After liming the soaked, softened and plumped stock is washed as thoroughly as possible. To do this it is necessary to supply repeated batches of clean cold water. Some manufacturers, however, use the warm water from the evaporators. Wooden vats or brick pits with arrangements for agitation, for draining off and for inspection, are used for this purpose. The agitation may be carried out by means of revolving shafts or drums with projecting curved spokes or vanes. An American patent (Hoeveler's glue stock washer) involves the use of a paddle wheel. It is combined with a settling tank to gather particles of stock. In the washing the chalk, excess lime, dirt, etc., are quickly removed and a slow deliming process is commenced. The sediment from the washers and wash waters has some value in making fertilizers. Deliming cannot be carried on further than certain limits by water alone. Hence acid is often added to finish off the process. Hydrochloric acid has the advantage of forming soluble salts, but if they are not removed completely their lyotrope influence is to weaken the gelatine. Sulphuric and sulphurous acids are even cheaper, and the lyotrope influence of their salts is in the opposite sense. The latter also has the advantage of destroying sulphides, an important advantage for food gelatines. Whatever acid is used, however, it is evident that an abundance of pure cold water is the fundamental requirement of a pure product. It is a sound maxim in gelatine manufacture to avoid, if at all possible, the addition of any soluble substance, for it is always present in a more concentrated state in the finished article. Thus if its solubility be even moderate, one is likely to attain supersaturation in the "cake" and consequently a dull product. Further, lyotrope influences can never strengthen a gel very much, but may and often do weaken it very considerably. Hence the aim of most manufacturers in the preliminary treatment is so to delime that a nearly neutral and salt-free product is obtained. An exception is the case of skin gelatine in which excess of sulphurous acid is used. This process has for its object not only deliming and purifying, but also a bleaching action. In the case of =bone gelatine=, the raw material is such that there are much longer and more elaborate preparatory processes. This arises from the fact that about half the bones of animals consists of mineral matter, chiefly calcium phosphate. Bones, of course, vary in composition to some extent, and those from younger animals contain distinctly less of the mineral constituents. Approximately speaking, bones have the following average composition:--