enamels in stained glass windows: preparation, chemical...

Spectrochimica Acta Part B 64 (2009) 812–820

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Spectrochimica Acta Part B

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Enamels in stained glass windows: Preparation, chemical composition,microstructure and causes of deterioration☆

O. Schalm a, V. Van der Linden a, P. Frederickx c, S. Luyten b, G. Van der Snickt a, J. Caen b, D. Schryvers c,K. Janssens a,⁎, E. Cornelis c, D. Van Dyck c, M. Schreiner d

a University of Antwerp, Dept. of Chemistry, Universiteitsplein 1, B-2610 Antwerp, Belgiumb University College of Antwerp, Conservation Studies, Blindestraat 9, B-2000 Antwerp, Belgiumc University of Antwerp, Dept. of Physics, Groenenborgerlaan 171, B-2020 Antwerp, Belgiumd Institute of Humanities, Sciences, and Technologies in Art, Academy of Fine Arts, Schillerplatz 3, A-1010 Vienna, Austria

☆ This paper was presented at the 19th “InternationaMicroanalysis” (ICXOM-19) held in Kyoto (Japan), 16published in the Special Issue of Spectrochimica Acconference.⁎ Corresponding author.

E-mail address: [email protected] (K. Janssens

0584-8547/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.sab.2009.06.005

a b s t r a c t
a r t i c l e i n f o
Article history:Received 19 March 2008Accepted 3 June 2009Available online 18 June 2009

Keywords:EnamelStained glassMicro-XRFTEMGold nanoparticles

Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject tosevere degradation while others from the same period survived the ravages of time. A series of dark blue,green–blue and purple enamel glass paints from the same region (Northwestern Europe) and from the sameperiod (16–early 20th centuries) has been studied by means of a combination of microscopic X-rayfluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim ofbetter understanding the causes of the degradation. The chemical composition of the enamels diverges fromthe average chemical composition of window glass. Some of the compositions appear to be unstable, forexample those with a high concentration of K2O and a low content of CaO and PbO. In other cases, thedeterioration of the paint layers was caused by the less than optimal vitrification of the enamel during thefiring process. Recipes and chemical compositions indicate that glassmakers of the 16–17th century had fullcontrol over the color of the enamel glass paints they made. They mainly used three types of coloring agents,based on Co (dark blue), Mn (purple) and Cu (light-blue or green–blue) as coloring elements. Blue–purpleenamel paints were obtained by mixing two different coloring agents. The coloring agent for red–purpleenamel, introduced during the 19th century, was colloidal gold embedded in grains of lead glass.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Figurative stained glass windows are assembled by fitting glasspanes of different shapes and colors in a network of grooved strips ofleadwith anH-shaped cross-section. The joints between the lead stripsare soldered with a Pb–Sn alloy. The finer details of the design arerendered by means of glass paints that are first applied as a powderdispersed in a painting medium (oil, gum water, etc.) and then firedonto the glass pane. Different types of glass paints can be distin-guished. Since the ninth century, grisaille has been used to paint trace-lines or shades on glass panes. It can be applied as a thick, opaque line(known as ‘grisaille à contourner’) or as a thin, uniform layer (‘grisailleà modeler’) in order to diminish the amount of light passing through apane [1]. From the end of the thirteenth century onwards, silver stain

l Congress on X-ray Optics and–21 September 2007, and ista Part B, dedicated to that

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has been used frequently to color glass panes in bright yellow [2,3].Finally, during the sixteenth century enamel glass paints weredeveloped to color glass panes in different shades of, e.g. blue, purple,red, etc. This article focuses on the latter type of glass paint.

Enamel glass paints are prepared on the basis of colored glassesthat melt at a lower temperature than the glass substrate on whichthey are applied. They are made by melting together the flux (e.g. alowmelting glass)with a coloring substance (e.g., smalt, copper oxide,etc). The colored and still liquid glass paste is quickly cooled down bypouring in it inwater and the resulting glass flakes are ground to a finepowder. This powder is mixedwith a small amount of gumwater or oilin order to obtain a paste that can be used to paint upon sheets ofglass. During firing, the applied glass powder transforms into a thinhomogeneous layer of glass (5–100 µm) coloring the glass pane intransmitted light. Usually, a sharp interface between paint layer andglass substrate exists. In Fig. 1, a schematic representation of such apaint layer before and after firing is shown in cross-section.

