Enamels in stained glass windows: Preparation, chemical

Spectrochimica Acta Part B 64 (2009) 812–820
Contents lists available at ScienceDirect
Spectrochimica Acta Part B
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
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, Belgium
University College of Antwerp, Conservation Studies, Blindestraat 9, B-2000 Antwerp, Belgium
University of Antwerp, Dept. of Physics, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
d
Institute of Humanities, Sciences, and Technologies in Art, Academy of Fine Arts, Schillerplatz 3, A-1010 Vienna, Austria
b
c
a r t i c l e
i n f o
Article history:
Received 19 March 2008
Accepted 3 June 2009
Available online 18 June 2009
Keywords:
Enamel
Stained glass
Micro-XRF
TEM
Gold nanoparticles
a b s t r a c t
Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject to
severe 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 same
period (16–early 20th centuries) has been studied by means of a combination of microscopic X-ray
fluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim of
better understanding the causes of the degradation. The chemical composition of the enamels diverges from
the average chemical composition of window glass. Some of the compositions appear to be unstable, for
example those with a high concentration of K2O and a low content of CaO and PbO. In other cases, the
deterioration of the paint layers was caused by the less than optimal vitrification of the enamel during the
firing process. Recipes and chemical compositions indicate that glassmakers of the 16–17th century had full
control 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–purple
enamel paints were obtained by mixing two different coloring agents. The coloring agent for red–purple
enamel, 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 glass
panes of different shapes and colors in 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 finer details of the design are
rendered by means of glass paints that are first applied as a powder
dispersed in a painting medium (oil, gum water, etc.) and then fired
onto the glass pane. Different types of glass paints can be distinguished. Since the ninth century, grisaille has been used to paint tracelines 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 (‘grisaille
à modeler’) in order to diminish the amount of light passing through a
pane [1]. From the end of the thirteenth century onwards, silver stain
☆ This paper was presented at the 19th “International Congress on X-ray Optics and
Microanalysis” (ICXOM-19) held in Kyoto (Japan), 16–21 September 2007, and is
published in the Special Issue of Spectrochimica Acta Part B, dedicated to that
conference.
⁎ Corresponding author.
E-mail address: [email protected] (K. Janssens).
0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.sab.2009.06.005
has been used frequently to color glass panes in bright yellow [2,3].
Finally, during the sixteenth century enamel glass paints were
developed 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 glasses
that melt at a lower temperature than the glass substrate on which
they are applied. They are made by melting together the flux (e.g. a
low melting glass) with a coloring substance (e.g., smalt, copper oxide,
etc). The colored and still liquid glass paste is quickly cooled down by
pouring in it in water and the resulting glass flakes are ground to a fine
powder. This powder is mixed with a small amount of gum water or oil
in order to obtain a paste that can be used to paint upon sheets of
glass. During firing, the applied glass powder transforms into a thin
homogeneous layer of glass (5–100 µm) coloring the glass pane in
transmitted light. Usually, a sharp interface between paint layer and
glass substrate exists. In Fig. 1, a schematic representation of such a
paint layer before and after firing is shown in cross-section.
Unfortunately, in several stained glass windows the dark blue and
purple enamel paint layers are subject to severe degradation while
others from the same period survived the ravages of time. An example
of a roundel showing deteriorated enamel paint layers is shown in
Fig. 2. This 17th century roundel, representing St. John the Baptist, was
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
813
green–blue and purple enamel glass paints from the same region
(Northwestern Europe) and from the same period (16–early 20th
centuries) have been studied from three different points of view.
These views are given below.
