By Jeffrey Levin

Its most impressive hour may be night.

When darkness removes most traces of modernity and lights from below shine upon it, Prague Castle looms over the red-tile-roofed buildings that surround it with a sweeping grandeur that knows few equals. The 20th century slips into the shadows, leaving behind the architecture of history.

Prague Castle

For a thousand years, the castle on Hradcany Hill has been the political and religious heart of Prague and the lands of Bohemia. The seat of government for kings, emperors, and presidents, it grew over the centuries, becoming the largest medieval castle in existence.

Just as Prague Castle visually dominates the city whose name it bears, St. Vitus Cathedral dominates the castle. Located in the center of the castle grounds, St. Vitus is massive and imposing, its Gothic spires towering above the 18th century buildings that mark the castle's perimeter. Begun in 1344, it is the third church to stand on the site. Here, in this place, Bohemian kings were crowned. Here, too, they are buried. Among those reposing within the cathedral is the nation's patron saint, St. Wenceslas, a prince of Bohemia murdered in 929 by his pagan brother.

When daylight returns to Prague, St. Vitus remains the city's most striking architectural landmark. Yet even the sun does not fully reveal on its exterior one of the cathedral's great artistic treasures—a rare medieval glass mosaic. Out in the open, but nevertheless barely seen, it is obscured as the result of a combination of weather, climate, and the elements of its own composition.

The Last Judgment

Six hundred years after the death of Charles IV—King of Bohemia and Holy Roman Emperor—Prague bears witness to the imperial ruler's vision of his chosen capital. A man of education, diplomatic skill, and religious feeling, Charles transformed the city into a political, cultural, and economic center, establishing the first university in central Europe, planning a new district for the city (Nové mesto, or New Town), and initiating numerous public works that still stand: the Carolinum, the university's central hall; the tower of the town hall in Old Town square; and the Charles Bridge over the Vltava River, which divides the city. Of all of Charles' projects, the most ambitious was undoubtedly St. Vitus Cathedral.

In 1370, the Emperor ordered the creation of an unusual work of art on the exterior of the cathedral's south entrance. The entrance, which faced the emperor's residence and served as the entry point for coronation processions, was to have over its three portals a glass mosaic depicting the Last Judgment.

Mosaic making was not a craft common to the Bohemian artisans of the time. Still, according to records, this large and complex glass mosaic was completed by the following year. In the center of the mosaic, Christ is encircled by a mandorla (an almond-shaped aura of divinity) surrounded by angels. Kneeling beneath this image are the saints of Bohemia and, below them, Emperor Charles IV and his fourth wife, Elizabeth of Pomerania. The panel to the left depicts heaven, while the right panel depicts hell.

The speed and quality with which the mosaic was executed fuels one of the lingering mysteries of its creation. Who performed the work on The Last Judgment? In technique and style, the mosaic resembles one found at Orvieto Cathedral in the Umbrian region of Italy, the single other Gothic building in Europe with a large exterior glass mosaic. Some experts believe that only Italian craftsmen (perhaps even those who worked on Orvieto) would have had the experience and skill necessary to accomplish in such a relatively short period the remarkable work in Prague. The fact that Charles IV himself had journeyed to Italy just a few years prior to the mosaic's construction adds weight to this contention.

But a close examination of the faces of the figures in the mosaic prompts a glimmer of doubt. The features are clearly Slavic in appearance, a physiognomy quite different from that found in the works of Italian mosaicists. How likely is it that Italians would have created facial characteristics such as these?

There are, unfortunately, no documents from the period that might have solved this puzzle. Such items as bills for the construction of the cathedral were archived beginning in 1372—one year after the mosaic's completion.

While questions may always remain regarding the artisans who made the mosaic, its extraordinary quality is not in doubt. "This is a unique object of visual art which has no companion east of the Rhine and north of the Alps," says Dr. Eliska Fucíková, Director of the National Heritage Department of the Czech Republic's Office of the President. But for much of its existence, this rare work of art has gone unseen. Sitting in her office in Prague Castle, the ringing of noonday bells heard through her open window, Dr. Fucíková explains the mosaic's "sad story." Although it has been cleaned and covered with protective coatings on a number of occasions in its history, over time the mosaic eventually always vanishes beneath a whitish, opaque layer of corrosion products that cover its surface. Such is the case today.

