Whether acid be used or not, the butts are now usually suspended in deep pits containing old and nearly exhausted tan liquors. These liquors contain a certain amount of lactic and acetic acids, derived by fermentation from the sugary matters of the tanning materials, and also in some cases, weak acids originally present in the materials themselves. These acids are most important to successful tannage, and their effect is twofold; in the first place, they neutralise and remove any lime which still remains in the butts; and, secondly, they bring the butt into a slightly acid condition, in which it remains plump and swollen in the liquors, while the tannin gradually penetrates and tans the fibre. If, as frequently happens, especially in modern yards where extracts are very largely used, the natural acid of the liquors is not sufficient for this purpose, the lime combines with the tanning matters, and the butts either become discoloured at once, or darken by exposure and oxidation, when they come to be dried, while the pelt remains flat and insufficiently swollen. To avoid this trouble, resort is sometimes had to artificial acidification of the liquors. As a general rule, it may be stated that it does not answer to mix the stronger mineral acids directly with the liquors, but lactic and acetic acids may be used, or even oxalic acid may be added to the suspenders in such quantities as to precipitate and remove the lime which they contain, setting free the organic acids with which it had been combined. The use of oxalic acid should never be pushed further than this, as it has a most powerful swelling action on the hide; and goods which are too much swollen by acids tan dark and brittle. After the hides have remained from ten days to a fortnight in the suspenders, they are usually laid in pits called “handlers” which are worked in series of 6, 8, or 10 pits, containing the same number of packs of goods. The weakest liquor from the youngest pack is run to the suspenders daily, a new and stronger liquor is run to the pit, which now becomes the head of the series, into which the oldest and most tanned pack of butts is moved; and the next takes its place and liquor, and so on down the series, the youngest pack finally occupying the place which had previously been taken by the last but one. In this way each pack receives a change of liquor of regularly graduated strength; and during the time which it remains in the handlers, passes from a strength of perhaps 20° Bkr. (sp. gr. 1·020) to one of about 40° Bkr. (sp. gr. 1·040). During this part of the process the butt is completely or nearly coloured through, and is then ready for the “layers.”