blue enamel on sixteenth - and seventeenth-century window glass

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Blue Enamel on Sixteenth- and Seventeenth-Century Window Glass: Deterioration,Microstructure, Composition and PreparationAuthor(s): Geert Van der Snickt, Olivier Schalm, Joost Caen, Koen Janssens and ManfredSchreinerSource: Studies in Conservation, Vol. 51, No. 3 (2006), pp. 212-222Published by: Maney Publishing on behalf of the International Institute for Conservation of Historic andArtistic WorksStable URL: http://www.jstor.org/stable/20619450 .

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212

Blue Enamel on Sixteenth- and

Seventeenth-Century Window Glass DETERIORATION, MICROSTRUCTURE, COMPOSITION AND PREPARATION

GeertVan der Snickt, Olivier Schalm,Joost Caen, Koen Janssens and Manfred Schreiner

It is still unclear why some of the blue enamel on sixteenth- and seventeenth-century stained window glass is faking off, while

enamel layers with other colours are still in relatively good condition. In order to obtain a better understanding of this conservation

problem, 3t historical recipes used for the fabrication of blue enamel were compared with results from the chemical analysis of 25

historic samples. The chemical composition and the microstructure of the enamels were analysed in cross-section by means of electron probe microanalysis (EPMA). This study demonstrated that the variation in chemical composition of the samples can be

explained by the use of the wide range of recipes existing at that time. Although this study gave an insight into the composition,

heterogeneity and use of colouring substances, no clear relation could be found between the parameters analysed and the

deterioration rate of the blue enamel paint layer.

INTRODUCTION

Figurative stained-glass windows are assembled by fitting

glass panes of different shapes and colours into a network

of grooved strips of lead with an H-shaped cross-section.

The joints between the lead strips are soldered with a

Pb-Sn alloy. The details of the design are rendered by means of glass paints, which are first applied as a powder

dispersed in a painting medium (oil or water) and then

fired onto the glass pane. Since the ninth century,

grisaille has been used to paint tracelines and shades on

glass panes. It was applied firstly as a thick, opaque line

(known as 'grisaille contourner), then as a thin, uniform

layer (grisaille modeller) in order to diminish the amount of light passing through a pane [1]. Since the end of the thirteenth century, silverstain has been used to colour

glass panes bright yellow [2, 3]. Finally, during the sixteenth century, enamel glass paints were developed to

add different shades of colour (for example, blue, purple or red) to colourless glass panes. This article focuses on

the enamel glass paints.

Enamel glass paint is a coloured glass that melts at a

lower temperature than the glass substrate onto which it

is applied. It is made by melting together the flux (a

low-melting glass) with a colouring substance (such as

smalt or a copper compound). The liquid glass is cooled

quickly and the resulting flakes are ground to a fine

powder. The powder is then mixed with a small amount

of water or oil in order to obtain a paste that can be used

to paint on sheets of glass. After drying, the painted pane is fired and the powder melts to form a thin coloured

layer on the glass substrate. A schematic representation of an enamel paint in cross-section, before and after

firing, is shown in Figure 1.

A previous study on the deterioration of nineteenth

century grisaille paint layers demonstrated that both

powdering and flaking of the paint are caused by a lack

of flux in the paint [4]. For enamel paint layers the cause

of degradation is still unclear, but conservators noticed

that peeling off is the dominant type of deterioration

mechanism and that blue layers appear to be more

affected by this type of damage than other colours.

Figure 2 shows an example of a seventeenth-century roundel with blue, degraded enamel. The loss of enamel Received April 2005

STUDIES IN CONSERVATION 51 (2006) PAGES 212-222



BLUE ENAMEL ON SIXTEENTH- AND SEVENTEENTH-CENTURY WINDOW GLASS 213

enamel powder in oil

support glass

Figure 1 Schematic representation of an enamel paint in cross-section, before and after firing.

500-600'C 25X

Figure 2 Seventeenth-century roundel from the private collection of

Prof. J. Caen. The blue enamel is partially flaked off.

appears to be a common phenomenon, as it was already

reported in the seventeenth and nineteenth centuries by Dossie [5] and Reboulleau [6] respectively. This subject remained almost unstudied during the twentieth

century. To gain a better understanding of the degrada tion of blue enamel, 31 historic recipes were studied and

25 historic glass fragments with blue enamel were

analysed by means of electron probe microanalysis

(EPMA). Both studies also gave an insight into the

chemical composition and the preparation of sixteenth

and seventeenth-century enamel glass paints.