Fig. 1. Schematic representation of an enamel paint layer in cross-section, before and
after firing.
not only broken in several pieces, also nearly all painted details of the
face 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 of
the enamel paint layer. Not only 16–17th century enamel paint layers
suffer from severe degradation, also 19th century enamel paint layers
(i.e., in windows less than 100 years old) are in some cases heavily
degraded. Problems with the stability of enamels were already
reported in the 17th century and in the 19th century. In order to
identify the causes of this difference in deterioration rate, dark blue,
(1) Historical documents and recipes provide an insight into the
fabrication of enamels in the past, how they were used and
applied. This study includes the most important recipe books of
the French, English, Dutch and German literature, assumed to
have been available to stained glass artists at that time;
(2) The chemical analysis of enamel paint layers on historical glass
fragments provides insight in the composition, microstructure and
the layer thickness of the final product. These results were
compared with recipes encountered in (1). It should be noted
that only the recipes resulting in the most stable enamel paint
layers can be studied in this manner; less stable paint layers are
most probably not present anymore in the set of analyzed samples;
(3) Remaining stocks of historical glass paint powders (mainly
from the 19th century) were applied on modern glass panes
and subsequently fired. The powder and the resulting paint
layers were characterized.
2. Historical background
In this study, original recipes concerning the production of enamel
glass paints were collected from several historical source books. One
Fig. 2. Roundel (St. John the Baptist, 17th century) showing deteriorated blue and purple enamel paint layers. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
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 books
related to this subject. At the end of the 17th century and certainly
during the 18th century the interest in stained glass windows
diminished mainly on the continent. During that time of decline,
several authors tried to preserve the skill of coloring techniques by
publishing books with recipes about glassmaking, coloring glass and
production of glass paints. An important example of such an author is
Pierre Le Vieil [5]. His intention was to preserve the knowledge of his
forefathers (well-known glass painters) by writing down their
knowledge and by combining it with recipes published in the past.
Some authors wrote comments about the recipes they copied and/or
added new ones, which indicates that they experimented to some
extent with the recipes they copied. The relationship between the
different publications and the influence of the authors on later ones is
shown in Fig. 3.
In order to obtain a low melting glass paint, the recipes prescribe
the use of high amounts of fluxing agents such as lead oxide (usually
minium, Pb3O4), one or several alkali rich sources (wood ash, salts of
tartaric acid, potassium nitrate, sea salt) and/or borax. Except for
wood ash, calcium rich ingredients were rarely mentioned. In some
recipes unusual ingredients such as tin oxide, bone ash and even
ultramarine blue as coloring substance were mentioned, all of them
resulting in opaque glasses. Other unusual fluxing agents are arsenic,
bismuth and mercury compounds. Many of these ingredients were
never detected during the chemical analysis of historical paint layers.
Fig. 3. Relation between authors writing about glass recipes and/or glass paint recipes. Black box: important publications; Light gray box: encyclopedic information; dark gray box:
important manuscripts; white box: less important publications; full line between publications: literal copy referring to the source; dotted line: literal copy but with no reference from
where it has been copied.
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
Indeed, a flux of the Na2O–SiO2–SnO2–PbO type is mentioned in the
recipes of Neri and in later copies but SnO2 is a flux constituent that is
never detected. However, the resulting enamels resemble majolica
glazes 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 of
these recipes the amount of K2O exceeds that of PbO and one can
wonder if stable glass can result from such a recipe. Finally, there is a
group of fluxes containing borax resulting in glass of the B2O3–Na2O–
SiO2–PbO type. This type can mostly be found in the 19–20th century
literature but was already mentioned by Le Vieil in the 18th century.
The fluxes mentioned in the recipes are characterized by a large
variety of raw materials and by the variability of the proportions in
which the latter were supposed to be used. It is likely that the (nonoptimal) chemical composition of the flux is one of the reasons of the
accelerated 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 coloring substance. Only Neri and Dossie [8] suggested in some recipes the
use of calcinated copper (CuO). During the 19th century, the authors
prescribe pure, industrial produced cobalt-oxide (CoO). The combination of copper and cobalt ingredients was not often mentioned together
in the recipes. For the purple enamels [9], two types of coloring substances were identified in the recipes, associated with different periods.
The dominant type in the 16–17th century was based on manganeserich minerals (MnO2). A blue–purple variant of this glass paint was
obtained by the combination of MnO2 with CuO or by the combination
of MnO2 and cobalt-rich products (usually saffre). In order to obtain
blue–purple enamel, the colorants could be introduced in the same flux
or two existing enamel powders were mixed together.
