Glass Manufacture

by Walter Rosenhain

For the successful production of substances which are formed by a process of fusion, the use of refractory materials of a proper kind is of great importance. In the production of glass the double difficulty has to be overcome of finding substances capable of being formed into furnaces and crucibles which shall not only resist the softening and melting action of the furnace heat for long periods of time, but shall also resist the dissolving action of the molten glass itself. The refractory materials employed in connection with glass-making thus fall into two distinct groups, members of one group being those which meet both of the above requirements and can therefore be used in positions exposed to direct contact with molten glass, while members of the second group are materials which resist the action of the heat and flame gases but cannot resist the dissolving effect of the glass itself; these, of course, can only be placed where molten glass is not liable to touch them. We shall deal with the former group first.

Those portions of glass-melting plant which come into contact with molten glass are almost universally made of some form of fire-clay. To discuss in detail the composition and properties of the varieties of fire-clay best suited to this purpose would exceed the entire limits of this book, so that only a few leading principles can be stated. Taking first the clays intended for the production of crucibles or “pots,” we find that for the purposes of the production of such objects the prepared clay must possess a certain degree of plasticity while damp and a considerable degree of strength when dried. The dried and burnt material must be so refractory as to resist the high temperatures used in glass-melting without undergoing fusion or even serious softening. Clays of various composition and physical nature also differ very widely in their power of resisting the chemical attack of molten glass; all clays are more or less dissolved under these circumstances, but not only the rate, but also the manner, of dissolution is of importance, so that frequently a clay which dissolves rapidly but uniformly is preferred to one which dissolves more slowly but in such an irregular manner as to throw off particles of undissolved material which contaminate the glass in the form of opaque enclosures or “stones.” It is also to be noted that the best results in this direction can only be obtained by careful adaptation of the clay employed to the particular kind of glass which is to be melted in the crucibles in question. In England this question has not received the amount of attention it deserves, but in Germany and America the available fire-clays of the country have been systematically studied and exploited. As a result the glass-maker has at his disposal a large selection of materials of accurately known physical and chemical properties. By carefully correlating these with the performance of his “pots” in the furnaces, the manufacturer is able to select the most suitable material, and is, moreover, in a position to know in what direction to look for improvement or for replacement if the supply of a satisfactory brand should cease.

We may now follow briefly the process of manufacture of a fire-clay pot or crucible. The size and shape of the crucible will depend upon the particular purpose for which it is intended. Crucibles varying in capacity from 4 cwt. to 2½ tons of glass are used for various kinds of glass, but the more usual sizes lie between 30 in. and 50 in. in diameter. For many kinds of glass the shape of the pot is simply that of an open basin, circular or oval in plan and larger in diameter at the brim than at the base ([Fig. 1]), but for the production of flint glass, and of other glasses which are to be protected from contact with the flame and gases of the furnace, so-called “covered” pots are used. In these the basin—here of a more nearly cylindrical shape—is covered over by a dome, and access is allowed only by a relatively small hooded opening ([Fig. 2]). Covered pots are built up on wooden moulds, which are made collapsible, and are removed before the drying of the pot is begun.

Fig. 1.—Open “pot” or crucible for glass melting.

Fig. 2.—Covered pot for glass melting, as used for flint glass and optical glass.

The material for pot-making is first prepared with great care. The proper variety of clay having been selected, it is ground to a fine powder in suitable mills and carefully sieved; with this fine clay powder is mixed, in accurately determined proportions, a quantity of crushed burnt fire-clay. In some works this burnt material is obtained by simply grinding up fragments of old used pots, but the better practice is to burn specially-selected fire-clay separately for this purpose. The quantity of such burnt material added to the mixture depends upon the chemical nature and especially on the plasticity of the virgin clay employed; with so-called “fat” or very plastic clays up to 50 per cent. of burnt material is added, but with the leaner clays, such as those of the Stourbridge district in England, very much smaller proportions are used. The object of this addition of burnt material is to facilitate the safe drying of the finished pots and to diminish—by dilution—the total amount of contraction which takes place both when plastic clay is allowed to dry, and further when the dry mass is subsequently burnt; the burnt material or “chamotte,” having already undergone these shrinking processes, acts both as a neutral diluent and also as a skeleton strengthening the whole mass and reducing the tendency to form cracks.

The virgin clay and chamotte having been intimately mixed, the whole mass is “wet up” by the addition of a proper proportion of water and prolonged and vigorous kneading, usually in a suitable pug mill. The mass leaves this mill as a fairly stiff, plastic dough, but the full toughness and plasticity of such clay mixtures can only be developed by prolonged storage of the damp mass. In the next stage of the process, the plastic clay is passed to the “pot maker” in the form of thick rolls, and with these he gradually builds up the pots or crucibles from day to day, allowing the lowest parts to dry sufficiently to enable them to bear the weight of the upper parts without giving way. The building of large pots in this way occupies several weeks, and during this time the premature drying of any part of the pot must be carefully avoided. After the completion of the pot, drying is allowed to take place, slowly at first, but more vigorously after a time when the risk of cracking is smaller; when it is taken into use, the pot is usually many months old and is thoroughly air-dry. The clay, however, is still hydrated, i.e., contains chemically combined water, and this is only expelled during the early stages of the burning process. This process is carried out in smaller furnaces or kilns placed near the melting furnaces. In these the pot or pots are exposed to a very gradually increasing temperature, until a bright red heat is finally attained. This is a delicate process in which great care is required to secure gradual and uniform heating, especially during the earlier stages, otherwise the pots are apt to crack and become useless. Finally, when a bright red heat has been maintained for at least a day, the pots are ready to be placed in the furnace, and this is ordinarily done while both pots and furnace are at a red heat, the pots never being allowed to cool down again once they have been burnt.

Fire-clay is also used in the manufacture of bricks and blocks of various sizes required for the construction of glass-melting furnaces. Here fire-clay is only used in positions where contact with molten glass is expected, as in the walls of the basin or tank proper in “tank” furnaces, or at a level below that of the pot or crucible in pot furnaces; in the latter position leakage of glass from broken pots or overflow being liable to result in an accumulation of molten glass on the floors or walls of the furnace and passages. The fire-bricks used in these latter positions are usually of a much poorer quality of fire-clay than that used for the manufacture of pots, and this is justified in so far as certain of the requirements that apply to crucibles do not apply here—but on the other hand the use of more refractory bricks would result in a longer life for the furnace. Such bricks, it should be noted, are not laid in mortar when used for furnace construction, but are set in a thin paste of fire-clay in water, and these joints are kept as thin as possible. The part of the furnace known as the “siege” (French “siège”), i.e., the floor of the furnace upon which the pots are placed, is usually built of very large blocks of fire-clay, made of coarse materials calculated to give great strength. At or near the points where the flame enters the furnace, these blocks rapidly wear away, partly by melting but chiefly by a process of abrasion, for it seems that a rapidly moving flame has an abrading action of a very marked kind.