EXPERIMENTAL

Analysis was carried out on a set of 25 historic window

glass fragments containing blue enamel, which originate from a private collection. Most probably, all the

fragments come from windows from the Low Countries

and cover a period between the sixteenth and the early twentieth centuries.

Glass splinters of all the fragments were embedded in

resin. The orientation of the splinters in the resin was

such that the cross-section, perpendicular to the original

glass surface, could be studied. The surface of the resin

was ground flat with corundum paper and polished with

fine diamond pastes (up to a final diameter of 1 jam).

The samples were analysed by means of a Jeol 6300

electron microprobe system equipped with an energy

dispersive Si (Li) X-ray detector (Princeton Gamma

Tech). EPMA spectra were collected from the cross

sectioned surface. For the window glass, four X-ray

Spectra were collected at 20 kV, a magnification of

X40000, a beam current of 1 nA and a live time of 50 s.

Under these measurement conditions, no sodium

diffusion could be detected for ordinary soda-lime glass. A quantification algorithm based on thin film sensitivity coefficients was used to calculate the composition of the

window glass on the basis of the net intensities of the

characteristic X-ray peaks observed in the spectra [7]. A

data matrix was obtained, with each row consisting of

the average composition of the enamels and every

column of an oxide constituent of the coloured glass. Hierarchical clustering was performed on the data

matrix by means of the software package Statistical

Package for the Social Sciences (SPSS). The microstruc

tures of the enamel samples were studied by collected

secondary electron, backscattered electron and X-ray

images. Since EPMA is not able to detect trace elements

(<0.1 wt%), X-ray fluorescence (XRF) spectra were

recorded from the enamel surface. A Tracor X-ray

Spectrace 5000 was used to collect spectra during 100 s

at 8, 20 and 50 kV. Spectra at 8 and 20 kV were meas




214 G. VAN DER SNICKT, O. SCHALM, J. CAEN, K. JANSSENS AND M. SCHREINER

ured under vacuum conditions. A cellulose filter was

used for the spectra at 20 kV, and a palladium filter for

the spectra at 50 kV.

RESULTS AND DISCUSSION

Historic literature

Little is known about the origin of enamel on window

glass, but it might be expected that it arose in Western

Europe from a combination of the knowledge of

enamelling other materials and a flourishing art of

stained glass. The techniques of enamelling metal (such as mail cloisonn , champlev and mail peint), ceramics (for

example, majolica) or vessel glass (for example, mosque

glass) were known well before the sixteenth century. This hypothesis can be found in the historic literature

such as that by Le Vieil [8] or Levy [9]. Although enamel glass paints for window glass were

developed during the sixteenth century, the oldest

published recipe dates from the beginning of the seventeenth century, written by Neri [10]. During the

following centuries, several authors translated the work

of Neri and added their own experiences or remarks (for

example Merret [11], Kunckel [12], d'Holbach [13] and

Haudicquer de Blancourt [14]). In 1674, F libien [15] published a book concerning this matter independent of

Neri [10]. The most complete information concerning enamel was given by Dossie [5]. The paragraph about

enamel was translated by Le Vieil and published in his own book [8], which also included some recipes from

his ancestors. During the nineteenth century several

technical handbooks about glass paints were published as

a result of the strong development of chemistry and the

interest in previous trends in art. The oldest nineteenth

century work concerning this subject was written by Reboulleau [6]. Later recipes, such as those from the

author Karl Strele [16], are variations of the recipes of

Reboulleau. From this literature, 31 recipes were

collected.

All recipes are based on a low-melting glass (the flux) and a colouring substance. Most authors prescribed saffre

(CoO + contaminants) or smalt (Si02-K20-CoO

+

contaminants) to give the enamel a dark blue colour.

Only Neri [10] and Dossie [5] suggested in some recipes the use of calcined copper (CuO). During the

nineteenth century, the authors prescribed pure,

industrially produced cobalt oxide (CoO). The combination of copper and cobalt ingredients are not

often found together in the recipes. Dossie [5] also

mentioned two recipes with ultramarine, but it is very

unlikely that this pigment was actually used for enamel

paint on window glass, as it would not have produced a

blue colour in transmitting light. The flux was formed by melting sand or quartz (SiOz)

together with ingredients to lower the melting point of

the resulting glass. When studying the seventeenth

century recipes, one cannot overlook the variety of raw

materials and the changing proportions in which they were used. A wide range of raw materials containing

potassium, sodium and lead were introduced into the

batch. In addition to lead compounds and wood ash

(potash), two common ingredients in the fabrication of

ordinary glass, other ingredients like potassium tartrate

(HOOC-CHOH-CHOH-COO-K+), saltpetre

(KN03), sea salt (NaCl) and borax (Na2B4Oy40H2O) were used. In contrast to the older recipes, those of the

nineteenth century are very similar. Based on the

amount of PbO in the flux, three types of recipes can be

identified. The composition of all recipes are presented Table 1 and can be summarized as follows:

Type Rl: Although these fluxes do not contain any

lead, they still melt at a much lower temperature than

window glass. They contain a high amount of alkali

rich ingredients and a low amount of alkaline-earth

raw materials. Only seven recipes from the seven

teenth century are among this type.