The dominant coloring substance during the 19th and early 20th
century in red–purple enamel glass paints was colloidal gold, even
though this type of colorant was already mentioned by Piere Le Vieil,
the oldest published reference we found so far concerning for this type
of colorant. Colloidal gold, also known as Cassius Purple, was obtained
by 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).
815
3.2. Qualitative elemental analysis by µ-XRF
The series of 15 window fragments containing dark blue and/or
purple enamel paint layers were first subjected to a non-destructive
qualitative elemental analysis by means of microscopic X-ray
fluorescence analysis (µ-XRF). In most fragments, several colored
areas were analyzed. For this purpose, cleaned but otherwise
unprepared glass panes were irradiated under an angle of 45° relative
to the surface by means of a focusing X-ray beam produced by a
polycapillary X-ray lens mounted on a Mo micro-focus X-ray tube. The
latter tube was operated at 35 kV and 400 µA. The fluorescence
radiation was also detected under an angle of 45° relative to the
sample by a 80 mm2 Si(Li) energy-dispersive X-ray detector [10]. The
µ-XRF spectra allowed the identification of a limited number of
fragments 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 resin
blocks were then ground with silicon carbide paper and polished with
diamond paste down to 1 µm in order to obtain a smooth cross-section
on which bulk measurements can be done without interference of the
corroded surface layers of the glass fragments. Finally these resin
blocks were coated with a thin carbon layer to prevent charging of the
surface during EPMA measurements. These were performed with a
JEOL 6300 Scanning Electron Microscope, equipped with an energydispersive X-ray detector [11]. From 25 dark blue and 11 purple
colored cross-sectioned thin enamel layers (ca. 20 µm), X-ray spectra
at six different locations were collected at 20 kV, a magnification of
40,000, a beam current of 1 nA and a live time of 50 s. Under these
measurement circumstances, no sodium diffusion takes place when
ordinary soda–lime glass is analyzed; this implies that the concentration of Na2O and that of the other major constituents of the glass can
be determined in a reliable manner [12]. The net intensities were
calculated with the program AXIL (Analysis of X-rays by Iterative Least
squares [13]) and quantified by means of a standardless ZAF-program
[14].
3. Experimental
3.4. Microstructure on a micro- and nanometer scale determined by TEM
3.1. Collection of analyzed material
This study was performed on a collection of 63 historical stained
glass window fragments featuring painted decorations. Many of these
fragments are multi-colored as can be seen in Fig. 4. Fifteen fragments
covering a period between 1600 and 1920 contained dark blue and/or
purple enamel paint layers. Also some unused historic purple enamel
powders (19th century) were available.
The TEM investigation of remaining stock of a historical glass paint
powder (for purple enamel) was carried out with a Philips CM20
microscope, working at 200 kV, equipped with an Oxford energydispersive X-ray (EDX) detector and a Link Analytical System. Present
conventional and analytical applied TEM techniques require a sample
thickness of less than 150 nm. To reduce the sample size, the grains of
the glass paint powder were crushed in an agate pestle and mortar in
Fig. 4. Examples of some painted glass fragments on which this study was based.
816
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
colored paint layers were distinguished, three types of purple colored
paint layers and three types of blue enamel paint layers. In this study,
green–blue paint layer should not be confused with a dark blue paint
layer in combination with a (yellow colored) silver stain which results
in a green color as well. Also transparent grisaille à modeler with a red
to purple color in reflected light should not be confused up with
purple enamel paint layers. Since it was not known if this classification
was arbitrary (the colors were visually evaluated) or based on a
physical reality (different types of colorants), the relative abundance
of the coloring elements Mn, Fe, Co and Cu was studied by means of µXRF.
From the collected X-ray spectra it became clear that all paint
layers associated to one color had a similar elemental make-up and
that the different colors could be associated with different coloring
elements. In Fig. 5 the relation between the intensity of the X-ray lines
of the coloring oxides and the color of the paint layer is shown.
