Some of the chief qualifications, from a chemical point of view, of earths suitable for making pottery, is the proportion and potentiality of the colouring matters present. Where the pottery is to be glazed, that is not so important; but with ordinary unglazed ware, colour and uniformity are two highly essential desiderata. We know that the temperature employed will modify the tint, but under similar conditions the clays alluded to in the above table will give, approximately, the following results. Sample No. 1 is typical of an excellent blue pottery clay, which burns white. It contains more alumina than is commonly met with in such materials, in which respect it differs markedly also from the fire-clays just described. The proportion of oxide of iron is very small, not sufficient to perceptibly colour the finished product, though, no doubt, on careful examination it would be seen not to be perfectly white. The latitude of the term “white” is pretty considerable with clayworkers, as the reader is probably aware. The pottery clay (also used for bricks) referred to in the middle column, is brown in colour; it is an ordinary kind, used primarily for black and common red ware. The proportion of iron is high, and considerable quantities of both lime and magnesia exist. As might naturally be expected of such material, it will not bear exposure to great heat, though that might be regarded as a qualification in some brick and pottery yards. The proportion of silica is high in sample No. 3, which appertains to a common yellow clay, with, possibly, some siliceous sand in it. The amount of alumina is correspondingly low, but the iron oxide is not excessive--for a common pottery clay. It is used for the manufacture of coarse ware, and burns yellow. The chemical composition of earths used for terra-cotta and bricks of that substance is so variable, that without going into each case specifically it would be impossible to convey an adequate idea. It may be stated generally that it is not one whit less important to consider the composition of the raw earths for ordinary brickmaking, than in respect of that for high-class bricks and pottery. An excellent earth, from the neighbourhood of Ruabon, is of the following composition:-- _Chemical Composition of Ruabon Clay._ Silica 63.00 Alumina 20.10 Sesquioxide of iron 4.84 Protoxide of iron 1.51 Potash 2.37 Soda 3.10 Combined water 3.54 Moisture 1.54