Flint Clay.—§ 40. There is a class of clays of almost stony hardness having a conchoidal fracture and a structure so like flint that they are commonly known as flint clays. This flinty condition is believed to be due to a process of cementation, the clays having at some time stood below the level of ground-water under conditions which favored the pre cipitation of such salts as the ground-water carried in solution. This precipitated material cemented the clay particles and in connection with the weight of the overlying rocks induced the flint-like structure. There is then no necessary relation between the flinty structure and their chemical composition or any physical property except plasticity. All flint clays are non-plastic. Some are rendered more or less plastic by the ordinary grinding and kneading to which clays are subjected, but some are not. Some authorities assert that no mechanical or physical process to which these clays can be subjected will render some of them plastic. It is probable however that wet grinding and kneading if sufficiently prolonged will develop plasticity in any of them. It seems to be commonly accepted that all flint clays are refractory, but this is not true, for any clay will become flinty if placed under proper condi tions. It is true, however, that most of the deposits of flint clay which are used commercially are high-grade clays and are consequently quite refractory. The preparation of the poorer grades is too expensive to per mit their use.
Some very curious deposits are found among the eroded limestones of southeastern Missouri. • They appear to have formed in sink-holes like the ball clays of the same region, but owing probably to defective drain age the clay particles have been cemented and the flinty structure de veloped. Where these deposits have been exposed to the weather and to leaching they have developed plasticity and have become ball clays. Most of the high-grade flint clays could be used in the manufacture of white ware if it were not for the excessive shrinkage and the cost of prepara tion. As it is, they are rarely used except in the manufacture of fire brick or other refractory wares. The table of analyses (§ 17) shows the composition of some of the better known flint clays. It should not be inferred, however, that all flint clays are as pure as those here given.
Pottery Clays.—§ 41. This term does not stand for any particular group of clays or for those possessing any particular properties except that they must be sufficiently plastic to be formed into the ordinary pot ter's wares and must be capable of being readily burned to a pleasing color. The term then covers any high or low-grade clay that has these characteristics.
Vitrifying Clays.—§ 42. A vitrifying clay is one whose ingredients melt at widely different temperatures and in which the substances melt ing at relatively low temperatures are sufficient in amount when fused to fill all the voids and form a tough, nearly or quite impervious mass.
It is desirable that there be several of these substances which fuse at regularly increasing temperatures, for in that case the ingredient which fuses first at once attacks that whose melting point is next higher and reduces it to a pasty condition before its fusion temperature is really reached. As the heat is increased this pasty mass passes into a fused condition and immediately attacks that ingredient whose melting point is next higher, converting it into a paste. This process continues as the temperature rises• until enough of the pasty material is produced to fill all the voids. If the now solid mass is cooled slowly enough this almost fused material will become semi-crystalline and will show on fracture a dull stony rather than a lustrous or glassy surface. This semi-crystal line condition of the partially fused material is one of the factors which makes the body tough when burned.
It will be seen from the above that a vitrifying clay must contain enough relatively refractory material to form a skeleton or framework which will support the ware and keep it from changing its form dur ing the vitrifying process. The skeleton may be made up of grains of refractory clay; of relatively coarse fragments of quartz, which, however, must not be too large or the resulting burned body will be too porous; or even of coarsely ground particles of the same clay whose more finely pulverized portions become pasty and fill the vacant spaces between the grains, always provided that the range between the melting points of the more and less refractory or of the finer and coarser material shall be 300° or more. This range must be demanded until we have learned to build kilns which dis'6ribute heat more evenly than those now in use.
That portion of a vitrifying clay which is expected to form the paste to fill the voids should not contain too large a percentage of carbonates or other compounds which give off gases at high heats, because these gases in their effort to escape tend to form "blebs" or vacant spaces in the mass and thus undo the very thing we are trying to accomplish. Again all the particles of carbonate of lime tend to give up their car bonic acid at the same temperature. This disassociation leaves the cal cium in what is called the nascent state in which condition it attacks the other ingredients vigorously and tends to produce compounds which soften at relatively low heat, and so the presence of a large amount of nascent calcium at any one time has a tendency to cause the whole mass to soften suddenly and flow so quickly that it cannot be readily controlled. The silicates of lime and the various compounds of potash, soda and iron, although the last somewhat resembles lime in its action, soften more slowly and so give greater latitude in burning, and for this reason are considered more desirable.