"It's absolutely invisible," she says. "If you tell someone there is a mosaic there, the obvious question is 'Where?'"

Standing in the castle court near the cathedral's south entrance, one can see that the mosaic is indeed obscured, as if dusted with a coat of chalk. Of the 31 shades of colored glass that make up The Last Judgment, all that emerge are a dull, rusty looking red, a pale green dimly reminiscent of tarnished copper, and gold, primarily in the upper left and upper right corners, from an early 20th century restoration. Figures can be seen, but most details and facial features cannot.

All in all, the mosaic appears bleached and faded, vanishing beyond discernment.

The Mosaic's Conservation

conservation image

Since October 1992, the Getty Conservation Institute (GCI) and the Office of the President of the Czech Republic have been collaborating on a project to conserve the St. Vitus mosaic. This effort comes after a series of interventions on the mosaic, the first as early as the 15th century, followed by at least one or two in the next century. In 1619 the mosaic was reportedly plastered over, but it was later uncovered and restored. A series of restorations was conducted in the 19th century, the most drastic occurring in 1890, when the mosaic was removed from the cathedral wall in 274 sections and placed in storage until 1910, when it was reinstalled and repaired.

Beginning in the 1950s, the mosaic again underwent a number of treatments to deal with the continuing problem of clouding over. Each treatment proved successful only temporarily. Part of the problem was the Communist government's failure to heed the advice of the Czech conservators conducting the treatments who urged that a consistent regimen of cleaning be maintained.

By the mid-1980s, The Last Judgment had vanished again. When the GCI approached the government in Prague about developing a joint conservation project, the St. Vitus mosaic was at the top of the Office of the President's list of conservation priorities.

The objective of the project is the mosaic's thorough cleaning, followed by the application of a treatment that will dramatically slow the return of the opaque layer. However, as is standard with GCI special projects, intervention is being preceded by a complete review of existing documentation, a thorough study of the causes of the problem, and an extensive investigation of possible treatments.

A good deal of information on the mosaic's past restorations has been culled from the Prague Castle archives. The project team also consulted with Czech conservators who worked on the mosaic in recent decades, and reviewed their records. In addition, two Italian mosaic specialists who had worked on St. Vitale Church in Ravenna were brought in to examine The Last Judgment and report on its current condition. They concluded that while it will be a great challenge to avoid "a repetition, after restoration, of the corrosion process now in progress," the mosaic generally remains in good condition beneath the present corrosion layer.

It is already clear that the kind of glass used in the mosaic, which differs from Italian glass, is a factor in the corrosion process. The making of glass involves the melting of sand (i.e., silicon dioxide) with a flux such as soda (sodium carbonate). In central Europe, unlike Italy, soda was not easily available in medieval times, so glassmakers used potash (potassium carbonate), which was extracted from the ashes of burned wood.

"While soda and potash are similar compounds, glass made of potassium is less stable and more soluble than sodium-based glass," explains Dr. Dusan Stulik, Acting Director of the GCI's Scientific Program and project leader for the St. Vitus effort. "The potassium-based composition of the St. Vitus mosaic's glass is a real problem."

Another factor is the protective coatings of waxes and resins of various kinds that have been applied since the reinstallation of the mosaic. These coatings themselves have degraded and contributed to the mosaic's near opacity today.

The mosaic's problems are exacerbated by its location. While most Italian mosaics are situated in interiors or in some way sheltered, The Last Judgment is completely exposed in a climate with greater extremes than those found to the south. In addition to being subjected to wind, rain, and snow, the mosaic, on the south side of the cathedral, can heat to up to 52 degrees centigrade during the summer and drop to minus 20 degrees in the winter. An environmental monitoring station, installed in June 1993 by Shin Maekawa of the Getty Conservation Institute, recorded this information, as well as data on air temperature, relative humidity, dew-point temperature, wind speed and direction, and solar radiation. All the data are being integrated in the development of long-term conservation strategies for the mosaic.

