Traditional Glass Manufacturing Techniques (Until 19th Century)

BROAD SHEET. Molten glass is gathered on a blowpipe, and blown to an elongated balloon shape . The ends are cut off 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 blown, it was made into leadlights.

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 19th Century.

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, except that larger cylinders are produced by swinging the cylinder in a trench. The glass is allowed to cool before cutting the cylinder, which is then re-heated and flattened. Larger panes and a much improved surface quality result. Manufactured in the UK in the mid 19th Century,it had been manufactured in France and Germany (and imported to the UK) since the 18th Century.

Machine Manufactured Glass (Early 20th Century)

MACHINE DRAWN CYLINDER SHEET. The first mechanical method of drawing glass, 40 ft high cylinders of glass were drawn vertically from a circular tank. The glass was annealed and then cut into 7 - 10ft cylinders, which were then cut lengthways, reheated and flattened. This process was used in the UK up to the end of the 1920's.

FLAT DRAWN SHEET. The glass was drawn vertically in a flat sheet until it cooled sufficiently to allow the 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.

SINGLE AND TWIN GROUND POLISHED PLATE. Here the glass is cast and then subsequently ground and polished on a conveyor belt,to a fine quality without distortion.

FLOAT GLASS. A layer of molten glass is "floated" on to a bath of molten tin and produces a fine quality of glass, but with a mirror like reflection, without any wave or distortion. It is the standard modern method of producing window glass today.