Article - Schalm2010 - Manganese Staining of Archaeological Glass

8/13/2019 Article - Schalm2010 - Manganese Staining of Archaeological Glass

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M A N G AN E S E S TA I N I N G O F A R C H AE O L O G IC A L G L A S S :

T H E C H A R A C T E R I Z A T I O N O F M n - R I C H I N C L U S I O N S

I N L E A C H E D L A Y E R S A N D A H Y P O T H E S I S O F

I T S F O R M A T I O N *

O. SCHALM,1 K. PROOST,1 K. DE VIS,2 S. CAGNO,1 K. JANSSENS,1 F. MEES,3

P. JACOBS4 and J. CAEN2

1Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium2Conservation Studies, Artesis University College of Antwerp, Blindestraat 9, B-2000 Antwerp, Belgium

3Department of Geology and Mineralogy, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium4Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium

During the study of a large number of archaeological glass fragments, manganese-richinclusions in leached layers were observed in a limited number of cases. This phenomenon

occurs only in black-coloured leached layers. Since the formation mechanism of such

manganese-rich inclusions is still unclear, a combination of several analytical techniques was

used in order to investigate this phenomenon and, more specifically, to obtain more informa-

tion on (a) the composition and morphology of the inclusions, (b) the chemical state of Mn and

(c) the 3D morphology of the inclusions. A mechanism that might explain the formation of

these inclusions is proposed.

KEYWORDS:ARCHAEOLOGICAL GLASS, MANGANESE STAINING, HETEROGENEITIES,

LEACHED LAYERS

INTRODUCTION

Historic glasses contain usually a small amount of manganese (Sayre 1963). Most of this

manganese is present as the colourless Mn(II), while a small fraction may exist as the strongly

coloured Mn(III). Higher oxidation states are normally not present in glass because they are

unstable at the temperatures at which glass is produced. This element was introduced into the

glass batch from two different sources: (1) as an impurity in one of the raw materials, such as

wood ash (Misra et al. 1993; Stern and Gerber 2004); and (2) as a deliberate addition of

pyrolusite to the liquid batch, in order to decolorize the glass (Smith and Gnudi 1990). In the

liquid glass, the higher oxidation states of manganese transforms the colouring substance Fe(II)into the less colouring Fe(III).

The presence of Mn can also cause darkening of glass. It is known that the oxidation of Mn(II)

in colourless glass, initiated by sunlight, results in the formation of purple glass in the course of

time (Long et al. 1998; Kunicki-Goldfinger 2008). This phenomenon is known as solarization.

However, there is another mechanism in which glass can be discoloured by manganese. Several

authors have noticed that a darkening results from the intrusion of manganese from the environ-

ment into decaying glass (Newton and Davison 1989; Cox and Ford 1993; Domnech-Carb

et al. 2001; Silvestri et al. 2005). This effect is not only observed in medieval glass, but also in

Roman glass (Janssenset al. 2000). The brown areas in the corrosion layer contain manganese in

*Received 14 July 2009; accepted 29 January 2010

Archaeometry53 , 1 (2011) 103122 doi: 10.1111/j.1475-4754.2010.00534.x

University of Oxford, 2010


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the 4+oxidation state (Cooperet al. 1993; Domnech Carbet al. 2006). According to Silvestri

et al. (2005), the occurrence of zones rich in Mn, Ca, P and Fe in corrosion layers are charac-

teristic for underground alteration. It does not occur for marine alteration.

In nature, manganese oxides and manganates are formed by either a chemical or a biological

oxidation of Mn(II), in a variety of aquatic environments (Mandernacket al. 1995). Since theformation mechanism of manganese-rich inclusions is still unclear, a number of historical glass

samples containing this type of inclusion were characterized by means of several analytical

techniques.

EXPERIMENTAL

The samples in this study were selected from a larger set of samples of 14th17th century window

glass that was analysed during a quantification campaign (Schalmet al. 2007). Most of the glass

fragments studied in that work were excavated in Raversijde, Antwerp and Namur (Belgium).

Several fragments were embedded together in one block of resin. The fragments in the resinwere orientated in such a way that the cross-sections, perpendicular to the original glass surface,

could be readily studied. The surface of the resin was ground flat with corundum paper and

polished with fine diamond pastes (down to a final diameter of 0.25 mm). In many cases, the

sample preparation of extremely deteriorated glass was impossible. The slightest pressure on

these glasses resulted in breakage or even pulverization. Only fragments that were heavily

deteriorated but that were mechanically strong enough could be embedded and polished. An

Olympus SZX12 binocular optical microscope was used to study colour fluctuations in the

leached layers at a microscopic scale.

