Glass Manufacture

by Walter Rosenhain

As soon as the empty pot has been put into place, the melting furnace is carefully sealed up by means of temporary work built of large fire-bricks, the whole being so arranged that the mouth of the hood of the pot is left accessible by means of an aperture in the temporary furnace wall. This aperture can be closed by one or more slabs of fire-clay, and when these are removed an opening is left by which the raw materials are introduced, and through which the other manipulations are carried out.

When this stage of the process is reached, the wagons containing the mixed raw materials are usually wheeled into place in front of the furnace, but the introduction of the materials themselves into the pot is not begun until several hours later, when the furnace has been vigorously heated and an approach to the melting heat has been attained.

When the furnace and pot have attained the necessary temperature, but before the raw materials are introduced, a small quantity of the cullet, which has been reserved for this purpose, is thrown into the pot and allowed time to melt, and then only is the first charge of mixture put into the pot. The object of this proceeding is to coat the bottom and part of the walls of the pot with a layer of molten glass which serves to protect it from the chemical and physical attack of the raw materials during the violent action which takes place when they are first exposed to the furnace heat.

The gradual filling of the pot with molten glass is now carried out by the introduction of successive charges of raw material; as the mixture not only occupies more space than the glass it forms, but also froths up a good deal during melting, the quantities introduced each time must be carefully adjusted so as to avoid an overflow of half-melted glass through the mouth of the pot. As the pot is more and more nearly filled, the space left for the raw materials is proportionately diminished, and the later charges are therefore much smaller than the first few.

When, finally, sufficient material has been introduced to fill the pot completely, the next stage of the process commences. When the last charge of raw materials has melted, the glass in the pot is left in the state of a more or less viscous liquid full of bubbles of all sizes; it is essential that these bubbles should escape and leave the glass pure and “fine,” and this result can only be achieved by raising the temperature of the furnace and allowing the glass to become more fluid, while the rise of temperature also causes the bubbles to expand owing to the expansion of the gas contained in them. In both ways, rise of temperature facilitates the escape of the bubbles, and the furnace is therefore heated to the full, and this extreme heat is maintained until the glass is free from bubbles. In the case of the more fusible glasses the temperature required for this purpose is not excessively high, and, indeed, in the case of these glasses care is taken to avoid too high a temperature, as it entails other disadvantages. In the case of the harder crown glasses, however, the difficulty lies in producing an adequately high temperature without at the same time endangering the life of furnace and crucible. The difficulty of freeing the molten glass from bubbles constitutes one of the causes that limit the range of our optical glasses in one direction—still harder glasses could be melted, but it would not be feasible to maintain a temperature high enough to render them fluid enough to “fine.”

In the case of other kinds of glass, again, it becomes impossible to entirely remove the bubbles from the molten mass even when very hot and very fluid. The exact cause is not known, but in some kinds of glass the bubbles formed are so minute that even when the glass is perfectly mobile the bubbles show no tendency to escape, while in other kinds of glass there appears to be a steady evolution of minute bubbles as soon as the temperature is raised with a view to removing those already in the glass. As this property attaches to some of the most valuable of the newer varieties of optical glass, opticians and the public have learnt to put up with the presence of minute bubbles in the lenses and prisms made of these glasses. These bubbles are, however, very minute and do not interfere with the optical performance of the lenses, &c., except to the extent of arresting and scattering the very small proportion of light that falls upon them; their presence is therefore to be regarded as a small but unavoidable drawback to the use of glasses which offer advantages that completely outweigh this defect.

Returning to the melting process, we find that the extreme heating required for the purpose of “fining” the glass is continued for a considerable period of time, as long as thirty hours in some cases, the glass being examined from time to time to test its condition as regards freedom from bubbles. This is done by taking a small sample of glass out of the pot and examining it to see if it still contains bubbles. In some works this test is made by taking up a very small gathering of glass on the end of a small pipe and blowing it into a spherical flask; on looking at such a flask in a suitable light the presence of even minute bubbles is readily detected. In other works a simpler process is adopted, a small quantity of glass being ladled out of the pot on the surface of a flat iron rod. It is allowed to cool on the rod, and when pushed off forms a small bar of glass some eight or ten inches long and about an inch wide; in this also the presence of bubbles is easily detected. These test pieces are known among glass-makers as “proofs.”

When proofs, taken as just described, have shown that the glass is free from bubbles, the extreme heat of the furnace is allowed to abate, and the fire-clay slabs in front of the mouth of the pot are removed. The next step is that of skimming the surface of the glass. Since most of the materials liable to contaminate the contents of a pot are specifically lighter than the molten glass, they will be found floating on the surface, and the surface glass is therefore removed with a view to ridding the glass of anything that may have been accidentally introduced and that has not melted and become incorporated with the molten mass.

The next steps in the process are those of stirring the molten glass with a view to rendering it homogeneous and free from striæ. The stirrer used for this purpose is usually a cylinder of fire-clay, previously burnt and heated. This is provided with a deep square hole in one end, and it is held at first by means of a small iron bar passed into this hole. By this means the red-hot cylinder of fire-clay is introduced into the open mouth of the pot, and when it has attained approximately the temperature of the molten glass it is dipped into the glass itself, in which it ultimately floats. When stirring is to begin, the square, down-turned end of a long iron bar is introduced into the corresponding square hole in the upper end of the stirrer, and by this means the fire-clay cylinder is held in a vertical position in the glass and given the steady rotatory movement which constitutes the stirring process. For this purpose the long iron bar just mentioned is made to pass over a swivel-wheel, while a workman moves it steadily by the aid of a large wooden handle. This operation is always laborious and trying; the workman is necessarily exposed to the intense heat radiated from the open mouth of the crucible, so that men have to relieve each other at frequent intervals.

During the earlier stages of the stirring process the glass is very hot and mobile, but the stirring is continued, with short intervals, until the glass is so cold and stiff that the stirrer can scarcely be moved in it at all, so that the work of moving the stirrer becomes heavy towards the end of the operation. The actual amount of stirring required varies according to the nature of the glass, and the size of the pot or crucible in question. Some meltings are found to be satisfactory after as little as four hours’ stirring, while for others as much as 20 hours are required.