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 hexagonal50 (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 he51 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.
coprolites are impure varieties of phosphate of lime, and the term should, properly speaking, be restricted to a substance of organic origin,—the fossilised excrement of animals. but the name is now loosely employed to designate phosphatic concretions in general, such as are commonly found in stiff clays, in certain “greensands,” and in other sedimentary deposits. the dark brown phosphate of lime has formed on and often completely envelopes many fossils; in certain cases it has in fact been utilised as a petrifying medium, in which form it ordinarily occurs in the thick black clays of peterborough, cambridge, the gault of kent, surrey, etc.
summing up the effects of carbonates and other52 kinds of lime in the kiln, it may be at once said that when present in any other form than as extremely minute particles, they are distinctly to be avoided. the small pellets and large pebbles especially are to be avoided, for the following reasons. carbonate of lime is made up of lime and carbonic acid; if a lump of this be subjected to great heat and thus calcined, the carbonic acid is driven off, escaping by means of flues, the open chimney, or kiln. the product is lime pure and simple—ordinary builders’ lime. everyone knows that on the addition of water builders’ lime becomes “slacked,” and eventually, after a fashion, “sets.” precisely the same thing occurs in the brick-kiln. the raw brick is often composed of pieces of chalk or other limestone, in limestone districts and in areas where boulder clays are largely employed for brickmaking. on being subjected to the heat of the kiln these pieces are promptly reduced to the condition of lime. during the process of conversion considerable expansion takes place, and subsequently contraction, leading to the formation of cracks radiating from the fragments of limestone, the homogeneity of the bricks being at once destroyed. apart from this, when placed in the open air the lime becomes slacked, and the quality of the brick is seriously impaired.
lime is a highly refractory substance, strongly basic in character, and forms fusible compounds with silica and other acid bodies. it is, therefore, useful as a flux in many earths used in brickmaking, being added to them expressly for that purpose, to the general improvement of the brick. the celebrated dinas bricks, for instance, are composed of a highly refractory earth containing about 97 per cent. silica, the remainder being lime, oxide of iron, alumina, alkali and water. to53 render this material fusible and so as to make refractory bricks, from 1 to 3 per cent. of lime is added.
but what we more particularly desire to draw the reader’s attention to at the present stage, is not the employment of lime in making fire-bricks so much as its mixture with ordinary brick-earth, as in the manufacture of malm bricks. sometimes the mixture has been effected by nature, as is the case with true marls; but the brickmaker does not care so much for these, as without considerable and expensive artificial assistance they do not often make readily saleable bricks. the common practice is, briefly, to grind chalk or similar earthy limestone in the wet state, and then to introduce it to the brick-earth with which it is thoroughly incorporated; and there are many ways of doing this, which we shall not attempt to describe now. the object of adding chalk to the brick-earth is twofold; in the first place it assists in diminishing the contraction of the brick on drying, i.e., before burning; and secondly, it acts as a flux in the kiln by combining with the free silica, or the silicates, in the earth. undoubtedly the second is, theoretically, its chief function; but its beneficial effects in that direction are largely marred by insufficient burning, whereby a large proportion of the chalk is not actively engaged, as may be seen on examining the majority of malm bricks with the microscope. indeed, the eagerness to save fuel, and to turn out the bricks as rapidly as possible, often leads to the chalk particles being utterly useless. and, if we may judge from conversations with several brickmakers, it would seem that the real reason why the limestone is used at all is unknown to them, except that it produces bricks of a saleable colour. this question of colour is the all-predominating one with most malm brickmakers.
54 we said just now that the fragments of limestone in the raw brick are reduced to lime on being burnt; some of the latter, however, as may be anticipated from our subsequent remarks, is engaged in forming a flux wherever possible in the immediate neighbourhood of such fragments: it is the “kernel” that is left which becomes “slacked,” and weakens the brick. the object of utilising the smallest particles only of the carbonate of lime is thus obvious; and if it were possible to use ordinary builders’ lime instead of carbonate of lime, the result would be better still. the difficulty in utilising builders’ lime is, of course, its certainty of slacking during the preparation of the brick-earth with which it would have to be thoroughly incorporated.
selenite.
