Spires and towers of Cologne Cathedral with blue sky

Cologne Cathedral

Cologne Cathedral has many impressive features, but its stained glass windows are among the world’s oldest and finest. However, centuries of being exposed to the elements has meant restoration glass was required to restore and protect them for the future. SCHOTT AMIRAN® Heritage Protect provided the answer.

Background

One of the defining features of Cologne’s magnificent gothic cathedral is its stained glass windows, some of which date back to the 13th century. As well as providing spectacular decoration, their illustrations and decorative images serve as religious symbols telling biblical stories. More practically, the windows let light into the huge structure (for a brief period, the tallest in the world) and protect it from the elements. Over many years, however, the clarity of the windows has slowly dimmed due to deposits from water and toxins in the atmosphere – a common problem with medieval glass.

Stained glass window and stonework of Cologne Cathedral
Some of Cologne Cathedral's stained glass windows date back to the 13th century

Task

To preserve the unique beauty of the original glass, exterior protective glazing is essential. Mounted a few centimeters from the original panes to ensure air circulation, it shields the delicate glass from exterior pressures and impacts. However, the first set of protective float glazing installed around the cathedral in 1980 had the disadvantage of obscuring the visibility and vivid colors of the original stained glass due to reflections. This issue needed to be addressed while maintaining the original protective qualities of the float glass.

Solution

Since many of the cathedral’s windows are high up (the upper choir windows, for instance, are 17 meters tall and 27 meters from the ground), visitors experience steep viewing angles, which can cause colored residual reflections in standard anti-reflective glass. So SCHOTT’s experts in Grünenplan, Germany developed a customized coating design characterized by layers of different thicknesses and refractive indices. The result was a brand new glass called SCHOTT AMIRAN® Heritage Protect, which offers strong anti-reflective performance whatever the angle it's viewed from.

Long shot of Cologne Cathedral with stone towers and spires
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At one time, Cologne Cathedral was the tallest structure in the world

Close up of stained glass window and flying buttress of Cologne Cathedral
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The stained glass windows of Cologne Cathedral require constant protection from the atmosphere

Series of flying buttresses and roofs of Cologne Cathedral
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Cologne Cathedral features some of Europe's finest stonework

SCHOTT AMIRAN® Heritage Protect glass offers strong protection and anti-reflective views
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Close up of stained glass window and protective glazing of Cologne Cathedral

A brand new glass for a unique project

The SCHOTT AMIRAN® Heritage Protect glazing used on Cologne Cathedral was processed using a complex system of anti-reflective coating layers with different thicknesses and technical properties. These layers were specially engineered to prevent light reflections and maintain color rendering even at acute angles. Using a careful combination of optical interference coatings, the usual color and light reflections experienced when viewing at acute angles were minimized to achieve an impressive view of the colors and detail of the original, centuries-old stained glass.

An effect that captivates from all angles

One of the main viewing areas of the cathedral is in the nearby Roncallitzplatz, which offers 45-degree views of the spectacular windows. Looking through conventional float glass, reflections from the sky, the surroundings, and even the fixtures of the cathedral would significantly obscure the view, and even with standard anti-reflective glass, color reflection effects would still be a factor. When the first SCHOTT AMIRAN® Heritage Protect panels were installed in 2018, however, these obstructions were absent.

Used materials & similar products

SCHOTT AMIRAN® Heritage Protect is specifically designed for the protection of historic buildings, architectural detail, and precious artworks. Benefitting from extremely high light transmission and minimal (1 %) visible reflection, the glass also offers strong visibility from acute angles, while protecting against the weather, UV radiation, mechanical stresses, and impacts.

Master craftsmen working together

Working on such an important building with such valuable glass required high levels of experience and skill. SCHOTT’s restoration glass experts teamed up with the craftsmen and women of Glaswerkstatt der Dombauhütte Köln in Cologne to restore the windows to their original glory, before ensuring they remain as bright and vibrant for many years to come.


Glass made by
SCHOTT

Installed by
Glaswerkstatt der Dombauhütte Köln

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