Conservation of the St. Vitus Mosaic in Prague

For most of its existence, however, the brilliant colors of the mosaic have been rendered invisible, covered over by a layer of corrosion that has repeatedly formed after each past cleaning. Despite attempts to restore the mosaic to its original appearance—the first as early as the 15th century—the problem of corrosion continued into the 20th century without a long-term solution.

The mosaic's conservation problems were due to the composition of its glass. In Central Europe, the soda needed for glassmaking was not readily available, so glassmakers typically used potash (potassium carbonate), extracted from the ash of burned wood, as a flux in the preparation of the glass. Unfortunately, potassium glass is less stable than sodium-based glass. When exposed to water, the potassium in the St. Vitus mosaic's glass leaches out and interacts with pollutants in the air, resulting in the formation of the corrosion layer.

In 1992 the Getty Conservation Institute and the Office of the President of the Czech Republic began collaborating on the conservation of the mosaic.

Various methods of mosaic cleaning were researched, and the Department of Material Science and Engineering, University of California, Los Angeles, assisted in the development of a system of coatings for the protection of the mosaic surface after cleaning that would prevent the reforming of the corrosion layer. These steps included removal of the corrosion layer, conservation of the mosaic, regilding, coating of the mosaic glass tesserae, and implementation of a regular monitoring and maintenance plan.

Conservation of the central panel was completed in 1998, the right panel in 1999, and the left panel in 2000. Ongoing monitoring of the mosaic is undertaken at regular intervals and the team tracked developments in the coating industry to ensure the sustainability of the conservation system.

This project aimed to determine the causes and mechanism of the mosaic's corrosion and deterioration process; develop an appropriate and efficient system of protection for medieval glass; develop a conservation strategy for The Last Judgment mosaic; and restore and conserve the mosaic, developing a long-term plan for its monitoring and maintenance.

Conservation Institute: Dusan Stulik, Francesca Piqué, Chris Seki, Catherine Myers, Michael Aronowitz

Office of the President of the Czech Republic: Eliska Fucikova, Jana Zakova, Ivo Koukol, Ivo Velíšek, Ivana Kyzourova

Institute of Chemical Technology Prague: Petr Kotlik, Irena Kucerova

Consultants: Alois Martan, Martin Martan, Milena Necásková, Eva Skarolkova

Department of Material Science and Engineering, UCLA: John Mackenzie, Eric Bescher

Department of Material Science and Engineering, University of California Los Angeles