Type R2: These fluxes contain a small amount of

lead (10 20 wt% PbO present in the glass composi tion calculated from the recipe) in combination with

a high amount of alkali-rich ingredients: K20 (5-57

wt%), Na20 (0-33 wt%). Thirteen of the recipes from the seventeenth century belong to this type.

Type R3: These fluxes contain a high amount of lead

(30-80 wt% PbO present in the glass composition calculated from the recipe). The five recipes from the

nineteenth century and six of the recipes from the

seventeenth century are of this type. Recipes of the

latter period have variable concentrations of Na20

and K20. The nineteenth-century recipes prescribe a

flux of the Si02-PbO-Na20 type. CaO is absent

from all recipes of type R3.

Many of the flux compositions in the recipes deviated

from the expected Si02-PbO glass type. It is not clear if

all these compositions were actually used in practice. It

is possible that the chemical composition is one of the

reasons for accelerated deterioration of the blue enamel

paint layers.





Table 1 The composition (wt%) of all recipes in chronological order Ingredients (%)

Recipe Si02 PbO K20 KN03 Pot. Tar.

Na,0 NaCI Borax As90 SnO CaO Ca3 HgO Bi203 CuO CoO Mn02 Lazurite

(po4)2

N1 N2 N3 F KN1 KN2 KN3 K1 K2 K3 K4 K5 K6 K7 K8 D1 D2 D3 D4 D5 D6 D7 D8 LV1 LV2 LV3 Re1 Re2 Re3 Re4 S

35 35 35 53 19 19 19 34 14 22 17 60 10 29 40 44 43 31 29 64 59 48 44 43 43 47 29 8 20 24 28

21 21 21 13 21 21 21 14 14 46 17 20 80 57 43 21 18 36 32 0 0 0 0 0 0 0 29 64 41 48 56

4 0 1 4 0 1 4 0 1 0 20 0 22 0 0 22 0 0 22 0 0 0 0 14 0 0 57 0 0 4 0 0 17 0 0 0 0 0 0 0 0 0 1 0 3 13 0 0 12 0 0 24 0 0 22 0 0 23 0 0 15 0 0 18 0 0 110 0 13 0 0 13 17 0 12 0 0 0 0 0 0 0 0 0 7 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 29 0 0 0 0 1 19 0 0 33 0 0 0 0 0 0 0 0 0 0 0 0 0 7 4 0 7 0 7 0 7 0 0 0 12 0 3 6 0 6 11 0 5 3 0 4 9 33 0 0 0 0 17 2 0 10 0 0 29 0 0 16 0 0 20 0 0 16 0 0 14

21 21 21 0 21 21 21 0 0 4 0 0 0 0 0 0 0 0 0 0 0 13 13 0 0 0 0 0 0 0 0

14 14 14 0 7 7 7 1 0 0 0 0 0 0 3 4 4 0 0 0 0 0 0 0 0 1 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0

3 0 3 0 3 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 13 13 0 0 0 0 0 0 0 0

0 3 0 13 0 3 0 7 14 4 17 20 10 14 11 0 0 2 2 2 2 1 1 7 7 6 12 12 12 12 2

Pot. Tar.: (HOOC-CHOH-CHOH-COO~K+); Borax: Na2B4O7-10H2O; Lazurite: Na8_1(/\l6Si6024S2. N: Neri; F: F libien; KN: Neri in Kunckel; K: Kunckel; D: Dossie; LV: Le Vieil; Re: Reboulleau; S: Strele.

Microstructure of the enamels

If glass is melted at a sufficiently high temperature and

thoroughly mixed, it is normally a homogeneous material on the microscopic scale. One would also

expect this to be the case for enamel paint layers.

However, the backscattered electron images of several

enamel paint layers in cross-section demonstrate that

enamels can also be heterogeneous. Moreover, several

types of heterogeneities can be observed in Figure 3.

