It will be convenient to consider the behaviour of these two minerals in the kiln together, as the difference between them from that point of view is practically nil. Under the action of the intense heat met with there, they become partially decomposed; oxide of iron and basic sulphides of iron remain. When, at a subsequent period, bricks containing these substances are exposed to the action of the weather, oxidation takes place, sulphate of iron and sometimes of lime are formed, which on crystallizing expand with considerable force and split or crack the brick. From this it is evident that sulphide of iron in any form is not to be tolerated in brick manufacture, and if the earth used in the first place contains much, it must be removed in the preparing process. If permitted to remain, it is impossible to obtain either a durable, or a good coloured brick.
CHAPTER VI.
MINERALS: THEIR BEHAVIOUR IN THE KILN (continued).
CALCITE, ARAGONITE, &c.
Carbonate of lime may occur in a crystalline form, or as earthy substances, and many varieties of it are found in clays used by the brickmaker. The commonest are calcite, aragonite, and a white earth.
Calcite, known also as calc-spar, crystallises in the hexagonal system, though true hexagons are not very common. It occurs principally as rhombohedra and scalenohedra, with variations therefrom; also fibrous, lamellar, granular, compact, nodular, and stalactitic. When pure, calcite is colourless and usually transparent, but when mixed with iron or other mineral colouring matter it commonly assumes yellow and brown tints.
Aragonite is also a crystalline form of carbonate of lime, but is by no means as common in Nature as calcite. It crystallises in the rhombic system, which assists the mineralogist to distinguish it from the last-mentioned mineral, from which it differs also in being harder and of higher specific gravity. Aragonite may occur as globular masses, or as incrusting other substances, or in the stalactitic form. It is sometimes white, but more often yellowish, or grey, and it is not, commonly, as transparent as calcite, whilst it often possesses one to two per cent. of carbonate of strontia, or other impurity.
It is generally stated that carbonate of lime, when deposited from cold solutions, crystallizes in hexagonal (calcite), and when from warm solutions, in rhombic (aragonite) forms. No doubt, on the whole, that is the case; but we ought not to forget that many marine organisms make their hard parts of aragonite, which, under the circumstances, is certainly not obtained from warm solutions. These crystalline forms of carbonate of lime are both of them found in fossil shells and the like in clays, and in not a few instances the calcareous constituent found in the brick-earth is present almost exclusively in the fossils, which are ground up with the rest in preparing the material for the moulding machine.
When present as hard crystalline lumps or pebbles, they have been derived from the destruction of limestones, and are then the greatest nuisance imaginable to the brickmaker and the most dangerous constituent at the same time. With proper machinery these hard lumps may be ground down to fine particles, but they are even then only to be admitted into the earth on sufferance. The best plan, without doubt, is to remove them altogether from the raw earth. They are commonly met with in what the geologist calls “boulder clay”—a deposit owing its origin to glaciers and icebergs. Very often the pebbles alluded to are not crystalline, but of an earthy character, as is the case when made of chalk. In the semi-dry process of manufacture, it is next to impossible to incorporate the ground-up particles of carbonate lime sufficiently well to result in the production of such a homogeneous earth as is desirable for making a first-class brick.
In sandy clays or loams, and in a few stiff clays used for brickmaking, certain remarkable concretions called “race” are found, the deleterious properties whereof are so well known to the average brickmaker that he carefully avoids the particular strata in which they occur. It is fortunate that these concretions have a habit of being confined to narrow limits along definite horizons in the brickyard section, so that they may be readily discarded in working. But that is not always the case, and little nodules of “race” are usually more or less frequent also in the beds above and below the horizons referred to. They are composed wholly of carbonate of lime, and their general effect in the kiln, and afterwards, will presently be explained. Other forms of concretions are known as “septaria,”—tabular or rounded masses of argillaceous limestone found in practically all stiff clays. These are often of enormous size, and are disposed in regular lines which the field geologist takes to indicate bedding planes in the clay—otherwise often very difficult to make out. In certain stiff clays little pellets of the same substance are found. The larger septaria have commonly been cracked in various directions, the fissures being subsequently filled with calcite.