Potstone is used in some countries for the lining of stoves, furnaces, and ovens; and it is so durable as to have, in some instances, stood unimpaired for several hundred years.
On the banks of the Lake Como, there were some extensive quarries of potstone, which had been worked from the beginning of the Christian era. These quarries, however, fell in, on the 25th of August, 1618, and destroyed the neighbouring town of Pleurs; which had previously obtained by means of them an annual revenue of about sixty thousand ducats.
134. COMMON, or VENETIAN TALC, is an earthy stone, capable of being divided into plates or leaves, which are soft and unctuous to the touch, somewhat transparent, and usually of greenish silvery white colour.
It leaves a white trace when rubbed upon any object.
Mica and talc have a near resemblance to each other; but the plates of the former, when bent, are elastic, while those of the latter are not.
Venetian talc is very abundant in the Tyrol and the Valteline. In a state of powder it renders the skin soft and shining; a property which appears to have suggested the idea of employing it as the basis of the cosmetic named rouge. This is prepared by rubbing together, in a warm mortar, certain proportions of carmine, or extract of the flowers of carthamus tinctorius, with finely powdered talc, and a certain portion of oil of benzoin.
The Romans prepared a beautiful blue or purple colour, by combining pounded talc with the colouring fluid of some particular kinds of testaceous animals, that are found among the submarine rocks of the coasts of the Mediterranean. According to Tavernier, the French traveller, the Persians whiten the walls of their houses and gardens with lime, and then powder them with a silvery white kind of talc; which, he says, gives to them a very beautiful appearance. Talc is now used by the Chinese, and was formerly used by the Europeans, in medicine,
135. INDURATED TALC, or FRENCH CHALK, is a heavy mineral, of close texture, and generally of greenish grey colour; unctuous to the touch, and having a somewhat slaty fracture.
It is found in a massive state; and leaves a white trace when rubbed upon any object.
This is a well known substance, which is in great request by carpenters, tailors, hat-makers, and others, as the lines that are drawn with it are not so easily effaced as those that are made with chalk, and particularly as they remain unaltered even under water. If lines be traced with it on glass, they remain invisible, or at least are scarcely perceptible by the naked eye, till breathed upon. This, it has been conjectured, in part depends on the comparative softness of the substance with which the impression is made; the condensation of the breath taking place more readily on the glass than on the talc that covers it, and the impression of the talc becoming more apparent by the contrast.