Unfortunately, in several stained glass windows the dark blue andpurple enamel paint layers are subject to severe degradation whileothers from the same period survived the ravages of time. An exampleof a roundel showing deteriorated enamel paint layers is shown inFig. 2. This 17th century roundel, representing St. John the Baptist, was

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Fig. 1. Schematic representation of an enamel paint layer in cross-section, before andafter firing.

813O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820

not only broken in several pieces, also nearly all painted details of theface and the hands have vanished. In the blue colored sky and cloth,several transparent areas can be seen. This is due to the flaking-off ofthe enamel paint layer. Not only 16–17th century enamel paint layerssuffer from severe degradation, also 19th century enamel paint layers(i.e., in windows less than 100 years old) are in some cases heavilydegraded. Problems with the stability of enamels were alreadyreported in the 17th century and in the 19th century. In order toidentify the causes of this difference in deterioration rate, dark blue,

Fig. 2. Roundel (St. John the Baptist, 17th century) showing deteriorated blue and purple enareader is referred to the web version of this article.)

green–blue and purple enamel glass paints from the same region(Northwestern Europe) and from the same period (16–early 20thcenturies) have been studied from three different points of view.These views are given below.

(1) Historical documents and recipes provide an insight into thefabrication of enamels in the past, how they were used andapplied. This study includes the most important recipe books ofthe French, English, Dutch and German literature, assumed tohave been available to stained glass artists at that time;

(2) The chemical analysis of enamel paint layers on historical glassfragments provides insight in the composition, microstructure andthe layer thickness of the final product. These results werecompared with recipes encountered in (1). It should be notedthat only the recipes resulting in the most stable enamel paintlayers can be studied in this manner; less stable paint layers aremost probably not present anymore in the set of analyzed samples;

(3) Remaining stocks of historical glass paint powders (mainlyfrom the 19th century) were applied on modern glass panesand subsequently fired. The powder and the resulting paintlayers were characterized.

2. Historical background

In this study, original recipes concerning the production of enamelglass paints were collected from several historical source books. One

mel paint layers. (For interpretation of the references to color in this figure legend, the

814 O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820

of the most important books was “L'Arte Vetraria” of Antonio Neri [4]since it was the basis of almost all the 17th and 18th century booksrelated to this subject. At the end of the 17th century and certainlyduring the 18th century the interest in stained glass windowsdiminished mainly on the continent. During that time of decline,several authors tried to preserve the skill of coloring techniques bypublishing books with recipes about glassmaking, coloring glass andproduction of glass paints. An important example of such an author isPierre Le Vieil [5]. His intention was to preserve the knowledge of hisforefathers (well-known glass painters) by writing down theirknowledge and by combining it with recipes published in the past.Some authors wrote comments about the recipes they copied and/oradded new ones, which indicates that they experimented to some

Fig. 3. Relation between authors writing about glass recipes and/or glass paint recipes. Blackimportant manuscripts; white box: less important publications; full line between publicationwhere it has been copied.

extent with the recipes they copied. The relationship between thedifferent publications and the influence of the authors on later ones isshown in Fig. 3.