The most common types of heterogeneity in enamel are

gas bubbles, inclusions rich in metals (for example silver,

Ag) and concentration gradients resulting in a cloudy

appearance of the enamels in backscattered electron

mode. From the cross-sectioned paint layers, it can be

seen that the interface between glass substrate and paint

layers is sharp. The thickness of the paint layers varies

between 10 and 80 um. A microscopic study of the

enamel surface was performed by electron and optical

microscopy. The images showed that the surface varies

from relatively even with a fine granular texture to

particularly uneven with craters.

Degradation of the enamel

Two types of degradation were found:

Type A: The degraded enamel shows a network of

micro-cracks following a distinctive pattern. The

schematic representation in Figure 4 exemplifies how

a number of cracks run roughly parallel over the

entire surface, from one side to the other. These

'main cracks' are connected by short cracks dividing the enamel into compartments. When such a

compartment flakes off, it will take away a part of the

support glass. Where the enamel has disappeared,




216 G. VAN DER SNICKT, O. SCHALM, J. CAEN, K. JANSSENS AND M, SCHREINER

Figure 3 Secondary electron image of a cross-section. A crack in

the heterogeneous enamel penetrates the support glass.

concentric circles and the characteristic pattern are

visible in the support glass, as demonstrated by Fig ures 4 and 5. Hence, the degradation is not caused

by failure in the adherence between the two materi

als, but appears to be the result of a tension which is

present in both enamel and support glass. This ten

sion might originate from a difference in coefficient

of expansion, as suggested by the historic authors.

The secondary electron image (Figure 3) reveals a

crack across the enamel and penetrating the support

glass. This characteristic pattern of cracks can also be

observed in the glaze of some ceramics, where it is

either unintentional or initiated deliberately by

employing a glaze with a coefficient of expansion which is slightly different from that of the pottery [17].

Type B: This shows a different sort of degradation. Micro-cracks are absent; the support glass reveals a

slightly granular, matt and even surface (Figure 5). The secondary electron image and the X-ray

mapping in Figure 6 supply an explanation for the

degradation. Beneath the heterogeneous layer of

enamel, some silicon- and lead-rich zones are present in the support glass. The depletion of potassium and

calcium ions indicates glass surface corrosion.

Consequently, the loss of enamel is due to a loss of

adherence with the glass substrate. Whether this

surface corrosion occurred before application of the

enamel (indicating the use of degraded glass) or after

cannot be verified.

Figure 4 Schematic representation of the surface of the enamel.

Left: a number of cracks divide the enamel in compartments. Right: when such a compartment flakes off it takes away a part of the

support glass. Where the enamel disappeared, concentric circles and

the characteristic crack pattern are visible in the support glass.

Chemical composition of the enamels

The recipes prescribed a wide range of compositions.

Many of them deviated from the well-known Si02 PbO glass type, which was used for grisaille paint. It is

not known whether all these recipes were used in

practice, but it is possible that some of the recipes resulted in compositions that are prone to accelerated

deterioration. For these reasons, the chemical composi tion was determined for a set of 25 historic samples. The

compositions of the enamel paint layers analysed were

classified in three groups:

Type El: Two types of alkali sources were mixed

together in order to obtain high amounts of Na20 and K20.

The alkali sources must have been poor in

CaO since this oxide appears in low concentrations

in the paint. In addition to Na20 and K20,

it

contains 10-20 wt% PbO to lower the melting point. Most fragments belong to this type, of which the

average composition corresponds with the biggest

group of recipes, namely R2 (also 10-20 wt% PbO). From this point of view, the analyses confirm the

historic recipes. The Cl-Ka peak in the X-ray

spectrum interferes with one of the Pb-M lines, but

the high amount of Cl could be explained by the use of sea salt as a source for sodium, which is confirmed

by the recipes.

Type E2: The main difference from type El is that this contains more PbO (approximately 33 wt%).

Consequently, this enamel must have a lower

melting point than enamel of type El. Five fragments

belong to type E2.





Figure 5 Detail of degraded enamel. Left: Type A, the surface of the enamel shows a pattern of micro-cracks, dividing it into compartments. The

support glass reveals the same pattern. Right: Type B, no cracks are visible.

Figure 6 Secondary electron image and X-ray mapping of a cross-section of a degraded enamel, type B. The glass underneath the enamel is corroded: zones rich in silicon and lead are visible.

Type E3: This type represents two twentieth-century

fragments. It has fewer contaminants, as a result of

the pure and artificially produced ingredients. It is

characterized by a very high concentration of PbO

and Na20, and the absence of K?0. This type

matches the nineteenth- and twentieth-century

recipes grouped in type R3.

The average compositions of these groups are given in

Table 2, and they are compared with the recipes from

historic literature in Table 3.

