conservation and research of reverse paintings on glass
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art conservationTRANSCRIPT
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A German House Altar from the Sixteenth Century: CONSERVATION AND RESEARCH OFREVERSE PAINTINGS ON GLASS Author(s): Simone Bretz, Ursula Baumer, Heike Stege, Johannes von Miller and Dedo vonKerssenbrock-Krosigk Source: Studies in Conservation, Vol. 53, No. 4 (2008), pp. 209-224Published by: on behalf of the Maney Publishing International Institute for Conservation of
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209
A German House Altar from the Sixteenth Century CONSERVATION AND RESEARCH OF REVERSE PAINTINGS ON GLASS Simone Bretz, Ursula Baumer, Heike Stege, Johannes von Miller and Dedo von
Kerssenbrock-Krosigk
Prior to the conservation treatment of a late sixteenth-century South-German house altar with seven reverse paintings on glass, technical and scientific research was conducted on the glass substrate, binding media, colourants and metals. Microscopic examination revealed the
particularities of the so-called amelierung technique and the glass panel manufacture. For material analysis, gas chromatography?mass spectrometry, amino acid analysis, high performance liquid chromatography, as well as micro X-ray fluorescence spectrometry and
scanning electron microscopy with energy dispersive X-ray microanalysis were applied. The main component of the binder was identified as a heat-treated pine resin (colophony), modified with various softening resins. The red paint layer contains cochineal and small amounts of dragons blood. The original blue and green areas, which contain smalt and copper resinate, have undergone considerable
colour change. The reverse paintings on glass were in a poor state of preservation: due to high moisture (flooding at the museum) and
material degradation, the paint layers were severely delaminated, fractured into hundreds of small paint flakes and partially attached to
silverfoil and paper backing The detached paint flakes were transferred back to their original positions on the glass with a fine brush and secured using a hydrocarbon resin. This was followed by the application of microcrystalline wax which achieved a brilliant saturation
of the reverse paintings on glass. However, the discolouration remained in the blue and green areas. A photograph of the altar's central
panel after the conservation treatment was digitally manipulated to recreate the presumed original tonality.
INTRODUCTION
Almost 40 years went by from the time of the acquisition of a small house altar by the Corning Museum of Glass
in 1959 until its permanent display in the museum's
galleries. A conservation treatment grant approved by the Getty Foundation made it possible to exhibit this
masterpiece of Renaissance reverse painting on glass
(Figure 1). The poor preservation condition and severe damage
prompted a preliminary study of the technology and
materials of the altar's reverse paintings on glass. This
delicate and fascinating painting technique, called
amelierung, has been the subject of technological research, which was published by Ryser and Bretz in several monographs, papers and contributions to exhibition
catalogues [1?6]. Scientific analysis of colourants and
binding media of reverse paintings on glass has been
carried out by Hahn [7] and by Koller and Baumer [8], but more work is required to allow broader comparisons between periods or regions to be made. Regarding the actual state of the art in conservation on reverse
Received February 2008
paintings on glass, Bretz published an overview on
restoration treatments of the past 20 years [9]. This paper presents a summary of findings on the
technique and materials used for the Corning house
altar with special emphasis on the reverse paintings on
glass. Stereomicroscopy was used to examine closely the painting technique and to document the state of
preservation of the paintings. Various spectroscopic and
Chromatographie methods were used for the analysis of glass panels, metals, colourants and binding media..
The results of the analysis contributed to a better
understanding of colour changes and allowed the
appropriate choice of conservation materials to be
made. A description of the conservation/restoration treatment and its difficulties is given. The analytical and
conservation efforts yielded enough information for a
virtual reconstruction of the original appearance of one
of the reverse paintings on glass.
DESCRIPTION OF THE HOUSE ALTAR
The house altar from the Corning Museum of Glass
(acc. no. 59.3.39) is made of an ebony-veneered oak case
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210 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
Figure 1 House altar with reverse paintings on glass, Corning Museum of Glass (59.3.39), after conservation. ? Corning Museum of Glass, Corning (NY).
and is decorated with seven reverse painting on glass
panels (Figure 1). It originates from South Germany,
perhaps Nuremberg or Augsburg, and probably dates
from around 1560-1580. The central panel shows
the Crucifixion, with St Mary, St John, and probably Mary Magdalene (Figure 2). The wings, when open, depict the allegorical figures Pity and Charity, bearing the inscriptions 'MISERICORDI' and 'CHARITAS'
respectively. The wings, when closed, display St Bridget and St Dorothy, with the inscriptions 'S. PRIGIDA'
and 'S. DOROTHEA'. The oval panel on the pedestal
represents the Seven Sorrows of the Virgin in a medallion:
Figure 2 Crucifixion after treatment. ? Simone Bretz, Munich.
the Virgin Mary, with seven swords pointing toward her.
The panel in the lower niche shows the Virgin as Queen
of Heaven, holding the Christ Child. The back of the altar is flat. The dimensions of the house altar are: height 49.6
cm; width with wings closed 19.5 cm; depth 12.7 cm.
House altars (i.e., altars used for private devotion) with reverse paintings on glass are not rare. An altar of
this kind in the Bayerisches Nationalmuseum in Munich,
Germany (acc. no. R 1104) shows the Coronation of the
Virgin Mary, surrounded by other scenes related to Mary: the Seven Sorrows, the Piet?, the Flight into Egypt, and the
Assumption [10, pp. 59-60, fig. 14]. The artist followed closely a print by Johannes Sadeler after a composition
by Marten de Vos [10, p. 58, fig. 12]. The altar is 37 cm
high; its wings are not movable. A second comparable altar, showing the Assumption of the Virgin Mary with St Jerome, St Ambrose, St Gregory and St Augustine,
STUDIES IN CONSERVATION 53 (2008) PAGES 209-224
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 211
is in the Museo Civico in Turin, Italy (acc. no. V.O. 48
2941), and has a height of 39 cm [11]. A third altar is in the National Museum in Cracow, Poland (acc. no.
MNK XIII-2324).The case is made of ebony-veneered
pinewood (height 44 cm), and the central panel shows the Nativity, with the Annunciation and the Adoration
of the Magi on the side panels. These three altars also
originate from South Germany and date from the late
sixteenth or early seventeenth century. Of these three
altars, the Cracow altar most closely resembles the
Corning piece. Both have wings that are hinged on the
columns in a very sophisticated way. Also, the general
configuration, with a drawer at the bottom, is the same.
