Читать книгу The Science of Brickmaking онлайн

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If a marine clay in a specified locality is found to be unsuitable for bricks at one point, by reason of the presence of too much lime, it would be a phenomenon if clay along the same geological horizon did not present the same unfavourable features at every other point within the district. The homogeneous composition, both from mineralogical and chemical standpoints, of thick marine clays renders them of special use to the brickmaker. Having by sundry processes, after infinite labour, produced a certain class of brick from such an earth, he does not as a rule have to materially modify those processes as the earth is dug into to continue manufacturing the same brick. He is dealing with an earth which, comparatively speaking, is a constant quantity—when the clays are thick, and no lines of bedding are distinctly visible.

We find that a rooted conviction exists in many brickyards that clays of marine origin are no good for brickmaking, because (so the opinion runs) they always contain so much salt. It is wonderful that such ignorance prevails, when the slightest acquaintance with the subject would teach otherwise. It is perfectly true that such deposits might have contained salt during and for some time after deposition, but it is absurd to suppose that their marine origin has anything to do with the presence of common salt in the clay at the present time. Salt is soluble in water, and has been removed from such clays by the percolation of underground water in 99 cases out of a hundred. Indeed, as a matter of experience, we find that salt is most commonly found in beds of lacustrine origin, or those laid down in enclosed portions of the sea, for reasons we need not enter into at the present moment. Of course, when material is taken from the sea-shore to make into bricks, a considerable quantity of salt is manifest, but that is a totally different thing to the clays deposited—we should not like to say how many thousands of years ago. Clays of all kinds, however, may be impregnated with salt (as in parts of Cheshire), owing to the proximity of other beds containing that mineral; also by the percolation of underground water with much salt in solution.