From these scatter plots, it can be concluded that the dominant
colorant in dark blue enamel paint layers is CoO. In a limited number
of cases (5 areas) CoO and CuO were both employed to color the
enamel and in an exceptional case, only CuO in high concentrations
was responsible for the dark blue color. All purple and blue–purple
enamels contained high amounts of manganese, but the blue–purple
enamels appear to contain somewhat higher amounts of CuO or CoO.
Two types of purple enamel paint layers were found in the data set:
(1) 16–17th century enamel purple and blue–purple glass paints were
systematically colored with manganese, and (2) 19–20th century red–
purple enamel paint layers were colored with colloidal gold. These
analyses are in agreement with the coloring agents mentioned in the
historical recipes. The relation between color and colorants is
summarized in Table 1.
It can be concluded that the color of the dark blue and purple
enamels was produced by means of different ingredients. The two
different colors were not produced by using the same main colorant in
combination with others to obtain different color tones. For example,
saffre with varying amounts of MnO2 as impurity could result in
different colors between dark blue and purple.
The two types of purple glass paints (Mn and Au-based colorants),
which were identified in the recipes, could be recognized in the analyzed
samples. However, no obvious relation between color, colorant use and
deterioration pattern could be found.
4.2. Quantitative determination of the glass composition
Fig. 5. Scatter plots of the line intensity of several coloring elements as determined by
means of µ-XRF: (a) Co vs Mn, (b) Cr vs Co and (c) Cr vs Mn.
the presence of ethanol. The liquid containing the crushed grains was
transferred by means of a pipette onto a carbon coated holey film
supported by a copper grid. Heterogeneities in the grains such as the
presence 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 paint
layers of different hues can be distinguished. Therefore, it was decided
to classify these paint layers according to their color. Two types of red
When rigorously followed, the historical recipes would lead to
enamel layers with a wide range of compositions. Many of them
deviate from the well-known glass SiO2–PbO type that is used as flux
in grisaille glass paints. It is not known if all these recipes were used in
practice but it might be possible that some of the recipes resulted in
compositions that are prone to accelerated deterioration. Unfortunately, by means of µ-XRF operating in ambient air, it is not possible to
detect the low Z-elements Na, Mg, Al and Si; thus, it is not a suitable
Table 1
Relation between the coloring elements and the color of the enamel paint layer.
Color
Principal colorant
Small amounts of colorants
Opaque red
Transparent red
Red–purple
Purple
Blue–purple
Fe
Fe
Au (and Sn)
Mn
Somewhat less Mn
than purple enamels
Co, Cu in exceptional cases
Mainly Cu, sometimes with
smaller amounts of Co
Mainly Cu, sometimes
with smaller amounts of Co
Almost no Mn, Co and Cu
Almost no Mn, Co and Cu
Almost no Mn, Fe, Co and Cu
Cu and/or Co
Somewhat more Cu (or in some
cases Co) than purple enamels
Mn, Fe and Ni
Mn and Fe
Dark blue
Light blue
Green–blue
Mn and Fe
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
817
Table 2
Average 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
MgO
Al2O3
SiO2
Cl
K2O
CaO
Cr2O3
MnO
Fe2O3
CoO
NiO
CuO
ZnO
As2O5
Ag2O
SnO2
BaO
Au
Bi2O3
PbO
0.7 ± 0.3
–
1.1 ± 0.1
60 ± 2
0.43 ± 0.05