the “petrified water” of the brickmaker. it is a crystalline form of gypsum—a hydrous sulphate of lime, occurring in large quantities in the commonest clays used in brickmaking. large and beautiful crystals, some of them radiating from a central point, are found in the london clay, kimeridge clay, oxford clay, &c. by expelling the water from selenite, or gypsum, plaster of paris may be prepared. in the kiln, therefore, it is important that this constituent be as finely ground as possible, so as to localise the effects of the anhydrous sulphate on being moistened subsequently. in hard burnt bricks, no doubt, a great deal of it is effectively used as a flux to other constituents of the clay; but in by far the larger quantity of bricks this sulphate is reduced to fine powdery particles easily picked out as being softer and lighter in tint than the remaining constituents.55 the weather-resisting qualities of the brick are naturally, not improved when much baked selenite is present; and the colour of the whole is apt to become variegated—that is, in a fairly soft brick.
dolomite.
dolomite is, chemically, composed of the carbonates of lime and magnesia in about equal proportions. it is found as rhombohedral crystals, the faces of which are often curved; also in granular and massive conditions. its prevailing colour is light yellow both in crystals and rock masses, but, as with most other minerals, impurities occasionally make it assume other tints, principally red and green. carbonate of iron is frequently present, sometimes to such an extent as to entirely alter the character of the substance. as separate crystals dolomite has very little interest for us, though rarely it may take the place of calcite or aragonite in the fossils of brick-earths and clays. but in its massive condition, as magnesian limestone, it is of increasing importance to the brickmaker. for many years it has been utilised in the manufacture of basic bricks, though at the present moment the market in these materials is attentively looking at the possibilities of the next mineral to be described.
magnesite.
magnesite is pure carbonate of magnesia—that is, magnesia = 47.6, and carbonic acid 52.4 per cent. it usually occurs massive or fibrous, but sometimes granular, and its fine rhombohedral crystals are well known. like dolomite, its prevailing tint is yellow or light brown, but, when very pure, is as white as snow. it is usually56 associated with serpentine rocks. in the kiln it is highly refractory, and behaves very much in the same way as lime—forming fusible compounds with silica and silicates. for the higher grades of basic bricks it is at this moment largely exploited in the few localities where it occurs in paying quantities. a few years since, investigation to determine the best basic refractory material was actively prosecuted in germany, and magnesia, preheated at the highest white heat, was awarded the palm. magnesite, when calcined, yields magnesia, which, however, still contains the impurities that might have been present in the raw material. an average percentage composition of the magnesite of commerce shows it to contain magnesia 45, carbonic acid 50, lime 1.5, protoxide of iron 1.6, the remainder being silica, alumina, and protoxide of manganese. the presence of silica in magnesite is an objection, because it is liable to have a fluxing effect at high temperatures.
magnesite has been found in paying quantities in california, styria, and recently in greece. in eub?a, in the last-mentioned country, the mineral occurs in lodes which, near krimasi, are worked on two levels 30 to 40 feet from the top, and dipping at an angle of about 70 degrees. the general average of the lode gives 88 per cent. of carbonate of magnesia, and the substance is peculiarly suitable for the manufacture of basic bricks. a novelty with the raw material is that the proprietors sell either by guaranteed degree, or degree of analysis, the former being 95 per cent. of pure magnesia, whilst the latter often gives as much as 97.8 per cent. in inferior grades the principal increase is in the proportion of silica.
salt.
chloride of sodium, or common salt, is present in57 many natural clays, especially (in england) in that formation known to geologists as the trias, developed largely in cheshire. the influence of a salt-bearing bed is, naturally, not confined to the immediate vicinity of the formation; salt being so readily soluble in water, it comes forth from the rocks in springs, which, flowing over loams and other similar absorbent earths, impart a saline character to them. in this manner otherwise useful earths for brickmaking are rendered absolutely unfit for the purpose. salt is one of the most powerful fluxes known; when mixed even in very small quantities with clay it becomes impossible to make a good brick of the substance. but we must recur to this matter at a later period in another connection. the fluxing property is sometimes taken advantage of by mixing salt with sand in moulding, or in employing a sand already saline, as when dredged from the sea, or obtained between tide-marks. a species of glaze is produced on the brick by the action of such moulding sand.
we may ignore the presence of a number of minerals such as rutile, augite, and hornblende in brick-earths, as they only exist therein in such small proportion, and have no appreciable effect in the kiln.
