The “sandy-clay” or loam is of a very common type, and produces light-red bricks. There is much in common between this and the “red-brick clay” previously referred to.
The practice resorted to in various parts of the world of making bricks from slate débris, although not hitherto adopted to any large extent in this country, merits some description in this place. Slates may be regarded as a highly compressed clay, the original structure of which has been materially modified by the great pressure exerted during their manufacture in Nature’s laboratory. To all intents and purposes they are silicates of alumina, plus iron, lime, magnesia, and so on, and have, practically, the same range of variation as have ordinary clays. But during their manufacture, and subsequently, certain adventitious mineral matter has been frequently introduced, as may be gathered from the following results:—
Chemical Composition of Slates.
| 1 | 2 | 3 | 4 | |
|---|---|---|---|---|
| Silica | 60.50 | 60.15 | 48.00 | 50.88 |
| Alumina | 19.70 | 24.20 | 26.00 | 14.12 |
| Iron (protoxide) | 7.83 | 5.83 | — | 9.96 |
| „ (sesquioxide) | — | 1.82 | — | — |
| „ | — | — | 14.00 | — |
| Lime | 1.12 | — | 4.00 | 8.72 |
| Magnesia | 2.20 | — | 8.00 | 8.67 |
| Potash | 3.18 | — | — | .88 |
| Soda | 2.20 | — | — | — |
| Alkalis (not determined) | — | 4.28 | — | — |
| Carbon dioxide | — | — | — | 6.47 |
| Water, &c. | 3.30 | 3.72 | — | — |
Analysis No. 1 refers to a blue Welsh roofing slate of Cambrian age. It is quite certain that the large proportion of alkalis present would render this material unsuitable for brickmaking, except for the commonest kinds of bricks. The iron, again, is very large in quantity, whilst the amount of alumina is low. We could not recommend this slate for good bricks under any consideration.
Analysis No. 2 is of a dark-blue slate from Llangynog, in North Wales. The amount of iron present is high, but from the low content of alkalis this material, under proper treatment, should make fairly good bricks. The ferruginous constituent is too powerful, however, for fire-bricks to be made of this slate.
Analysis No. 3, of a purple slate from Nantlle, shows a remarkable diminution in silica and a corresponding increase in iron. Lime and magnesia being present to such an enormous extent, taken in conjunction with the iron, would render this slate absolutely useless for brickmaking. There is not a redeeming feature about it.
Analysis No. 4, which refers to a green Westmorland slate, has a low percentage of alumina and very large quantities of iron, lime, and magnesia. Only bricks of an exceedingly inferior quality could result from such material.
Summing up the general characteristics of these slates from the chemical aspect, one would say that none of them are very suitable for high-class bricks. No. 2 is the best. Several minor differences will be observed between the results quoted and those referring to ordinary brick-earths—in particular, the distribution of the alkalis. A general impression is abroad that any purple slate will do for brickmaking, and manufacturers do not yet seem to have realised that the chemical nature of slates is as variable as of brick-earths. That may account for the difficulties experienced in many cases in turning out a satisfactory material. The microscope is of much use in this connexion, however, and the practical effects of chemical analyses are not always as bad as they seem at first sight.
The remainder of this chapter will be devoted to the consideration of rarer kinds of brick-earth and other raw earths used principally in the manufacture of bricks for special purposes, or as pointing to certain anomalies. As an example of what some manufacturers can do, we may quote the chemical composition of a peculiar brick-earth employed in Zurich, in Switzerland:—