=Plate Glass= is used as mirror glass and in glazing shop windows and showcases. It may vary between 1/4 and 3/4 in. in thickness, and is more expensive to produce than crown or cylinder glass. In the manufacture of best plate glass, the materials are melted in open crucible-shaped pots of varying sizes; sometimes, in making large, heavy plate, their capacity reaches 25 cwts. of metal. When the metal is plain and clear from seeds it is either ladled out into smaller crucible pots for casting, or, as in the case of casting large sheets, the whole crucible of metal is lifted bodily out of the furnace by means of a crane, and, after being skimmed, is conveyed by an overhead travelling derrick to the casting table. This table is a level iron bench the size of the plate to be cast, the face of which consists of thick sheets of iron plate riveted together to form a level top; along the whole length of each side of this table is a raised flange of a height sufficient to give the thickness of the plate of glass to be cast: resting on these two outer edges a long, heavy metal roller runs, covering the full width of the table. The crucible of hot metal is brought to a convenient position and the contents poured out on the table in front of the metal rollers. These rollers then travel along and squeeze or roll out the hot metal over the surface of the table to the thickness regulated by the side pieces, which also prevent the metal from flowing over the sides. The empty crucible is then conveyed back to the furnace for refilling. The cast plate of glass is then trimmed from any excess of glass at the ends, and when set and stiff is lifted at one end slightly and pushed forward into a conveniently situated annealing oven, where it is re-heated and subjected to a gradually diminishing temperature to anneal it. The plate of glass, as delivered from the annealing oven, shows surfaces somewhat rough, wavy, and uneven, from the marks left by the table and the roller, and it has to be ground and polished level and smooth on both sides. This is done by fixing one face of the glass plate in a plaster of Paris bedding and setting it within a mechanical grinding machine. This machine carries several revolving arms, to which are attached other smaller plates of glass. These are used as the rubbers, a slurry or paste of sharp sand and water, or abrasive powder, being interposed between the two. The revolving circular motion of the arms causes a grinding action between the two plates, which wears down any irregularities and gives a more even face. After this, finer grades of abrasive materials are employed, and, finally, polishing powder, until the face of the glass plate is polished smooth and level. The large plate of glass is then reversed and the process of grinding resumed on the other side.