The materials used are silica (white sand), carbonate of soda (soda ash), and lime. Other materials, as arsenic and charcoal, are used in small proportions, but the main ingredients are the first three named. Probably it is little imagined that in the production of plate glass, mining is involved in two or more forms (namely silica and coal), also the quarrying of limestone, the chemical manufacture of soda ash on a large scale, the reduction and treatment of fire clay to its right consistency, an elaborate and expensive system of pot making; and the melting, casting, rolling, annealing, grinding and polishing of the glass. In special uses, as in beveled plates and mirrors, two more elaborate processes must be added--beveling and silvering--all of which are performed under the direction of experts aided by a large amount of labor and expensive machinery. Pots of fire clay take so important a part in the successful manufacture of plate glass that the subject deserves especial notice. The different clays after being mined are exposed to the weather for some time to bring about disintegration. ~THE CLAY MUST BE TRAMPLED WITH BARE FEET~ At the proper stage finely sifted raw clay is mixed with coarse, burned clay and water. This reduces liability of shrinkage and cracking. It is then “pugged,” or kneaded in a mill; kept a long time (sometimes a year) in storage bins to ripen; and afterwards goes through the laborious process of “treading.” Nothing has thus far been found in machinery by which the right kind of plasticity can be developed as does this primitive treading by the bare feet of men. The clay must be treaded, not once or twice, but many times. The building of pots is a slow, tedious and time-killing affair; but this is most essential. ~HOW MELTING POTS ARE MADE~ Without extreme care, some elements used in the making of the pots might be fused into glass while undergoing the intense heat of the furnace; or they might break in the handling. The average pot must hold about a ton of molten glass, and the average furnace heat necessary is about 3,000° Fahrenheit. The work is not continuous. Each workman has several pots in hand at a time, and passes from one to another adding only a few inches a day to each pot, so that a proper interval for seasoning be given. After completion, comes the proper drying out of the pots; and this is another feature in which the greatest scientific care is required. No pot may be used until it has been left to season for at least three months, and even a year is desirable. And after all this trouble, the pot has but 25 days of usefulness. The pots form one of the heavy items of expense in plate glass manufacture; and upon their safety great things depend. [Illustration: POT MAKING.] [Illustration: MIXING THE CLAY. TRAMPLING THE CLAY.] [Illustration: SKIMMING THE POT.] [Illustration: CASTING PLATE GLASS.] ~HOW THE HUGE PLATES OF GLASS ARE CAST~