Of the greatest importance to the glass manufacturer are the refractory materials upon which the life of his furnace and pots depends. A few notes giving a description of them and dealing with the manufacture of the fire-resisting blocks used in building the furnaces will be of interest.
The chief and most generally used of such materials are the fire-clay goods. The best known deposits of fire-clays in this country are those in the Midlands, Stourbridge, Leeds, and Glasgow districts. In each of these districts the mining of fire-clays and the manufacture of fire-resisting goods for furnace work forms an important industry. The theoretical composition of a true fire-clay would be a double silicate of alumina, and in this pure state it would be of a very refractory nature. But, naturally, fire-clays show the presence of other bases, such as iron, lime, magnesia, titanium, and alkalies, which, if present to any appreciable extent, lower the degree of resistance to heat or refractoriness of the clay. These other bases may be considered as impurities or natural fluxing agents. The characteristics of a highly refractory clay suitable for glass manufacturers' requirements would be: (a) that such a clay should show no signs of softening at the highest heat of the furnace; (b) a melting point not below Cone 31 or 1690° Centigrade; (c) a high alumina content not below 30 per cent.; (d) the greatest freedom from impurities; (e) a fine grained texture; and (f) a high degree of plasticity. These are the qualities most 36 essential for glass house work. The figures given by the chemical analyses of good fire-clays would probably fall within the following limits:—
Silica . . .
49% to 65%
48% „ 31%
Ferric Oxide ....
0-5% „ 1.5%
nil „ 1.5%
nil „ 0.5%
nil „ .2%
Total Potash and Soda
0.5% „ 1.8%
Clays of higher silica content than 70 per cent, would not be considered suitable as pot-clays owing to the ease in which glass attacks silicious clays. It is important that chemical analyses of fire-clays should be compared with results obtained from the analysis of fired or burnt samples, or they should be recalculated to allow of such comparison, so as to exclude the figures for the hygroscopic and chemically combined water of the clays.
The writer gives the following particulars of a fireclay very suitable for glass house pot-making. It is plastic and highly refractory, and is now being considerably used by the trade. The clay is supplied by Mansfield Bros., Church Gresley. The figures are from a report made by Mr. J. W. Mellor, D.Sc, of the County Laboratorv. Stoke-on-Trent, and are as follows:—
46.45 per cent.
Titanic Oxide .....
Manganese Oxide ....
Loss when calcined over 109* Cent.
The melting point is given as equal to Seger Cone 33 or 1730° Centigrade.
The physical properties of fire-clays vary as well as their chemical properties. The analysis alone of a fire-clay is not always sufficient indication as to its ultimate behaviour when in use. Many physical tests have to be carried out before a clay can be proved satisfactory for a particular purpose, and much information can be gained by engaging the services of a specialist upon refractory materials to carry out petrographic, pyrochemical, and physical tests, and report upon the suitability of the material for its specific purpose. Fire-clays should be plastic, and this plasticity should be developed to its utmost to increase the binding properties of the clay when used. To develop the plasticity, fire-clays should be weathered or exposed in thin layers to the action of atmospheric influences. The heat of the sun and rain, and the action of frosts have a direct influence in breaking up the clay and developing its better properties. The use of new unweathered clay is the cause of much trouble to the glass manufacturer who makes his own pots and furnace goods, and on this account he should insist upon having his clays weathered for some time before use, so as to have them thoroughly matured. Before fire-clays are weathered or used for important work they should undergo a process of selection and cleansing. When first raised from the mines all foreign and inferior portions, carbonaceous matter, vegetation, iron pyrites, and stones are removed. The best and cleanest portions of the fire-clay are sorted out and removed to the weathering beds, where the lumps are broken down to small pieces about the size of an egg, and left to mature and season by weathering.
This fire-clay is then spread out in a layer about 2 ft. deep, and, after a period of exposure to the action of the weather, the heap is turned by men shovelling the clay from one side to the other. The clay, under the continued action of the wind, frost, and rain, disintegrates and slacks down until it is reduced to a mild, fine grained mass, which condition shows it to be well seasoned and ready for use. Fire-clays vary in this respect : some clays season quickly in the course of a few months, others take years to develop their proper nature. The former may be classed as mild fire-clays, the latter as strong fire-clays.
After weathering, the clay is carted or conveyed to the clay-grinding plant, where it is stored under cover until it is dry enough to be ground on the clay-mill. Here the clay is fed into a revolving pan, and crushed under heavy iron runners, and, after passing through perforations in the bottom of the pan, it is elevated on to screens which sieve the clay to a requisite degree of fineness. It is then admixed with a large proportion of ground burnt fire-clay and the mixture is tempered with water until it forms a plastic mass or dough, which is conveyed to the workshops where the furnace blocks or pots are to be made. These making and drying shops have false or double floors, under which steam or heated air is passed by means of pipes or flues below the floors, giving the steady and uniform heat which is necessary to dry the fire-clay goods as they are made. Heavy fire-clay goods should on no account be hurried in drying, lest trouble should occur through the goods cracking or warping.