The "sweating-pit" now in use is sometimes of wood, but usually consists of a building of brick or stone, protected from changes of temperature, both above, and at the sides, by thick banks of soil or spent tan. If soil be used, it will form an excellent bed for vines, &c., which are fertilised by the ammonia penetrating from below, which is evolved in large quantities and which assists the unhairing process by its action on the epidermis.[S] Though called a "pit," it is undesirable that it should be actually below the level of the ground, but should be arranged so that the hides can be wheeled in and out in barrows. It is lighted and ventilated by a lantern roof above a central passage, and should be divided into chambers, each capable of suspending a pack of hides. By means of sprinklers above and steam-pipes below, the chambers may be cooled or warmed, as required, and the air kept so moist that globules of condensed water collect on all parts of the hides, which are suspended from tenterhooks. [Footnote S: Hides have been unhaired by the action of gaseous ammonia alone, but the method does not seem suited for technical use.] The process is principally used in America for dried hides, but may be employed either for wet or dry salted, after complete removal of the salt. It is imperatively necessary that dried hides should be completely softened before sweating. As the sweating process advances more rapidly in the upper than in the lower part of the pit, and as the thick portions are more resistant than the thin ones, the hides, after about 3 days' sweating, require constant attention in changing their positions, and in checking the forward ones by taking down and laying in piles on the bottom of the pit. The usual treatment for sweated hides, when the hair is sufficiently loosened, is to throw them into the stocks, and work out in this way the slime and most of the hair. This has the disadvantage of working out too much of the dissolved gelatin, and of fulling the hair so firmly into the flesh, that it is difficult again to remove it. To overcome these evils, some American tanners now pass the hides, after sweating, through a weak lime. This, to a great extent, prevents the hair fixing itself in the flesh, and tends to counteract the injurious effect of the vitriol (which is almost invariably used in plumping sweat stock) on the colour of the leather. By this process, 10,000 Texas and New Orleans wet-salted hides gave an average yield of leather of 73 per cent. on their green weight, and the leather was excellent in quality (Schultz). If sweated or very lightly limed hides are imperfectly worked on the grain, greasy spots are apt to remain, which will not colour in the liquors ("white spots"). These may be made to colour by scraping and working the grain with a knife, or by the application of a solution of soda or soda ash, and would probably be avoided by the use of soda ash in the soaks on greasy parcels of hides.