In order to obtain a low melting glass paint, the recipes prescribethe use of high amounts of fluxing agents such as lead oxide (usuallyminium, Pb3O4), one or several alkali rich sources (wood ash, salts oftartaric acid, potassium nitrate, sea salt) and/or borax. Except forwood ash, calcium rich ingredients were rarely mentioned. In somerecipes unusual ingredients such as tin oxide, bone ash and evenultramarine blue as coloring substance were mentioned, all of themresulting in opaque glasses. Other unusual fluxing agents are arsenic,bismuth and mercury compounds. Many of these ingredients werenever detected during the chemical analysis of historical paint layers.

box: important publications; Light gray box: encyclopedic information; dark gray box:s: literal copy referring to the source; dotted line: literal copy but with no reference from


Indeed, a flux of the Na2O–SiO2–SnO2–PbO type is mentioned in therecipes of Neri and in later copies but SnO2 is a flux constituent that isnever detected. However, the resulting enamels resemble majolicaglazes more than they do to those used for stained glass windows [6].Other recipes prescribe a flux of the SiO2–K2O–PbO type. For some ofthese recipes the amount of K2O exceeds that of PbO and one canwonder if stable glass can result from such a recipe. Finally, there is agroup of fluxes containing borax resulting in glass of the B2O3–Na2O–SiO2–PbO type. This type can mostly be found in the 19–20th centuryliterature but was already mentioned by Le Vieil in the 18th century.The fluxes mentioned in the recipes are characterized by a largevariety of raw materials and by the variability of the proportions inwhich the latter were supposed to be used. It is likely that the (non-optimal) chemical composition of the flux is one of the reasons of theaccelerated deterioration of some the enamel paint layers.

For dark blue enamels [7], most authors prescribe saffre (CoO+contaminations) or smalt (SiO2–K2O–CoO+contaminations) as color-ing substance. Only Neri and Dossie [8] suggested in some recipes theuse of calcinated copper (CuO). During the 19th century, the authorsprescribe pure, industrial produced cobalt-oxide (CoO). The combina-tion of copper and cobalt ingredients was not oftenmentioned togetherin the recipes. For the purple enamels [9], two types of coloring sub-stances were identified in the recipes, associated with different periods.The dominant type in the 16–17th century was based on manganese-rich minerals (MnO2). A blue–purple variant of this glass paint wasobtained by the combination of MnO2 with CuO or by the combinationof MnO2 and cobalt-rich products (usually saffre). In order to obtainblue–purple enamel, the colorants could be introduced in the same fluxor two existing enamel powders were mixed together.

The dominant coloring substance during the 19th and early 20thcentury in red–purple enamel glass paints was colloidal gold, eventhough this type of colorant was already mentioned by Piere Le Vieil,the oldest published referencewe found so far concerning for this typeof colorant. Colloidal gold, also known as Cassius Purple, was obtainedby dissolving gold in aqua regia (i.e., mixture of concentrated HNO3

with concentrated HCl) and precipitated by adding a solution of SnCl2(i.e., SnCl2 is formed when metallic tin is dissolved in aqua regia).

3. Experimental

3.1. Collection of analyzed material

This study was performed on a collection of 63 historical stainedglass window fragments featuring painted decorations. Many of thesefragments are multi-colored as can be seen in Fig. 4. Fifteen fragmentscovering a period between 1600 and 1920 contained dark blue and/orpurple enamel paint layers. Also some unused historic purple enamelpowders (19th century) were available.

Fig. 4. Examples of some painted glass frag

3.2. Qualitative elemental analysis by µ-XRF

The series of 15 window fragments containing dark blue and/orpurple enamel paint layers were first subjected to a non-destructivequalitative elemental analysis by means of microscopic X-rayfluorescence analysis (µ-XRF). In most fragments, several coloredareas were analyzed. For this purpose, cleaned but otherwiseunprepared glass panes were irradiated under an angle of 45° relativeto the surface by means of a focusing X-ray beam produced by apolycapillary X-ray lens mounted on a Momicro-focus X-ray tube. Thelatter tube was operated at 35 kV and 400 µA. The fluorescenceradiation was also detected under an angle of 45° relative to thesample by a 80 mm2 Si(Li) energy-dispersive X-ray detector [10]. Theµ-XRF spectra allowed the identification of a limited number offragments to be sampled for EPMA and TEM investigations.

