If we assume that the minute particles of clay substance possess the round shape claimed by Dr. Koenig, this behavior is easily explained, as the " water of shrinkage " envelops these particles, and upon evaporation will allow them to approach, until each touches the other at six points on the surface. The intervals between all other points will still be filled with "water of the pores ;" and its evaporation cannot produce any shrink age of the clay. Now, Dr. Koenig claims that it is an important rule for the potter that the number and sizes of the pores are independent of the water contained in the clay, and is constant for all plastic clays ; and, further, that the cubical shrinkage is equal to the volume of water lost by evaporation up to the limit of "linear shrinkage." Aron contends, further, that if the pur est clay is mixed with fine quartz sand, the shrinkage will in crease up to a certain point, which he terms the " point of greatest density." From this point the shrinkage decreases again, with increasing leanness, while the porousness increases. On submitting clay to a red heat, two molecules of water are driven off from the silica of alumina at that temperature, and, as a result, the clay shrinks a second time. This is known as the " fire shrinkage," and can be neutralized by the addition of finely-divided sand, and may even be made to produce a slight expansion. Finely pulverized chalk is also an excellent mater ial to counteract the "fire shrinkage" in clays.
The ordinary yellow brick clays contain iron in the state of oxide and carbonate chemically combined with water, forming what are known in chemistry as hydrates. The expulsion of this water in the process of burning imparts a red color, due to the conversion of the hydrated oxides of iron into the an hydrous form. The principal constituent in brick clay, and that upon which its plasticity depends, is the chemical combi nation of silica and alumina, more particularly described under the head of " Kaolin." This constituent used alone shrinks and cracks in drying, warps and becomes very hard when baked. Silica is also present in nearly all clays in an uncom bined state, such as sand. A proper proportion of sand pre vents cracking, shrinkage and warping, and furnishes silica necessary for a partial fusion of the materials which increases the strength of the brick. The sand also makes the brick
more shapely and equable in texture.; but an excess of sand in clay renders the brick made from it too brittle. A small quantity of carbonate of lime has a beneficial effect upon brick clay in two ways—it lessens the contraction of the newly-made brick in drying, and acts as a flux in the kiln by the formation of silicate of lime, which binds the particles together. It is evident from this that an excess of carbonate of lime in the clay would cause the brick to melt. and lose its shape. Iron pyrites in a brick clay are objectionable ; also the presence of carbonaceous matter to any considerable extent, as a black discoloration, similar to that produced in brick in proximity to chimney flues, is likely to occur.
Common salt is nearly always present in minute quantities in clay. In that near the seashore the amount is apt to be so great that brick made from it are certain to be of a poor qual ity. Salt melts readily and glazes the outside of the brick, and the heat cannot be raised or maintained sufficiently long to burn them to the core, or into good hard brick ; as a conse quence they are soft, and from the presence of the decomposed salts of magnesia and soda, are always damp, owing to the tendency of these salts to absorb moisture from the atmosphere. The presence of the alkaline carbonates in clay to any notable extent, prevents its being used as a brick clay, the alkali caus ing the material to melt readily.
Rare minerals containing such metals as cobalt, copper, zinc, and such salts as phosphate of iron, are met in clays, but are exceptions and are of no importance in practical work. Thus it appears that mineralogically, clays are kaolinite, mixed with sand, colored by iron and organic matter, and showing varying amounts of feldspar, mica, and other silicates and titanates. The chemical investigation of a clay should en deavor to present these facts, besides grouping those bodies together which are similar in action and effect.