The first thing that will strike the reader on looking at these results on Welsh materials, is their uniform composition as compared with the clays from Newcastle. Yet there is as much as 10.40 per cent. of iron in sample No. 1, which cannot be a first-rate clay. Its proportion of silica to alumina is, however, excellent, and, as in sample No. 3, the amount of soda and magnesia is not excessive. The soda in sample No. 2 (which acts somewhat like lime in the kiln) taken together with the magnesia and iron in the same material, is too much for a first-class clay, and would have to be suitably modified before good results could be obtained. On the whole, it is possible that sample No. 3 would yield the best results from the chemical standpoint.
We should not forget that remarkable substance of which the well-known Dinas bricks are made. The proportion of silica present ranges from about 96 to 99 per cent., the remainder consisting principally of alumina, though traces of iron, lime, and magnesia frequently occur. There is not, of course, sufficient natural flux for this “clay,” so a small proportion (2.5 to 3 per cent.) of lime is added, which produces the desired effect. In other words, if we can obtain a pure siliceous sand, with hardly any lime, iron or magnesia in it, we have the material of which the better kinds of fire-bricks are made. Such sandy earths are not uncommon in the South of England, but strange to relate, they are not used for the purpose indicated.
The earths from which the superior Stourbridge bricks are made, are approximately of the following chemical composition:—Silica, 64.10; alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00 per cent. It will be observed that the proportion of iron and magnesia here is very small, whilst lime is altogether absent. It is a most excellent earth for the purposes for which it is used, and the chemical results may be taken as a standard for that class of material. Another Stourbridge earth yields as much as 4.14 per cent. of iron, however, whilst its proportion of silica is lower, 51.80, and alumina higher, 30.40, which serves to remind us of the variability of even good earths used in the manufacture of fire-clay goods.
Let us now turn to the consideration of pottery clays, of which the following results may be taken as typical:—
Chemical Composition of Pottery Clays.
| 1 | 2 | 3 | |
|---|---|---|---|
| Silica | 46.38 | 49.44 | 58.07 |
| Alumina | 38.04 | 34.26 | 27.38 |
| Iron oxide | 1.04 | 7.74 | 3.30 |
| Lime | 1.20 | 1.48 | .50 |
| Magnesia | trace | 1.94 | trace |
| Water | 13.57 | 5.14 | 10.30 |
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.