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Stained glass is a particularly sensitive medium that in many churches has stillnot received sufficient protection against destructive influences of many andvarious kinds. Over long periods of time, nearly all materials are subject tochemical and physical change. The dramatic scale on which the processes of decayoften occur results from the influence of corrosive gases that are part of themix of today’s atmosphere. A basic requirement in the preservation of anyhistorical monument is therefore interdisciplinary collaboration; only the jointefforts of art historians, restorers and technicians, as well as scientists,offer a chance to protect our artistic and cultural heritage from decline.
Measures to protect, conserve and restore stained glass are carried out by glassstudios. The project on which this chapter reports provides support for thispractical work through scientific back-up. In order to clarify what are verycomplex issues, a brief explanation of the scientific/technical context followshere, to preface the exposition of the basic questions posed.
To improve the adhesives used for restoring stained glass windows, various adhesives were tested and modified. Stained glass windows often show gaps that are too wide to be glued with conventional adhesives, and complementing the gap with spare glass is very time-consuming.
To solve this problem, a filler with improved adhesive strength was developed to bridge wider gaps and cracks. The new filler combines glass adhesives with specially treated glass powder.
Three different glass adhesives were selected and mixed with the specially treated glass powder. The adhesive – glass powder mixture developed was then subjected to accelerated aging and analysed with Fourier transform infrared analysis, scanning electron microscopy, colourimetry, and bending and tensile tests. The results were excellent and the mixture was subsequently used to restore glass windows from four 19th-century churches and chapels. The restoration process was described in conjunction with the advantages of using this approach.
This new adhesive – glass powder mixture provides an adhesive process that can be adapted to the special needs of any object being restored and thus provides a ready answer to the specific damage of each object.
The initial focus of this research was on the development of a general workflow for the documentation and monitoring of historical stained glass windows using structured light scanning. Therefore windows from different churches, time periods and with different corrosion and damage phenomena were scanned before and after conservation measures. To evaluate the execution of the restoration measures the data was compared using 3D inspection software to examine the differences in geometry between the two scans. Various problems had to be solved, for example, how to deal with heavily reflective surfaces and the extreme contrast between light and dark surfaces, as seen in the borders between ‘Schwarzlot’ painting and plain glass. The application of materials for matting the surfaces, such as Cyclododecane spray, was impossible due to the high accuracy of the surface measurement required for 3D inspection. Regarding the contrast differences of the surfaces, the creation of exposure fusions and the use of polarization filters to reduce reflections were tested. In addition to the general problems encountered when recording translucent surfaces, the historical glasses caused additional problems in calculating surface comparisons. For example, the windows have to be moved and turned around several times, both during the conservation process and while scanning, causing deformations of the geometry due to the flexible lead rods allowing a certain degree of movement.
Cleaning - A balancing act
(2006)
This text originally appeared as chapter 5, 'Reinigung - Eine Gratwanderung' (pp. 101-28) in A. Wolff (ed.), Restaurierung und Konservierung historischer Glasmalereien, Mainz, 2000.
Within the context of this research programme, scientists and conservators collaborated to assess various cleaning methods for historical stained glass. For cleaning glass surfaces there are the mechanical methods already being applied in most workshops, as well as the new chemical cleaning possibilities. Laboratory experiments were conducted initially on model glass in order to test the effectiveness and damage potential of these various methods; the model glass was examined both before and after cleaning by means of light microscopy and infra-red spectroscopy. In addition to this, cleaning tests were carried out on samples of original glass; electron-ray micro-analysis was also used to evaluate the results of these tests. The model-glass samples were corroded in advance by accelerated weathering in an attempt to simulate the damage seen on medieval glass. Enough model glass was prepared to allow any number of series of experiments to be carried out, in order to compare the effectiveness and suitability of the various chemical and mechanical methods and techniques.
For the mechanical cleaning tests, a wide range of selected tools was tested by different conservators on pre-corroded model glass. This showed clearly that the success (or lack of it) of a cleaning episode was determined not just the choice of tool, but by the skill with which conservator wielded it. This observation was later confirmed by research on samples of original glass. Insensitive handling of tools or unsuitable tools could damage or completely destroy the gel layer. The gel layer should not be damaged, as this leads to increased levels of corrosion subsequently, though the encrustations should be reduced, as their hygroscopic qualities accelerate the corrosion processes.
At the next stage, chemical cleaning methods were tested, and the results of cleaning with organic solvents (toluene, ethanol, acetone, etc.) and water-based solutions (oxalic acids, EDTA, ammoniac, etc.) compared with one another and evaluated. It became clear that strongly acidic and alkaline solutions, depending on the condition of the surface, could cause damage or have uncontrollable consequences.
In the specific case of the glass from Erfurt Cathedral, for whose firmly fixed, very hard, opaque encrustations mechanical methods proved to be either ineffective or damaging to the glass, new chemical cleaning possibilities had to be found. Following comprehensive preliminary tests on model glass, various types of ion-exchanger and ammonium-carbonate solution were chosen. Both methods were successful in improving the transparency of some (but not all) of the samples.
The experiments with the original material from Erfurt Cathedral clearly showed that successful cleaning was dependent on the particular composition of the encrustation. Really gentle cleaning can be achieved by chemical means, though only when the chemical agents are tailored to individual damage situations; this can only be achieved when scientific analysis is undertaken in tandem with the cleaning processes.
The assessment of these mechanical and chemical cleaning methods was oriented towards treating corrosion on sensitive medieval glass. Nineteenth-century glass is generally less sensitive, though cleaning of this material must also be careful and very gentle. Scientific analysis alone is not enough to determine the choice of cleaning method, but can act as an aid to the conservator in selecting the right cleaning agent and method.
19th and 20th centuries glass paint layers consist of a colour body and a colourless lead silicate flux, in which borax or boric acid was added as further component to improve the paint ability and to reduce the firing temperature for multiple layers of paint. Model glasses were used in laboratory tests to investigate the stability of glass paints with additions of boron oxide. To determine boron in paint layers, a LIBS-system with pulsed NdYAG-laser was used.
Medieval stained-glass windows are protected by an outside gazing system in many churches.The interspace between the original and the protective glass is ventilated with air from the indoor or exterior environment, where dust and particles can move.Cascade impactor measurements and SEM/EDX analyses of sampled dust were carried to determine the dust composition. The effect of particles on model glass samples was investigated in climate chambers under accelerated weathering conditions. The results from in situ measurements and laboratory tests were combined to evaluate the potential effect of particulate matter on historic stained-glass Windows.