Period

Qualification / diploma obtained

Principal subjects / occupational skills

covered

Name and type of education institution /

training provider

Level in national or international

classification

10.2012-present PhD studay, Faculty of wood ingineering, Transilvania

University of Brasov

10.2010–07.2012 master– Advanced Wood Structures and Innovative

Technologies , Faculty of wood ingineering, Transilvania University of Brasov

12.2009–06.2010 Student, preparation of romana language in Babeş-Bolyai

University of Cluj-Napoca.

09.2005-07.2009 Inginer – Wood science and Enginering, Beijing Forestry

University

Personal skills and

competences

Mother tongue

Other language

Social skills and competences

Organisational skills and

competences

Technical skills and competences

Computer skills and competences

Chinese

Romana, English

Foreign language Understanding Speaking Writing

Romana good good good

English good good good

Sociable, organizational, open, kind. Computer Auto CAD, Autodesk 3dsMax, Photoshop, Word, Excel, Power Point.

Repair and maintenance of machinery.