An interesting observation is the fact that all frag ments contained at least some lead; 4.73 wt% was the

lowest concentration measured." Thus, the type of reci

pes in which lead is absent, namely type Rl, was not

confirmed by the fragments. Surprisingly, no correlation

was found between the composition of the fragments and whether or not they showed degradation. Conse

quently, further investigation is needed to establish the

cause of the degradation of type A. The preparation of

replicas could help to find out whether a difference in

coefficient of expansion between the support glass and





Table 2 Average composition of the blue enamel in wt%, per type

Code Na20 MgO Al203 S 02 CI K20 CaO Cr203 MnO Fe203 CoO NiO CuO ZnO As205 Ag20 Sn02 BaO PbO

TypeEl 4.14 0.00 2.27 59.83 0.40 11.68 2.27 0.04 0.14 2.43 1.69 0.62 1.77 0.16 1.87 0.08 0.04 0.13 10.54

TypeE2 2.43 0.00 1.07 44.30 0.32 9.50 1.49 0.00 0.07 1.95 1.17 0.41 2.48 0.34 1.53 0.00 0.00 0.09 32.84

TypeE3 7.74 0.63 5.99 35.59 0.27 0.52 1.79 0.52 1.17 3.18 1.46 0.09 0.22 0.90 0.03 0.00 0.79 0.10 39.02

Table 3 Classification of the recipes and the chemical composition of the fragments

Historic literature

Type of material Classification Number of recipes

Flux Type R1 : no PbO 7

Type R2: 10-20 wt% PbO 13

Type R3: 30-80 wt% PbO 11

Colouring substance Group A: CoO 25

Group B: CuO/CoO 0

Group C: CuO 6

Chemical composition of the fragments Type of material Classification Number of fragments

Flux Type E1 : 10-20 wt% PbO 18

Type E2: 32 wt% PbO 5

Type E3: 39 wt% PbO 2

Colouring substance Group A: CoO 19

Group B: CuO/CoO 3

Group C: CuO 3

the enamel actually instigates the flaking off. If this

appears to be the case, it would also be interesting to

study the parameters that affect the coefficient of

expansion of the glass and the enamel.

An alternative method of classifying the enamels is to

group them according to the colouring substances used

(the proportions of CoO and CuO). Analogous with the

recipes, three groups can be distinguished (see Figure 7) :

Group A: Only CoO is present (1.5-2.5 wt%). Most

fragments (19) come under this group.

Group B: Three fragments contain less CoO and

some CuO. It is not clear whether this enamel is

coloured by cobalt, copper or both.

Group C: Only CuO is present (6.5-9.5 wt%).

The graph also shows that there is no connection

between the colouring ingredient and the dating of the

enamel.

The amount of CoO required to obtain dark blue

window glass of a few millimetres thickness is

approximately 0.1 wt%. This is close to the detection

limit of EPMA. For enamel paint layers, the amount of

CoO is much higher. This means that relatively high

amounts of smalt or saffre were employed. As a result,

the typical impurities accompanying the CoO in smalt

or saffre, such as As205 and NiO, could be detected with

EPMA [18]. The concentration of nickel (NiO) and arsenic

(As205) appeared to be related to CoO, thus

proving that smalt or saffre was used. No correlation was

found between the amount of copper and the visual

colour of the enamel.

In spite of the fact that no quantification of the XRF

spectra could be carried out, the interpretation of the

peak areas supplied some interesting information. For

example, the presence of bismuth (Bi) (related to

cobalt), titanium (Ti), rubidium (Rb), strontium (Sr) and zirconium (Zr) in the enamel could be proved.

Chemical composition of the glass substrate

The support glass can be classified in three types (Table 4 presents the average composition of each type):

Type Gl: Most samples belong to this type with a

relative high amount of potassium oxide (K20, 6.5

wt%) and a low concentration of sodium oxide

(Na20, 1.4 wt%). The presence of phosphorus oxide





10

9

8

7 ]

6

5

4

3

2

1

i !

GROUP C much Cu

GROUP B Cu/Co mix

GROUPA much Co

*

0,5 ,0 2,5 1,5 2,0

weight% CoO

16-17th-C, type El 16-1 7th-C, type E2 A20th-C, type E3

3,0 3,5

Figure 7 Copper content versus cobalt content (wt%). Three groups can be distinguished: group A coloured by cobalt, group C coloured by copper and group B containing both metal oxides.

