The effect of fluxes on a clay depends also on the fineness of grain of the clay as a whole and of the fluxes in particular. A coarse-grained clay will stand more fluxes than a fine one, especially if the fluxes be coarse. It will be seen then that a fire clay is likely to be refractory in propor tion as the percentage of alumina is high and that of the fluxes low. It is evident also that a single flux will have less proportionate effect than a mixture and that fineness of grain is a determining factor of consid erable importance. A study of the table of analyses of fire clays (§ 17) will bring out these facts.
§ 38. The temperature of fusion also depends upon the character of "the fire to which it is subjected. Clays which withstand a high temper ature when exposed to a strongly oxidizing flame will give way at a much lower degree of heat if exposed to reducing conditions.
Second, a fire clay must sustain rapid changes of temperature without shattering. This property depends on its homogeneity and openness of structure. If some of the ingredients of a clay expand more with a given increase in temperature than others the tendency will be for those which expand most to push the others away and weaken the whole structure. For this reason the more homogeneous clays are more valuable in this respect. It should also be noted that the larger the grains of the im purities the more harmful will they be in this way.
Again, so far as this property is concerned, it is desirable that the clay when burned should have an open porous structure. Clay is a poor conductor of heat, and if its structure is dense it takes a long time for the center of a block to become as hot as the surface. The difference in temperature between the outside and inside of a block is sometimes considerable, and the difference in expansion has a strong tendency to shatter it. If, on the other hand, the block is porous, the heat finds its way through much more readily and the tendency to fracture is less. No generally accepted test has been proposed to demonstrate the proper ties of a clay in this respect although some prominent French engineers claim that no clay should rank as a fire clay whose products shatter when heated to redness and immediately plunged in cold water—a very severe test.
§ 39. Third, fire clays are often made into crucibles in which glass or metals are to be fused. They are also often so placed that they are exposed to the action of ashes and various furnace gases. In order to withstand these conditions they require a certain degree of density and must have a chemical composition such that they will not be readily attacked by the substances to which they are exposed. First, for the
above uses the clay should contain an amount of flux just sufficient to cause it to contract and fill the pore spaces during burning so that the wares will not be readily penetrated by the liquids or gases to which they are to be exposed. It will be noticed that this density of structure is a quality which is not desirable in a clay which is to be subjected to violent changes of temperature. Second, if the liquids or gases to which the clays are exposed are strongly basic they will attack a clay rich in silica much more readily than one rich in alumina. On the other hand, if they are strongly acid the aluminous clays will be more easily attacked than the siliceous ones.
From the above it may be seen that the term fire clay does not stand for a single kind of clay, but for a group whose only common property is the ability to withstand relatively high temperatures, which means that they must be high-grade clays relatively free from all fluxing in gredients. Any clay that is sufficiently pure may be used as a fire clay, but the better grades of kaolin and ball clay are generally too valuable for such use. Again, as shales or slates are only clays whose physical structures have been somewhat changed by pressure or heat or both acting together, some of them should be and are sufficiently pure to be used for the of refractory wares.
As weathering and leaching tend to remove the most active of the fluxing materials in clay they must improve its refractory properties also, except in so far as they make its component particles finer. It is then a question of balance between these two effects whether its refrac tory properties will be improved by these processes or otherwise. There is a method of purification which is thought to be responsible for the formation of many fire clays. Many of those most widely used lie im mediately below deposits of coal, and it is generally believed that they were laid down in swamps before the coal was formed; that they were soils which supported and nourished the coal plants; and that these plants abstracted from the soil considerable portions of the alkalies and alkaline earths as well as iron and other fluxing materials and built them into the coal, thus materially increasing the refractoriness of the clay. Probably more is made of this process in accounting for the refractoriness of clays than should be, but nevertheless it is true that any process which removes fluxes from clay makes it more refractory, and growing plants have this effect to a degree.