At the end of this period, the butts are taken to the "layers" or "bloomers," in which they are laid down with stronger liquors and much larger quantities of "dust"; the latter is usually bark or valonia, though mimosa is occasionally used. The liquors vary from 40° to 60° or 70° Bark. in strength in mixed tannage, and the duration of each layer from 10 days in the earlier stages to a month in the later ones. For the best heavy tannages, 6-8 layers are required. Each time the butts are raised, they should be mopped on the grain, to remove dirt and loose bloom. Strong valonia liquors, or heavy valonia dusting, causes a brown sandy crust to form on the freely exposed parts of the butts. This is removed in striking, but is sometimes very troublesome on rough dried dressing leather. In pure bark tannage, which, however, is gradually becoming extinct, the liquors used are of necessity much weaker, as it is extremely difficult to obtain liquors of more than 25°-30° Bark. from this material. The last layer, however, should always have liquors of the greatest strength which can possibly be obtained, or the leather will be deficient in firmness. After receiving their last layer, the butts are well mopped or brushed and washed up in a clear liquor, and thrown over a horse to drain before going into the shed. In America, the Howard scrubber (Fig. 43) is generally employed instead of hand labour at this stage. It consists of 2 rotating wooden frames at the top of a pit, provided with brushes or birch-brooms, and, when in use, enclosed by a cover A, through a slit G in which the sides are inserted and drawn back, while water is supplied by the pump B. Sometimes the brush-drums are placed one above another, and the leather is passed in at the side. [Illustration: PL. VI. _E & F. N. Spon, London & New York._ "INK PHOTO" SPRAGUE & CO. LONDON OPENING, STRETCHING AND STORING WHITE SKINS.] [Illustration: Fig. 43.] In mixed tannages, where the colour is dark, the leather is frequently handled or suspended in a warm sumach or myrobalanes liquor, and occasionally in dilute sulphuric or oxalic acids. If these acids are not effectually removed before drying, the toughness of the leather will be destroyed, and in extreme cases the leather will become brittle and refuse to take black. In any case, strong acids are prejudicial to the durability of the leather. In America, alternate baths of vitriol and sugar of lead are frequently used for bleaching and weighting the leather, but the colour given is not durable.