Medieval glass mosaic at St. Vitus Cathedral under scrutiny

The mosaic consists of about a million cubes, some brightly colored, others less so

One of the most valuable medieval monuments at Prague Castle, the glass mosaic of the Last Judgement at Saint Vitus Cathedral, is undergoing a close inspection. Two decades ago, the 650-year-old glass mosaic of the Last Judgement was completely renovated, but some of the glass pieces have now started showing signs of corrosion.

Experts are looking for a way to clean the mosaic from corrosion | Photo: Eva Kézrová,  Czech Radio

The monumental mosaic of The Last Judgement, located on the south façade of St. Vitus Cathedral at Prague Castle, was completed in 1371 at the request of King of Bohemia and Holy Roman Emperor Charles IV.

The triptych is made of approximately one million glass pieces in over thirty shades of coloured glass. Its central panel depicts Christ surrounded by angels with six saints kneeling below, while the two side panels are images of heaven and hell.

Milena Nečásková | Photo: Eva Kézrová,  Czech Radio

In the past, the brilliant colours of the mosaic were hidden beneath a layer of corrosion that repeatedly formed after each cleaning. The problem was solved by a renovation 20 years ago, when a protective varnish was applied on its surface, explains restorer Milena Nečásková:

“The touch of a human hand cannot harm it because it is protected by a layer of protective varnish that is indistinguishable to the touch. But if it wasn't on the mosaic, grey crystals would have gradually started to cover it after every cleaning. In spite of the protective varnish, the mosaic is still exposed to the elements and needs to be regularly inspected.”

Glass mosaic of the Last Judgement at Saint Vitus Cathedral | Photo: Eva Kézrová,  Czech Radio

Working on a platform suspended above the Golden Gate of Saint Vitus Cathedral, at a height of around 15 metres, Mrs Nečásková scrutinizes the tiny glass pieces, some of which are brightly coloured, while others less so:

“This is due to the physical structure of the glass. The greyish ones have lots of bubbles that occurred during their production, while the smooth cubes have white bubbles, but only on the surface, trapped inside the protective varnish.

“Sometimes, the bubble in the varnish bursts and rain water gets into contact with the glass. The water starts to wash away the alkalis present in the glass and by reacting with carbon dioxide they create corrosion.”

The vast majority of the pieces on the mosaic are the original ones | Photo: Eva Kézrová,  Czech Radio

The restorer can also easily distinguish between the newly added pieces and the original, medieval ones:

“The medieval glass pieces are less smooth because of the way they were produced. At the time, glass contained unmelted pieces, so it was rougher. The vast majority of the pieces on the mosaic are the original ones.”

To assess the state of the valuable mosaic, Mrs Nečásková works side by side with her colleague from the University of Chemical Technology. They take detailed pictures so as to be able to compare the state of the mosaic over time. One of their main goals this year is to come up with an effective treatment of the isolated corroded pieces in the mosaic.