Klaus Dickmann, Jens Hildenhagen and Peter Mottner
Historical glass can suffer a tremendous loss of transparency from atmospheric or biogenic corrosion resulting from long-term exposure to air pollutants and humidity (Figure 1). Surface layers and deposits can present serious obstacles to efforts to preserve stained glass. Although conventional cleaning methods can remove most surface layers, it is difficult to remove more complex layers such as dense, well-adhering encrustations of corrosion products or aged and insoluble organic polymers.
Attacks on the glass surface produce ion exchanges that form a gel layer, which then acts as a protective "skin" for the sensitive glass. Cleaning should not disturb this layer, but available mechanical and chemical methods, rather than preserving it, can scratch it or even remove it entirely. Feasibility studies have shown that a KrF excimer laser operating at 248 nm might be able to remove encrustations and to ablate organic polymers and biolayers on glass without harming the gel layer...