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Brightening up old church windows

Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. : 08 December, 2002  (Company News)
Somber semi-darkness, flickering candles and the smell of incense, visitors to old churches are delighted by this atmosphere, especially at Christmas time. Yet only a few of them will have noticed that Gothic churches are growing darker inside with every passing year.
The reason: Deposits of soot and pollution on the window panes combine with dampness and gases in the air, ingredients of the corroding glass and microorganisms to form a thickening and coarse layer. A good example is the Mariendom in Erfurt . For years, curators and preservation experts have been looking for the ideal means of cleaning the grimy Gothic windows which have badly darkened over the centuries. The problem: The cleaning methods used must under no circumstances scratch the glass or damage the original pigmented layers.

The first attempts to clean up the windows were carried out prior to German reunification. After 1989, scientists set about tackling the problem in an interdisciplinary project funded by the German Bundesstiftung Umwelt. 'Since we cannot experiment on the original windows, we first simulated the damage on model panes in a controlled-environment cabinet in accelerated mode,' explains Dr. Sandra Gerlach, who works on the conservation of historical artifacts at a branch laboratory of the Fraunhofer Institute for Silicate Research ISC. It took the researchers seven years to find the optimum cleaning method for the windows: a paste that looks rather like toothpaste. Ammonium carbonate dissolves the gypsum crust which was formed in combination with sulfur dioxide from exhaust emissions. The curators were thrilled with the result.

Laborious restoration work has been going on in Erfurt for three years now. An entire window was carefully removed for cleaning. Compresses spread with the cleansing paste were laid on the sections of glass. 'Depending on how long the paste is left to work, the encrusted dirt can be either partially or completely dissolved. The residue is then rinsed away with a great deal of water,' explains Gerlach. Since this method is only a mild form of chemical cleaning, the glass suffers no physical damage. In the case of Erfurt cathedral, it was important not to make the windows too clean, so as not to influence too stark a contrast in the lighting. In any case, it is preferable to keep a certain amount of the gel layer, because it protects the glass. The paste has the advantage of being much cheaper than laser cleaning methods, and requiring less specialized skills. Nevertheless, it still takes about a year to restore an entire window to its former splendor.
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