The use of ferrocyanides has been suggested as a means of precipitating iron and copper present in extracts, and it may also be pointed out, that with many red-coloured tanning materials, such as hemlock and quebracho, the addition of small quantities of alum to the tanning liquor effects considerable improvement in colour, not only by precipitating a part of the difficultly soluble “reds,” but by developing the yellow colour of certain colouring matters (quercetin, myricetin, etc.) which may be present. Such an addition does no harm in the case of soft leathers, but would probably be injurious in a sole-leather tannage. The liquors, whether direct from the leaches or from the decolorising vats, must next be concentrated by evaporation (Chap. XXVI.), to sirupy consistency for liquid extracts, or until they will become nearly solid on cooling, if a solid extract is required. As has already been stated, the action of heat tends to cause a loss of tannin and a darkening of colour by decomposition and the formation of insoluble reds. To reduce this loss to a minimum, the weak liquors are evaporated with as little access of air and at as low a temperature as possible, and these conditions are best obtained by the use of steam-heated vacuum pans. [Illustration: FIG. 83.--Triple-effect Yaryan Evaporator.] For concentration to gravities not exceeding 1·200, the Yaryan apparatus made by Mirrlees, Watson and Yaryan, of Glasgow, is that most employed. The general arrangement of a “triple effect” machine of this make is shown in Fig. 83, and the internal construction in Fig. 84. Each body consists of a strong casing into which steam is admitted, and which is traversed by copper tubes which terminate in a separating chamber at the further end, which is maintained at a low pressure by an air-pump. The liquid to be evaporated is admitted into the tubes, and is immediately converted into spray by the steam generated from it, and swept forward into the separating chamber, from which it is withdrawn by a pump. The steam before going to the air-pump (or, in the case of “multiple effects,” to the next body), is passed through a “catch-all,” to separate any spray still retained in the steam. Thus the liquor to be evaporated will pass through the entire apparatus in four or five minutes, and may be concentrated from a gravity of 1·02 or 1·03 to that of 1·20 without ever having been heated above 70° C. (160° F.). Unless fuel is very cheap, which is often the case where the spent tanning material can be used to raise steam, it is advisable to use a double or triple effect, in which the steam from the evaporation of the weakest liquor in the first body is used to heat the second, which is maintained at a lower vacuum, and so on. In this way the steam is made to do nearly double or triple duty. As the steam from the extract-liquors contains acids which corrode iron, it is necessary to have the casing as well as