TECHNICAL EXAMINATION
Glass manufacture
The technique by which the glass panels of the house altar were made is as yet not entirely understood. The
glass is very glossy and thin (between 1.7 and 3 mm) and slightly uneven, which makes it unlikely that the panels were cast and polished. The presence of mostly
perfectly spherical and only a few oval air bubbles in the glass indicate that the panels were made by the cylinder
blowing process [5, pp. 189-190]. Judging from the
length of the oval bubbles, the cylinder must have been short, otherwise the bubbles would have been more
elongated due to lengthening of the hot glass cylinder
during blowing. Once the cylinder had been cut along its length and allowed to flatten, the panels were cut to
size.
Amelierung technique
Well-executed reverse paintings on glass do not reveal
the complexity of their manufacture. Since the design is applied to the back of glass panels, the painting
must be built up in reverse order, starting with the
details and foreground and working 'backwards'. This
technique makes corrections virtually impossible. Reverse paintings on glass are known under the German term hinterglasmalerei in many countries. If the technique
makes use of gold and silver leaf applied to the glass, the painting is often referred to as verre ?glomis?. However, in
the case of the house altar, the German term amelierung (or gamalierung) is more appropriate. The term is not
translatable but in old German gamal means colourfully decorated. This speciality had been introduced in
Nuremberg, Germany, before 1532 by applying metal leaf and/or metal powder (both gold and/or silver) onto
the glass panel, after which the decoration was etched
with a needle or stylus [5, pp. 196-201]. A second layer of colourful translucent lacquers (lustre) was then applied and covered with a layer of silvery foil (silver or tin), creating sparkle and luminosity. In the case of the house
altar a thin silver foil was attached to paper, which was
then glued to the edges of the glass panel. In addition to the technique of amelierung, the altar shows a special treatment of the flesh layer, consisting of silver powder and pigments that were painted on, engraved afterwards
and highlighted by applying black paint.
Delamination of paint layers
The most typical disfigurement occurring on reverse
paintings on glass, which was also observed on the house
Figure 3 Crucifixion before treatment. ? Corning Museum of Qlass, Corning (NY).
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212 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
Figure 4 Detail of Virgin as Queen of Heaven, showing delamination, cupping and loss of paint. ? Simone Bretz, Munich.
altar, is the separation of paint from the glass, which
when seen from the front appears as patches of greyish, less saturated areas of paint. An estimated 80-90%
of the paint surface of the house altar was no longer
securely attached to the glass substrate (Figures 3 and 4). Hundreds of small, dislodged paint flakes and splinters were loose and needed to be reintegrated.Various factors
like normal ageing mechanisms under the influence of
changing climate conditions caused brittleness, flaking and resulting glass-like paint fragments. Degradation
processes were further aggravated when the Corning Museum of Glass was inundated during a flood in 1972.
Lastly, heat, perhaps from accelerated drying after the
flood, had caused softening and fusing of some of the
paint layers. The glass itself was found to be in good condition and showed no signs of degradation like
crizzling.
In general, a reverse painting on glass comprises a
non-porous glass substrate and a multi-layered paint
system. Internal and external factors influence the
degradation. A cleaned, degreased glass surface and a
well-chosen binding medium are crucial factors for
good physical bonding at the paint/glass interface. Poor
surface wetting produces voids that restrict contact and
trap air. The level of adhesion of the paint layer to the
glass support depends also on the composition of the
paint (colourants and binding media) and its drying mechanism, the process of film formation and the
method of application. The brittleness or flexibility of the organic coating is in correlation with the thickness
of application: the thicker a paint layer, the more prone it
is to delamination. The instability can also be caused by a
warm environment that results in faster volatilization of
the solvent and softeners. With changes in temperature
and, as consequence, changes in relative humidity, the
paint film takes up and releases moisture. The stress of
shrinking and expanding will lead to hairline cracks and
voids within the paint matrix, to blistering and cupping, and ultimately to the loss of paint fragments.
MATERIAL ANALYSIS
Glass
To avoid taking samples from the undamaged glass, a
semi-quantitative, and for some elements qualitative determination of the glass composition was undertaken
by energy dispersive micro X-ray fluorescence spectro
metry ^XRF). Six of the seven panels were examined
after local cleaning with ethanol/water, but without
additional polishing, thus taking into account that values
obtained for light elements, mainly sodium to silicon, are
affected by leaching processes. The six panels showed a
very similar composition with only small variations. In
general, the glass is rich in potassium (~ 16-17 wt% K20) with medium calcium content (~ 8-10% CaO) and
only traces of lead. In addition, the absence of bromine
in the spectra proves that ash from halophytic, soda-rich plants can be excluded as the fluxing agent.
Using a similar spectrometer type, M?ller was
able to show that traces of bromine can be regarded as
a characteristic marker for soda ash glasses [12, p. 122]. The ratio of K90/CaO in the glass panels was found to be around 1.7 to 2.0 and therefore at the upper limit
for so-called 'colourless wood ash glasses', a modified
typification suggested by M?ller [12] following the chemical classification of medieval glass by Wedepohl
[13]. However, a partial addition of purified wood ash
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 213
(potash) to the batch of glass used for the house altar
panels cannot be excluded. Very low iron oxide content
(~ 0.1?0.15% Fe203) indicates that good quality sand
was used as raw material. The presence of manganese dioxide (~ 0.5% MnO) suggests the addition of'glass makers' soap' (Mn02)
as decolourizer.
Recent analytical research by M?ller on medieval
and post-medieval glasses [12] has shown that the glass
composition found for the house altar panels is the
most typical for sixteenth-century colourless (vessel)
glass from South Germany. However, it could have
originated elsewhere in Central Europe, for example
Saxony, Bohemia or Eastern Austria. An import of the
glass panels from regions producing mainly sodium-rich
glass at that time, e.g. Italy and also Tyrol (Austria), can be
ruled out [12, especially pp. 194-220].