A scanning electron microscope energy-dispersive X-ray system (SEMEDX) was used for

the compositional study of small zones (down to 100 mm3

) in samples and for visualization ofelemental distributions on a microscopic scale. The instrument was equipped with a thin-window

Princeton Gamma Tech energy-dispersive Si(Li) detector. The analyses were performed on the

samples after the resin blocks were coated with a thin layer of carbon. The system allows the

recording of X-ray emission lines for elements down to carbon; detection limits of approximately

0.1wt% for elements with an atomic number between 11 and 20 are obtained. For the quantitative

analyses, X-ray spectra were collected with a 2 nA electron beam at an energy of 20 keV. The

spectra were collected at a maximum magnification of 3000 over a time period of 400 s. Under

such circumstances, sodium migration was not observed during the measurements. The chemical

composition was calculated by using a thin-film elemental yield approach (Schalm and Janssens

2003).Synchrotron radiation was employed to obtain information on the oxidation states of manga-

nese in the samples. These experiments were performed at the micro-focus beam line L of

the Hamburger Synchrotronstrahlungslabor (HASYLAB) in Hamburg (Germany), using a

polycapillary-based set-up (Falkenberget al. 2001).A polycapillary half-lens produced a focused

beam on the sample surface with a diameter of approximately 20 mm (Proost et al. 2003). The

sample was mounted on a sample stage that could be remotely translated in the x,y,zdirections.

Fluorescent photons, produced by the sample, were detected by an energy-dispersive Si(Li)

detector in a 45 geometry. When performing microscopic X-ray absorption near-edge structure

(m-XANES) experiments on the manganese edge, the energy of the primary beam is varied in

small steps (1 eV) around the MnK edge at 6.435 keV. During such an energy scan, the numberof manganese fluorescence photons was recorded by the energy-dispersive detector during

typically 520 s per energy value, in order to collect a MnK XANES spectrum. The XANES

104 O. Schalmet al.

University of Oxford, 2010, Archaeometry 53, 1 (2011) 103122


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spectra of original glass, the leached layers and the dark Mn-rich inclusions glass were qualita-

tively compared to those of the following reference compounds for, respectively, Mn(II), Mn(III)

and Mn(IV)that is, MnSO4.H2O, Mn2O3and MnO2in order to obtain information on the

oxidation state of manganese in the corroded glasses. The origin of the energy scale of the

XANES spectra was arbitrarily set to the first inflection point of the MnSO4.H2O referencespectrum. The reference compounds were diluted with boron nitride (about 1% of manganese in

BN) and pressed into pellets in order to minimize self-absorption effects.

Three-dimensional information on the Mn-rich inclusions in some of the corrosion bodies was

obtained by absorption tomography, using a SKYSCAN-1072 high-resolution desktop micro-CT

instrument (www.skyscan.be). In this instrument, an X-ray tube with a tungsten anode produces

a beam of polychromatic radiation. The sample of interest, a heavily corroded glass fragment,

was rotated in the beam while a CCD camera behind the sample recorded the transmitted beam,

visualizing the attenuation by the sample under different rotation angles. A mathematical recon-

struction algorithm allows the calculation of the absorption in arbitrary slices within the object of

interest. This was done using Cone-Beam Reconstruction software delivered with the instrument.The X-ray tube was operated at 78 mA and 137 kV.

THEORETICAL BACKGROUND

Just beneath the surface of a glass object, the chemical composition and the structure of the glass

network are affected by reactions with substances from the environment surrounding the glass.

Moisture is the most important factor initiating and sustaining the different reactions leading to

glass decay. Usually, chemical deterioration is described as a combination of three simultaneous

partial processes (Silvestriet al. 2005; Pollard and Heron 2008). The processes, described below,

explain how a leached layer is formed and why the local concentration of mobile cations such asNa+, K+, Mg2+, Ca2+, Mn(2+,3+) and Fe(2+,3+) decreases.

