The Building Conservation Directory 2020

117 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 0 METAL, WOOD & GLASS 3.3 rooms, for outbuildings such as the Orangery at Schloss Hof in Vienna (Fig 5) the greenish ‘forest glass’ was deemed sufficient. Nowadays, raw materials are chemically cleaned, so for restorations, ‘discoloration’ must be specifically introduced into the molten glass. By adding various metal oxides and salts, the most varied colour tones and nuances can be matched. Besides the need for consistency in colour, transparency and surface composition, it was the format of the glazing which perhaps posed the greatest technical challenges for the early glassworks. To glaze bigger window openings the glass panes, which in early times were much smaller than they are now, had to be trimmed and assembled with lead cames or wooden glazing bars (Figs 6 & 7). Through the centuries constant developments in firing and cooling technology among other improvements, enabled the production of increasingly large flat panes of glass. However, ultimately, the size of mouth-blown glass that can be produced was restricted by the limitations of human performance in terms of lung capacity and physical strength. Only with the development of machine-made flat glass (drawn glass and float glass) in the 20th century could sizes be created that did not depend on physical strength. The subsequent increase in production was substantial, starting a price decline which led to the closing of countless glassworks in the 1920s. As a result, only three glass factories (Fig 8) survive worldwide which continue to produce mouth-blown glass by the traditional methods for use in works of art and heritage conservation. References Mark Spoerer, Adalbert Busl, Heinz W Krewinkel and Reinhard Holsten: 500 Jahre Flachglas , 1487–1987: von der Waldhütte zum Konzern , Schorndorf 1987 Manfred Mislik, ‘Fensterglas’, Restaurator im Handwerk : the trade magazine for restoration practice, 9th Volume, No 4, Pages 22–25, 2017 MICHAEL BRÜCKNER studied the conservation and restoration of glass at the Erfurt University of Applied Sciences with an affiliated craftsman training as a glass finisher specialising in glass painting and artistic glazing. Since 2017 he has been working at Glashütte Lamberts as a technical consultant and is responsible for the company’s historical preservation work. Fig 5 The 18th-century orangery at Schloss Hof near Vienna: the glazing was restored using slightly greenish mouth-blown window glass made to match fragments of the original. Fig 7 The meeting room in Eidsvoll House, Norway where the Norwegian Constitution was drawn up and signed in 1814: the fine wooden glazing bars were a technological necessity because it would have been impossible to make panes of mouth-blown flat glass large enough to glaze the entire opening. (Photo: Eidsvoll 1814, Guri Dahl) Fig 8 The production hall of a traditional glass factory that is still operating today where mouth-blown cylinder glass, crown glass and even hand-cast flat glass are all produced using historic techniques (Photo: Glashütte Lamberts) Fig 6 A leaded light windows in the Quaker meeting house at Come to Good, Cornwall which was built in 1710: the small panes or ‘quarries’ are heavily seeded and may include some of the original glass (Photo: Jonathan Taylor)