Binding media
The technical literature on reverse paintings on glass mentions a wide variety of possible binding agents, such
as drying oils, natural resins and gums [4, pp. 39?40, 14]. Yet in-depth analyses of binding media are rare. For the
identification of binding media of the house altar, five
small samples (~ 0.5 mg) from the St Bridget panel were used.
The binding media were first analysed by gas chromatography (GC) and gas chromatography?mass
spectrometry (GC-MS). A special solvent extraction
scheme for aged and altered materials was used for sample
preparation [15]. With the help of various polar solvents
complex mixtures of components with different polarity, such as waxes, oils and resins can be pre-separated, thus
yielding more easily interpretable chromatograms. The
solvent extracts were directly injected into the GC
or GC-MS without prior derivatization. Instead of
derivatization, oxalic acid (10% w/v) was added to the
methanol extracts (see Appendix). The transparent samples of red, brownish-red and
green paint were easily soluble in methanol and gave
very similar GC-MS results with only slight variations
in the quantitative composition. The main compo nents in these samples were diterpenoid resin acids of
the abietane type (dehydroabietic acid), apart from a minor variety of oxidized and dehydrogenated diter
penoid resin acids (e.g. 7-oxo-dehydroabietic acid,
7-oxo-dehydrodehydroabietic acid). Resin acids of the
abietane type are the most abundant components of
diterpenoid natural resins from the Pinaceae family. In
particular, colophony, the resin of pine trees (Pinns), was widely used in art and craft. Due to isomerization
and disproportionation reactions during the distillation
process of the pine resin, abietic and particularly
dehydroabietic acid are formed from a varying number
of diterpene resin acids [16]. On long exposure to air and
light the resulting relatively stable dehydroabietic acid will oxidize to form several oxidized products [17, 18]. In contrast to these ageing processes, the high content of
oxidized and dehydrogenated resin acids in the analysed
samples is only explicable by strong heating of the resin [19]. Research on resin and tar production revealed
that certain types of resins can be obtained by melting at high temperatures. The resins obtained after indirect
heating of the resinous wood contain isomerized and
dehydrated products including dehydroabietic acid and
dehydrodehydroabietic acid (abieta-tetraenoic acid) as
well as oxidized products like 7-oxo-dehydroabietic acid and 7-oxo-dehydrodehydroabietic acid. The main
components now present in the Corning altar samples are therefore the degradation and heating products of
former pine resin (Figure 5,Table 1). In addition, neutral diterpenoid components from
larch turpentine (epimanool, larixol, larixylacetate, Figure
6) could be identified [20, 21]. Balsamic larch turpentine originates from the Larix species (e.g. Larix decidua Mill.), which grow in the Austrian-Tyrolean region. Larch
turpentine belongs to the group 'fine turpentine', which
does not tend to crystallize and retains a long-lasting
elasticity. Even today, larch turpentine (in combination
with Canada balsam) plays a special role in the glass and
optical industry, where it is used as high-grade glue for
glass prisms and glass panels [22]. Only minor amounts of a third resin could be found
in the triterpenoid region of the chromatogram (Figure
6). Small amounts of noroleanenone and other related
oleanane structures were identified, which serve as
useful compounds in the identification of aged mastic
resin [23]. Mastic is the resin of the mastic tree (Pistacia lentiscus L.), of the Pistachio family. Its natural distribution
area encloses the coastal regions of the Mediterranean
Sea, but most mastic comes from the Greek island of
Chios.
After nearly 400 years, some volatile compounds were
still detectable in the samples. Due to the thickness of the
paint layers, small amounts of monoterpenoids camphor and cineole are preserved. Camphor is originally contained in the bark of the camphor tree (Cinnamomum
camphora) and was added as softener [24]. 1,8-cineole
(eucalyptol) is no constituent of the resins analysed but
derives from the addition of an essential oil as solvent for
the resin mixture. A cineole-rich essential oil (spike oil) was produced from the lavender subspecies Lavendula
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214 S. BRET2, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
Oxidation
DA
Dehydroabietic acid 7-Oxo-dehydroabietic acid ODA
DDA ODDA
Figure 5 Green lacquer layer, gas chromatogram of the methanol extract. The addition of oxalic acid leads to a slightly changed polarity of the non-polar separation column and the resin acids could be separated. The structures of the most stable pine resin compound (dehydroabietic acid) and its oxidation
product (7-oxo-dehydroabietic acid) are given. Abbreviations: C = camphor, DA = dehydroabietic acid, DDA = dehydrodehydroabietic acid, ODA = 7-oxo
dehydroabietic acid, ODDA = 7-oxo-dehydrodehydroabietic acid. ? Doerner Institut, Munich.
Table 1 Summary of binding medium components identified with their origin and colourants of the paints (Saint Bridget panel)
Components identified Origin Red sample Red-brown sample 'Green' sample 'Blue' samples (now brown) (now greyish-black)
dehydroabietic acid
dehydrodehydroabietic acid
7-oxo-dehydroabietic acid
7-oxo-dehydrodehydro abietic acid
epimanool larixyl acetate larixol 28-nor-olean-17-en-3-one
camphor 1,8-cineole (eucalyptol) amino acids
(mainly serine and glutamic acid) fatty acids
(mainly palmitic and stearic acid) colourants
heated and oxidized pine resin
larch turpentine
mastic
camphor spike oil
aged egg protein
] (fat)
cochineal, dragon's blood
(+)
dragon's blood copper resinate smalt, lamp black
(thin, black layer)
Abbreviations: +++ = main component, ++ = minor component, + = small amounts, (+) = trace amounts, - = not detected.
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 215
Larjxyl acetate
L2
DA
ODA
Figure 6 Red lacquer layer, gas chromatogram of the methanol extract. In the pure methanol extract, without further derivatization agents, the neutral
diterpenoid markers for larch turpentine (L1 and L2) are clearly visible. The structure of the main component of larch turpentine, larixylacetate, is given. Abbreviations: C = camphor, L1 = epimanool, L2 = larixylacetate, DA = dehydroabietic acid, ODA = 7-oxo-dehydroabietic acid, M = noroleanenone (mastic). ? Doerner Institut, Munich.
latifolia (Spike lavender) [25], Hence, spike oil was used as a solvent for this resin mixture (Table 1).
