GLASS POTS.
The practical success of the tank system " of glass manu facture has not as yet curtailed the demand for glass melting pots. These pots are composed of clay, which is required to be as free as possible from lime and iron. A clay obtained from the carboniferous shales of Wbrcestershire, in the neigh borhood of Stourbridge, England, is highly esteemed for the manufacture of glass pots. There are also several American and German clays which are suitable for the purpose of pro ducing the large pots in which glass is melted and worked.
These clays are, 1st, Gross Almerode, near Coblentz, Ger many, for bond clay; 2d, Christy clay, from near St Louis, Mo., used for calcine and bond ; 3d, Blue Ridge, Missouri, clay, used for bond and calcine ; 4th, Mineral Point, Ohio, flint clay, used as flint and calcine ; 5th, old pot shells for cal cine.
The German clay is shipped as ballast in the holds of ves sels, and hence transportation costs but little. It is an ex cessively fine-grained and heavy clay, and is very plastic, mak ing a better bond than any native clay. It comes in blocks 9x6x6 inches, which have to be pared with a draw-knife, and then broken and inspected and all irony spots removed. No pieces larger than a walnut are allowed to go into the mixture_ The work involved in getting the clay ready for use is exces sive, and it is the opinion of those at the works that it is much overrated. It is an excellent bond clay, it is true, but its re fractory properties are excelled by the Christy clay of Missouri.
These Missouri clays come in blocks, either calcined or raw. They are pared and broken, but not sorted over. They are washed before shipping, so that they are much finer than in nature. The Blue Ridge is the finer-grained of the two. The Mineral Point calcined clay is not now largely used, because the old pot-shells, being already in the desired composition of the mixture, make a better calcine than any single clay.
These shells are chipped with small hammers until no part of the surface remains and only the clean interior is left. The charge is composed quite largely of calcine with a little flint clay, and the remainder German and Missouri bond clays. The mixture is ground in a dry pan and sifted in a jig bolt, and the coarse part re-ground. It is then pugged five or six times in succession, and then is stored and blanketed. It remains in this state until it sours and smells offensively, which the men claim is necessary to its proper working. It is wedged by hand and is ready for use.
When required for forming the pots, a sufficient quantity of of the clay is taken and kneaded with one-fourth of its quantity of the material of old pots, which are ground to fine powder and carefully sifted ; this material gives firmness and consistency to the paste, and renders it less liable to be affected by the heat.
The pots are of two kinds, the open and the covered. The first are used for melting common glass, such as window and bottle glass ; the other for flint glass. In each case, the pots are made entirely by hand, and require great skill and care.
The pots are large structures about five feet high, four feet wide, and four feet long, bounded on top and side by covered walls, and on the bottom by a flat face. They weigh from 2,000 to 3,00o pounds, and sometimes as much as 3,50o pounds. They are made from three to five inches thick with a thicker floor, and are each built on a small platform covered with gravel, so that the air may circulate beneath them and dry them faster.
The flint glass pots are only from two to three inches thick. Each builder has on hand twelve or fifteen pots at once, on which he daily builds a little more, until at the end of three weeks or a month he finishes them all together. The buildings in which glass pots are made are provided with elevators so that the heavy pots can be handled without danger of injuring them.
When the bottom is finished, the workman begins to build up the side of the pot by first forming a ring of the same height all round, taking care to round off the upper edge to a semicircular curve of great regularity ; upon this he begins bending over other lumps of the paste until another equal layer is formed, and these are continued until the pot is com plete ; the workmen spread wet clothes over the edges when they discontinue working. This is necessary, to admit of a certain amount of drying, otherwise the large weight of clay used would prevent the form being kept, and the pot would either fall to pieces or lose shape ; the building of the pot is consequently extended over several days. After the potter has finished his work, the pots are removed into the first dry ing floor, where they are only protected from draughts, so that the drying may be conducted with the greatest possible uni formity. When they have progressed sufficiently, they are removed to the second drying floor, which is heated with a stove, and the drying is here completed. They are then placed in the store, where usually a good stock is kept on hand, as time improves them, and they are seldom kept less than six or nine months.