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 usually 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 Eubœa, 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 in 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.
CHAPTER VII.
THE CHEMISTRY OF BRICK-EARTHS.
Introduction: THE BLOWPIPE.
It is not our intention to write an elementary treatise on chemistry; but we know it is the custom for brickmakers to have chemical analyses of their raw earths made, and we are aware also that the precise meaning to be attached to these analyses is very little understood. Our principal aim in introducing this subject, then, is to interpret, in an elementary manner, certain typical analyses of earths and substances used in brickmaking; but before doing so we shall explain some easy methods of examining earths by means of the blowpipe, which will not merely give some insight into their chemical constitution, but will afford the intelligent brickmaker a means of investigation which he can himself put into practice.
The results of a chemical analysis of a compound earth, as ordinarily used by the brickmaker, widely differ from those obtained by a mineralogical or petrological examination. The petrologist views the earth as a mineral aggregate, the constituents of which may be ascertained on appeal to a properly-constructed microscope—that is, in the majority of instances. By noting the relative proportions of the different minerals, he is enabled to state, with approximate accuracy, what is the ultimate chemical composition of the whole. From this it would appear that a rough chemical analysis could be drawn up by the petrologist without having recourse to the ordinary methods of chemical investigation. And in a limited sense that is true. But we should not lose sight of the fact that there is, in too many cases, an amorphous residuum in earths, the nature whereof the microscope is powerless to reveal. It is upon this remnant that the chemist should direct his most careful attention.