Treating the Problem

conservation image

Early in the project, the GCI team met with a group of leading Czech conservators to help develop a consensus on intervention. Prior to the Institute's involvement, there were two schools of thought within the Czech conservation community: one group believed that the mosaic should be removed, new housing found, and the original replaced by a copy; another group maintained that the mosaic should be treated in situ. After a series of discussions, the Czech group came to the consensus that treatment in situ was the appropriate approach.

Encompassing 84 square meters (904 square feet), the mosaic contains about one million tesserae, most approximately one centimeter in size. In April 1994, photographic documentation of the entire mosaic was performed. Three hundred images were made, detailing the mosaic's present condition. Transferred to photo compact discs, the images will enable conservators to create high-resolution condition maps of the mosaic.

Investigation has already begun into possible treatments to be applied to The Last Judgment once cleaning is complete. The GCI is working with the Fraunhöfer Institute for Silicate Research in Würzburg, Germany, testing several treatments including a composite treatment that has been used on medieval stained glass.

Part of the treatment testing program are ten small test mosaics and one large one. Each test mosaic contains several different tesserae including: (1) test glass, developed by the Fraunhöfer Institute, with a known corrosion rate; (2) original tesserae from the mosaic (discovered in archives and presumably left from the mosaic's removal in 1890); and (3) three types of modern mosaic glass. The test mosaics have all been treated with different materials being considered for use at St. Vitus. In May 1994, the large test mosaic was installed in the vicinity of the cathedral. The smaller test mosaics are being subjected to accelerated aging experiments at the GCI and in Würzburg; these experiments simulate cycling of temperature, humidity, UV radiation, and pollutant concentrations. The results will determine whether coatings can protect the mosaic from the extremes of climate to which it is exposed and, if so, what the maintenance cycles of cleaning or recoating will have to be.

While actual treatment of The Last Judgment is anticipated in early 1996, all involved, including the Office of the President, are emphasizing thoroughness over speed.

"They want us to make sure that everything is done properly, so there isn't any pressure applied on us time-wise," says Dr. Stulik. "This kind of patience is unusual, but very appropriate given the mosaic's past problems."

Born and raised in Prague, Dr. Stulik well understands the desire of the Czech government to be certain that the St. Vitus mosaic is returned to visibility with great care. If the cathedral is the heart of Prague Castle, then the mosaic is arguably at its most precious spot. In a chamber of the cathedral behind the mosaic is the royal treasury where the Bohemian coronation jewels are stored. Beneath the chamber is the Chapel of St. Wenceslas. From the standpoint of history, observes Dr. Stulik, "this was the center of power and religion."

An Emperor's Legacy

Charles Bridge, Prague

When Charles IV died in 1378, St. Vitus Cathedral had been under construction for 34 years. But despite three and a half decades of work, the cathedral was centuries from completion. War, internal strife, and other problems interrupted the building's progress, and it was not until 1929, exactly a thousand years after the death of St. Wenceslas, that the cathedral was finished. By then, The Last Judgment mosaic had passed from sight numerous times, only to be resurrected by new generations seeking to recover Prague's medieval masterpiece.

Today in the Prague Castle courtyard in front of the cathedral's south entrance, tourists wander through, mostly oblivious to the unique creation close at hand. Occasionally a guide will point out the mosaic to the visitors gathered around him or her, and they will look up, squinting, trying to make out the stunning art that lies beneath the white layer of corrosion.

Perhaps some day soon they will need no one to show them The Last Judgment. Instead, as they turn the corner into the courtyard, the glass mosaic, its colors brilliant in the sun, will capture their vision without coaxing, and they will gaze with awe and amazement at an emperor's shimmering legacy.

Jeffrey Levin is Editor of Conservation, The GCI Newsletter.