Stained Glass - Art at the Glass Surface
Cambridge - Monday 4th and
Tuesday 5th September 2017

Manfred Torge

article posted 18 May 2017

Dr. Torge is a scientist at the Federal Institute of Materials Research and Testing (BAM) in Berlin, working in the department for “Material and Environment”. Since obtaining his PhD in material science in 1990 he has focused on projects regarding the restoration and conservation of stained glass windows, glass characterization and on ancient glass technology using a range of scientific techniques by working in collaboration with colleagues at the BAM. At the BAM research department, it is possible to analyze the composition of historical glass; to melt model glass according to historic composition and to perform environmental simulation with the use of climate chambers. Dr. Torge has been a project manager and expert for protective glazing in projects for restoration and conservation of stained glass windows in churches and cathedrals in Germany, France, Great Britain and Poland. During these international projects the protective glazing had been monitored and evaluated by comparing climatic measurements and environmental data. He is a member of ICOM-CC “Glass and ceramic conservation”.

Determination of lost paint layers by Scanning Electron Microscopy
Manfred Torge*1, Ines Feldmann1 & Tom Küpper2

1Bundesanstalt für Materialforschung und -prüfung, Berlin
2Lincoln Cathedral, Glazing Department

Paint layers consist of a flux and the appropriate coloured oxides. In the literature (see list in MÜLLER 1997) one can find 16 books from 1700 to 1900, which contain recipes for glass colours. According to the recipes, the flux in medieval times contained 1 part of SiO2 and 2 parts of PbO (until about 1850) and thereafter it consisted mainly of 1 part of SiO2 and 3 parts of PbO to reduce the melting point. The flux components and the coloured oxides are fused together to produce a paint layer and were often applied to the inside and the outside of stained glass windows. Today, these medieval paint layers can exhibit various degrees of damage because of glass corrosion. The damage phenomena can range up to the total loss of larger parts of painted detail, where the original image can only be imagined from the faded negative details left behind on the inside of the window. To the outside of the window paint layer are often covered by corrosion crusts. To determine the existence of external paint layers research was carried out on 4 medieval glass samples which were provided by Lincoln Cathedral. The samples date from the 13th Century and all of them showed a significant external corrosion crust. Analysis of glass composition by Scanning Electron Microscopy (SEM/EDX) and cross section images of the glass samples were performed, together with EDX analysis indicating the distribution of elements. As a result, no lead has been found within the glass composition. However, further analysis of the surface corrosion deposits suggests that a high concentration of lead is present. The gel layer underneath is wider than 20 µm and is covered with micro cracks. Within these cracks a high concentration of lead (Pb) was found (see figures). This leads to the speculation that this is possibly due to remnants of external back painting or thin washes. The application of external paint layers on medieval stained glass windows in Lincoln Cathedral is not only of interest for art historians but is also important for the development of restoration proposals. For the gradual reduction and removal of corrosion layers conservators have to take care to preserve remaining paint layers.

Fig, 1, 2:
Electron scanning microscope image of a cross section of a medieval glass sample of Lincoln Cathedral, including the layer of the corrosion deposits and the gel layer (left). The EDX analysis indicating the distribution of elements including the concentration of lead (Pb) in this layer

Wolfgang Müller, Manfred Torge, Detlef Kruschke, Karin Adam; „Sicherung, Konservierung und Restaurierung historischer Glasmalereien“(Protection, preservation and restoration of historical stained glass windows)
BAM Research Report 217, Berlin 1997, p. 78