The first process is the penetration of molecular water into the glass. This is due to molecu-

lar diffusion and/or through reversible hydrolysis and condensation reactions. When water

tagged with O18 diffuses into the glass, O18 exchanges with O16 atoms of the siliconoxygen

glass network. During this process, the silicate network is subject to structural transformations

(Bunker 1994; Doremus 1998). For buried medieval glass, the diffusion of ppm-amounts of

molecular water into the glass surface precedes the ion exchange (Sterpenich and Libourel

2006):

+ ( ) Si O Si H O aq 2 Si OH2

2 Si OH Si O Si H O2 glass + ( )

The process of ion exchange is based on the leaching of mobile cations from the glass and their

substitution with protons following the reaction given below (Scholze 1982; Schreiner 1991;

Melcher and Schreiner 2004):

+ + +( )+

( ) +

( )+

( )Si O M H Si O H Mglass aq glass aq

In this process, the silicate network remains intact. It is driven by the concentration gradient

between the glass and the environment. The mobility of the cations depends on their charge,

their size and on the composition of the initial, uncorroded glass. As a result, some glass

compositions are more resistant to leaching than others. For example, it is well known that(silica-rich) Roman glass is usually in a better condition than (silica-poor) medieval glass, in

spite of its more advanced age. Since in this process, (heavier) cations are replaced by (lighter)

Manganese staining of archaeological glass 105



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protons, while the volume of the leached glass remains very similar, the density of the leached

layer becomes smaller than that of the original glass. The leached-out ions are able to form

weathering products such as sulphates, carbonates, hydro-carbonates, chlorides or nitrates on

top of the glass surface. These products are formed when the solubility products of these

substances present in the water film on the surface are exceeded (Schreiner et al. 1999;Woisetschlger et al. 2000; Domnech-Carb et al. 2006; Melcher and Schreiner 2006).

If small amounts of water in close contact to the glass are not regularly replenished, its pH will

increase as a consequence of the ongoing ion-exchange. This is the case when water is entrapped

in cracks or in surface scratches. When pH values exceed the value of 9, hydroxyl ions in solution

will attack the silicate network:

+ + ( )Si O Si OH Si OH Si Oaq

+ + ( )

( )Si O H O Si OH OH2 aq aq

A consequence of these three reactions is that a leached layer is formed, characterized by a

cation-depleted and structurally weakened SiO network. This so-called leached layer gradually

increases in thickness until the complete glass is transformed into a deteriorated product. The

leached layer can be transformed into a silica-rich film in which the number of SiOSi bonds has

been increased (Hench 1975).

Although the above-described reactions clearly fail to predict the transformation of homoge-

neous glass into heterogeneous leached layers, many types of heterogeneous leached layers have

been reported in the past (Cox and Ford 1993; Sterpenich and Libourel 2001). Alteration zones

show a typically finely laminated structure in which hydrated phosphate compounds might be

found. Freestone (1985) reported that Ca phosphate can be precipitated inside the leached

layer of archaeological glass, probably in the form of apatite (Ca5(PO4)3OH) or brushite

(CaHPO4.2H2O). Bioglass is another example that shows how glass containing P2O5is able tointeract with Ca-rich compounds from the environment. In that case, the P2O5in the glass is able

to form a strong and stable bond with newly growing bone. This is due to the formation of a stable

calcium phosphate film on the surface of the material (Hench 1975). A special form of hetero-

geneities in leached layers is the presence of manganese-rich inclusions. In the list of corrosion

products formed during precipitation reactions, manganese-rich compounds are usually not

mentioned.

RESULTS

Morphology of some heavily deteriorated glasses

Among the ~500 archaeological window glass fragments examined, several heavily or even

completely deteriorated samples containing dark inclusions were identified. This type of inclu-

sion was only encountered in blackened fragments. Moreover, the dark colour was always

restricted to the leached layers themselves, and was never present in the original, unaffected parts

of the glass. Optical and backscattered electron images of cross-sections of the samples that were

examined in detail are shown in Figures 1 and 2. A number of the heavily deteriorated glass

fragments still contained a core of original glass, with a chemical composition very similar to that

of other, less corroded, glass fragments. For buried archaeological glass, two types of manganese-rich inclusions could be distinguished: (1) dark inclusions in the low-density lamellae of stratified

leached layers, and in cracks such as encountered in a 15th century artefact excavated in

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Raversijde and shown in Figure 1; and (2) dendritic invasion of the leached layer by brownish-

black material, such as found in the glass fragments recovered in Namur and Antwerp. Figure 2

