Conservation concepts for architectural glass

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Glass related R&D at the Fraunhofer Institute for Silicate Research ISC covers a broad spectrum, from specialty glass for technical applications, to glass surface finishing with functional or decorative coatings. It also includes concept designs for the restoration and preservation of historic glass paintings or other glass items of cultural heritage.

A recent project involved the conservation of the glass façade of the public indoor swimming pool located at Stuttgart Feuerbach. The concept was designed by a team of researchers from the International Convention Center for Cultural Heritage Preservation IZKK, which is part of the Bronnbach Branch of the Fraunhofer ISC.

The façade of the public swimming pool building at Stuttgart Feuerbach features double glazing which is displaying the obvious signs of corrosion. Of particular concern are the coloured glass elements, which were painted by the artist HAP Grieshaber between 1959 and 1964.

In order to prevent further damage to the artistically valuable façade, the authorities in charge (Hochbauamt Stuttgart) commissioned the Fraunhofer ISC to design a suitable conservation concept, and also to develop an effective protection coating for subsequent conservation of the restored façade.

Analysis undertaken

To find the best suited long-term concept for cleaning, restoring, and conserving the glass paintings, the IZKK researchers analysed sample panes of the façade glazing.

Analysis methods included light microscopy, cross section preparation and a combination of X-ray spectroscopy and scanning electron microscopy. An unpainted sample glass pane was used to determine the composition of the glass, and to analyse the glass surface. The composition of the paint was analysed from a painted glass pane.

The investigation revealed crystalline deposits of calcium and sodium silicate compounds, which fogged the pane like a white veil, and so not only impaired the view but also covered the colour paintings. The leaching took the form of massive glass corrosion between the double glazing, with the corrosion reaching depths of 20 micrometers in some areas.

The edge seal of the insulating glass had also given way and made room for condensation to form between the panes of the double glazing. Glass components had begun to leech out and corrosion products to accumulate, until the surface of the glass was partly dissolved.

Tailored solution

Based on these results, a tailored concept was designed to carefully clean the façade without affecting the colour of the paintings. The IZKK team then went on to develop a special protective coating, which they tested on self-made glass samples matching the façade elements.

The measures are meant to preserve the transparency of the façade and to improve the ‘readability’ of the glass paintings. In order to prevent further corrosion, the façade panes are to be reincorporated into double glazing using the latest standards and technology.

Glass finishing and coating

Apart from such glass conservation concepts, Fraunhofer ISC develops functional coatings for architectural glazing. Dust-repellent and anti-reflective properties can be combined in one single coating. The coating creates a special glass surface structure, which provides for self-cleaning through wind and rain. This finishing system has already stood the test of time on external glazing of the Cologne Cathedral. Another special coating was developed for window panes to improve the lighting condition of indoor rooms. The coated window panes promote the passage through the glass of those wavelengths of light that govern our hormonal balance and have a positive effect on our biorhythm.

Indoor climate control is another important research topic of the Fraunhofer ISC. A recent development features highly porous glass flakes which can absorb, store, and release excess moisture to regulate indoor air humidity. Incorporated into plaster or wall paint, the glass flakes help maintain a comfortable indoor climate and prevent mildew and mould.

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