Samples taken from an area that was originally blue
were found to contain the pigment smalt (see below) and revealed similar resin mixtures (larch turpentine, oxidized pine resin, mastic), but also small amounts of
saturated fatty acids in a ratio which is not characteristic
of drying oils but rather of compounds from protein aceous media. Hence the non-soluble residues of the
blue samples were further investigated by amino acid
analysis [26]. The amino acid profiles of the hydrolysed samples clearly showed the presence of an aged and
degraded egg binding medium. In coherence with the fatty acids detected in the solvent extracts, these markers
hint toward a 'fat' egg binding medium (entire egg or
egg yolk). It is possible that egg was first used to help grind the pigment, while the resin mixture served as
painting medium.
Colourants and metals
Five colourants (Table 1) were identified in red, brownish-red, green and blue paint samples from the
St Bridget panel. Scanning electron microscopy/energy
dispersive X-ray microanalysis (SEM-EDX) and ^, high performance liquid chromatography (HPLC) with ultraviolet-visible (UV-VIS)-detection and GC-MS
were used for the analysis. No samples of the flesh tone
were available for analysis. The red lake contained carminic acid, which was
identified by its retention time and UV-VIS spectrum
using HPLC. Carminic acid is the main colourant of
cochineal, one of the most valuable organic lakes used
for paintings in the late sixteenth century [27, 28]. Due to the minute sample size, no minor components of the
lake could be detected, which left no means to determine
the provenance of the cochineal lake more precisely.
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216 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
Small amounts of aged dracorhodin and characteristic
triterpenes from dragon's blood, a red plant resin, here of
the Daemonorops draco species, were detected in red and
brownish-red samples by GC-MS [28, pp. 142f., 29, 30]. This reddish-brown resinous plant exudation originates from Indonesia (Sumatra/Borneo). Although dragon's blood is frequently mentioned in historical sources, its
analytical identification on sixteenth century art works
has rarely succeeded so far.
The bluish paint layer was found to contain smalt
as the main pigment (it also contains traces of dragon's blood, presumably from an adjacent red paint). Significant amounts of arsenic and iron, and traces of bismuth,
copper, nickel and zinc were detected by SEM-EDX and
E They are typical for smalt of the second half of the sixteenth century, which was mostly extracted from
cobalt ores in the Erzgebirge (Ore Mountains between
Saxony and Bohemia) [31, 32]. Using smalt, the artist of the house altar may have tried to minimize the costs of
materials. The low refractive index of smalt (1.49-1.53) is close to that of the resinous binding medium that was
used, and makes it an ideal translucent colourant for this
paint composition. Whereas the use of smalt in easel
painting is somewhat difficult because of floating and
dripping of the wet paint [33], in reverse paintings on
glass the blue pigment advantageously settles down on
the glass while the paint dries, and thus becomes highly visible through the glass.
The greenish-brown paint layer was found to have
a high content of copper. SEM-EDX analysis showed
that copper is homogeneously distributed throughout the lacquer without showing distinct pigment particles.
Keeping the results of the binding medium analyses in mind, this is one of the rare cases where copper resinate could unambiguously be identified in such a
glaze. The colourant may have been produced in the
artists' workshop by boiling a copper compound such
as verdigris with the resin(s) as described in the De
Mayerne manuscript (c. 1620?1646, e.g. fol. 31 and
31 [34, 35]), which explicitly mentions the use of this
compound on gold and silver foil as well as on glass. However, it is also possible that the copper resinate
slowly formed over the centuries as a reaction product of
verdigris and the resinous medium. The green paint layer is noticeably thicker than the other lacquers, and it has
an even surface with no visible brushstrokes.
The silver area that represents the flesh was, as men
tioned earlier, coated with a black, opaque paint, which
contains a finely dispersed black pigment, presumably lamp black.
Gold and silver foils were used to create the amelierung effect. Both metals were found to be of high quality with
only about 5 wt% Ag in the gold and no other metal in the silver in amounts that could be detected by SEM
EDX. A heavily corroded metal foil on the reverse side
of one of the panels (Seven Sorrows), which protected the
silver layer, was identified as pure tin foil (thickness about
7 ). 6 size used to apply the gold and silver leaf on
the glass could not be identified because the coating was
extremely thin.
DISCOLOURATION AND DIGITAL RECONSTRUCTION
Although the paint layers were protected against UVB
and UVC light by the glass substrate, the binding medium was aged. It had become brittle and changed from the original light yellow to a deeper yellow or even
darker tone. The latter is most obvious in the original blue paint layer (see below), whereas the optical effect of
yellowing is far less evident in the red lacquer. The red paint colour is in comparably good condition,
although some uneven fading has been observed in
transmitted light (Figure 7). The light-fastness of the two
organic colourants identified is different: cochineal on
alum substrate is considered one of the reasonably stable
red lakes available in the sixteenth century [36], whereas
dragon's blood is usually described as being prone to
fading. Artificial ageing tests of this resin has confirmed
the low light-fastness but showed considerable difference
in stability between the various botanical sorts and
binding media [37]. The intense green areas depicting meadows (Figures
2 and 3) are greenish brown today. These areas only reveal their original green colour in transmitted light.
The discolouration is most likely due to yellowing of
the medium and to a greater extent to the browning of
the copper glaze. This impermanence of copper resinate
and also verdigris glazes is a well-known phenomenon in painting [38], yet its discolouration mechanisms and
the accelerating factors are not yet entirely understood.
However, the final formation of brown CuO is
suggested by various authors as a possible cause for the
discolouration [35, pp. 150f, 39]. Deterioration and colour change of the formerly blue
areas of the sky and the background of the medallions
(Figures 2 and 3) are comparably severe. Smalt has
partially discoloured due to the well-known process of leaching of potassium from the pigment particles.
Interestingly, the condition of the binding medium and
thus its effect on the optical appearance of the blue paint
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 217
Figure 7 Virgin as Queen of Heaven in transmitted light, with reintegrated paint flakes. Fading of carmine and dragons' blood can be seen. ? Simone Bretz, Munich.
shows variations: in some areas the binding medium, which was identified as a mixture of resins with egg,
appears extremely darkened or even black. Similar
darkening is typical for oil paint layers that contain
degraded smalt. Saponification products are mentioned
as a possible factor [40], yet the chemical processes in the
medium used here may be different. In other blue areas
of the Corning altar, the paint is totally opaque, giving a milky, light-blue appearance. A cross-section revealed
strong degradation of the binder in the back-scattered
electron image with several micro-cavities probably
acting as light-scattering sources (Figure 8). In order to have a better idea of the original appear
ance of the house altar, given the severely obscured
tonality of the glass paintings, a digital reconstruction of
the panel Crucifixion was made. Only the original green and blue parts, which were found to be most heavily
affected by colour changes, were digitally rendered.
