The Building Conservation Directory 2020

116 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 2 0 C AT H E D R A L COMMU N I C AT I ON S was generally unaffordable even for wealthy families, so its use remained restricted to the most important buildings, especially churches. The production of flat glass with fire- polished surfaces on both sides was made possible thanks to the development of glass- blowing from about the 2nd century AD. In comparison to the cast glasses available until then, this was a substantial step forward in terms of both quality and technology. Instead of pouring the hot, viscous glass mass on a flat, fireproof surface and spreading it as evenly as possible to form a flat glass panel (Fig 2), the new technology took a detour, creating a hollow body as an intermediate step. For this purpose, a certain quantity of hot glass is picked up by the tip of the blowpipe, by simultaneously turning and blowing with the mouth, and shaping the hot glass with the help of wooden moulds into an elongated balloon. Opened on both sides, a hollow cylinder is created, and once cool it is cut longitudinally and then heated up again so it can be unfolded and flattened (Fig 3). The result is a flat glass panel with a slightly uneven and distorted surface. Trapped air bubbles and fine streaks are characteristic features of this handmade flat glass called ‘mouth-blown cylinder glass’. A variation of this method which was widely used in Britain is the so called ‘crown glass’. The technique produces a particularly large flat disc with a partly sharp-edged stub in the centre, the ‘crown’ (Fig 4). The first step in the process is to blow and shape a glass sphere which is then transferred to a metal rod on the opposite side, and the blow pipe is knocked off, leaving an opening. The sphere is then heated once again. Through fast rotation of the rod in front of the fire, the glass sphere is opened out and extended by the centrifugal force to form a flat disc. Small rectangular or diamond-shaped panes could then be cut from this disk for use as window glass in lattice windows. Although the remaining middle piece is obscured by the stub, it still lets in light and was occasionally used to save money. At the end of the 17th century another technique began to develop in parallel to the mouth-blown technique. Hand casting could produce far larger panes of glass, but the glass was not clear because one side was textured. Now, with improvements in grinding and polishing techniques, larger sizes of completely transparent flat glass could be produced, free from distortion. However, the process was extremely laborious and even more expensive than mouth-blown window glass. These ground and polished cast glass panes were mainly used for mirrors in palaces and stately homes, but in some cases also as glazing where customers demanded bigger windows without glazing bars or lead cames. Although highly transparent glass surfaces were achievable with more or less effort in the techniques described above, completely colourless window glass in any size remained rare for many centuries. Both the procurement of raw materials and the firing technology played crucial roles in this respect. Quartz sand, soda and lime, the basic components of clear glass, are abundant almost everywhere, but naturally occurring forms are rarely pure. Contaminants can affect the stability and amount of colour in the glass which can lead to very different qualities depending on the region. A light tint of green in the glass, caused by ferrous sands, was widespread and a reason for a price reduction. In the 19th century, classification into different purity classes was still usual, and the classes were deliberately employed by builders according to where they were to be used. While the much more expensive crystal- clear glass was often used in representative Fig 2 The production of hand-cast flat glass (Glashütte Lamberts’ table cathedral glass); the underside has a pronounced texture from contact with the table which is typical of cast glass. (Photo: Glashütte Lamberts) Fig 3 Making mouth-blown flat glass (cylinder glass): left, picking up the viscous glass mass onto the blowpipe; centre, the elongated glass balloon is formed by simultaneously turning and blowing; and right, after being reheated, the cylinder is cut lengthwise, opened and flattened to create the flat glass panel. (All photos: Glashütte Lamberts) Fig 4 Crown glass with typical concentric ripples around a partly sharp-edged stub in the centre (Photo: Glashütte Lamberts)