(a) shows an optical image of a polished cross-section of a fragment of 16th17th century

window glass excavated in Namur, while Figure 2 (b) shows a cross-section of a 14th century

window glass sample excavated in Antwerp. The optical microscopy image of Figure 1 demon-

strates that the leached layer just below the glass surface has a brownish to black colour. While,

optically, no particular structure can be discerned in this dark-coloured area, the corresponding

backscattered electron microscopy image (Fig. 3 (b)) does reveal a layered microstructure in this

region. In Figure 3 (b), the light grey region at the bottom of the image is the original glass, the

dark grey region is the leached layer and the bright inclusions within the leached layer are thebrown-black coloured inclusions. The black area at the top of the image is the surrounding resin,

in which the samples are embedded. The stratified structure of the leached layer consists of

alternating layers of lower and of higher densities. This image suggests that the dark inclusions

were preferentially formed in the darker (i.e., low-density) lamellae. The optical photographs

shown in Figure 2, on the other hand, clearly show that here the blackbrown structures were not

formed in low-density lamellae of stratified leached layers but in more homogeneous leached

layers, forming dendritic structures.

The phenomenon of manganese staining was also found in a 15th century free-standing stained

glass window. In an originally colourless pane, manganese-rich inclusions were found in the

dark-coloured lamellae of the stratified leached layer. This is shown in Figure 4. Next to thesemanganese-rich inclusions, the leached layer contains regions rich in calcium and phosphorus as

well.

Figure 1 An optical image of a deteriorated region in a cross-sectioned 15th century sample excavated in Raversijde.

In this sample, Mn-rich inclusions can be found between the lamellae of the stratified leached layer. This can be seen in

Figure 3.




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Compositional analysis of the glass samples

It is striking to note that almost all of the ~500 15th17th century window glass fragments that

were analysed have a composition that is very similar to the average composition, given in

(a)

(b)

Figure 2 An optical image of a dendritic invasion of manganese in cross-sectioned samples excavated in (a) Namur

(16th17th century) and (b) Antwerp (14th century).

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Figure 3 X-ray images (a) and a backscattered electron image (b) of the cross-sectioned 15th century sample of

Raversijde. In this sample, manganese migrated along the cracks and between the low-density regions within the lamellarstructure.




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Table 1. Window glass produced before the 15th century has a clearly different composition from

that of later periods, especially regarding its SiO2 content. Despite this great compositional

similarity of the original glass, among the analysed set were encountered:

(a) fragments with almost no signs of deterioration,

(b) heavily deteriorated fragments but still with a core of original glass, and

(c) fragments that no longer contained any original glass.

Clearly, in such a sample set, it was not possible to find a relation between the chemical

composition of the original glass and its resistance against corrosion. It can be noted in Table 1

that, on average, the original glass contains a small amount of MnO (0.85%). Although other

metals such as copper, zinc and lead are present in the inclusions (see below, Figs 3 and 5), theseconcentrations are too low to allow detection in the original, uncorroded glass by means of

SEMEDX.

Figure 4 A detail of the leached layer of a glass fragment from a 15th century free-standing stained glass window: (a)

optical image; (b) backscattered electron image; (c) X-ray images.

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Figure 3 (a) shows SEMEDX X-ray images of the Raversijde fragment (shown in Fig. 1).

The dark inclusions that formed along the low-density lamellae in stratified leached layers are

rich in manganese. The backscattered electron image of the central part of the corrosion body (see

Fig. 3 (b)) demonstrates that manganese has penetrated the darker lamellae in the leached layer,

indicating that the (lateral) diffusion in these layers is much faster than in the high-density

brighter ones. There is also an Mn-enrichment in cracks perpendicular to the lamellae.Figure 5 shows a number of SEMEDX X-ray spectra collected from the Raversijde fragment.

