By Francesca Piqué, Giora Solar, and Dusan Stulik

Ever since the "Velvet Revolution" brought democracy to Prague in 1989, a growing number of travelers have made the city a popular destination in central Europe. The reasons are easily apparent. Walking the narrow and winding streets of this great capital, one can see a stunning panoply of architecture that reflects the centuries of dramatic history and rich culture the city has witnessed. With landmarks from the medieval to the modern, Prague is a place of great churches and synagogues, university buildings, civic structures, family palaces, royal residences, and historic cemeteries.

No matter where you are in Prague, the dominant landmark is St. Vitus Cathedral, in the center of Prague Castle on Hradcany Hill. The third church to stand on the site, the cathedral was constructed beginning in 1344. Work was interrupted early in the next century by the Hussite revolution, and the cathedral ultimately was not fully completed until 1929. Today St. Vitus Cathedral is not only a house of prayer but also a center of Czech art and culture. Within its walls of magnificent stone and ironwork are six hundred years of history and heritage, including the ornate St. Wenceslas Chapel and the chamber housing the Bohemian crown jewels.

Among the most extraordinary of the cathedral's artistic treasures is The Last Judgment mosaic on the facade above the south entrance of the building, often called the Golden Gate. Considered one of the great monuments in the Czech Republic, the 84-square-meter (904-square-foot) mosaic was completed within one year—1371. It is the most significant exterior medieval mosaic north of the Alps, embodying history and extraordinary art.

Unfortunately, for most of the mosaic's existence, its brilliant colors have been rendered invisible, covered over by a layer of corrosion that would repeatedly form after each cleaning. The problem has persisted for centuries, despite a series of attempts to restore the mosaic to its original glory. But now—perhaps for the first time since the mosaic's creation—visitors can finally see the full brilliance of the colors in this magnificent 14th-century masterpiece, thanks to a collaboration between the Czech government and the Getty Conservation Institute.

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As mosaics were not typically used in Gothic architecture, The Last Judgment probably reflects the personal taste of Charles IV of Bohemia, the Holy Roman emperor who commissioned the work after returning from a visit to Italy. Charles had made Prague his capital, and he spared no effort to ensure that the new cathedral would be a fitting symbol of the city's power and prestige.

The Last Judgment is divided into three panels placed above three large archways. In the central panel is Christ ascendant, above the patron saints of Bohemia. Below the saints, Charles IV is depicted with his fourth wife, Elizabeth of Pomerania. The left panel shows souls ascending into heaven, while the right depicts the damned being cast into hell. The mosaic is made up of nearly one million pieces of cut glass cubes (tesserae), along with natural quartz and chalcedony pebbles. The pebbles were used exclusively to create the flesh tones, while the glass, in over 30 hues, makes up the rest of the work.

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The return, after every cleaning, of the grayish layer of corrosion products covering the mosaic has confounded for centuries those who sought to reclaim the work. Prague Castle records indicate that the first restoration attempt occurred one hundred years after the mosaic's completion. Other cleaning and restoration efforts followed, including one beginning in 1890 in which the mosaic was removed from the cathedral, restored, and then reinstalled 20 years later. Four decades after that, the surface of The Last Judgment was again heavily corroded.

In 1953 Czech experts began a comprehensive scientific study of the mosaic's deterioration and determined that the problem was the potassium-rich mosaic glass. When exposed to water, the potassium in the glass leaches out, then interacts with pollutants in the air, resulting in the formation of a corrosion layer. This research led to the cleaning, restoration, and regilding of the mosaic and, importantly, to the application of a protective multilayer polymer coating. However, because of the inadequacy of materials available at the time and the lack of periodic maintenance recommended by the restorers, the coating failed to prevent subsequent deterioration. In the 1970s, Czech conservators reported the delamination of the protective layer and the renewed growth of the corrosion layer.

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Since then the mosaic has been cleaned for important events. However, each cleaning involves the removal of a very thin layer of the glass tesserae—between 0.5 to 1.0 millimeters. Because each tessera is approximately one centimeter thick, theoretically, the mosaic would no longer exist after perhaps 10 more cleanings. What was needed to save the mosaic in situ was a coating that could prevent the formation of corrosion, and a program for regular inspection and maintenance of this coating.

This conservation challenge was undertaken in 1992 when the Office of the President of the Czech Republic and the GCI began a collaborative project to find a solution to this centuries-old problem and to restore and conserve the mosaic in its original location.