Table 4 Average composition of the glass substrate in wt%, per type

Code Na20 MgO Al203 Si02 P205 S03

Typed 1.41 3.41 2.82 58.55 2.44 0.38 0.18 6.57 21.45 0.96 1.30 0.53

TypeG2 4.63 2.99 3.13 61.84 1.68 0.29 0.67 2.06 20.49 0.66 1.12 0.46

TypeG3 14.54 2.13 1.40 69.25 0.01 0.54 0.14 0.33 8.92 1.93 0.41 0.41

(P205, 2.4 wt%) indicates the use of wood ashes. This

composition, which is typical for the glass production in Western Europe from the tenth to the seventeenth

century, confirms the dating of the fragments [4]. It

is known that this type of potassium-based glass is

susceptible to corrosion. Corrosion of the glass substrate affects its adherence with the enamel layer,

resulting in losses.

Type G2: Only three samples belong to this type, which is characterized by a relatively low potassium oxide (K20,

2 wt%) concentration and a higher sodium oxide (Na20, 4 6 wt%) concentration.

Besides wood ash, soda-based ingredients must also

have been used.

Type G3: This type contains a particularly high con

centration of sodium oxide (Na20, 14.5 wt%). The

two fragments of this type were dated as twentieth

century. During the nineteenth and twentieth

centuries sodium was added in the form of artificially

produced borax in order to lower the melting point.

As a result of better purification of the raw materials, this type contains almost no ferric oxide

(Fe203).

CONCLUSIONS

The composition of sixteenth- and seventeenth-century blue enamel on window glass was found to be much

more complex and variable than expected. In spite of

the large variety of ingredients and the differing

quantities in which they were applied, the research

showed that this type of enamel was always prepared by

melting an ingredient rich in silicon (sand or quartz)

together with components for lowering the melting

point, containing silicon, potassium, sodium and/or

lead. A metal oxide was added to the batch to colour it.

All recipes from the seventeenth century are

noticeably different from one another, while the recipes from the nineteenth century are very similar. Based on

the concentration of lead, the recipes were divided in

three types: most recipes are type Rl with 10 20 wt%




220 G. VAN DER SN GKT, O. SCHALM, J. CAEN, K. JANSSENS AND M. SCHRE NER

of lead. Type R2 contains no lead. Type R3 contains

30-80 wt% of lead. The nineteenth- and twentieth

century recipes, in particular, are characterized by a high concentration of sodium (borax) and the absence of

potassium, and also belong to the type R3 group. In general, the results from the EPMA and XRF

analysis of the fragments matched the outcome of the

study of historic literature. An interesting finding is that

lead appeared to play a less important role than would

have been expected from the composition of grisaille

paint. Unlike grisaille paint, relatively more ingredients

containing potassium and sodium were added to lower

the melting point. Three types were distinguished. Most

fragments (18) belong to type El with an average

composition of 11.7 wt% potassium, 4.1 wt% sodium

and 10.5 wt% lead. This relatively high concentration of

ingredients for lowering the melting point was expected, as the enamel has to melt at a lower temperature than

the support glass. This type El matches the group of

recipes R2. Type E2 contains considerably more lead

(32.8 wt%) and a smaller concentration of potassium (9.5

wt%) and sodium (2.4 wt%). As a result, this type has a

lower melting point. Type E3 consists of two twentieth

century samples, characterized by an even higher amount of lead (39 wt%) and sodium (7.7 wt%) and the absence of potassium. As a result, this composition confirms the recipes (type R3) from the nineteenth and

twentieth centuries. However, the group of recipes Rl,

without lead, was not confirmed by analysis, as all

fragments contained a considerable amount of lead.

Most authors prescribed the use of the pigment smalt

or saffre for colouring the enamel, sometimes in

combination with copper. The analysis showed that

most samples indeed contained cobalt (CoO). The

conclusion that the concentration of nickel, arsenic and

bismuth is related to the concentration of cobalt, proves that smalt or saffre was used, as these materials are typical contaminants of these pigments. Analogous with the

recipes, only three fragments were coloured by copper; three other fragments contained both cobalt and copper.

No correlation was found between the amount of cop

per and the visual colour of the enamel. The following elements were also found in the enamel as trace

elements: aluminium, zinc, chlorine, manganese, iron,

barium, titanium, rubidium, strontium and zirconium.

The secondary electron images showed that the

microstructure of the enamel can differ severely. Two

types of degradation were found. Type A is most likely to be due to a tension caused by a difference in the

coefficient of expansion of the paint layer and glass substrate. The enamel shows a characteristic pattern of

micro-cracks dividing the enamel into compartments. These are also revealed on the support glass after the

enamel has flaked off. Surprisingly, no correlation could

be found between the presence of degradation and the

composition of the enamel. Consequently, further

investigation is needed to establish the cause of type A

degradation. The preparation of replicas could help to

find out whether a difference in coefficient of expansion between the support glass and the enamel actually

instigates the flaking off. If this appears to be the case, it

would also be interesting to study the parameters that

affect the coefficient of expansion of the glass and the

enamel. Type B is caused by the corrosion of the sup

port glass underneath the enamel. In this case, the defect

is due to a loss of adherence between both materials.