Figure 5 (a) is a spectrum of the original glass, where an initial amount of ~0.8% manganese is

Table 1 The average composition of 396 15th17th century

calco-potassic window glasses

Oxide Concentration [min; max]

Na2O 1.55 10.06 [0; 5.7]

MgO 3.57 10.04 [0.04; 7.4]

Al2O3 3.58 10.05 [0.6; 5.7]

SiO2 58.7 10.1 [51.5; 67.4]

P2O5 2.33 10.04 [0.14; 4.7]

SO3 0.20 10.01 [0; 1.03]

Cl 0.29 10.01 [0; 0.81]

K2O 6.2 10.1 [1.4; 18.6]

CaO 21.3 10.1 [8.8; 26.3]

MnO 0.85 10.02 [0.01; 4.7]

Fe2O3 0.93 10.01 [0.14; 2.15]

BaO 0.50 10.01 [0.02; 1.42]

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9 10

Energy (keV)

Relativeintensity

O

Al

Si

P

Pb-M

KCa

Ba

ZnCu

Mn FeMg

Na

(a)

(b)

(c)

Figure 5 SEMEDX X-ray spectra of (a) original uncorroded glass, (b) a leached layer and (c) a manganese-rich

inclusion from the Raversijde sample shown in Figure 3.




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present, next to other elements that are common in glass, such as Al, Si, P, K, Ca and Fe. Figure 5

(b) shows the SEMEDX spectrum of the leached layer. It is clear that the intensity of the X-ray

lines of most elements decreased as a result of the leaching process, while that of Al and Si

remained as dominant lines. The spectrum in Figure 5 (b) also indicates that more or less the

same concentration of Mn is present in the leached layer as in the original glass. In Figure 5 (c),

the spectrum of one of the manganese-rich inclusions is presented. While a lower Si signal is

observed, the Mn peak is substantially more intense than in Figure 5 (b). The spectrum in

Figure 5 (c) demonstrates that inside the manganese-rich bodies there is still a considerableamount of silicon present, while other metals such as copper, zinc and lead also appear to have

co-precipitated together with the Mn. This sample suggests that manganese migrated from the

environment into the glass.

X-ray images collected by means of SEMEDX of the Namur and Antwerp fragments (cf.,

Fig. 2) are presented in Figures 6 and 7. These X-ray images demonstrate that the Mn is

concentrated in dark-coloured inclusions, of ~30200 mm in size. In Figure 6, just beneath the

surface, at the bottom of the image, the inclusions appear to be isolated and surrounded by the

leached glass layer. However, the images acquired with light microscopy suggest that the isolated

inclusions in the leached layer have a tree-like appearance, spreading out from a central position

at the surface of the leached layer. These images indirectly suggest that the inclusions are notisolated inclusions, but are part of a three-dimensional structure that has grown into the leached

layer from the surface.

Figure 6 SEMEDX X-ray images of the cross-sectioned sample shown in Figure 2 (a).

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Internal or external source of manganese?

When a volume of 1 mm3 of healthy glass contains less manganese than 1 mm3 of corroded glass,

then it can be assumed that manganese has migrated from the environment into the glass (i.e.,

external source). In the opposite case, the formation of inclusions is caused by concentrating the

manganese from the glass itself into a limited number of inclusions (i.e., internal source).

In principle, the type of source causing the formation of manganese inclusions can be deter-

mined by measuring the volume fraction, Vinclusions, occupied by the inclusions and by the volume

fraction of the leached layer, Vleached, inside the corroded glass. This can be done by means of

tomography. Also, the average manganese concentrations in the healthy glass,Whealthy(Mn), in the

leached layer,Wleached(Mn), and in the inclusions,Winclusions(Mn), need to be determined. However,

the major problem with SEMEDX is that the quantitative analysis requires a normalization of100 wt%, which is hampered by the unknown amount of water in the corroded glass. Two

methods were used to overcome this problem:

SiO2content as an internal standard.The number of Si atoms per cm3 in the healthy glass and

in the leached layer can be considered as the same. For this reason, the concentration W(Mn) in

wt% is proportional to the ratio of peak intensities I(Mn)/I(Si). However, for the inclusions this

is not always the case, because they can be formed in zones that are poor in SiO2, such as cracks

or low-density lamellae. As a result of this, the concentration W(Mn) for the inclusions will be

overestimated.

Peak-to-background ratio for the MnKa peak.Although this ratio is only approximately

proportional to the manganese concentration, it is an easy way to suppress the effect of live time,beam current or self-absorption of X-ray peaks. This method is used for the quantitative analyses

of various mineral particles (van Grieken and Markowicz 2002):

Figure 7 X-ray images of a cross-sectioned 14th century sample excavated in Antwerp. This image illustrates the

dendritic appearance of the manganese invasion.




