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Glass goes round the bend

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In principle, the technique of bending glass is uncomplicated. To create bends in one or two planes, annealed glass is heated to the point where it becomes malleable (630- 690degreesC), and is then pressed or sag-bent over formers. This method, using float glass, was practised in English Regency architecture of the early nineteenth century .

Making curved glass today is restricted by safety legislation (Building Regulations 1991: Document N, bs 6262) which requires safety glazing in virtually all areas of a building. This requires the use of laminated or toughened glass, which is not so easy to curve.

Glass can be curved and toughened in the same operation, traditionally by using tongs to clamp the top edge of a sheet of glass and carry it to the kiln. After being heated to about 630degreesC, the glass is press-bent, then quenched on both sides to toughen it. The method has drawbacks: tong marks are visible along one curved edge and often create visual distortion, and tong-toughened bent glass is difficult to laminate as the curvature of two sheets is not compatible.

Alternative methods have been developed by specialist manufacturers. Romag curves and laminates glass by sag-bending, Firman laminates with poured acrylic resin, and Saint-Gobain Solaglas curves and toughens glass in a roller hearth.

Romag Security Laminators is based in the North-east near Consett. The firm originally specialised in producing security glass screens such as those used or post office and bank counters, but is now a major producer of flat and curved laminated glass for buildings - including the four- layer laminate roof of the new National Glass centre in Sunderland (see page 31). The factory contains two large autoclaves which can laminate glass with polyvinyl butyral (pvb) to produce many types of protective glass, ranging from safety glass (one layer of pvb between two sheets of glass) to bullet-resistant and blast-proof glass (four or more sheets of glass layered with pvb). The factory even has a firing range to test the efficiency of its bullet-proof products. To produce curved laminated glass, the sheets to be laminated are stacked on top of one another in the kiln, so that their curvature is identically matched and optically perfect. Size is limited by the kiln size to a maximum (precurved) of 2900 x 1950mm, with a maximum drop of 500mm. The glass is then layered with pvb, heated and compressed, and sealed together in an autoclave.

Firman, in Romford, Essex, is well known for its innovative work with structural engineer Tim Macfarlane - the firm fabricated the first glass beams and columns ever used (aj 22.7.92), and tested the cantilevered glass canopy in Tokyo (aj 9.1.97). Firman works on a bespoke basis, with an emphasis on craftmanship rather than mass production, and offers a complete service, drawing glass construction details, supplying the components and fixing on site. John Hodgson, md of Firman, says: 'We have a closer relationship with architects and structural engineers than most; we like to work with them to push the boundaries of glass technology a bit further.' Firman's kiln can curve annealed (float) glass; sheets to be laminated are then taped at the edges and laminated with cold-poured acrylic resin, then cured under ultra-violet light. Firman will also laminate other manufacturers' toughened glass.

Saint-Gobain Solaglas in Bishop Auckland, County Durham, hasthe largest toughening/bending furnace in Europe. Glass can be bent and toughened in one continuous operation. After a sheet of glass is heated in a furnace to about 630degreesC it moves on to a roller hearth which curves the glass to the required shape - a process which takes about three seconds. Jets of air, operating between the rollers, then quench the glass to stress and toughen it. Glass can be produced in this way in a variety of curves, to a maximum size of 3650 x 2130mm.

At the same Bishop Auckland plant it can also be pre-drilled with great accuracy, fritted with coloured ceramic inks, usually to reduce solar gain, and tested by heat soaking. A supply and fix service is also offered.

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