Talc
Although this mineral is not used as a pigment by itself, it must be mentioned here because it is not infrequently employed for mixing with other colours, and is also used in the wall-paper industry. It also serves to distribute certain pigments in a state of fine division, the “rouge végétal” of the perfumer, for example, usually consisting of talc and a small quantity of very fine carmine.
In commerce the name talc is sometimes applied to two separate minerals, true talc and steatite or soapstone. The former is rarely met with native as well-defined crystals, mostly occurring as scaly masses in primitive rocks. Thin pieces exhibit a certain degree of flexibility. The hardness of this mineral is so small that it can be scratched with the finger-nail; and its sp. gr. is 2·9–2·8. Talc is easily scraped, and the powder remains sticking to the knife, a property which renders the substance difficult to reduce to powder, because it balls together and takes a very long time to convert into a fine flour. The process is facilitated by calcining the talc and quenching it in cold water, this treatment increasing the hardness and at the same time making it more brittle, and thus more easy to pulverise.
A characteristic feature of all the talc minerals is their peculiar greasy appearance and feel. The colour varies, white pieces alone being of any use to the colour manufacturer. The yellow- or green-tinged varieties owe their shade to the presence of ferric and ferrous oxides. In chemical composition, talc consists of a combination of magnesium silicate with hydrated silica, the supposed formula being: 4MgO . SiO2 + H2O . SiO2, and the percentage composition: silica, 62·6%; magnesia, 32·9%; water, 4·9%.