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R V W V W

W=

( ) + ( )leached leached inclusions inclusions

health

Mn Mn

yy Mn( )

For one tomographed archaeological sample, the volume fraction occupied by the inclusionsinside the corroded glass is 0.085. When the SiO2content is used as internal source, the ratio R

is equal to 1.78. For the peak-to-background method, Ris equal to 1.29. Both methods suggest

that manganese originates from an external source. However, neither quantification method could

be validated with the samples that were analysed and, therefore, the results should be used with

care. Fortunately, from an experimental point of view, the X-ray spectra collected from the

Raversijde sample and shown in Figure 5 indicate that it is possible for manganese to migrate

from the environment into the corroded glass because the intensity of the Mn-Kapeak in the

leached layer is similar to that of the healthy glass.

Manganese speciation by means ofm-XANES

The dominant oxidation state of manganese in the original glass, in the leached layer and in the

inclusions can be determined by means ofm-XANES, because a positive relation exists between

the redox state and the position of the MnK edge (Farges 2005). Such a relation was also noticed

for the MnL3edge (Schofieldet al. 1995). In uncorroded glass, Mn is predominantly present as

Mn2+ and, therefore, typically XANES spectra are obtained resembling that of the MnSO4reference compound shown in Figure 8.

Figure 8 (a) shows MnK XANES spectra from the original glass and the leached layer of the

Raversijde sample, while Figure 8 (b) shows the XANES spectra of the Mn-rich inclusions in

comparison to the reference spectra. The XANES profiles of the original glass and the leachedlayer indicate that mainly Mn2+ is present. The fact that the spectra of original glass and leached

layer profiles are nearly identical indicates that during the leaching process, no changes in the

(a) (b)

Figure 8 (a) Manganese K-edge XANES profiles of the original glass core and the leached layer of a heavily corroded

glass, excavated in Raversijde. Leached glass: profiles A and B; original glass: profile C. (b) Manganese K-edge XANES

profiles of a manganese-rich inclusion between lamellae of the stratified leached layer, excavated in Raversijde and

shown in Figure 3.

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oxidation state of the Mn took place. The XANES spectra of the manganese-rich areas show a

strong resemblance to the MnO2reference profile. However, on the left edge of the absorption

jump an additional shoulder is visible in spectra AD of Figure 8 (b), indicating that next to

Mn(IV) lower-oxidation states of manganese can also be present. This might be the result of the

presence of Mn(II) and Mn(IV) in close proximity. Between the Mn(IV) inclusions, insoluble

Mn(II) hydroxides or carbonates might be present as well (Tonner et al. 1999).In Figure 9, the XANES spectra from the original glass, the leached layer and the Mn-rich

inclusions of the Namur samples are displayed, showing similar spectra as found for the Raver-

sijde samples. The same applies to the XANES spectra recorded from the Antwerp fragment

shown in Figure 10.

It can be concluded that in all cases, Mn(II)-like profiles are observed in the original glass and

in the leached layer, while the XANES spectrum of the dark inclusions or inclusions mostly

resembles that of Mn(IV). This similarity suggests that with respect to the Mn redox state, there

are no fundamental differences among the Mn-rich inclusions found in the corrosion bodies

studied. Therefore, it can be assumed that both types of inclusions (Mn inclusions between

lamellae and dendritic-shaped inclusions) are formed by identical, or at least very similar,processes.

The three-dimensional form and structure of the manganese inclusions

Although the two-dimensional images collected by optical and electron microscopy (cf.,

Figs 17) suggest that in some cases the Mn-rich inclusions form complex three-dimensional

dendrites or other shapes, these techniques only allow the visualization of the cross-sections of

these structures. A suitable method for studying these structures in their entirety is X-ray

absorption computed micro-tomography (CMT). The resolution and sensitivity obtainable from

present-day desktop X-ray micro-tomographs is sufficiently good to permit a detailed visualiza-tion of the MnOxnetwork within the corrosion bodies without physically slicing them (Roemich

et al. 2005).

Figure 9 Manganese K-edge XANES profiles of heavily corroded glass, excavated in Namur: profile A, Mn-rich

inclusions; profile B, leached layer; profile C, original glass.




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Figure 10 Manganese K-edge XANES profiles of dendritic Mn-rich inclusions present in the leached layer of the glass,

excavated in Antwerp (see Fig. 2 (b)): profiles A and B, Mn-rich inclusions: profile C, leached layer.

Figure 11 A tomographic slice of a sample, excavated in Antwerp. This cross-section clearly shows the dendritic

invasion. Crusts on the surface of the leached layer are also visible.

