Geo. I think I have heard of bricks being baked by the sun’s heat alone in very hot countries.
Tut. True; and they may serve for building in climates where rain scarcely ever falls; but heavy showers would wash them away. Fire seems to change the nature of clays; for after they have undergone its operation, they become incapable of returning of themselves to a soft and ductile state. You might steep brick-dust or pounded pots in water ever so long without making it hold together in the least.
Geo. I suppose there are many kinds of clays?
Tut. There are. Argillaceous earths differ greatly from each other in colour, purity, and other qualities. Some are perfectly white, as that of which tobacco-pipes are made. Others are blue, brown, yellow, and in short of all hues, which they owe to mixtures of decaying vegetable substances or metals. Those which burn red contain a portion of iron. No clays are found perfectly pure; but they are mixed with more or less of other earths. The common brick-clays contain a large proportion of sand, which often makes them crumbly and perishable. In general, the finest earthenware is made of the purest and whitest clays; but other matters are mixed in order to harden and strengthen them. Thus porcelain or china is made with a clayey earth mixed with a stone of vitrifiable nature, that is, which may be melted into glass; and the fine pottery called queen’s ware is a mixture of tobacco-pipe clay, and flints burnt and powdered. Common stone ware is a coarse mixture of this sort. Some species of pottery are made with mixtures of burnt and unburnt clay; the former I told you before, being incapable of becoming soft again with water like a natural clay.
Har. Are clays of no other use than to make pottery of?
Tut. Yes, the richest soils are those which have a proportion of clay; and marl, which I have already mentioned as a manure, generally contains a good deal of it. Then clay has the property of absorbing oil or grease, whence some kinds of it are used like soap for cleaning clothes. The substance called fullers’ earth is a mixed earth of the argillaceous kind; and its use in taking out the oil which naturally adheres to wool is so great, that it has been one cause of the superiority of our woollen cloths.
Har. Then I suppose it is found in England?
Tut. Yes. There are pits of the best kind of it near Woburn in Bedfordshire, and Nutfield in Surrey, England. The different kinds of slate, too, are stones of the argillaceous class; and very useful ones, for covering houses, and other purposes.
Har. Are writing slates like the slates used for covering houses?
Tut. Yes; but their superior blackness and smoothness make them show better the marks of the pencil.