Page:Popular Science Monthly Volume 34.djvu/622

604 a definite chemical compound, the proportions in which the ingredients of the "batch" are mixed vary in every establishment. Sand is the basis of the operation. It is the commercial representative of silicic acid. With this are mixed lime and alkali (either carbonate or sulphate of soda, or both) in sufficient quantity to furnish an easily fusible mass that, on solidifying, shall be both clear and transparent. There is very wide range in the choice of materials. The purest grades have the disadvantage of costliness, while the inferior glass has the equal disadvantage of commanding but an indifferent price. Between these two considerations swings the balance of expediency.

When the batch has been made up, it is melted in large clay crucibles, or glass pots, as they are commonly called. The manufacture of the pots is the most tedious and exacting process connected with glass-making. It is one of the few industrial operations in which machinery has not been able to supersede man. A mixture of raw and burned fire-clay is employed. It is necessary that this should be prepared with the greatest care.

Once a day for at least four weeks the mass must be turned and worked, m order to get it free from air and give it the proper toughness. For this kneading process no tool has been found equal to the bare foot. There are a warmth and an elasticity about it that better than anything else develop the required plasticity in the clay. Bare-footed men, pacing up and down in lead-lined troughs, present a very primitive industrial picture. The impression is not removed when one goes up-stairs and watches the transformation of this much-worked material into crucibles. The hand here occupies the place that the foot does below-stairs. By equally slow stages the crucible is built up. First the bottom is formed, a circular slab about four inches thick and some forty inches in diameter; then the sides are gradually raised, a little addition being made each day, until at the end of about six weeks the work is completed, and a heavy, tub-shaped crucible is the result. Meanwhile the incompleted walls are kept constantly covered with damp cloths to prevent premature hardening. The temperature and humidity of the work-room are also objects of unremitting attention. But, though the crucible has now taken form, and its material been under treatment for more than ten weeks, it is not yet ready for the trial by fire. Several months must pass before it is considered sufficiently dried to withstand even a