Within moderate limits, the strength of the rising liquor is not of great importance, since the skins will only absorb a certain amount of acid (increasing with the concentration of salt). In the second or falling liquor the large excess of salt forces all the acid present into the skins, none diffusing into the bath. Skins may be effectively pickled with very much smaller quantities of acid than those prescribed above, or ordinarily used, and are much easier to tan satisfactorily; but it is said that they are more liable to suffer from mildew. Pickling may also be done by placing the skins in a concentrated brine-bath, and adding a calculated quantity of acid, not exceeding 0·1 grm.-molecule of sulphuric acid per kilo. of dry hide substance, but the method is not economical in practice from the dilution of the bath produced by the water brought in by the skins and the necessity of constant large additions of salt. Pickled skins must not be brought in contact with water, which by diluting the brine they contain, allows the excess of acid to act upon and destroy the fibre. Even drops of water, accidentally sprinkled on the skins produce this effect, and it is said that it spreads to parts which have not been wet. For similar reasons, it is necessary in tanning pickled skins, at least to begin the process in liquors to which salt has been added, the quantity required being dependent on the amount of acid used in pickling the skins, and where this is reduced to a minimum, it is even possible to tan without further addition of salt than that contained in the skins.[58] The pickling process converts the skins into a species of white leather, and skins tanned in salted liquors after pickling, or by addition of both acid and salt to sumach liquors give good colour, and tough leather with a much diminished consumption of sumach. The permanency of such leather is somewhat doubtful, but the writer was unable to detect free sulphuric acid in a sample which he examined, and it may be that when no acid is added to the later liquors, that derived from the pickling is expelled by the tannin; but this is very doubtful. [58] Instead of using salted liquors, the skins maybe “depickled” by a bath of whitening and water, borax, or some other mildly alkaline solution before tanning.