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 mould, 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 fire-clay quarries. When the mould is filled the surplus clay is cut off and the wooden frame is lifted up, leaving the clay block on the floor. The empty mould 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 tempera-tuie rises and approaches red heat, the rate of heating should be retarded to allow proper oxidisation to take place throughout the structure of the fire-clay blocks, and prevent black cores 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 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 oxidisation period, during which any organic carbonaceous matter present in the clay is expelled. During this stage in burning, fire-clay goods require an extended time, so as to allow for the heated air to permeate and get to the interior portions of the blocks and oxidise the cores; otherwise the blocks are badly burnt.
After the oxidisation 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 fire-clay 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, fire-clay 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 fire-clay may show a 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 more suitable chemical constitution. The size of grain in both the burnt clay and raw clay used in the mixtures for making glass house 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, so as 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 own 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 fire-clay firm replied to an inquiry for samples of fire-clays to be sent for important research work then being undertaken upon the fire-clay resources of the country, stating " that, as their clay product was perfect, any 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 fire-clay 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.