IMPACT OF MOISTURE SORPTION STABILIZATION AS A PREVENTIVE CONSERVATION APPROACH

Pavel Zítek, Tomáš Vyhlídal, Ivana Kopecká

Centre for Applied Cybernetics, Czech Technical University in Prague,

Department of Technology, National Institute for the Preservation of Cultural Heritage

Outline of the presentation

• Unheated historical buildings and moisture originated damage of the deposited collections.

• Modelling the equilibrium moisture content (EMC).

• Compensating the impact of temperature drift on EMC by means of humidity adjustments

• Equal-sorption humidity control, implementation experience

Introductory Statements

To remove the harmful impact of moisture consist inaverting the moisture content changes,

i.e. in maintaining the equilibrium moisture content (EMC) constant.

Main concern: Unheated historical buildings are peculiarwith great temperature and humidity changes necessarily bringing heavy sorption transport phenomena.

Motto of preventive conservation : Not to remedy the consequences but

to remove the causes of damage as soon as they emerge.

Primary Considerations

Challenge: A great deal of European cultural heritage is preserved in unheated historical buildings where the

air humidity impact is a prime cause of deteriorating the deposited artistic or historical collections.

Problem: EMC is not available as a measured variable – hence only an EMC model can provide information needed for EMCcontrol. Moreover: EMC is not even a microclimate parameter but a property appropriate to a specific material.

A chance: Basically, not the steady state but first of all the changes of moisture content bring about the damaging impact on the

artefacts. Therefore certain air humidity correction cancounteract these changes and become a potential means to

prevent the exhibits from this impact.

The Karlštejn Castle – the Main Tower

The Holy Cross Chapel – collection of medieval paintings

Three-parameter Henderson model of moisture content

Henderson: EMC – relationship to relative air humidity and temperature T

Three parameters: distinct for various materials A in 1/°C, B in °C, C dimensionless

1,0,1,0)()1ln(

uBTA

uC

Character of dependence: EMC increases with growing relative humidity φ and decreases with growing temperature T.

Saturation – apparently, the model is not applicable in cases near the state of saturation (ln(0) does not exist). However, such

conditions are inadmissible for the deposits.

Equal-Sorption Humidity Adjustment

Hence: If an air state change has satisfied this condition the materialwith the sorption parameter B does not change its EMC.

For a temperature change froma specific humidity change can be provided that both these changes

cancel each otherif they satisfy the following condition

1. Henderson model suggests a principle to maintain EMC

constant by means of humidity corrections :

)C()()1ln(

)()1ln(

2

2

1

1

BTBT

2. However: Condition (C) holds for only one material in fact.For another material this condition requires a distinct humidity change due to a distinct B.

21 to TT

Crucial Find: Humidity Adjustments for Various Materials

The temperature parameter B takes on relatively high values compared to the temperature range supposed:

Take two materials M and N with sorption parameters and suppose an air state change fromto a different temperature T. The desired new humidity values result as follows

Result: The humidity adjustments for M and N are rather lowand therefore, in fact, cannot differ significantly from each other.

)1ln(

)1ln(,

)1ln()1ln(

0000

N

N

NM

M

M

BT

BT

BTBT

00 ,TNM BB ,

TB

Implementation in Třeboň Archives – Historica Collection

The environment yearly course – before implementation

The environment yearly course – after implementation

Equal Sorption Control Implementation

Relative humidity controller adjusting the interior humidity towards the desired value

)()1ln(exp10

0 RBTBT

D

provides permanently an interior air state which does not causeany change of EMC in the material with the sorption parameter B from its value appropriate to the reference state 00 ,T

Wide validity of humidity adjustment: Due to negligible humidity differences required for various materials, the control according to(R) is valid for a wide class of preserved materials.

Conclusions

• Decisive experience: The humidity corrections inhibitting theEMC changes are only slight and for the most part of various materials their mutual differencies are negligible.

• Equal-sorption humidity control: due to approximately equal humidity corrections for various organic materials – the humidity adjustment can be considered as microclimate control in fact.

• Predictive character. Due to very slow response of the moisture content the humidity control is easily able to provide its adjustment in advance, i.e. actually to remove the causes of the EMC changes before their consequences emerge.

• Power consumption. Due to minimizing the heating demands the proposed equal-sorption control considerably reduces the power consumption.