The wall paintings of the Chapel of the Holy Rood were analysed in Chemical Laboratories of the National Gallery in Prague in 1980 and 1982. The samples for analysis were taken by Academic Painter Jan Pasálek. Here we shall summarize only the main findings. The full text of the laboratory reports, including a description of methods used, is available for study in the Archives of Historical Art Technology and in the National Gallery.

Sample No 1 – 24 Old Men, background

In the lowest position on the cross-section is a grey siliceous layer 0.092mm thick. Microscopically it is identical to the siliceous layers on the panel paintings of the same period, e. g. the panels from Bubeček and the Alterpiece from Zátoň. The histochemical test for binding media was positive both for protein and for oil. The supplementary solubility test showed the presence of glue with an admixture of oil.

Above the siliceous layer lies an ochre base 0.025 mm thick. The microrystal-line substance with particles less than 0.001 mm in diameter contains colourless particles as well as yellow and red ones. The ground layer is bound with oil. The histochemical test for the presence of proteins was negative. The layer of white colour 0.025 mm thick consists of lead white with traces of ochres. The histochemical test for binding media in the colour layer was positive both for proteins and for oils. The supplementary tests – the foam test, solubility in 10 % potassium hydroxide, 396 nitric acid, reaction with Mil-Ion 's reagant – were carried out on a layer of paint also including the ochre ground. In the evaluation of the results we had to take this circumstance into consideration. We are of the opinion that the binding agent in the colour layer is egg tempera with the addition of oil.

Results of spectral analysis:
Ca Mg Pb Si Al Mn Cu Fe Ag (Sn)

Further samples taken from the same scene (the 24 Elders of the Apocalypse) gave the following composition of colour layers:

– blue cloak of the Elder in the middle of the front row: ultramarine, azurite and lead white

– blue-green cloak of the Elder in the second row on the left: azurite, lead white and a tiny amount of red organic lake (under this layer is a very thin layer of charcoal black)
yellow hair of the same figure:
In the microscopic fragments (grey siliceous layer, microcrystalline ochre ground and thin yellow paint), which it was impossible to separate, there were found yellow and red ochres, colourless siliceous particles, lead white and charcoal black

– the Lamb – grey modelling in the highlights: lead white, charcoal and a negligible admixture of azurite

Sample No. 2 – the God of the Apocalypse – blue background

The grey siliceous layer 0.185 mm thick and the ochre base 0.016 mm thick are identical with the layers of sample No. 1. Over the ochre base is a thin insulating layer consisting of a thin aplication of oil. The painting consists of two layers. The lower is light blue and contains lead white, azurite and charcoal black. The tiny admixture of red was considered under the microscope to be vermilion. This layer is 0.066 mm thick. The upper layer, dark blue in colour, is made of azurite crystals 0.010 to 0.017 mm in diameter. This layer is 0.140 mm thick.

The test for binding media in the paint was carried out in the same way as for sample No. 1. The reactions were again positive both for protein and for oil. Even though the supplementary tests cannot differentiate the nature of the proteins, we presume egg tempera with the addition of oil.

The supplementary samples from the same picture gave us the following determination of pigments:

– the red layer from the monochrome painting of the Angels around God contains minium as the dominant pigment and a small amount of ferrous red is also present.

– the blue layer from the same monochrome painting (the »Blue Angels«) consists of azurite and ultramarine (lapis lazuli) in proportions ofl:l. The layer also includes lead white.
In another sample two colour layers can be seen under the stereo-microscope. The upper layer lies on a blue-green layer, the shade of which is created by a greater amount of azurite, the robes of God: lead white and charcoal black

– the right hand of God: lead white, charcoal black, admixture of yellow and red ochres and azurite

– brown background bottom right (powder sample):
Fragments prevail with a grey siliceous layer, ochre base and traces of oil gilding. The brown tone is not formed by brown pigment (only yellow and red ochres are present), but by the general surface dirt. The oil gilding on an ochre base was analysed on the sample from the Throne of God:
Above the siliceous layer lies an ochre base 0.020 mm thick. To this is applied a thin layer of oil 0.008 mm thick. On the oil base is a strip of gold 0.006 mm thick. This is oil gilding which was identified histochemically and by solubility tests.