Based on the visual colour estimation of the remaining
green and blue spots visible under magnification in
transmitted light and on the knowledge of the original colourants through analysis, paint trials with smalt of
dark quality and copper resinate, bound in a natural resin
mixture (colophony, mastic in ethanol) were prepared.
Using Photoshop? CS2, the blue and green areas were
isolated and their colour values selectively adjusted until they visually matched the paint samples provided.
Figures 2, 3 and 9 compare the condition and tonality of the central panel of the house altar before and after
the conservation treatment and as a result of the digital reconstruction.
CONSERVATION
All seven panels with reverse painting on glass are set into
an oak frame and held in place by ebony veneer. The
construction is simple, but finely detailed. The original
joints were made with animal glue.The glass panels were
dismantled by removing the veneer frames mechanically without any use of solvents to avoid possible damage to
the paint layers. The surface of the ebony veneer was
cleaned with distilled water and white spirit afterwards
to remove accumulated grime and dirt.
In order to gain access to the decorative layers of the
reverse paintings on glass of the house altar, the paper
covering the reverse sides of the glass panels had to be
removed, together with the silver foil. Unexpectedly, up to 30% of the paint was found to be stuck to the foil (Figure 10). The edges of some of the flakes were not
sharp and thus seemed to have softened due to heat. The
paint transfer was probably caused by exposure to heat
when the house altar had to be dried after the flood at
Corning in 1972. The most time-consuming part of the
conservation work was the transfer of paint back onto
the glass. Paint flakes were trapped under lifting paint and needed to be retrieved. Where possible, each flake
of paint was reattached in its original position. At the
beginning it seemed that the decoration had suffered
extensive loss. After conservation only minor losses of
not more than 1-2% were apparent (Figure 2). Since there are many different types of reverse paint
ing on glass, each one requires an individual approach to
its treatment. Several conservation materials have been
tested within the last 20 years to identify suitable materials
for the adhesion of flaking paint to a glass substrate, such
as acrylic resin, polyvinyl acetate, hydrocarbon resin, ketone resin, cellulose, natural resin, animal glue and wax
[9]. The requirements for the consolidating material are
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218 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
(b)
Figure 8 (a) Light microscopical image of a blue cross-section, 200*. The sample was extremely brittle and difficult to polish. Residues of the
polishing material in surface fractures render the sample whitish, (b) Back scattered electron image of the blue paint layer at higher magnification showing a highly degraded binding medium between the smalt particles. ? Doerner Institut, Munich.
as follows: the coating has to provide excellent adhesion to the glass and should have some flexibility (adequate transition temperature, Tg). It should not have any visible
impact on the artefact. The refractive index should be
equal to the paint medium to minimize reflection at the
interface. It must be compatible with organic materials
to avoid discolouration of the original paint. The advantage of solvent-based consolidant systems
for reverse paintings on glass is their ability to bond
the delaminated paint in low concentrations as a pre
Figure 9 Crucifixion, digitally reconstructed colour appearance. ? Andrew M. Fortune, Corning Museum of Glass, Corning (NY).
liminary application. Where loose paint fragments have
been dislodged and have to be reintegrated in the paint layer, resin solutions are very useful to make them
adhere to the glass again. The consolidation resin used
for this purpose on the house altar paintings was the
hydrogenated hydrocarbon resin Regalrez 1094? in a 20% (w/v) solution in white spirit. The disadvantage of such a conservation material is that after the evaporation of the solvent a high proportion of voids become visible after a period of time. In order to eliminate this, a subsequent treatment with an adhesive-like wax was
experimented with on delaminated reverse paintings on
glass and proved to be successful. In the case of the house
altar a sheet of silicone paper was coated with molten
TeCerowax 30445 (a microcrystalline hydrocarbon wax) and placed face down on the damaged paint layer. Pressure and heat applied by a heated spatula needed
to be adjusted depending on the level of damage. The
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 219
Figure 10 Detail of St Dorothy: (a) reverse side during conservation; (b) large parts of the paint layer are attached to the silver foil and paper. ? Simone Bretz, Munich.
softening of the original resin medium during thermal
treatment enabled the closing of gaps between paint islands and the flattening of concave flakes, yielding a
uniform paint layer which was not achievable by using a solvent-based consolidant alone. A brilliant saturation
of the original paint was the result. A thin wax coating was left on the paint layer, which can be reactivated by
reheating in case of further delamination from the glass.
Finally, the conserved glass panels were set into their
original positions and the ebony veneer frames were
reattached with animal glue. Pieces of missing wood
were replaced with new ebony veneer.
CONCLUSION
The technical and scientific research, conducted prior to the conservation treatment of the reverse paintings
on glass of the Corning altar gave valuable insight into
the still rather little-investigated amelierung technique. The colourless glass panels, which are not significantly altered, are of good quality and might well be a
regional product. The binding medium is a mixture of
various resins with good adhesion properties, which is
important for its application to a non-porous substrate
like glass. Quite unexpectedly, no drying oils or gums were found, and only blue samples contained some
egg. The most abundant components derive from aged and oxidized pine resin (colophony), which had most
likely been heat-treated. The processed pine resin was
modified by the addition of different softeners, i.e., larch
turpentine (also known under the trade name 'Venetian
turpentine'), mastic and camphor. In some layers remnants of spike oil were preserved. The identified resin
mixture is therefore composed of regional materials
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220 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
(pine resin, larch turpentine) and imported, exotic resins
(mastic, camphor). For the colourants, this situation is
much the same: the artist used costly and exotic goods
(cochineal, dragon's blood, high-quality noble metals) as well as cheaper and locally available materials (smalt,
copper resinate or verdigris, lamp black). The colour
changes observed on the reverse paintings on glass most
heavily affect the formerly blue and green parts of the
composition. These are due to chemical reactions of
the pigments in conjunction with degradation of the
resinous medium. Fading was observed in the red organic colourants. It is hoped that future scientific research on
reverse paintings on glass will allow discussion of the
technical and material particularities of the Corning house altar in a broader context.
