Tut. That is only on account of its age. Burning lime and making mortar are as well understood now as ever: but in order to have it excellent, the lime should be of a good quality, and thoroughly burnt. Some sorts of lime have the property of making mortar which will harden under water, whence it is much valued for bridges, locks, wharfs, and the like.
Geo. Pray, is not plaster of Paris a kind of lime? I know it will become hard by only mixing water with it, for I have used it to make casts of.
Tut. The powder you call plaster of Paris is made of an earth named gypsum, of which there are several kinds. Alabaster is a stone of this sort, and hard enough to be used like marble. The gypseous earths are of the calcareous kind, but they have naturally a portion of acid united with them, whence they will not effervesce on having acid poured on them. But they are distinguished by the property, that after being calcined or burnt in the fire, and reduced to powder, they will set into a solid body by the addition of water alone. This makes them very useful for ornamental plasters, that are to receive a form or impression, such as the stucco for the ceiling of rooms.
Well—we have said enough about calcareous earths; now to another class, the argillaceous.
Geo. I think I know what those are. Argilla is Latin for clay.
Tut. True; and they are also called clayey earths. In general, these earths are of a soft texture and a sort of greasy feel; but they are peculiarly distinguished by the property of becoming sticky on being tempered with water, so that they may be drawn out and worked into form like a paste. Have you ever, when you were a little boy, made a clay-house?
Geo. Yes, I have.
Tut. Then you well know the manner in which clay is tempered, and worked for this purpose?
Har. Yes—and I remember helping to make little pots and mugs of clay.
Tut. Then you imitated the potter’s trade; for all utensils of earthenware are made of clays either pure or mixed. This is one of the oldest arts among mankind, and one of the most useful. They furnish materials for building, too; for bricks and tiles are made of these earths. But in order to be fit for these purposes, it is necessary that clay should not only be soft and ductile while it is forming, but capable of being hardened afterward; and this it is, by the assistance of fire. Pottery-ware and bricks are burnt with a strong heat in kilns, by which they acquire a hardness equal to that of the hardest stone.