3.3. Major and minor element composition determination by EPMA

Small pieces of the selected glass fragments were embedded,perpendicular to the original surface, into acrylic resin. The resinblocks were then groundwith silicon carbide paper and polished withdiamond paste down to 1 µm in order to obtain a smooth cross-sectiononwhich bulk measurements can be done without interference of thecorroded surface layers of the glass fragments. Finally these resinblocks were coated with a thin carbon layer to prevent charging of thesurface during EPMA measurements. These were performed with aJEOL 6300 Scanning Electron Microscope, equipped with an energy-dispersive X-ray detector [11]. From 25 dark blue and 11 purplecolored cross-sectioned thin enamel layers (ca. 20 µm), X-ray spectraat six different locations were collected at 20 kV, a magnification of40,000, a beam current of 1 nA and a live time of 50 s. Under thesemeasurement circumstances, no sodium diffusion takes place whenordinary soda–lime glass is analyzed; this implies that the concentra-tion of Na2O and that of the other major constituents of the glass canbe determined in a reliable manner [12]. The net intensities werecalculated with the program AXIL (Analysis of X-rays by Iterative Leastsquares [13]) and quantified by means of a standardless ZAF-program[14].

3.4. Microstructure on a micro- and nanometer scale determined by TEM

The TEM investigation of remaining stock of a historical glass paintpowder (for purple enamel) was carried out with a Philips CM20microscope, working at 200 kV, equipped with an Oxford energy-dispersive X-ray (EDX) detector and a Link Analytical System. Presentconventional and analytical applied TEM techniques require a samplethickness of less than 150 nm. To reduce the sample size, the grains ofthe glass paint powder were crushed in an agate pestle and mortar in

ments on which this study was based.

Fig. 5. Scatter plots of the line intensity of several coloring elements as determined bymeans of µ-XRF: (a) Co vs Mn, (b) Cr vs Co and (c) Cr vs Mn.

Table 1Relation between the coloring elements and the color of the enamel paint layer.

Color Principal colorant Small amounts of colorants

Opaque red Fe Almost no Mn, Co and CuTransparent red Fe Almost no Mn, Co and CuRed–purple Au (and Sn) Almost no Mn, Fe, Co and CuPurple Mn Cu and/or CoBlue–purple Somewhat less Mn

than purple enamelsSomewhat more Cu (or in somecases Co) than purple enamels

Dark blue Co, Cu in exceptional cases Mn, Fe and NiLight blue Mainly Cu, sometimes with

smaller amounts of CoMn and Fe

Green–blue Mainly Cu, sometimeswith smaller amounts of Co

Mn and Fe


the presence of ethanol. The liquid containing the crushed grains wastransferred by means of a pipette onto a carbon coated holey filmsupported by a copper grid. Heterogeneities in the grains such as thepresence of colloidal gold particles could be studied in this way.

4. Results

4.1. Qualitative results

Among the set of historic window glass fragments, enamel paintlayers of different hues can be distinguished. Therefore, it was decidedto classify these paint layers according to their color. Two types of red

colored paint layers were distinguished, three types of purple coloredpaint layers and three types of blue enamel paint layers. In this study,green–blue paint layer should not be confused with a dark blue paintlayer in combinationwith a (yellow colored) silver stainwhich resultsin a green color as well. Also transparent grisaille à modeler with a redto purple color in reflected light should not be confused up withpurple enamel paint layers. Since it was not known if this classificationwas arbitrary (the colors were visually evaluated) or based on aphysical reality (different types of colorants), the relative abundanceof the coloring elements Mn, Fe, Co and Cu was studied bymeans of µ-XRF.

From the collected X-ray spectra it became clear that all paintlayers associated to one color had a similar elemental make-up andthat the different colors could be associated with different coloringelements. In Fig. 5 the relation between the intensity of the X-ray linesof the coloring oxides and the color of the paint layer is shown.

From these scatter plots, it can be concluded that the dominantcolorant in dark blue enamel paint layers is CoO. In a limited numberof cases (5 areas) CoO and CuO were both employed to color theenamel and in an exceptional case, only CuO in high concentrationswas responsible for the dark blue color. All purple and blue–purpleenamels contained high amounts of manganese, but the blue–purpleenamels appear to contain somewhat higher amounts of CuO or CoO.