13.2 ± 0.8
0.73 ± 0.03
–
2±2
0.6 ± 0.2
–
–
7±2
–
–
–
–
–
–
–
14.6 ± 0.3
–
–
0.8
44
0.5
9.3
0.9
–
0.15
0.5
0.03
0.05
6
–
–
–
–
0.16
–
–
38
4±2
–
2.2 ± 0.4
60 ± 4
0.4 ± 0.2
12 ± 2
2.3 ± 0.8
0.1 ± 0.2
0.1 ± 0.2
2.4 ± 0.9
1.7 ± 0.7
0.6 ± 0.3
2±2
0.15 ± 0.09
1.9 ± 0.8
0.1 ± 0.2
0.1 ± 0.3
0.13 ± 0.05
–
–
11 ± 5
2 ± 1.4
–
1.1 ± 0.2
44 ± 5
0.3 ± 0.1
10 ± 2
1±1
–
0.07 ± 0.04
2±1
1.2 ± 0.7
0.4 ± 0.2
2±3
0.3 ± 0.3
2 ± 1.5
–
–
0.09 ± 0.02
–
–
33 ± 6
8±5
0.6 ± 0.9
6±1
36 ± 1
0.3 ± 0.3
0.5 ± 0.2
1.8 ± 0.3
0.5 ± 0.7
1 ± 1.6
3±4
1.5 ± 2
0.09 ± 0.08
0.2 ± 0.3
0.9 ± 1
0.03 ± 0.04
–
1±1
0.10 ± 0.08
–
–
39 ± 2
5.5
–
2
63
–
10
1.8
–
2
1
1.8
0.8
0.5
–
8.3
–
–
–
–
2
–
2±2
0.2 ± 0.6
1.4 ± 0.3
55 ± 4
0.5 ± 0.2
14 ± 3
1±1
–
6.3 ± 0.9
0.6 ± 0.6
0.2 ± 0.3
0.1 ± 0.1
1.3 ± 0.8
0.04 ± 0.09
0.1 ± 0.2
0.3 ± 0.7
–
0.1 ± 0.1
–
–
17 ± 7
9±3
–
0.3 ± 0.4
31 ± 2
0.1 ± 0.2
–
4±2
–
0.03 ± 0.04
0.6 ± 0.8
–
–
0.02 ± 0.02
–
–
–
4.20 ± 0.04
–
0.2 ± 0.3
–
51 ± 1
Listed uncertainties refer to 1s standard deviation on the average concentration expressed in weight percentage. Boldfaced number indicated major coloring elements.
technique for the analysis of the flux used in enamel glass paints. For
this reason, a quantitative analysis of cross-sectioned historic enamel
paint layers was performed by means of EPMA. The composition of the
analyzed enamel paint layers was used to classify them in a limited
number of groups. The average compositions of these groups are given
in Table 2.
Almost all fluxes of the 16–17th century examined here consist of
glass of the Na2O–SiO2–K2O–PbO type with different K2O:PbO ratios. It
is noteworthy that the concentration of K2O is much larger than that of
CaO. The presence of Na2O and K2O and the absence of MgO, CaO and
P2O5 indicate that no untreated wood ash was employed as alkali
source. Instead, the soluble part of wood ashes or another source of
K2O such as potassium nitrate was used. It is hard to determine the Cl
concentration in these samples with some confidence due to the
overlap of the Cl-Kα line with one of the Pb–M lines. Nevertheless, in
some cases relative high amounts of chlorine could be observed.
Consistent with the recipes, this might indicate the use of sea salt as a
source of sodium. In general, the results of chemical analysis agree
well with the recipes, except for the unusual ingredients regularly
mentioned in them that were not detected. This could mean that they
were never employed or that the paint layers made with recipes
containing these ingredients did not survive the ravages of time.
In dark blue enamel paint layers, where CoO is the principle
coloring oxide present, the concentration of CoO is much higher than
that in dark blue window glass (usually ca. 0.1 wt.%). This means that
relatively high amounts of smalt or saffre were employed. As a result
of 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 and
CuO; CoO appears to be employed less frequently than CuO, probably
because of its much higher coloring strength.
For the 19–20th century enamels, a flux of the Na2O–SiO2–PbO
type with relative high amounts of PbO was employed. The dark blue
enamels of this period were made with a CoO that contains significantly lower levels of NiO, CuO, ZnO and As2O5; the purple enamels
were colored with Au and contained considerable amounts of SnO2,
probably as a consequence of the use of Cassius Purple. The same
samples also allowed the determination of the major composition of
the supporting window glass but there was no relation between its
composition and the quality of the paint layers.