ACKNOWLEDGEMENTS

This article is based on the thesis of the same title,

submitted (in Dutch) to obtain the title 'Master in

Conservation/Restoration' at the Hogeschool Ant

werpen. This thesis was promoted by Prof. J. Caen of

the Hogeschool Antwerpen, who supplied the historic

samples and literature. Co-promoter was Dr O. Schalm

from the University of Antwerp, who was responsible for the quantification of the EPMA spectra. The XRF

spectra were obtained in cooperation with Prof. Dr M.

Schreiner of the Akademie fur Bildenden K nste Wien

(Academy of Fine Arts, Vienna), also co-promotor of

the thesis.

REFERENCES

1 Schalm, O., Janssens, K., and Caen, J., 'Characterization of the

main causes of deterioration of grisaille paint layers in the 19th

century stained-glass windows by J.-B. Capronnier', Spectro chimica Acta B 58 (2003) 589-607.

2 Jembrih-Simbiirger, D., Neelmeijer, C, Schalm, O.,

Frederickx, P., Schreiner, M., De Vis, K., M der, M.,

Schryvers, D., and Caen, J., 'The colour of silver stained glass -

analytical investigations carried out with XRF, SEM/EDX,

TEM, and IBA', Journal of Analytical and Atomic Spectrometry 17

(2002) 321-328.

3 Frederickx, P., Transmission Electron Microscopy for Archeo-materials

Research: Nanoparticles in Glazes and Red /Yellow Glass and Inor

ganic Pigments in Painted Context, PhD thesis, Antwerp (2004). 4 Schalm, O., Characterization of Paint Layers in Stained Glass

Windows: Main Causes of the Degradation of Nineteenth Century Grisaille Paint Layers, PhD thesis, Antwerp (2000).

5 Dossie, R., Handmaid to the Arts, A. Millar, London (1758). 6 Reboulleau, M., Nouveau Manuel complet de la Peinture sur Verre,

sur Porcelaine, et sur l'Email, Roret, Neuchatel (1825).





7 Schalm, O., and Janssens, K., 'A flexible and accurate algo rithm for electron probe X-ray microanalysis based on thin

film element yields', Spectrochimica Acta B 58 (2003) 669-680.

8 Le Vieil, P., VArt de la Peinture sur Verre et de la Vitrerie par Feu,

Imprimerie de L.-F. Delatour, Paris (1774). 9 L vy, E., Histoire de la Peinture sur Verre en Europe et

particuli rement en Belgique, Tircher, Brussels (1860). 10 Neri, A., Arte Vitraria, Firenze (1612). 11 Merret, C, In libros Antonii Neri de Arte Vitraria observations et

notae, Andream Frisium, London (1622). 12 Kunckel, J., Ars Vitraria experimentalis oder volkommene

Glasmacherkunst, Leipzig (1674). 13 d'Holbach, P.H., l'Art de la Verrerie, Durand, Paris (1752). 14 Haudicquer de Blancourt, J., l'Art de la Verrerie. Jean Jombert,

Paris (1753). 15 F libien, A., Des Principes de l'Architecture, de la Sculpture, de la

Peinture et des autres Arts qui en dependent, Veuve & Jean Baptiste

Coignard, fils, Paris (1676). 16 Strele, K., Handbuch der Porzellan- und Glasmalerei. Enthaltend:

Die Technik des Kolorierens und Dekorierens von echtem und Fritten

Porzellan, Steingut, Fayence, Glas, Email, Bernh. Friedr. Voigt,

Leipzig (1883). 17 Leach, B., Het pottenbakkersboek, Cantecleer, Amsterdam

(1978). 18 Harley, R.D., Artists' Pigments: 1600-1835, Butterworth

Scientific, London (1990).