Two types of purple enamel paint layerswere found in the data set:(1) 16–17th century enamel purple and blue–purple glass paints weresystematically colored withmanganese, and (2) 19–20th century red–purple enamel paint layers were colored with colloidal gold. Theseanalyses are in agreement with the coloring agents mentioned in thehistorical recipes. The relation between color and colorants issummarized in Table 1.

It can be concluded that the color of the dark blue and purpleenamels was produced by means of different ingredients. The twodifferent colors were not produced by using the samemain colorant incombination with others to obtain different color tones. For example,saffre with varying amounts of MnO2 as impurity could result indifferent colors between dark blue and purple.

The two types of purple glass paints (Mn and Au-based colorants),whichwere identified in the recipes, couldbe recognized in theanalyzedsamples. However, no obvious relation between color, colorant use anddeterioration pattern could be found.

4.2. Quantitative determination of the glass composition

When rigorously followed, the historical recipes would lead toenamel layers with a wide range of compositions. Many of themdeviate from the well-known glass SiO2–PbO type that is used as fluxin grisaille glass paints. It is not known if all these recipes were used inpractice but it might be possible that some of the recipes resulted incompositions that are prone to accelerated deterioration. Unfortu-nately, bymeans of µ-XRF operating in ambient air, it is not possible todetect the low Z-elements Na, Mg, Al and Si; thus, it is not a suitable

Table 2Average composition of the different groups of historic green–blue, dark blue, blue–purple, purple and red–purple enamel paint layers.

GB1 GB2 DB1 B2 B3 BP1 P1 RP1

Color Green–blue Green–blue Dark blue Dark blue Dark blue Blue–purple Purple Red–purple

Period 16th–17th C. 16th–17th C. 16th–17th C. 16th–17th C. 20th C. 16th–17th C. 16th–17th C. 19th–20th C.

Number of samples 2 1 19 5 2 1 7 2

Na2O 0.7±0.3 – 4±2 2±1.4 8±5 5.5 2±2 9±3MgO – – – – 0.6±0.9 – 0.2±0.6 –

Al2O3 1.1±0.1 0.8 2.2±0.4 1.1±0.2 6±1 2 1.4±0.3 0.3±0.4SiO2 60±2 44 60±4 44±5 36±1 63 55±4 31±2Cl 0.43±0.05 0.5 0.4±0.2 0.3±0.1 0.3±0.3 – 0.5±0.2 0.1±0.2K2O 13.2±0.8 9.3 12±2 10±2 0.5±0.2 10 14±3 –

CaO 0.73±0.03 0.9 2.3±0.8 1±1 1.8±0.3 1.8 1±1 4±2Cr2O3 – – 0.1±0.2 – 0.5±0.7 – – –

MnO 2±2 0.15 0.1±0.2 0.07±0.04 1±1.6 2 6.3±0.9 0.03±0.04Fe2O3 0.6±0.2 0.5 2.4±0.9 2±1 3±4 1 0.6±0.6 0.6±0.8CoO – 0.03 1.7±0.7 1.2±0.7 1.5±2 1.8 0.2±0.3 –

NiO – 0.05 0.6±0.3 0.4±0.2 0.09±0.08 0.8 0.1±0.1 –

CuO 7±2 6 2±2 2±3 0.2±0.3 0.5 1.3±0.8 0.02±0.02ZnO – – 0.15±0.09 0.3±0.3 0.9±1 – 0.04±0.09 –

As2O5 – – 1.9±0.8 2±1.5 0.03±0.04 8.3 0.1±0.2 –

Ag2O – – 0.1±0.2 – – – 0.3±0.7 –

SnO2 – – 0.1±0.3 – 1±1 – – 4.20±0.04BaO – 0.16 0.13±0.05 0.09±0.02 0.10±0.08 – 0.1±0.1 –

Au – – – – – – – 0.2±0.3Bi2O3 – – – – – 2 – –

PbO 14.6±0.3 38 11±5 33±6 39±2 – 17±7 51±1

Listed uncertainties refer to 1s standard deviation on the average concentration expressed in weight percentage. Boldfaced number indicated major coloring elements.