4.3. Microstructure of enamel paint layers
The optical properties of enamel paint layers are not only
determined by their chemical composition but also by their layer
thickness and their homogeneity. Since window glass is homogeneous
at a microscopic scale, it was initially expected that it would be the
case for this type of material as well. However, backscattered electron
images and X-ray images collected from several cross-sectioned
enamel paint layers demonstrated that enamels can also be strongly
heterogeneous. The most common types of heterogeneities that were
encountered in enamel paint layers are gas bubbles, inclusions
(containing e.g. Al2O3–SiO2–K2O, SiO2, Fe2O3, Ag, AgCl, etc.) and
fluctuations in concentrations resulting in a cloudy appearance of the
enamels in backscattered electron mode. A schematic overview of the
heterogeneities is given in Fig. 6. In one case, an enamel paint layer
was not well vitrified during the firing process. In the X-ray images of
this sample, shown in Fig. 7, it can be clearly seen that the enamel
paint layer was made by mixing a dark blue enamel glass paint (rich in
Co) with a purple enamel glass paint (rich in Mn). Another type of
heterogeneity that was regularly observed was the presence of a thin
grisaille paint layer between the glass substrate and the enamel paint
layer. This is the case for the paint layer shown in Fig. 8.
Fig. 6. Overview of all kinds of heterogeneities in enamel paint layers.
818
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
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.
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
819
17th century ruby glass made by J. Kunckel [15]. These crystals have
sizes of about 100 nm. From these measurements, it can be concluded
that 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 glass
grains that do not contain any gold nanoparticles.
5. Conclusions
Fig. 9. (a) Grain of lead-rich glass containing gold nanoparticles, (b) gold nanoparticle
displaying internal twinning.
4.4. TEM visualization of the Au-containing pigment particles
Several grains of the unused “Pourpre Rubis V Vitraux” of A. Lacroix
and Company were investigated by TEM. This company was a 19th
century manufacturer of glass paints in Paris. The powder consists of a
variety of different grains. There are grains of an unidentified organic
material, likely to be some type of gum Arabic. This product was most
probably added to the mixture to form a viscous paste when mixed
with water. Also particles of CaCO3 and TiO2 were found. Lead glass
particles appeared to be sensitive to the electron beam: under
irradiation with electrons there is a clear rearrangement of the matrix,
causing intensity variations in the images that can be mistaken for
color-inducing nanoparticles. However, some of these lead glass
grains did not contain nanoparticles. There were also grains that
contained nanoparticles as seen in Fig. 9. The average size of these
nanoparticles is 15 ± 1 nm; they are essentially gold particles, as can
be 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 density
in the matrix is higher than in transparent red glass [15].
The glass matrix of the grains containing gold nanoparticles is rich
in lead and contains a small amount of tin, whereas the glass matrix of
the particle-free grains clearly contains less tin. In some of the grains
containing gold nanoparticles, larger iron-rich crystals have been
found. 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
The study of recipes and the analytical investigation of historical
enamel paint layers demonstrated that their chemical composition
diverges from the average chemical composition of window glass.
Some of these compositions appear to be unstable, for example those
with a high concentration of K2O and a low content of CaO and PbO. In
the case of other, deteriorated paint layers, it is clear that the layers
were not well vitrified during the firing process.
Recipes and chemical compositions indicate that glassmakers of
the 16–17th century had a full control over the color of the enamel
glass paints they made. They used three types of coloring agents: (1) a
cobalt-rich product such as saffre or smalt resulting in a dark blue
color, (2) a manganese-rich product such as pyrolusite resulting in a
purple color, and (3) a copper-rich product such as brass resulting in a
light-blue or green–blue color. Only in some exceptional cases the
dark blue color was obtained by means of CuO. Blue–purple enamel
paints were obtained by mixing the two different coloring agents in
one batch or by mixing enamel glass paints with two different colors.
The usage of red–purple enamel glass paint was introduced during the
19th century. The coloring agent for this type of paint was colloidal
gold embedded in grains of lead glass. The glass contained small
amounts of tin oxide.
