Deposits of ball clays occur as outwash aprons at the base of highlands containing deposits of kaolin. When running water passes over places where beds of kaolin come to the surface the clay will be picked up and carried to the base of the slope. On the way the lumps will disintegrate and the flakes and granules of kaolin be separated and more or less broken up, and these processes will develop plasticity in the mass. On the way also the residual kaolin will be assorted and at the base of the slope the quartz and fragments of undecomposed minerals will be de posited before the clay particles, and these. last will build up aprons of 'pure plastic clay derived from non-plastic kaolin mixed with a vary ing proportion of undecomposed material.
In a preceding section (§ 25), we have explained how clay may come to enter into the composition of limestone, and in another (§ 28), how this limestone when it decomposes leaves the clay behind as a residual product. If, as sometimes happens, the clay which was built into the .limestone was nearly or quite pure kaolin and the limestone itself was pure carbonate of lime, the residual clay would be a fine quality of ball clay. If, as is often the case, the limestone decomposed unevenly and cavities or caverns were formed in the mass which increased in size until the roof was too heavy to support its own weight and consequently broke down forming more or less conical depressions on the surface, called sink-holes, these clays as they were formed would be washed into and collect in the sink-holes making considerable deposits of the pur est ball clay.
When beds of kaolin or of flint clay (described in § 40) are exposed to the action of the weather for a sufficient length of time disintegration takes place and the non-plastic material becomes plastic, but in this case any undecomposed granules in the original mass will be retained in the ball clay.
It rarely happens that when deposits of clay containing impurities which can be made soluble by weathering are leached, the foreign mat ters are carried away and pure ball clays left behind.
As may be inferred, deposits of a good grade of ball clay are not very common because the very agents which produce them tend to bring into them impurities of many kinds and so render them less pure than the beds from which they were derived. It is but seldom, although it sometimes occurs, that earth-water or surface water is pure enough to cause it to work the other way and produce a purer instead of a less pure deposit.
Fire Clays.—§ 36. A fire clay is one which will withstand a high temperature without softening to such an extent as to become mis shapen even when subjected to considerable pressure; which will endure rapid changes of temperature without shattering; whose wares have sufficient density to impede the passage of gases or liquids which would attack it, and a chemical composition such that it will not readily unite with the gases which it is likely to meet in use.
It will be seen that each of these qualities is variable and that in consequence no fixed definition of a fire clay can be formulated. Let us
consider them in order. First it must withstand high temperatures without material softening. Authors assign widely different meanings to the expression high temperatures as applied to fire clays, and no agreement has been reached as to what temperature a clay must be able to withstand in order to merit a place in this class. Each user re gards any clay which will bear the highest degree of heat which he uses as a fire clay.
The ability of a clay to withstand a high temperature depends upon its chemical composition and upon its physical constitution.
The temperature at which a:clay softens is governed by the presence or absence of impurities which soften or melt at a temperature lower than that which would produce a like effect in the clay itself. It is a curious fact that the softening or melting of one substance often brings about a similar change in contiguous materials which would remain un affected except for the presence of the more easily melted material.. These less resistant ingredients of a clay are called fluxes. The more common fluxes are in order the alkalies, potash and soda, the alkaline earths, lime and magnesia, protoxide of iron, and to a certain extent sesquioxide of iron and silica.
§ 37. It has been found that pure alumina will withstand a tempera ture higher than that required to fuse Seger's cone 36, and pure silica withstands cone 35, but if a small amount of finely pulverized silica be mixed with alumina it induces slight fusion at a temperature less than that required to fuse either pure alumina or pure silica. If we increase the proportion of silica this effect increases also, but so slowly as to be hardly perceptible until the mixture contains 25 per cent of silica. From this point the effect increases rapidly with the increase of silica, until the proportion of alumina 10 to silica 90 is reached. This mix ture melts at cone 30. Further addition of silica causes the mixture to become more and more refractory. It is thus seen that silica al though a very refractory substance in itself becomes a flux when finely divided and added to alumina. In the same way lime and magnesia, which are among the most refractory substances known when pure, be come exceedingly active fluxes when mixed with silica or alumina or both. Iron when in the condition of the sesquioxide or iron rust does not act vigorously as a flux but the protoxide is very active. Potash and soda are the most active of the common fluxes. Another curious fact is that a given percentage of mixed fluxes will produce a more marked effect than the same amount of any one of them and that the effect will be greater the larger the number of different fluxes contained in the mixture. It will thus be seen that the fluxing effect of im purities in a clay depends not only on the amount of fluxes present or upon their amount and kind, but upon the number of different kinds as well.