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 |
The proportion of silica in this is higher than in many clays used for brick- or terra-cotta making, but the alkalis, potash and soda, are in strong force, so that any refractoriness on the part of the silica is soon subdued in the kiln. The iron, also, is in abundance. The principal colouring ingredient is the sesquioxide, and we can quite understand the manufacturer when he informs us that, in spite of the rich tint of the goods produced, nothing is artificially mixed with this clay to produce such a result. We may call attention to the method of expressing the chemical analysis in this case, which might be copied to advantage. In the first place, the combined and the uncombined iron are separately shown, or rather the degree of combination is indicated; and secondly, the proportion of water chemically combined is differentiated from that which has simply soaked into the clay, though expelled, following a well-known practice of chemists, prior to commencing the analysis proper. It is of very little use giving the amount of water, unless the proportions are divided in this manner. In the result given above we learn that there is very little chance of the clay shrinking, as it only contains moisture to the extent of 1.54 per cent.; but if that had been added to the water combined, we should have had a result of 5.08 per cent., which is not nearly so clear in its meaning. We may add that the Ruabon earth referred to is utilised also in the manufacture of tesselated and encaustic tiles.
In regard to the composition of earths employed in the manufacture of the commoner kinds of bricks, we may give the following examples:—
Chemical Composition of Common Brick-earths.
| Silica. | Alumina and Iron. | Lime. | Magnesia. | Manganese. | Water and Loss. | |
|---|---|---|---|---|---|---|
| Reddish-brown brick clay | 52.6 | 30.8 | 3.4 | 2.8 | 1.4 | 9.0 |
| Red-brick clay | 50.4 | 24.0 | 2.7 | 1.3 | — | 21.6 |
| Common brick-earth | 33.0 | 11.2 | 39.8 | 6.0 | — | 10.0 |
| Sandy-clay (loam) | 60.2 | 24.0 | 2.4 | 1.6 | — | 11.8 |
Reviewing these results, it will be noted that the brown colouring imparted to the brick in the first-mentioned example is due, to a large extent, to the presence of manganese, a rather uncommon feature in brick-earths, except where these have resulted from the denudation of iron-producing rocks rich in manganese. It will be noticed also that the proportion of water is not high for a common earth, and it must be a fairly easy material to deal with. There seem to be some possibilities in it that might, in competent hands, lead to higher things. The amount of lime and magnesia is, however, a rather serious one for a first-class clay.
In regard to the “red-brick” clay, an essential feature is the comparative absence of lime, and it would, no doubt, make “rubbers” of an ordinary kind. Unfortunately, in the results given, the iron is not separated from the alumina, but clearly the latter is very small in amount, and the results refer to a sandy material. The proportion of water is disastrous for the employment of this earth by unskilful hands. In drying, the greatest care would have to be exercised to prevent undue shrinking, and, in any case, the earth would have to be very thoroughly incorporated to make a really serviceable brick. It is with earths of this character that the majority of brickmakers in embryo come to grief; they know not how to handle them successfully, and twisting, warping, cracking, and “bursting” follow as a natural consequence. It is a common and treacherous material, that could only be made to succeed by perseverance and wide experience.
The “common brick-earth,” as will be seen, contains an abnormal quantity of lime, and doubtless refers to a marl, though not much alumina is shown. Malm bricks could be made from it, and the product would have to be burned at a low temperature. For bricks useful to the “jerry-builder” this earth could be strongly recommended. It was, no doubt, mainly derived from limestone rocks; and, judging from the high proportion of magnesia, probably from within a watershed composed to some extent of magnesian limestone.