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A typical absorption tomogram is presented in Figure 11, showing the density variation within

the Antwerp fragment. The dark area is the remaining original glass, featuring a higher density

(and therefore a higher linear absorption coefficient) than the surrounding leached layers. The

dark spots inside the leached layer and close to the surface are the manganese-rich inclusions. In

this image, the dendrites just below the surface, the cracks (i.e., the dark lines), and tubular

inclusions (elliptical inclusions in the middle of the leached layer) can clearly be observed.

Another advantage of the use of tomographic imaging in this context is the fact that, simulta-

neously with vizualizing the density variations induced below the surface by the leaching

process, the precipitate formation on top of the original glass surface can also be studied; inFigure 11, the crust-like material attached on the lower glass surface has a dark tone, indicating

a high density/strong X-ray absorption, probably due to the presence of the cations that were

leached out of the glass. This material is usually not observed during electron microscopy

analyses, as it disappears during sample preparation.

The manganese inclusions in the 3D reconstruction (the dark grey pixels in Fig. 11) were

isolated from the surrounding material (the light grey pixels) by removing all pixels that did not

correspond with the grey value of the Mn-rich inclusions. Small regions were selected from the

data cube in order to evaluate the shape of the inclusions. In this manner, different shapes of

manganese-rich inclusions can be distinguished, as shown schematically in Figure 12:

(a) dendrites,(b) planar shapes, and

(c) tubular structures.

Figure 12 Manganese inclusions in different shapes: (a) penetration of dendrites from the glass surface, seen from the

centre of the glass towards the surface; (b) manganese in planar shapes; (c) tubular manganese enrichments just below

the dendrites; (d) a schematic overview of the different shapes.




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The dendrites are located just below the glass surface and have a maximal penetration depth of

~600 mm. The dendrites shown in Figure 12 are seen from the centre of the glass towards the glass

surface. Due to a lack of resolution, only the contour of the dendrites can be observed, resulting

in droplet-shaped dendrites. The planar and tubular-shaped Mn-rich regions (see Figs 12 (b) and12 (c)) are mostly located deeper in the glass and are often oriented parallel with the glass surface.

A hypothesis explaining inclusion formation

For all of the glass analysed, manganese-rich inclusions occur only in the leached layer and never

in the original remaining glass. This indicates that the formation of such inclusions occurs in two

steps, where the first step involves the formation of a leached layer, followed by the formation of

Mn-rich inclusions in the leached layer. The reason why these inclusions were systematically

found in the leached layer and never in the original glass might be explained by a much higher

mobility of ions in the low-density leached layer. In stratified leached layers, which are usuallyparallel to the glass surface, diffusion in the low-density lamellae is easier than in the higher-

density lamellae, resulting in planar manganese inclusions. The formation of such parallel planes

of Mn-rich inclusions might be explained using the mechanism shown in Figure 13. The Mn

inclusions are formed along the interface between the healthy glass and the leached layer. When

the leached layer grows, the Mn inclusions will be formed at the new interface, deeper into the

glass. This mechanism suggests that the formation of Mn-rich inclusions can occur simulta-

neously with the formation of the leached layer formation. In that case, the formation of Mn

inclusions is influenced by the slow movement of the border between the leached layer and the

healthy glass. This explains the occurrence of Mn precipitates in elongated cracks perpendicular

to the surface, as can be seen in Figure 3 (b). In other cases, these inclusions can also be formedat later times, when the leached layer is already present.

It is remarkable that the Mn compounds are concentrated in a limited number of inclusions,

surrounded by Mn-poor leached layer. For one tomographed sample, the inclusions occupied

only 8.5 vol% of the corroded glass. Such a local enrichment can be the result of the migration

of all the manganese in the corroded glass (i.e., the internal source) towards a limited number of

centres. This mechanism might explain the presence of black inclusions in stained-glass windows

(Perez y Jorba et al. 1980). However, in one archaeological inclusion-containing sample, the

leached glass and the original glass feature similar Mn concentrations. In that case, the presence

of inclusions can only be explained by an intrusion of soluble Mn2+ from the surrounding

environment (i.e., the external source), where the glass was buried, into the glass surface.Experiments in which highly sensitive glasses were immersed in a solution of Pb2+, Cu2+ and Mn2+

demonstrated that these cations are able to penetrate the leached layer but not the healthy glass.