AUTHORS

Geert Van der Snickt is a researcher/conservator, who

worked on the SALUT project (Study of Advanced Laser techniques for the Uncovering of polychromed works of arT) for the Hogeschool Antwerpen, Depart ment of Conservation Studies and is currently working on a PhD at the University of Antwerp, Belgium. Address: Universiteit Antwerpen, Universiteitsplein , B-26 0

Wilrijk., Belgium. Email:[email protected]

Olivier Schalm is a researcher working for the Uni

versity of Antwerp, Department of Chemistry and the

Hogeschool Antwerpen, Department of Conservation

Studies. He is currently involved in the Smartplasma

project, in which a prototype for the plasma cleaning of

works of art will be developed. Address: as Van der

Snickt. Email: [email protected]

Joost Caen is Professor of Glass Conservation at the

Hogeschool Antwerpen, Department of Conservation

Studies. Address: Hogeschool Antwerpen, Conservation

Studies, Blindestraat 9, 2000 Antwerpen, Belgium. Email:

[email protected]

Koen Janssens is Professor of Analytical Chemistry at

the University of Antwerp, Department of Chemistry, and works on the non-destructive analysis of various

materials of environmental, archaeological and artistic

origin using microscopic X-ray and electron beams.

Address: as Van der Snickt. Email: [email protected]

Manfred Schreiner studied chemistry at the Vienna

University of Technology, Austria, did his PhD in materials science and made his Habilitation in the field

of analytical chemistry in art and archaeology. Since

2000 he has been a full professor at the Academy of Fine

Arts in Vienna and, since 2001, Head of the Institute of

Science and Technology in Art. His research projects

mainly deal with the non-destructive analysis of objects of art as well as the long-term behaviour (corrosion and

degradation processes) of materials used in art. Address:

Akademie der Bildenden K nste Wien, Schillerplatz 3, 1010

Wien, Austria. Email: [email protected]

R sum On ne sait toujours pas pourquoi une partie des maux bleus des vitraux des XVIe et XVIIe si cles s' caille, alors

que les maux d'autres couleurs demeurent en relativement bonne condition. Afin de mieux comprendre ce probl me de

conservation, on a compar 31 recettes historiques de fabrication de l' mail bleu avec les r sultats de l'analyse chimique de 25

chantillons historiques. On a d termin la composition chimique et la microstructure des chantillons d' mail sous forme de coupes

stratigraphiques par microanalyse lectronique (EPMA). Cette tude a d montr que la variation de la composition chimique des

chantillons peut s'expliquer par l'usage d'un large ventail de recettes en usage a la m me poque. Bien que cette tude ait fourni un aper u de la composition, de l'h t rog n it et de l'usage des substances colorantes, aucune relation claire n'a pu tre tablie

entre les param tres analys s et le degr de d t rioration de la couche maill e bleue.

Zusammenfassung Es ist ein immer noch ungekl rtes Problem, warum sich blaue Glasschichten aufgef rbten Fenstergl sern

des sechzehnten und siebzehnten fahrhunderts abl sen, w hrend anders gef rbte Schichten in weit besserem Zustand sind. Zum

besseren Verst ndnis dieses Konservierungsproblems wurden 31 historische Rezepte zur Herstellung blauer Schmelzen genutzt und die Ergebnisse mit 25 historischen Proben verglichen. Die chemische Zusammensetzung und die Mirkrostruktur der





Schmelzen wurden anhand von Querschliffen mit Elektronenstrahlmikroanalyse (ESMA) untersucht. Die Studie zeigt, dass die

gro e Heterogenit t in der Zusammensetzung durch die gro e Bandbreite der zeitgen ssischen Rezepte erkl rt werden kann.

Obwohl die Studie Einblicke in die Zusammensetzung, die Hetereogenit t und die Verwendung f rbender Substanzen geben kann, konnte keine klare Beziehung zwischen der analysierten Parametern und den Abbaurate der blauen Schmelzen gefunden werden.

Resumen No est a n claro porqu ciertos esmaltes azules, en las vidrieras de los siglos XVI y XVII, tienden a perder cohesi n y desprenderse, mientras que las capas de esmalte de otros colores se mantienen en un estado relativamente bueno. Con el

fin de comprender mejor este problema de conservaci n fueron comparadas treinta y una recetas hist ricas empleadas en la

preparaci n de esmalte azul con los an lisis obtenidos de 25 muestras hist ricas. La composici n qu mica y la microestructura de

los esmaltes se analizaron en las estratigraf as por medio de microan lisis por sonda de electrones (EPMA). Este estudio

demuestra que las variaciones en composici n qu mica de las muestras pueden ser explicadas por el empleo de una amplia variedad

de recetas existentes en la poca. A pesar de que esta investigaci n aporta una visi n clara sobre la composici n, heterogeneidad y uso de las materias colorantes, no se detecta una relaci n clara entre los par metros de los an lisis y el grado de deterioro de la

pintura de esmalte azul.




blue enamel on sixteenth - and seventeenth-century window glass

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