Chapter VI: Appendix: III

GRANITE TEMPER AND LIMONITE PIGMENT EXAMINATION

By

PROFESSOR CHARLES MEYER

The piece of granite, no. 4326, used for temper is high in quartz (20-25 per cent) and potash feldspar (35-40 per cent), with perhaps 10 per cent of black mica now chloritized. The remainder is probably soda-rich plagioclase, a feldspar. This is a very acid granite, silica probably constituting around 70 per cent of the total mass. As a result, as the rock surface weathered, it would not wash off as clay but would maintain hard spicules and sharp angles of quartz useful as temper.

The limonite pigment, no. 4295, Fe{2}O{3}·n(+)H{2}O, has mostly crystallized on exposure to become toethite, Fe{2}O{3}·nH{2}O. If originally derived from a sulphide, none of this seems to remain. Some clay is contained and a little quartz silt; also some carbonate in the form of calcite, which acts as a cement for the whole; but the total of silicates and carbonates, that is, noniron oxide, is not over 10 per cent. On roasting, the water content is driven off, and the remaining Fe{2}O{3} is red. A reducing heating with carbon however produces magnetic powder Fe{3}O{4}, a black pigment.