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History

Prof. John Parker

Casting a light on glazing Prof Parker highlights early ways to make flat glass sheets for windows

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lthough some early Roman mansions were already glazed with glass, manufacturing of glass sheets significantly post-dates the production of glass vessels. Initially glass for glazing was in competition with less expensive materials, such as thin layers of alabaster, waxed linen, shell, horn and sheets of mica. This remained the case throughout the time of the Roman Empire and for much longer in most of Europe. In the brilliant sunshine of the Mediterranean region, limiting the light transmission and reducing solar heat gain were crucial considerations in window design. But as the Romans extended their domain northwards, achieving sufficient internal lighting levels became increasingly important, requiring more transparent media. Glass eventually became (for those who could afford it) the material of choice, and so the resultant demand stimulated window glass production, motivating process improvements and lowering its cost. Initially glass panes were made by casting molten glass into a rectangular arrangement of iron bars, forming a tray, with a stone base, perhaps covered by clay. Based on surface markings left on the glass after casting, some have even suggested that a table of wet wood may have been used. Large cast sheets have been found in the glazing in Pompeii. A second century bath-house window, found in a RomanoBritish complex in the UK, measured 235mm by 255mm and was made by this technique. Inevitably such glass slabs would have had a seriously disfigured surface where the glass contacted the solid casting surface. Rapid chilling of the surface of the slab on the table accompanied by slower cooling of the glass through its thickness would give rise to ripples; sand or grit on the casting surface, or any asperities, would have

caused pock marks. Improvements in the casting process, and consequently the quality of the glass surfaces, from the Middle Ages onwards included: the use of copper and then iron lined tables; the ability to suspend and control much bigger crucibles over the tables so that larger sheets could be poured; the rolling of the hot glass; and the introduction of mechanical grinding and polishing. Even in the early 20th century, when continuous sheet production was already in place, Pilkington developed a twin grinder using large rotating metal discs for simultaneously grinding the top and bottom surfaces and, further down the line, polishing them to achieve maximum optical quality. In early Roman times, the introduction of blowing irons for shaping glass containers gave a major stimulus to glass production. The origin of the technique is unclear but evidence suggests that it was first used in ancient Phoenicia, the coastal region of modern Syria and Lebanon, and dates back to somewhere between the fifth and the first century BC. From there, the technology spread along the trading routes within the Roman Empire. Before long artisans discovered how to create sheets using the same approach. Two methods dominated, namely the cylinder and crown methods; both had roots in Roman technology. For the first process a blown bulb was stretched by swinging into an elongated cylinder whose diameter was controlled by marvering (rolling a glass gather on a marble table) and by manipulation using various hand working tools, before finally being cut longitudinally and opened to make a flat sheet. At different points in its history this was variously known as cylinder, broad, sheet, spread or muff glass. The crown method will be described in a later article. Sheets made by the

cylinder technique can be recognised by the elongation of internal bubbles in the direction of the axis of the cylinder. From the fourteenth century, Lorraine had become a key European centre in the production of cylinder glass, based on immigrant labour from Bohemia where no doubt the early Roman traditions had been maintained and developed. Hence it was often termed Lorraine glass. This was linked to major improvements in quality, for example less variable thicknesses. Also improved control of temperature and other operating parameters meant that both surfaces of the cylinder glass were glossy, unlike the early Roman production. Cylinders were up to 1.5m long, making them suitable for large windows. Replacement glass sheets for ancient buildings are still being made using this method. In the past people did not expect perfect images of the panoramic views from their mansions nor did they want strong reflections of the sun from perfect panes. Now the use of cylinder glass adds authenticity to building restorations. Meanwhile the technique of casting has returned to fashion. In the 1930s in Paris, Jean Gaudin began casting rectangular coloured glass slabs 20cm x 30cm in size and up to 3cm thick. These were shaped by breaking with hammers or cutting by saw, creating edges that were chipped or faceted. Particularly during the 1950s and 1960s they were assembled into complex images for decorative windows and sculptures using cement or epoxy resin to retain them in place. A lack of watertightness of some early constructions cast a shadow on the technology. ďż˝

* Curator of the Turner Museum of Glass, The University of Sheffield, UK www.turnermuseum.group.shef.ac.uk j.m.parker@sheffield.ac.uk

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