Long-term protection of the last judgment mosaic in Prague

A sol-gel coating was used in the long-term protection of the 14th Century mosaic situated above the gates of St. Vitus cathedral, in the center of Pr ague Castle. The choice of a sol-gel coating is the result of a research ef fort involving both on-site testing in Prague and accelerated ageing of var ious polymers and sol-gel materials in the laboratory. The coating selected for treatment of the entire 13 m x 10 m mosaic is a multi-layer system in which an organic-inorganic sol-gel layer is placed between the glass substr ate and a fluoropolymer coating. This coating combination performed very we ll in an accelerated weathering chamber as well as during onsite testing. S ince the top layer is scheduled to be removed and re-applied periodically, it is hoped that this concept will allow long term protection to the larges t outdoors mosaic north of the Alps. On-site application of the coatings on the entire mosaic started in 1998 and will be completed in 2000.