Pastiglia from the picture of Christ and Madgalene
The plastic ornament is on two layers. The lower layer of ochre paint contains yellow and red ochres and lead white. It is bound with oil. Over it is a grey-brown layer recalling the grey siliceous layer of the panel paintings from the Bohemian Gothic period. It contains siliceous and lime particles with an admixture of yellow and red ochres. The binding agent in this layer is starch, the grains of which, 0.006 to 0.041 mm in diameter, were identified as rye starch. On the basis of the solubility tests we also presume the presence of glue.

Sample No. 3 – the Red Book (Christ, Mary, Martha) Spectral analysis:

Ca Si Al Fe Hg Mg Pb (Ag) (Mn) (Cu) (Sn) Over the ochre base, 0.020 mm thick, is a layer of lead white (0.107 mm). Over this again is a thin layer (0.016 mm) of vermilion and lead white. The binding media in all the layers are identical to those in sample No. 2. (Sample No. 2/supplements from 1982) from the scene of Christ with Mary and Martha, not published here (carnation of Christ) brought the following findings:

The layer closest to the plaster is a siliceous layer of light grey colour, 0.041 mm thick. It contains siliceous parts of white, grey and black colour, pigmented with yellow ochres The histochemical analysis was positive both for the presence of oils and proteins. We presume that the oil soaked through from the upper ochre layer, which is strongly bound with oil. In the siliceous layer the presence of starch was also indicated. Above the siliceous layer there is an ochre layer 0.020mm thick, made up of yellow and red ochres and lead white. The painting over the ochre layer consists of two equally thick (0.024mm) applications of lead white, of which the upper ispigmented with vermilion, yellow ochre and charcoal black.)

Sample No. 4 – grey cloak (Christ, Mary, Martha)
Over the ochre ground (0.158 mm) there is grey panting (0.044 mm) of lead white and charcoal black. The sample applies to the binding media as given for the previous samples.

Conclusion
From the point of view of painting technology the wall painting is composed as follows: on a thick layer of plaster lies grey siliceous layer, microscopically identical to the silicious layers of the panel paintings, e. g. the panels from Dubeček and the Altarpiece ofZátoň. Over the siliceous layer there is thin ochre ground containing yellow and red ochres with lead white. Through supplementary analysis the presence of lead white in the ochre ground was also shown in the scenes of the »24 Elders« and the »God of the Apocalypse«. The coat of oil over the ochre ground was evident only in sample No. 2. The painting contains the pigments of the period-lead white, yelow and red ochres, vermilion, ferrous, red, azurite, natural ultramarine and charcoal black.
The analysis of the binding media, due to the very thin layers, was mainly carried out microchemically with a compact sample and to the partial results were confirmed histochemically on the cross-sections. The siliceous layer is bound with protein (glue) and a smaller amount of oil (not even with fluorescent analysis is it possible to decide whether this is not due to the soaking through of oil from the ochre ground) The layer also contains rye starch, which was subsequently identified also in the sample in previous reports. The ochre ground is bound with oil. The painting contains an emulsion bonding medium – protein with a small amount of oil. The supplementary solubility tests lead us to suppose egg tempera with the addition of oil.

  author
Jindřich Tomek,
Dorothea Pechová, AHVT I 010 (Q. D.)

24 Elders background
1  white layer – lead white with traces of ochres 0,025 mm
2  ochre ground, 0,025 mm
1  0,092 mm grey siliceous layer

The God of the Apocalypse – blue background
6  thin layer of varnish or impurities, 0,021 mm
5  blue azurite and lead white, 0,140 mm
4  light blue azurite, lead white,vermilion, carbon black, 0,066 mm
3  insulating interlayer,0,005 mm
2  ochre ground,0,016 mm
1  šedá křemičitá vrstva, 0,185 mm

Christ, Mary and Martha – red book
3 vermilion and lead white 0,016 mm
2 lead white, carbon black, 0,044 mm
1 ochre ground – yellow and red ochres, lead white, 0,020 mm

garment of the same picture
2 lead white, carbon black, 0,044 mm
1 ochre ground – yellow and red ochres, lead white, 0,058 mm