Lime is provided from native limestone, if sufficiently pure. It must not contain more than 3 per cent of iron, when used for white glass. There is a large saving of heat in the glass furnace by burning the limestone beforehand, and grinding it just before using, although most of the burnt lime used is previously slacked. Natural minerals of the feldspar class are em ployed in common bottle glass, in which mix tures their iron contdnt is desirable as a flux. Granite and basalt are other available sources of silica, soda and alumina, in glasses where all three may be used together. Magnesia is added to such glass mixtures as call for it, either as the carbonate or the oxide. The former is found sufficiently pure in nature, and has only to be calcined to produce the oxide. Barium is employed only in special glasses. It is added preferably as the carbonate, and in some barium glasses the natural mineral with erite is suitable after being ground to powder. For fine optical glasses the nitrate and the hy drate of barium are made use of. Manganese is widely used in the form of peroxide, occa sionally as the sesquioxide, as a decolorizer in all white glasses. It always contains a percent age of iron and a little silica. In making opal glass, alumina is used in the form of hydrate, which is found in a very pure state in the markets. Zinc, as the oxide, is employed in heat-resisting glass. Lead is largely employed to impart brilliancy. In some plants litharge is used, but the form preferred is the red oxide. Boric acid and borax and arsenic are employed in some glasses. They are bought in the open market. In addition to the primary materials, a bulk of broken glass, known as "cullet" is always put in with a batch, sometimes more, sometimes Less. As the cullet melts at a lower degree of heat it aids materially in breaking down the other raw materials.
Preliminary preparation is concentrated on the sand, which is washed again and again, then drained for several days, and finally dried— so that no moisture shall be carried into the furnace. Thorough mixture of the ingredients is of first importance. In some plants it is done by hand, hut generally it is done mechani cally. a large revolving drum with internal paddles being perhaps the most effective. The exact amount of each constituent used varies with the practice of each manufacturer, and, indeed, depends to a considerable extent upon the plant itself — the degree of heat generated by the furnace, the quality of the goods to be turned out, the methods of hand or machine production, etc. It is impossible to tell by testing any particular sample of glass just what ingredients were put into the furnace to pro duce it, for some of them are dissipated by the intense heat, and others remain in the slag.
Crucibles and Tanks.— The melting of the glass mixture is conducted either in crucibles or in shallow tanks. The crucibles are used for small batches, and the tanks for large batches. The crucibles, or "glass pots" as they are called, are of various sizes, to hold from 400 or 500 pounds up to two tons, or more. They are usually from 30 to 48 inches in diam eter at the top, and somewhat smaller at the bottom, and their height is about equal to the diameter. They are generally round, but some times oval in outline. For the ordinary kinds of glass the pots are open at the top. For the finer kinds, which must be carefully protected from actual contact with the furnace flames, as well as from possible dropping of clay or other substance from the. roof of the oven, the pots arc covered over with a dome, and the opening is at the side, under a projecting hood with a little °hearth" below it. These pots are made
of choice selected fire-clay, a part of which has beenpreviously burnt. The clay is made into a stiff dough with a little water, and the pots are built up gradually, a segment at a time, the process occupying some weeks for large and heavy pots. When pot is com plete it is allowed to dry for several months. It is then placed in a kiln, and brought slowly to a bright red heat, which is maintained for at least 24 hours, when it is filled with glass mix ture without being permitted to cool.
The tanks are simply basins, 20 inches to 3 feet in depth and often of great size, holding up to 200 tons of glass. They are con structed of large slabs of fire-clay material similar to that used for crucibles but somewhat coarser. These slabs are set dry, with no cementing material between. During the melt ing the liquid glass penetrates into the aper tures between the slabs until it congeals from the lower temperature and thus renders the tank tight. The top edge of the tanks thus constructed is on a level with the floor of the furnace, and all parts not to be covered with the molten glass are made, of silica brick.
Furnaces and Fuel.— Furnaces for melt ing glass have to be of the most highly refrac tory materials. Even fire clay will not stand the intense heat of the glass-furnace flame, and the dome or roof of the furnace, and also all of the walls not covered by the molten glass when in operation are constructed of bricks of almost pure silica, the 2 per cent allow able admixture being of lime and alumina. The side walls of the furnace are of large blocks of fire-clay, and it is the practice with some builders to put into the material a generous proportion of quartz pebbles. Two types of furnace are in use, one in which the fuel is burned directly in the furnace itself, the other in which an outer chamber is used to convert the fuel into gas which is then mixed with air and ignited, the flame surging through the furnace proper. In the former, the fuel is small coal or coke, and the heat passes upward through a short flue into the furnace chamber above it. The surge of heat strikes against the vaulted roof of the furnace and is thrown back upon and around the glass pots standing on the furnace floor. For a furnace of this kind several flues are usually provided in order to distribute the heat evenly. The burnt gases are taken out by numerous small openings around the sides so that there shall be no de cided current of flame in any one direction. Very few of such furnaces are in operation, nearly all glass being now made in gas-fired furnaces, in which much higher temperatures are attained. Two types are in favor: the regenerative and the recuperative. In the for mer there are two regenerative chambers, one on each side of the furnace proper. These chambers are of fire-brick with cross walls of loosely piled fire-brick which while they do not wholly obstruct the passage of the gases, delay them while the fire-bricks are absorbing their heat. The waste gases from the furnace are passed through one of these chambers on their way to the chimney, and when it has be come very hot, these gases are turned through the other regenerative chamber, while the un burned fuel-gas is passed through the hot chamber, in turn absorbing the heat of the fire-brick walls. This reversal of the flow of the gases is alternated from time to time as the melting goes on, the course being changed about every half hour. The fuel-gas and the air required for combustion are thus heated very economically by the waste heat of the burnt gases.