Work began with a review of all existing written, photographic, and scientific documentation, as well as with an assessment of the state of the mosaic. Scientific tests confirmed the conclusions of the Czech scientists regarding the causes of the mosaic's deterioration. The project's two main tasks were to develop a method for gently removing the surface corrosion and to identify a coating that could provide long-term protection against moisture and air pollutants. Both the Office of the President and the GCI agreed that the project should not have any artificially imposed deadlines. The restoration treatment would be executed only when the cleaning and coating methodologies were fully tested in laboratories and in situ.

Several mechanical, chemical, and laser-assisted cleaning methods were considered. Laboratory tests and examination of cleaned tesserae under optical and electron microscopes indicated that cleaning could best be accomplished with a stream of compressed air and microscopic glass particles. Using glass particles harder then the corrosion layer but softer than the mosaic tesserae would thoroughly clean the mosaic surface while automatically stopping the process once the surface corrosion was removed.

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The development of a mosaic protective coating proved most challenging. Inorganic coatings could provide adequate protection against water, but these coatings are not fully reversible and must be applied at a very high temperature that would be incompatible with in-situ treatment of the mosaic. Organic coatings are reversible and easily applied, but they offer less protection against water and pollutants.

In collaboration with the Materials Science and Engineering Department of the University of California, Los Angeles (UCLA), the GCI conducted a three-and-one-half-year investigation of a preservation technology for mosaics, including testing of protective coating methods and materials. This research identified sol-gel materials—a hybrid of organic and inorganic materials that have properties similar to glass but do not require very high temperatures for production or application—as the best coating materials currently available for the mosaic. The Materials Science and Engineering Department, which includes one of the leading laboratories for sol-gel research, prepared the sol-gel material; it was then tested in custom-designed aging chambers at the GCI.

The mosaic coating selected includes three layers of organically modified sol-gel with embedded gold leaf where needed. In the application process, all layers are heat-treated using computer-controlled banks of infrared lamps. The top layer is a "sacrificial layer" that protects the underlying layers and will be replaced during scheduled periodic maintenance of the mosaic. The coating is designed to protect the mosaic surface for a long period, but can be removed when advances in science and technology provide even more durable materials.

One complicated issue involved the ethics of regilding the mosaic. In mosaics, gilding consists of an extremely thin layer of gold leaf applied to individual tesserae, then protected by a thin layer of glass. In the case of The Last Judgment, in which the entire background was gilded, corrosion damaged the gilded tesserae, destroying both the glass protective layer and the gold leaf itself. All that was left were the red or blue tesserae in which the gilding had originally been embedded.

The golden color in the mosaic has significance that goes beyond aesthetics. It is a representation of heaven and also has the functional role of glowing and shining as it reflects sunlight. For these reasons—and because the entrance beneath the mosaic is called the Golden Gate—previously gilded parts had been regilded in past restorations.

For reasons of significance and aesthetics, regilding had to be considered. It was also clear that the ethical implications of regilding required thorough discussion. Was it ethical to complete missing color? What should be done in the case of existing traces of old gold? What about places where it was not clear which tesserae were gilded? How much regilding should be applied, if any?

To resolve these questions, an international advisory committee was formed. Meeting in Prague in October 1996, the committee reached full consensus—the once-gilded background of the mosaic should be regilded. Among the committee's additional suggestions were that nonbackground parts be regilded only if there was certainty regarding their past color; tesserae with any traces of previous gold should not be regilded; new gold leaf should not be applied to the whole surface of each tessera; and some glass should be left ungilded.

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Conservation of the central panel started in June 1998, beginning with the removal of the corrosion layer, using the methodology developed at the GCI. Working tessera by tessera, two teams of conservators took a month to clean the mosaic. The cleaning immediately revealed the beauty of the medieval artwork, with its magnificent range of colors. The second phase of the conservation treatment—the application of the protective coating to the glass, with gold leaf applied where appropriate—was finished in September. Conservation of the remaining two panels will be carried out by Czech conservators, who will complete the work by the year 2000, with the GCI continuing to provide technical support and guidance. A long-term maintenance and monitoring program is being developed and will be implemented to ensure the mosaic's preservation. The project has been thoroughly documented, and the results of the work will be published.

On October 29, 1998, the central panel was unveiled outside the cathedral's Golden Gate. The timing of the ceremony was propitious—not only because it was held just a day after the celebration marking the 80th anniversary of the founding of an independent Czechoslovakia. This "resurrection" of The Last Judgment mosaic occurred exactly 650 years after Charles IV—the monarch who commissioned the work—established Prague's university (the oldest university in central Europe) and the New Town district of the city. It is fitting that yet another one of his legacies has been renewed in a manner that he would surely recognize as true to his original vision.

Francesca Piqué is a project specialist at the GCI. Giora Solar is the GCI's group director of conservation. Dusan Stulik is a GCI senior scientist.