Conservation and Research of Reverse Paintings on Glass

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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

STUDIES IN CONSERVATION 53 (2008) PAGES 209-224


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,



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).




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




(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



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.




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.




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




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




(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




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




(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




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




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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the use of the green glaze called "Copper resinate" and its colour changes', in Looking Through Paintings, ed. E. Hermens, A. Ouwerkerk and . Costaras, de Prom and Archetype Publications, Baarn and London (1998) 133-146.

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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]




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.



Article Contentsp. 209p. 210p. 211p. 212p. 213p. 214p. 215p. 216p. 217p. 218p. 219p. 220p. 221p. 222p. 223p. 224

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]

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Conservation and Research of Reverse Paintings on Glass

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