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 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 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 squatting 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 essential for glass house work. The figures given by the chemical analyses of good fire-clays would probably fall within the following limits—

Clays of higher silica content than 70 per cent. would not be considered suitable as pot-clays owing to the case 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 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 Laboratory, Stoke-on-Trent, and are as follows—

The physical properties of fire-clays vary as well as their chemical properties. The analysis alone of a is not always sufficient indication as to its ultimate behavior 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 the action of frosts and rain 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 cleaning. 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 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 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 shoveling 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 and the mixture is tempered with water until it forms a plastic mass of 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 using pipes or flues below the floors, giving the steady and uniform heat which is necessary to dry the goods as they are made. Heavy goods should on no account be hurried in drying, lest trouble should occur through the goods cracking or warping.

In making the blocks for the furnaces the workman takes a portion of the prepared clay and tramps the plastic mass into a wooden frame, or mold, the shape and size of the block required, with due allowance made for shrinkage. The blocks are made on the warm floor, which is of cement or overlaid with quarries. When the mold is filled the surplus clay is cut off and the wooden frame is lifted up, leaving the clay block on the floor. The empty mold is then cleaned and refilled. The blocks are left until they attain considerable stiffness from the evaporation of the water present by the heat of the room. They are then dressed and cut to the final shape desired, after which they are further dried until they become quite hard and white. When thoroughly dry the blocks are removed from the drying sheds to the kiln for burning.

In burning thick and heavy blocks much care and vigilance is required in expelling the chemically combined water present in the clay, and, as the temperature rises and approaches red heat, the rate of heating should be retarded to allow proper oxidation to take place throughout the structure of the blocks, and prevent black cores from being formed. In all fire-clays, besides the mechanically admixed water used in preparing the clay to a plastic mass, which is mostly driven off whilst in the drying shed, there exists water in a chemically combined state. This the combined water is not expelled below 250° Centigrade, and is tenaciously held by many varieties of mild fire-clays. Due care has to be exercised in dehydrating goods made from such clays; therefore the man in charge of the burning regulates his fires, keeping the kiln at a moderate heat for some time to allow this chemically combined water to be properly and completely expelled. This dehydration stage in burning clay goods occurs between the temperatures of 300° and 650° Centigrade.

After the dehydration stage of burning is completed, the fireman raises the temperature within the kiln to a dull red heat, when the next stage in the process of burning begins. This is the oxidation period, during which any organic carbonaceous matter present in the clay is expelled. During this stage in burning, goods require extended time, to allow for the heated air to permeate and get to the interior portions of the blocks and oxidize the cores; otherwise the blocks are badly burnt.

After the oxidation stage is completed, the fireman raises the heat quickly until he obtains a high temperature, sufficient to eliminate and complete the shrinkage of the goods. When this heat is sufficient to complete the fire-shrinkage, the kiln is finished and is allowed to cool down. The blocks, when cold, are then withdrawn and delivered to the furnace builder.

For the erection of the furnaces several grades of blocks are used, according to the conditions and nature of the heat they have to resist. In the presence of reducing agents, fuel ash, or glass, goods vary greatly as to their suitability. So the local conditions to which they are to be subjected whilst under heat should be first ascertained, and the mixtures for the blocks adapted accordingly. So many differences exist in the pyrochemical and physical properties of clays that their misuse is often apt to occur if the conditions under which they are to be used are not properly understood and allowed for. A may show a the high degree of refractoriness under a fusion test, and yet be less suitable for a specific purpose than one of less refractoriness showing better physical properties and of the more suitable chemical constitution. The size of grain in both the burnt clay and raw clay used in the mixtures for making glasshouse furnace blocks is of the greatest importance. In many cases it is necessary to grade the ground-burnt material used, so that the proportion of coarse grains to the fine flour can be regulated to suit requirements. The burnt clay used in making the furnace blocks should be hard and well burnt, to prevent any after-shrinkage of the goods when they are used in the furnace. Fire-clay goods for glass house furnaces should not be burnt at a lower temperature than Cone 12, and in the construction of gas-fired furnaces and tanks, burning the blocks at a higher temperature, Cone 14 would give much better results.

On the Continent the glass manufacturers usually grind and mix their fire-clays, with the result that they know exactly what they are using in making their pots and furnace goods, and they are not then dependent upon outside firms to carry out their wishes. English glass manufacturers usually buy their clays ready mixed, and as often as not have perforce to take the mixtures offered by the clay firms. Unfortunately, in Great Britain many of the firms who supply the refractory requirements of the glass trade are exceptionally backward in applying technical knowledge to their trade; consequently, progress is somewhat retarded in the glass trade as far as the refractory materials are concerned. So obstinate is this ignorance of science that quite recently one well-known firm replied to an inquiry for samples of fire-clays to be sent for important research work then being undertaken upon the resources of the country, stating “that, as their clay product was perfect, and research work was quite unnecessary.” It often turns out that their conservatism is simply a cloak to hide ignorance, as it is quite evident to any technicist that there is ample scope for improvement in the present goods on the market, and such an open opportunity for a scientific investigation into the nature of their fire-clays, however well known they may be, should be welcomed with delight, and every facility and assistance offered for research chemists to improve their material, and apply tests with the object of developing the best properties of such refractories for special purposes.