The technique and state of preservation of each
reverse painting on glass is unique and thus requires an individual approach for the choice of material and
treatment. The speciality of the amelierung technique which only uses lacquers enabled the conservation as
described. For the house altar panels the use of a single consolidant would have been preferable. However,
without any pre-fixation of the loose paint particles with the resin (Regalrez 1094?,), the reintegration of the
flakes back onto the glass support would not have been
possible. The fragments would have been displaced by the treatment using molten wax (TeCerowax 30445) alone. Only by combining these two conservation
materials could a consolidated and saturated amelierung
paint layer be achieved.
APPENDIX: EXPERIMENTAL DETAILS OF MATERIAL ANALYSES; MATERIALS AND SUPPLIERS
Sample treatment for binding media analysis
Samples were pre-treated for GC-MS by step-wise extraction with different solvents: isooctane, methanol, chloroform/methanol (7:3 v/v), oxalic acid in methanol
(10% w/v) (all solvents of gas chromatogaphic grade, supplied by Merck chemicals, and anhydrous oxalic
puriss by Fluka company). The solvent extracts were
injected into the GC-MS without derivatization. Aged resins are unstable even in common solvents and show
considerable compositional changes within a few days; therefore the extracts were analysed within a few hours.
The most straightforward identification of natural resins
is by non-acidic marker compounds that can be detected
in the underivatized methanolic extract. Resins are also
very sensitive to unsuitable preparation procedures,
and drastic methylation methods, in particular those
using strong acid or basic catalysts, should be avoided.
Therefore, anhydrous oxalic acid (5?10% w/v) was
added to the methanolic extract in an additional analysis
step. In the gas chromatogram of these acidified solu
tions, non-acidic diterpenes are strongly attenuated and
no longer detectable, while diterpenoid acids elute with
fairly good response factors and are separated well. In
that step, resins with a high amount of diterpenoid acids, such as pine resin and colophony, can be identified. The
approach described here does not provide a full analysis of all chemical compounds in a sample. Instead it is
optimized to obtain patterns and marker compounds that enable the identification of the specific types of
natural resins that are used in art [30]. Moreover, similar
resins can be distinguished in mixtures, and conclusions
on treatments during the manufacturing process (e.g. thermal treatments) can be drawn even from aged
samples. Proteinaceous binding media were identified based
on their amino acid composition. In order to release
amino acids from proteins, the sample residues from
the solvent extraction described above were hydrolysed with boiling HCl (6 ) for 24 hours under vacuum. If necessary, metal ions from pigments were precipitated with 8-hydroxy quinoline at pH 9 and removed by filtration with a C18 cartridge. The amino acids were
collected by an acidic buffer and separated in an amino
acid analyser (see below).
Gas chromatography
Gas chromatography was performed using an Agilent GC 6890 with autosampler equipped with a DB-5ht column (-40 to 400?C, 15 m, 0.32 mm ID, 0.1 film
thickness, J & W company). Measurement conditions
were the following: carrier gas helium 5.0 (purified with
heated high-capacity gas purifier, Supelco company), constant-flow mode, 1.7 mL per minute; split/splitless
injector: injection temperature =
250?C, splitless mode, 1 . injection volume, 0.5 minutes splitless; purge flow
36 mL per minute; oven programme: TI =
55?C, tl =
1 minute, Rl = 11?C per minute, T2
= 150?C, R2
=
10?C per minute, T3 =
360?C, t3 = 5 minutes; flame
ionization detector (FID), detector temperature =
360?C.
Gas chromatography-mass spectrometry
GC-MS analysis was performed using a gas Chromato
graph of the Hewlett-Packard (now Agilent) HP 5890
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A GERMAN HOUSE ALTAR FROM THE SIXTEENTH CENTURY 221
series II coupled to a Hewlett-Packard quadrupole mass spectrometer type 5989B, MS Engine, in the
electron impact (El) mode, scan range 40-500 m/z.The
Chromatograph was equipped with a DB-5ht column
(5%-phenyl-methyl-polysiloxane), 30 m, 0.25 mm ID, 0.1 film thickness (J & W). Measurement conditions
were as follows: carrier gas helium 5.0 (purified with
heated high-capacity gas purifier, Supelco company), constant-flow mode, 1.7 mL per minute; split/splitless
injector: injection temperature =
250?C, splitless mode, 1?2 . injection volume, 0.5 minutes splitless; purge flow 36 mL per minute; oven programme: Tl
= 55?C,
tl = 1 minute, Rl = 14?C per minute,T2
= 150?C,R2
= 10?C per minute, T3 =
360?C, t3 = 5 minutes; US
National Institute of Standards and Technology (NIST) library, version 05.
Amino acid analyses
For amino acid analyses (AAA), ion exchange liquid chromatography with post-column derivatization
(reagent ninhydrin, supplied by Merck chemicals) and photometric absorption detection (570 nm, 440 nm) was
applied using a Biotronik LC 5001 with autosampler and a separation column with cation exchange resin (resin
type BTC 2710, 210 mm length, Biotronik company). Measurement conditions were as follows: hydrolysate buffer system with sodium citrate buffers (pH 2.2-13),
injection volume 50 ., flow rate 30 mL per minute. For a full description see [41].
High performance liquid chromatography
The red lake was identified using a Hewlett-Packard
1050 HPLC system consisting of a 7125 injection system (20 . sample loop), gradient pump, diode array detector 1100 with microcell and HP Chemstation.
UV-VIS spectra were measured between 200 and 600
nm; detection wavelength was set at 275 nm, reference
wavelength at 550 nm (band with 4 and 100 nm). For separation a C18 column (column type HyperClone 5 u ODS (C18) 100 x 2.00 mm 5 micron, supplied by Phenonemex) was used. Eluents were distilled
water/0.1% trifluoroacetic acid (A) and acetonitril/0.1%
tetrafluoroacetic acid (B). Gradient elution A:B from
95:5 to 5:95 within 40 minutes, then kept for 5 minutes.
Flow rate 0.8 mL per minute (method after Halpine
[42]).The sample was dissolved in oxalic acid/methanol
(10% w/v), the reference samples in methanol/HCl
(1% v/v).
