The yellow-brown sludge deposited in the pans during the concentration of crude green vitriol liquor, constitutes the product termed vitriol ochre, which contains varying amounts of ferric oxide, sulphuric acid and water, according to the quantity of ferric oxide resulting from the oxidation of the pyrites and the character of the latter, _e. g._:-- Ferric oxide 65–70% Sulphuric acid 14–16% Water 13–16% Although the colour of these ochres is not particularly handsome, they can be transformed, by firing, into colours of fairly good quality. As this subject will be more thoroughly gone into when dealing with the preparation of the red iron pigments, the applicability of these ochres will only be casually referred to here. During the burning process, these ochres, of course, part with the whole of their contained water; and by protracted, high calcination, the whole of the sulphuric acid can also be expelled, so that finally nothing but pure ferric oxide is left. _Alum Sludge._--Solutions of crude alum always contain a certain amount of ferric oxide which settles down at the bottom of the pans during concentration. This sludge, too, consists of basic ferric sulphate, but is inferior in covering power to vitriol ochre, the crystals being of coarser grain. On the other hand, the ochreous sediment from the alum concentrating-pans has the valuable property of being readily transformable into red-brown to pure red tones by burning. For this reason, particular attention has been devoted to this sludge in a number of alum works. Since the products are only of value when burned, and the shades thereby obtained are always red, they will be dealt with more fully along with the red earth colours. _Pit Ochre._--Springs containing small quantities of ferrous sulphate and other salts are met with in many iron mines, but, in most cases, the amounts are too small for their recovery by artificial concentration to be contemplated. If, however, the conditions allow of the springs being easily diverted, they may often be utilised for the preparation of low-grade ochre. The chemical composition of these pit ochres varies considerably, and depends on the geological character of the locality. Water can only dissolve such minerals as occur in the form of fairly readily soluble compounds; and for this reason pit waters are always solutions of the metals which are found in the mine. The variety of compounds that may be present in an ochre can be seen from the subjoined analyses of ochres deposited from pit waters at Rammelsberg. As elsewhere, two distinct classes of ochre are met with, having a conchoid and an earthy fracture respectively. The latter usually contain rather more ferric oxide, and, in particular, a higher content of foreign substances, the most important of which is quartz sand. In the Table, the ochres with conchoid fracture are marked A, and those with an earthy fracture, B.