Fig. 6. Overview of all kinds of heterogeneities in enamel paint layers.


technique for the analysis of the flux used in enamel glass paints. Forthis reason, a quantitative analysis of cross-sectioned historic enamelpaint layers was performed bymeans of EPMA. The composition of theanalyzed enamel paint layers was used to classify them in a limitednumber of groups. The average compositions of these groups are givenin Table 2.

Almost all fluxes of the 16–17th century examined here consist ofglass of the Na2O–SiO2–K2O–PbO typewith different K2O:PbO ratios. Itis noteworthy that the concentration of K2O is much larger than that ofCaO. The presence of Na2O and K2O and the absence of MgO, CaO andP2O5 indicate that no untreated wood ash was employed as alkalisource. Instead, the soluble part of wood ashes or another source ofK2O such as potassium nitrate was used. It is hard to determine the Clconcentration in these samples with some confidence due to theoverlap of the Cl-Kα line with one of the Pb–M lines. Nevertheless, insome cases relative high amounts of chlorine could be observed.Consistent with the recipes, this might indicate the use of sea salt as asource of sodium. In general, the results of chemical analysis agreewell with the recipes, except for the unusual ingredients regularlymentioned in them that were not detected. This could mean that theywere never employed or that the paint layers made with recipescontaining these ingredients did not survive the ravages of time.

In dark blue enamel paint layers, where CoO is the principlecoloring oxide present, the concentration of CoO is much higher thanthat in dark blue window glass (usually ca. 0.1 wt.%). This means thatrelatively high amounts of smalt or saffre were employed. As a resultof this, the typical impurities accompanying CoO in smalt or saffre,such as MnO, CuO, NiO and As2O5 could be detected with EPMA.

The purple enamels contain much more MnO2 than CoO and CuO.Blue–purple enamels are usually colored with a mixture of MnO2 andCuO; CoO appears to be employed less frequently than CuO, probablybecause of its much higher coloring strength.

For the 19–20th century enamels, a flux of the Na2O–SiO2–PbOtype with relative high amounts of PbO was employed. The dark blueenamels of this period were made with a CoO that contains signifi-cantly lower levels of NiO, CuO, ZnO and As2O5; the purple enamelswere colored with Au and contained considerable amounts of SnO2,probably as a consequence of the use of Cassius Purple. The samesamples also allowed the determination of the major composition of

the supporting window glass but there was no relation between itscomposition and the quality of the paint layers.

4.3. Microstructure of enamel paint layers

The optical properties of enamel paint layers are not onlydetermined by their chemical composition but also by their layerthickness and their homogeneity. Sincewindow glass is homogeneousat a microscopic scale, it was initially expected that it would be thecase for this type of material as well. However, backscattered electronimages and X-ray images collected from several cross-sectionedenamel paint layers demonstrated that enamels can also be stronglyheterogeneous. The most common types of heterogeneities that wereencountered in enamel paint layers are gas bubbles, inclusions(containing e.g. Al2O3–SiO2–K2O, SiO2, Fe2O3, Ag, AgCl, etc.) andfluctuations in concentrations resulting in a cloudy appearance of theenamels in backscattered electron mode. A schematic overview of theheterogeneities is given in Fig. 6. In one case, an enamel paint layerwas not well vitrified during the firing process. In the X-ray images ofthis sample, shown in Fig. 7, it can be clearly seen that the enamelpaint layer wasmade bymixing a dark blue enamel glass paint (rich inCo) with a purple enamel glass paint (rich in Mn). Another type ofheterogeneity that was regularly observed was the presence of a thingrisaille paint layer between the glass substrate and the enamel paintlayer. This is the case for the paint layer shown in Fig. 8.

Fig. 7. Cross-section of an enamel paint layer, which was not well vitrified. The glass paint consists of a mixture of blue and purple enamel grains.

Fig. 8. Cross-section of an enamel paint layer on top of a grisaille paint layer.


Fig. 9. (a) Grain of lead-rich glass containing gold nanoparticles, (b) gold nanoparticledisplaying internal twinning.