Preparation of cross-sections
Paint cross-sections were embedded in Technovit? 2000
LC (UV-hardening acrylic resin, supplied by Heraeus Kulzer) under water-jet vacuum, pre-ground with Leco
Grit 400 and 1200 (Leco corporation) and polished with Micro-Mesh? cushioned abrasives (supplied by Dick
GmbH) down to grade 12000.
Light microscopy
Light and fluorescence microscopy was carried out on an
Zeiss Axioskop with filter 'DIC for direct light. Images were taken with a digital AxioCAM MRc microscope camera and AxioVision 3.1 software (Zeiss).
Scanning electron microscopy/X-ray microanalysis
SEM-EDX analyses were undertaken with a Philips (now FEI company) XL 20 electron microscope
with an EDAX (now AMETEK) Si(Li) detector and Genesis software (EDAX / AMETEK) at the following measurement conditions: 25 kV, 30 A, 1000 cps, 100 s
live-time, standardless ZAF quantification.
Micro X-ray fluorescence spectrometry
For energy-dispersive XRF, a portable Mikro TAX
spectrometer (Bruker company) with Mo-microfocus
tube (30 W), poly capillary, peltier-cooled SDD detector and helium purging was made available by the Federal
Institute of Material Testing and Research, Berlin.
Measurement conditions: 45 kV, 600 A, 100 s live
time, semi-quantitative calibration with various standard
glasses of NIST, The Corning Museum of Glass and
the Federal Institute for Material Testing and Research
(BAM).
Materials for conservation and paint trials
Smalt (No. 10000), copper resinate (No. 12200) and Regalrez 1094?: Kremer Pigmente, www.kremer-pigmente.de
TeCerowax 30445:Tromm Wachs, www.wax-tromm.de
ACKNOWLEDGEMENTS
The authors would like to acknowledge the conservation
treatment grant approved by The Getty Foundation and are very grateful for the support. The authors would like to thank Cornelia Tilenschi, Karin Lutzenberger and
Irene Fiedler, Doerner Institut in Munich for carrying
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222 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
out the SEM/EDX, HPLC and amino acid analyses; Oliver Hahn, Federal Institute for Material Testing and Research (BAM) in Berlin for his support with the XRF measurements in Munich; and Andrew M.
Fortune, assistant photographer and digital image special ist at The Corning Museum of Glass for the digital
rendering of the house altar. Sandra Davison, private
glass conservator in Thame, UK, is warmly thanked for
valuable corrections and comments on the manuscript.
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AUTHORS
Simone Bretz is a private conservator based near
Munich, Germany. Since 1985 she has specialized in
the conservation and restoration of reverse paintings on glass, and has acquired a unique knowledge of the
painting technology and published on the subject. She has worked for a number of major museums and
private collections in Europe and the United States and
contributed her expertise to research projects both in
Germany and abroad. Address: Schmiedeweg 28, 82496
Oberau, Germany. Email: [email protected]
Ursula Baumer received a degree in chemistry from the
Chemieschule Elhardt, Munich, Germany. She joined the Doerner Institut in 1988 and is working as a research
chemist, responsible for organic analysis. She specializes in the analysis of ancient and modern binding media of
paintings and works of art, archaeological resins and tars,
lacquers and varnishes on furniture and altars. Address:
Doerner Institut, Bayerische Staatsgem?ldesammlungen, Barer Strasse 29, 80799 Munich, Germany. Email:
baumer@doernerinstitut. de
Heike Stege received her diploma (1994) and PhD (1998) degree in chemistry from the Technical University of Berlin, Germany. She has a postgraduate degree
(habilitation in 2005) in the field of modern analytical techniques especially suited to works of art made of glass and enamel. Since 2002 she has specialized in pigment research at the Doerner Institut, where she is currently head of the scientific department. Address: as Baumer.
Email: stege@doernerinstitut. de
Johannes von Miller has been working as a private conservator of furniture and wooden objects for more
than 15 years. He specializes in the research of historic
lacquer and the reconstruction of historic surfaces of
wooden objects. Address: Dorfplatz 9, 83707 Bad Wiessee,
Germany. Email:johannes@vonmiller. de
Dedo von Kerssenbrock-Krosigk is head of the
Glasmuseum Hentrich at the Stiftung museum kunst
palast, D?sseldorf, Germany. After receiving his doctorate
from Humboldt University, Berlin, Germany he worked
at the Br?han-Museum in Berlin, and from 2004 to 2008
as curator of European glass at the Corning Museum of
Glass in Corning, NY, USA. Address: Stiftung museum
kunst palast, Ehrenhof 4-5, 40479 D?sseldorf, Germany. Email: [email protected]
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224 S. BRETZ, U. BAUMER, H. STEGE, J. VON MILLER AND D. VON KERSSENBROCK-KROSIGK
R?sum? ? Lors de la pr?paration du traitement de conservation d'un autel domestique de la fin du XVIe si?cle du sud de
l'Allemagne, au Corning Glass Museum, orn? de peintures fix?es sous verre, on a men? des ?tudes techniques et analytiques sur
le substrat de verre, les liants, les colorants et les ?l?ments m?talliques. Un examen au microscope a r?v?l? les particularit?s de la
technique appel?e amelierung et de la fa?on dont le verre a ?t? manufactur?. Pour l'analyse des mat?riaux, la Chromatographie gazeuse coupl?e ? la spectrom?trie de masse, l'analyse des acides amin?s, la Chromatographie liquide haute performance, la micro
spectrom?trie de fluorescence des rayons X et la microscopie ?lectronique ? balayage coupl?e ? la spectrom?trie des rayons X ont
?t? utilis?es. Le principal composant des liants ?tait une r?sine de pin chauff?e (colophane), modifi?e par l'ajout de diverses r?sines assouplissantes. La couche de peinture rouge contient de la cochenille et de petites quantit?s de sang de dragon. Les zones
originales bleues et vertes, qui contiennent du smalt et du r?sinate de cuivre, ont subi d'importants changements de couleur. Les
peintures fix?es sous verre ?taient en pi?tre ?tat. En raison de l'humidit? tr?s ?lev?e (? la suite d'une inondation au mus?e) et
des m?canismes de vieillissement, la couche de peinture ?tait s?rieusement soulev?e, fractur?e en centaines de petites ?cailles et
partiellement coll?e ? la feuille d'argent et au fond en papier. Les fragments de peinture ont ?t? remis en position sur le verre
? l'aide d'un fin pinceau et adh?r?s ? l'aide d'une r?sine hydrocarbure. Ce traitement a ?t? suivi de l'application d'une cire
microcristalline qui a permis d'obtenir une saturation brillante des peintures fix?es sous verre. Cependant, le changement de
couleur subsistait dans les zones bleues et vertes. Apr?s le traitement on a manipul? des images num?riques pour restituer la
tonalit? d'origine pr?sum?e.
