100 THE BUILDING CONSERVATION DIRECTORY 2025 CATHEDRAL COMMUNICATIONS CONTROLLING THE LIGHT An alternative to adhesive film was developed in 2011 in Germany by Lamberts Glass, manufacturers of mouthblown traditional glass sheets for stained glass windows. Lamberts Restauro UV™ incorporates inorganic nanoparticles in its molecular structure which filter out all UV radiation while allowing most of the visible daylight to pass through. Unlike applied films or coatings, such glass does not require periodic replacement. It is therefore ideal for use in historic buildings that house UV-sensitive materials such as paper and textiles. This specialist glass was first used in Britain in 2014 as protective glazing for the Great East Window of York Minster. There it was used to ensure the longevity of the UV-sensitive epoxy resin repairs to the medieval stained glass. On its own, however, UV glass does not significantly lower the transmission of visible light and therefore does little or nothing to reduce infrared radiation (IR). That would have to be addressed in a different way. The idea was to use Lamberts Restauro UV™ glass to create a new stained glass window that would not just block UV light but also significantly lower the levels of IR radiation by diffusing the sunlight while still leaving the effigies sufficiently illuminated to be enjoyed. No-one had attempted this before. In many medieval windows, glass corrosion erodes the surface of the affected glasses, giving them a whitish dull appearance on the outside. This results in a significant scattering of the transmitted light, much like a thin cloud layer or parasol would do. This all but eliminates the direct projection of sunlight onto interior fabric, avoiding rapid changes in its temperature. One might expect this to reduce the vibrancy of the stained glass, but in fact corrosion layers can enhance the luminosity of colours, almost as if they trap sunlight within themselves. Modern glass does not corrode in the same way as medieval glass but a visually similar effect can be achieved by applying white vitreous enamel to the surface as a diffuser. So, it was decided to investigate the design of a new stained glass window with a diffuser layer to the rear; one that would be aesthetically and thematically appropriate without overwhelming the main attraction – the Culpeper effigies. This approach raised several challenges: • Any form of painted decoration would In many medieval windows, a whitish dull appearance on the outside is caused by glass corrosion. The new stained glass window above the effigies, seen from the outside, showing the faint white enamel coating require the UV glass to be fired. Nobody had tested the effects of firing on this glass before so we did not know if firing would affect its UV-filtering quality • We did not know if the glass could be coloured with silver and copper stains to provide lively yellows and reds • If we included heraldic elements, they would have to be researched to ensure they were correctly depicted. LABORATORY TESTS: WOULD KILNFIRING AFFECT THE UV FILTER? If we were to use Lamberts Restauro UV™ to make a stained glass window, we had to make sure that its special properties would survive the kiln-firing necessary to fix the colours and painted decoration. We therefore fired a sample of glass at a typical kilning temperature for vitreous glass paints. This was then sent to Germany to be subjected to testing by the same independent laboratory which had provided the initial transmission tests on the same batch. The results showed that kiln-firing did not diminish the UV filtering ability of the glass. This gave us the confidence that we could use this glass for our purpose. CONTROLLING IR RADIATION: SCATTERING THE LIGHT We therefore painted white enamel onto the outside surface of each glass piece and fired it. This enamel’s scattering effect was so successful that a person standing immediately outside the window was completely invisible and very little direct sunlight now impacted the effigies. One worry was that the white enamel would give the new glazing a very different appearance from the neighbouring stained glass windows when seen from the outside. We tested this first with an enamelled glass sample and, while it does indeed look a little paler, judged it to be perfectly acceptable. COLOURING THE GLASS Restauro UV™ glass is almost colourless with a very faint yellow tint. Our design for the new window, however, required the addition of three colours: red, yellow and blue. In a traditional stained glass window those colours would be cut from glass sheets coloured during the manufacture of the glass. We needed to use the Restauro glass throughout, so needed another solution. For blue, we fired a blue transparent enamel onto the surface. For yellow and red, we used a technique known as ‘staining’. This is usually done by applying silver-rich clay (for yellow) or copper-rich clay (for red) to the glass surface. During kiln-firing, the metal ions migrate into the glass surface and cause it to change colour. Silver and copper stains are temperamental and unpredictable. The actual colour can only be determined after firing when the clay carrier is washed off and the piece is held up to the light. The tonal value depends not only
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