Figure 13 The formation of specific morphologies when the movement of the border between the leached layer and the

healthy glass is hampering the formation of Mn inclusions.

118 O. Schalmet al.



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However, during such experiments the pH close to the glass surface increased, which could result

in a precipitation of the soluble Mn2+ into Mn(OH)2. It should be remarked that this phenomenon

might have affected the conclusions of Watkinsonet al. (2005). It is clear that for both internal

and external Mn sources, the formation of darkened glass requires a migration of Mn2+ towards

the inclusions. This migration is needed in order to explain the heterogeneous distribution of

manganese throughout the leached layer.

The Mn-rich inclusions appear to consist of Mn4+. Also, for dendritic pyrolusite intrusions in

sandstone and for manganese rock varnish the Mn valences are between 3+and 4+(McKeown

and Post 2001). Since the solubility of oxides and hydroxides of Mn4+ in aqueous systems is very

low, migration of Mn4+ from the surrounding environment into the glass cannot explain thepresence of Mn4+-rich inclusions. Moreover, Mn4+ does not exist in the original glass and in the

leached layer. This means that during the formation of the inclusions, Mn2+ must have been

oxidized into Mn4+. Oxidation of Mn2+ requires oxygen gas and water. This means that a flux of

both constituents, and in some cases also Mn(II), migrates from the external environment into the

leached layer. At the same time, a flux of Mn(II) migrates out of the glass. The interdiffusion of

both fluxes causes an oxidation and precipitation of Mn(IV) compounds as a direct consequence.

This reaction mechanism is visualized in Figure 14.

This explains why the inclusions are always in contact with the glass surface. Although the

oxidation of manganese appears to be driven by a spontaneous chemical reaction, a biological

driving force is not excluded. It should be mentioned that some micro-organisms live on theoxidation of manganese (de la Torre and Gmez-Alarcn 1994):

2Mn O 2H Oleached glass 2 leached glass 2 leached glass2+

( ) ( ) (+ + ))

( )+

( )

+2MnO 4H2 leached glass leached glass

Once nucleation centres are formed, a depletion of manganese around these centres occurs and

a concentration gradient is set up. As a consequence, more Mn diffuses down the gradient towards

the nucleation centres. Most probably, only in these centres are the cations transformed into Mn 4+.

The presence of a limited number of nucleation centres suggests that earlier formed inclusions

catalyse the oxidation of Mn

2+

. It is known that reactiondiffusion systems result in the formationof complex patterns (Chopard et al. 1999). A particular example is the formation of mineral

dendrites such as pyrolusite intrusions in sandstone and manganese rock varnish. The dendrites

Figure 14 The formation of Mn-rich inclusions due to the interdiffusion of a flux of water, oxygen and, in some cases,

also Mn(II) from the external environment into the weathered layer and a flux of manganese(II) out of the fresh glass.




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are formed by a diffusion of reagents coupled to a precipitation reaction. Aggregates are formed

when saturation is reached or when the precipitation is catalysed by aggregates that were formed

previously (Chopard et al. 1991). This mechanism appears to be very similar to the inclusion

formation in leached layers.

CONCLUSIONS

In (partially) corroded black archaeological glass fragments, manganese enrichments were exclu-

sively found in the leached glass and never in the original glass. They do not invade the leached

layer uniformly; instead, they are concentrated in well-defined inclusions. The formation of such

inclusions is probably a secondary degradation phenomenon, occurring after a leached layer has

been formed. The inclusions have a planar appearance in the low-density lamellae of stratified

leached layers and in cracks. In other cases, dendritic or tubular shapes were observed, gradually

developing from the glass surface into the deteriorated glass.

It was shown that the manganese-rich inclusions contain black-coloured Mn(IV)-rich com-

pounds. This oxidation state does not occur in the original glass or in the leached layer. Mobile

Mn(IV) ions do not occur in natural waters. Therefore, mobile Mn(II) ions originating in the

surrounding soil or in the leached layer itself must be oxidized in accumulation centres, forming

Mn(IV)-rich substance precipitates in the micropores of the leached glass. This explains why the

inclusions contain manganese-rich compounds as well as leached glass. Migration of Mn(II)

towards the inclusions is coupled with a redox reaction, leading to the formation of complex

patterns such as dendrites. Such a diffusion-reaction coupling is possible when previously formed

Mn(IV) nuclei catalyse the reaction.

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