Zusammenfassung ? Zur Vorbereitung f?r die Konservierung eines s?ddeutschen Hausaltares mit Hinterglaseinlagen aus dem
sp?ten I6. Jahrhundert wurden technische und naturwissenschaftliche Untersuchungen des Glases, der Binde- und Farbmittel
sowie der Metallfolien durchgef?hrt. Die mikroskopische Untersuchung zeigte die Besonderheiten der so genannten Amelierungs Technik und der Herstellung der Glastafeln auf. F?r die Materialanalysen wurden Gaschromatographie / Massenspektrometrie,
Aminos?ureanalyse, Hochleistungsfi?ssigkeitschromatographie sowie Mikror?ntgenfluoreszenzspektrometrie und Ras t?relektron?
nmikroskopie mit energiedispersiver R?ntgenmikroanalyse verwendet. Als Hauptbestandteil des Bindemittels wurde ein thermisch
vorbehandeltes Kiefernharz (Kolophonium) nachgewiesen, das durch Zusatz verschiedener Weichmacherharze modifiziert wurde. In den roten Malschichten wurden Cochenille und geringe Reste von Drachenblut gefunden. In ehemals blauen und gr?nen Bereichen, die Smalte bzw. Kupferresinat enthalten, waren betr?chtliche Farbver?nderungen festzustellen. Die Hinterglas tafeln waren in schlechtem Erhaltungszustand: Wegen hoher Luftfeuchte (?berflutung des Museums), vermutlich nachfolgender
W?rmebehandlung und Materialver?nderungen war die gesch?digte Malschicht gro?fl?chig vom Glas abgel?st, in zahlreiche lose
Farbpartikel zerbrochen und klebte an den Hinterlagen aus Silberfolie und Papier. Nach dem Transfer der Malschichtfragmente in ihre originale Position auf dem Glas und Fixierung mittels eines Kohlenwasserstoffharzes und, in einem zweiten Schritt, mit
einem mikrokristallinen Wachs, zeigten sich die Hinterglastafeln wieder strahlend. Allerdings blieben die Farbver?nderungen in den
blauen und gr?nen Partien irreversibel. Ein Photo nach der Konservierung der Mitteltafel des Hausaltars wurde digital bearbeitet, um die angenommene urspr?ngliche Farbigkeit zu visualisieren.
Resumen ? Como fase previa a los procesos de restauraci?n de un altar de devoci?n privada procedente del sur de Alemania del siglo XVI tard?o, que incluye siete pinturas bajo vidrio, se realiz? una investigaci?n t?cnica y cient?fica del substrato vitreo, de
los aglutinantes, metales y colorantes. El examen microsc?pico inform? sobre la t?cnica llamada uamelierung" y la manufactura de
los paneles de vidrio. Para los an?lisis de materiales se emplearon las siguientes t?cnicas: cromatograf?a de gases-espectrometr?a de
masas, an?lisis de amino ?cidos, cromatograf?a l?quida de alta resoluci?n, as? como micro espectrometr?a por fluorescencia de rayos X y microscop?a electr?nica de barrido combinada a microan?lisis por energ?a dispersiva de rayos X. El principal componente del
aglutinante se identific? como resina de pino precalentada (colofonia), modificada con otras resinas "plastificantes". La capa roja de pintura contiene cochinilla y peque?as cantidades de sangre de drag?n. Las ?reas que en principio deb?an haber sido azules
y verdes, que se componen de esmalte y resinato de cobre, han sufrido un dr?stico cambio crom?tico. Las pinturas bajo vidrio se
encontraban en un delicado estado de conservaci?n: debido a los altos niveles de humedad (inundaciones en el museo) y ala
degradaci?n de los materiales, las capas pict?ricas se encontraban gravemente exfoliadas, fracturadas en cientos de peque?as escamas
y parcialmente adheridas a la l?mina de plata y a la trasera de papel. Las part?culas de policrom?a desprendidas se transfirieron a sus localizaciones originales con un pincel fino y aseguradas mediante la aplicaci?n de una resina hidrocarbonada. A ello sigui? la aplicaci?n de cera micro-crisialina con la que se consigui? una buena saturaci?n de las pinturas bajo vidrio. Sin embargo se
mantuvo la decoloraci?n de las ?reas azules y verdes. Una fotograf?a de despu?s del tratamiento fue manipulada digitalmente para recrear el probable cromatismo original.
STUDIES IN CONSERVATION 53 (2008) PAGES 209-224
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Issue Table of ContentsStudies in Conservation, Vol. 53, No. 4 (2008) pp. 209-304Front MatterA German House Altar from the Sixteenth Century: CONSERVATION AND RESEARCH OF REVERSE PAINTINGS ON GLASS [pp. 209-224]The Conservation of Glass Ingots from the Bronze Age Uluburun Shipwreck [pp. 225-237]Thermal Stress as a Possible Cause of Paintwork Loss in Medieval Stained Glass Windows [pp. 238-251]Approaches for Conservators to the Identification of Plant Material used in Mori Artefacts [pp. 252-263]Calcium Carbonate on Bronze Finds: SAFE SEQUESTERING WITH SODIUM TRIPOLYPHOSPHATE? [pp. 264-272]A Review of the Colour and Condition of Lindow Man 20 Years After Conservation [pp. 273-284]In MemoriamLyuben Prashkov 19312007 [pp. 285-286]
Climate Change and Museum Collections [pp. 287-297]Book ReviewsReview: untitled [pp. 298-300]Review: untitled [pp. 300-301]Review: untitled [pp. 301-304]
Back Matter