Project Objectives
For most of its existence, the brilliant colors of The Last Judgment mosaic on St. Vitus Cathedral in Prague 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 the corrosion continued into the 20th century without a long-term solution. The objectives of this project included the following:

• determining the causes and mechanism of the mosaic's corrosion and deterioration process;
• developing an appropriate and efficient system of protection for medieval glass;
• developing a conservation strategy for The Last Judgment, the mosaic on St. Vitus Cathedral;
• restoring and conserving the mosaic;
• developing a long-term plan for monitoring and maintenance of the mosaic.

Project Overview
In October 1992, the Getty Conservation Institute and the Office of the President (OP) of the Czech Republic began collaborating on the conservation of The Last Judgment mosaic. Located on the south facade of St. Vitus Cathedral in Prague Castle, The Last Judgment is considered the most important exterior monumental medieval mosaic north of the Alps.

The mosaic's conservation problem is due to the composition of the mosaic's glass. In Central Europe (and other parts of 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, then interacts with pollutants in the air, resulting in the formation of the corrosion layer.

As part of the GCI-OP project, various methods of mosaic cleaning were researched and the Department of Material Science and Engineering, University of California, Los Angeles (UCLA), 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. All proposed technological steps and planned interventions were discussed in detail with an advisory group of leading Czech art historians, historians, and conservators. 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.

The St. Vitus mosaic encompasses 84 square meters (904 square feet) and depicts the Last Judgment in triptych form. It was completed in 1371 at the request of Charles IV, king of Bohemia and Holy Roman Emperor, who, during his reign, made Prague the empire's center of power, religion, and knowledge. In the mosaic's center panel is the figure of Christ surrounded by angels; kneeling beneath them are the figures of six saints of the Czech lands. On the triptych's two side panels are images of heaven and hell. Thirty-one shades of colored glass, plus gilded tesserae, can be found in the approximately one million glass pieces that compose the mosaic. Originally, the entire background of the mosaic was gilded, hence the name of the southern portal of the cathedral—The Golden Gate.

Conservation of the central panel was completed in 1998. Conservation of the right panel was finished in 1999, and was followed by the left panel's conservation in 2000.