1938. Pilkingtons developed the twin

ground POLISHED PLATE system.

1959 FLOAT GLASS was launched on the

UK market, invented by Sir Alistair

Pilkington.

blown, it was made into lead lights. CROWN GLASS: Molten glass is gathered on a blowpipe, and a balloon shape is blown. The blowpipe is removed, a solid ‘punty’ rod is attached and the glass is spun rapidly until a disc is formed. The outer portion beyond the central knob is then cut into panes. By the 18th century quality was often very good with an almost unmarked fire-finished surface. Crown was the preferred choice for window glass, together with some imported cylinder glass until the mid-l9th cntury.

BLOWN PLATE: Produced from Broad Sheet, each sheet of glass was laboriously

hand ground and polished on both surfaces. The plate was of a sufficient quality and size for mirrors or coach glasses.

POLISHED PLATE: Produced by casting glass onto a table and then subsequently grinding and polishing the glass, originally by hand, later by machine. An expensive process requiring a large capital investment.

CYLINDER BLOWN SHEET: A similar process to Broad Sheet, exceptthatlarger cylinders are produced by swinging the cylinder in a trench. The glass is allowed to coolbefore cutting the cylinder, which is then re-heated and flattened. Larger panes and amuchimproved surface qua!ity result. Manufactured in the UK in the mid-l9th century, it had been made in France and Germany (and imported to the UK) since the 18th century.

The above methods lasted at least until the end of the 19th century. The early 20th century marks the move away from hand blown to machine manufactured glass.

MACHINE DRAWN CYLINDER SHEET:

The first mechanical method of drawing glass; 40 ft high cylinders of glass were drawnverticallyfrom a circulartank. The glasswas annealed and then cut into 7-10 ft cylinders, which were then cut length- ways, reheated and flattened. This process was used in the UK up to the end of the 1920s.

FLAT DRAWN SHEET: The glass was drawn vertically in a flat sheet until it cooled sufficientlyto allowthe glass to be cut. The Belgians invented the original process but it did not reach the UK until 1919. Horticultural Sheet is produced by a later variation of this process. The glass was noted for having a wave in one direction only.

SINGLEANDTWINGROUND POLISHED PLATE: Here the glass is cast and then subsequently ground and polished on a conveyor belt, to a fme quality without distortion.

FLOAT GLASS: A layer of molten glass is ‘floated’ on to a bath of molten tin and produces a fme quality of glass, but with a mirror like reflection, without anywave or distortion. It is the standard modern method of producing window glass today.

PRESERVING AND RE-USING ORIGINAL GLASS

Where old glass exists in good condition, it should, if at all possible, be retained. When this is not possible, care should be taken to select the correct glass.

Old glass can be fragile and care needs to be taken when removing it from a frame. The 19th century method of removing valuable old glass from its Georgian sash, was to dump frame and glass in a heap of farmyardmanurefor6-8weeks. The glass

BROAD GLASS: Fig. 2 shows part of the process of Broad Sheet manufacture. ‘d’ shows the muff about to be sheared open, ‘h’ shows the sheet after flattening

MANUFACTURING PROCESS BROAD SHEET: Molten glass is gathered on a blowpipe, and blown to an elongated balloon shape. The ends are cutoff and the resulting cylinder is split with shears while still hot, then flattened on an iron plate. This is the forerunner of the cylinder process. The quality of the glass was not good, with many imperfections. Because of the relatively small sizes

PLATE GLASS: Fig. 3 This shows the French process of Cast Plate manufacture. The box of molten glass on the jib has been turned over and the molten glass is poured onto the casting table and the large iron roller will be passed over the glass to make a uniform thickness prior to polishing and grinding

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