The chemical compositions are in agreement with the historical
recipes, except for the use of fusing agents such as arsenic, bismuth or
mercury compounds which were recommended in several recipes but
never encountered experimentally. It is unclear if these recipes were
ever applied in the past or not. Also the presence of considerable
amounts of tin in the recipes is not reflected in the set of analysis
results.
Acknowledgements
The authors gratefully acknowledge the University of Antwerp and
Hogeschool Antwerpen for their funding of this BOF-research project. This research was supported by the Interuniversity Attraction
Poles Programme-Belgian Science Policy (IUAP VI/16). The text also
Fig. 10. EDX spectrum of a gold nanoparticle (left) and of a hematite particle in a gold nanoparticle-containing grain (right). The Cu peaks arise from the grid.
820
O. Schalm et al. / Spectrochimica Acta Part B 64 (2009) 812–820
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 and
G.0704.08.
References
[1] O. Schalm, K. Janssens, J. Caen, Characterization of the main causes of deterioration
of grisaille paint layers in 19th century stained glass windows by J.-B. Capronnier,
Spectrochim. Acta Part B 58 (2003) 589–607.
[2] D. Jembrih-Simbürger, C. Neelmeijer, O. Schalm, P. Frederickx, M. Schreiner, K. De
Vis, M. Mäder, D. Schryvers, J. Caen, The colour of silver stained glass — analytical
investigations carried out with XRF, SEM/EDX, TEM, and IBA, J. Anal. At. Spectrom.
17 (2002) 321–328.
[3] Frederickx, P., Transmission Electron Microscopy for archeo-materials research:
nanoparticles in glazes and red / yellow glass and inorganic pigments in painted
context, PhD thesis, Antwerp (2004).
[4] A. Neri, L'Arte Vetraria, Florence (Italy), 1612.
[5] P. Le Vieil, L'Art de la Peinture sur Verre, Paris (France), 1774.
[6] R. Padilla, O. Schalm, K. Janssens, R. Arrazcaeta, P. Van Espen, Microanalytical
characterization of surface decoration in Majolica pottery, Anal. Chim. Acta 535
(2005) 201–211.
[7] G. Van der Snickt, “Blauwe email op 16de en 17de eeuws vlakglas: Onderzoek naar de
samenstelling en bereiding”, Thesis, Hogeschool Antwerpen, Antwerp (Belgium), 2003.
[8] R. Dossie, Handmaid to the Arts, London, 1758.
[9] S. Luyten, “Paars email op vlakglas: onderzoek naar de oorzaak van het verschil in
degradatiesnelheid”, Thesis, Hogeschool Antwerpen, Antwerp (Belgium), 2005.
[10] K. Janssens, G. Vittiglio, I. Deraedt, A. Aerts, B. Vekemans, L. Vincze, F. Wei, I. Deryck,
O. Schalm, F. Adams, A. Rindby, A. Knöchel, A. Simionovici, A. Snigirev, Use of
microscopic XRF for non-destructive analysis in art and archaeometry, X-ray
Spectrom. 29 (2000) 65–73.
[11] S.J.B. Reed, Electron Microprobe Analysis, 2nd EditionCambridge University Press,
Cambridge, 1993.
[12] O. Schalm, PhD Dissertation: Characterization of paint layers in stained glass
Windows, University of Antwerp, pp. 45–71, 2000.
[13] P. Van Espen, K. Janssens, J. Nobels, Axil-Pc, software for the analysis of complexX-ray spectra, Chemometr. Intell. Lab. Syst. 1 (1986) 109–114.
[14] O. Schalm, K. Janssens, A flexible and accurate quantification algorithm for electron
probe X-ray microanalysis based on thin-film element yields, Spectrochim. Acta
Part B 58 (2003) 669–680.
[15] P. Fredrickx, D. Schryvers, K. Janssens, Nanoscale morphology of a piece of ruby red
Kunckel glass, Phys. Chem. Glasses 43 (4) (2002) 176–183.