4.4. TEM visualization of the Au-containing pigment particles

Several grains of the unused “Pourpre Rubis V Vitraux” of A. Lacroixand Company were investigated by TEM. This company was a 19thcentury manufacturer of glass paints in Paris. The powder consists of avariety of different grains. There are grains of an unidentified organicmaterial, likely to be some type of gum Arabic. This product was mostprobably added to the mixture to form a viscous paste when mixedwith water. Also particles of CaCO3 and TiO2 were found. Lead glassparticles appeared to be sensitive to the electron beam: underirradiationwith electrons there is a clear rearrangement of the matrix,causing intensity variations in the images that can be mistaken forcolor-inducing nanoparticles. However, some of these lead glassgrains did not contain nanoparticles. There were also grains thatcontained nanoparticles as seen in Fig. 9. The average size of thesenanoparticles is 15±1 nm; they are essentially gold particles, as canbe seen from the energy-dispersive X-ray spectrum (EDX) in Fig. 10,although in some particles a small peak of silver can be observed.Many of the gold particles are twinned (see Fig. 9b) and their densityin the matrix is higher than in transparent red glass [15].

The glass matrix of the grains containing gold nanoparticles is richin lead and contains a small amount of tin, whereas the glass matrix ofthe particle-free grains clearly contains less tin. In some of the grainscontaining gold nanoparticles, larger iron-rich crystals have beenfound. An EDX spectrum of such iron-rich particle is shown in Fig. 10.This is very similar to the iron-rich particles that have been found in

Fig. 10. EDX spectrum of a gold nanoparticle (left) and of a hematite particle in a

17th century ruby glass made by J. Kunckel [15]. These crystals havesizes of about 100 nm. From these measurements, it can be concludedthat the gold nanoparticles were embedded in the lead glass grains.They were not present as individual particles in the paint powder.Moreover, the coloring grains appeared to be diluted with lead glassgrains that do not contain any gold nanoparticles.

5. Conclusions

The study of recipes and the analytical investigation of historicalenamel paint layers demonstrated that their chemical compositiondiverges from the average chemical composition of window glass.Some of these compositions appear to be unstable, for example thosewith a high concentration of K2O and a low content of CaO and PbO. Inthe case of other, deteriorated paint layers, it is clear that the layerswere not well vitrified during the firing process.

Recipes and chemical compositions indicate that glassmakers ofthe 16–17th century had a full control over the color of the enamelglass paints they made. They used three types of coloring agents: (1) acobalt-rich product such as saffre or smalt resulting in a dark bluecolor, (2) a manganese-rich product such as pyrolusite resulting in apurple color, and (3) a copper-rich product such as brass resulting in alight-blue or green–blue color. Only in some exceptional cases thedark blue color was obtained by means of CuO. Blue–purple enamelpaints were obtained by mixing the two different coloring agents inone batch or by mixing enamel glass paints with two different colors.The usage of red–purple enamel glass paint was introduced during the19th century. The coloring agent for this type of paint was colloidalgold embedded in grains of lead glass. The glass contained smallamounts of tin oxide.

The chemical compositions are in agreement with the historicalrecipes, except for the use of fusing agents such as arsenic, bismuth ormercury compounds which were recommended in several recipes butnever encountered experimentally. It is unclear if these recipes wereever applied in the past or not. Also the presence of considerableamounts of tin in the recipes is not reflected in the set of analysisresults.

Acknowledgements

The authors gratefully acknowledge the University of Antwerp andHogeschool Antwerpen for their funding of this BOF-research proj-ect. This research was supported by the Interuniversity AttractionPoles Programme-Belgian Science Policy (IUAP VI/16). The text also

gold nanoparticle-containing grain (right). The Cu peaks arise from the grid.


presents results of GOA programs “Atom” and “XANES Meets ELNES”(Research Fund University of Antwerp, Belgium) and of FWO(Brussels, Belgium) projects no. G.0177.03, G.0103.04, G.0689.06 andG.0704.08.

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