2. The best material for making brick is what in the English language is called loam, a term usually applied to a natural mixture of sand and clay. Such a mixture may be converted into brick with out any addition whatever. Marl likewise answers the purpose of common bricks very well,--indeed better than Most other mixtures. Marl is a natural mixture of limestone and clay in variable propor tions. Now, the more lime it contains, the better does it answer for a manure ; and the less lime it contains, the more suitable it is to the brickmaker.
It would be in vain to attempt a particular detail of the constituents of clays, because they vary too much from each other to admit of any correct gene ralization. We believe, however, that clays very frequently consist of decomposed felspar, in which case we may conceive them as composed of about three parts of silica in the state of 'a very fine pow der, and one part of- alumina. This is the case with porcelain clay. , Indeed, the porcelain clay of Corn wall appears incontrovertibly to be nothing else than decayed felspar, or perhaps felspar which never had assumed any other form than that of clay. The rock from which it is taken is an agglutinated mix ture of quartz and this clay. The quartz is sepa rated by washing. Such a rock might probably be converted into most beautiful brick, merely by cut ting it out in the proper shape, and subjecting it to the requisite heat ; or rather by kneading the whole Into a paste with the requisite quantity of water, moulding it into bricks, and drying and burning them, When the clay proceeds from the decomposition of hornblende, as is likewise often the case, it con. tains about s parts of silica, I of alumina, 1 of liine, and about If of oxide of iron. Sometimes the of sand which exist in clay consist of fragments of felspar. In such cases the clay may_ be fused by heat.
No mixture oraluraina and silica, in any propor tibns whatever, can be fused by the strongest heat which can be raised in our furnaces. Hence such mixtures are best adapted for making fire-bricks, crucibles, and glasshouse pots. Stourbridge clay is suck a inixtnre, blackened by coaly matter. It an swers these purposes better than any other clay in England. Neither can a mixture of lime and alu mina be fused, in whatever proportions the ingredi ents be mixed. But a mixture of silica, lime, and alumina, is very fusible, and the fusion is most rea dily effected when we employ two parts of silica to one of lime. The presence of oxide of iron also renders clay fusible, but not unless its proportion be much greater than ever is likely to occur in any clay , used for the manufacture of bricks.
For making common bricks, the most durable mix ture -ought to be common clayand limestone or chalk. Perhaps the best proportions would be three parts of " clay, and one part of limestone or chalk in powder. i When such a mixture is exposed to heat, it would experience an incipient fusion, and would thereby be rendered much harder and denser than common bricks. The consequence would be, that it would imbibe much less water, and would therefore be much less liable to crack and fall to pieces in winter than common bricks. For when water has insinuated it. self into the pores of a common brick, and is con verted into ice, it undergoes an expansion which dis locates the parts of -the brick, and reduces it to fragments. This is often conspicuously the case
with tyles, which, from their exposed situation, are more liable to be soaked with water than common bricks. Hence also covering the surface of the brick with 'a coating of paint has a great tendency to preserve them from cracking and breaking. This practice is frequently followed in England.
It would be foreign to the object of this article to enter into any long details respecting the chemi cal investigations, and the opinions entertained at different periods respecting the nature of clay. At first, it was supposed to be a species of earth, but Hellot demonstrated that it consisted at least of two constituents ; for sulphuric acid had the property of destroying its plastic nature, and of ren dering it scarcely more adhesive than land. The , portion that remained behind did not effervesce with acids. It was not, therefore, of a calcareous nature. Mr Pott went a step farther; he showed in the con tinuation of his Liihogeognosis that sulphuric acid formed, with the portion of clay which it dissolved, a salt possessing the properties of alum. In the year 1769, Baum6 published his Dissertation on Clays, which he had drawn up in consequence of a premium offered by the Academy of Sciences at Bourdeaux, for the best solution of the following question : What are the principles and constituents of clay, and the natural changes which it experiences, and what are the methods of rendering it fertile ? The Academy did not consider Bourne's solutions as satisfactory. He published his Memoir, in conse quence, as a kind of defiance. He had been em ployed along with Macquer in making numerous ex periments on clay, with a view to the improvement of the porcelain manufacture in France. Guided by these experiments, he drew as a conclusion that clay is a mixture of two different substances; 1. Si lica in a state of purity ; 2. Silica combined with an underdose of sulphuric acid. It was the second of these constituents that gave to, clay its fatty and plastic nature. Margraaf had long before (in 1756) demonstrated that the ingredient of clay which Baume took for a salt, and which he affirmed was soluble in water, was a peculiar 'species of earth, dif ferent from every other, which constitutes the basis of alum, which dissolves in sulphuric acid, but which oloesnot form alum unless a portion of potash added to the solution. Thus, by the labours of Hahn, Pott, Bourne, and Margraaf, the nature of clay was completely developed. It was ascertained to be a mixture of alumina and silica, in variable proportions. It was shown, also, that it sometimes contained sulphuric acid, and not unfrequently pot ash. Hence the reason why, in some cases, it could be converted into alum by digestion in sulphuric acid, without the necessity of adding any potash to the solution. Modern chemists have added consi derably to these facts. They have shown that chalk, felspar, mica, hornblende, oxide of iron, coal, bitumen, &c. are not unfrequently mixed with it ; and that these additions after its qualities consider ably, and render it fit or unfit for the different pur poses to which clay is usually applied.