In the case of hide or skin gelatine, the raw material is a bye-product of the leather industry. After the hides or skins have passed through the preparatory processes which convert them into "pelt" (see Part I., Section II.), they are so trimmed that all that is left will make a useful leather. These "trimmings" or "roundings" include ears and noses, the udders of cows and heifers, and also include parts from the butt, belly and shanks which are collectively termed "pieces." The operation of fleshing (Part I., Section II., p. [22]), in which fat and flesh are cut from that side of the hides and skins which was next the flesh, also involves cutting into the collagen to some extent, and these "fleshings" comprise another very large class of raw material. The fleshings obtained by hand labour contain distinctly more hide substances than those obtained by machine work, and their commercial value to the gelatine manufacturer is of course proportionate to the collagen content. Some hides and skins are split in the pelt (Part I., Section IX.; Part II., Sections II., III. and IV.), and the "flesh split," though sometimes made into leather, is also used in making gelatine, a high quality being obtained from such material. Minor sources of material are tendons and cartilages, and also hides and skins which have been too much damaged by partial putrefaction or by accidents to make sound leather. Of course the material from the hides for heavy leathers form the greater bulk of raw material for skin gelatine which is thus derived principally from ox hides but sheep and goat skin pieces have also an important place. The skins of other animals, such as dogs, cats, hares and rabbits not usually made into leather can also be depilated and used for making skin gelatine and glue. Horse hide fleshings and pieces are sometimes used, but are notorious for the poor quality of their product. They seem to contain less α gelatin. All these materials are of course readily putrescible and must be put "into work" without much loss of time. When it is impossible to convey them from the tannery to the gelatine factory quickly enough, e.g. foreign material, the "glue stock" is dried out completely and sold in that condition. In the manufacture of pickers from limed pelt there is some superfluous material, and this is cut into shavings and dried. This "picker waste" also forms a useful source of raw material. Skin gelatine material is not very strong in gelatine-substance. The fleshings, pieces, etc., contain much water, even up to 80 per cent. This, however, is very variable, and only a practical test or a hide substance determination can indicate the commercial value of any particular material. This value, moreover, is determined not only by the yield and quality of the gelatine which can be obtained, but also by the yield of grease, the valuable bye-product.

The preliminary treatment of material for skin gelatine consists essentially of liming and of washing. The object of each process is to purify. Liming has much the same action on hide pieces, etc., as on hides, and indeed the liming treatment is somewhat superfluous on cuttings from well-limed hides. The material is plumped up and the partially hydrolyzed products are taken into solution. Lime also acts as mild antiseptic, stops any putrefaction and liberates ammonia formed by fermentation in transit to the factory. When plumping is particularly wanted (as in wetting in dry stock) caustic soda is sometimes used as an assistant (cf. dried hides, p. [18]). Sodium sulphide has also been used for this purpose. The liming is in brick pits, an excess of undissolved lime being always used. It is advantageous frequently to disturb or agitate the goods in the lime pits. Up to ten weeks liming has sometimes been given, but about three weeks is now generally considered sufficient, and the tendency is to shorten the time. The lime and soda have also a detergent action on soiled stock, and they probably assist in hydrolyzing the pigments of the hair roots and sheaths. They also saponify and emulsify the grease, and it is obvious, therefore, that liming can be carried too far. Slaked lime, of course, must always be used.

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:—

Gelatinous matter 21½per cent.
Fat 12½" "
Calcium phosphate 48" "
Calcium carbonate 3" "
Alkali salts, silica, etc. 2½" "
Water 12½" "
100" "

It will be seen, therefore, that the manufacture of bone gelatine and of a comparatively large proportion of phosphate involves the recovery and purification of much fatty matter. The manufacturing processes are naturally subject to considerable variation. One respect in which they differ is the stage in which grease is removed. Sometimes this is simply done as the need and occasion arise, and it is skimmed out in the acid or water extractions, but it is now more usual to have a special "degreasing" process. There are, moreover, two quite distinct types of manufacture. In one of these (the boiling process) the routine bears some resemblance to that for skin gelatine. In this process the bones are washed and cleansed and then immediately subjected to extraction with water. This removes the gelatinous matter and leaves the phosphate and earthy matters behind. Grease may be removed before the water extraction, but is also sometimes removed by skimming off during the extraction, as is usual in the case of skin gelatine. This procedure is now not much favoured unless only a low-grade glue is required. In the other type of manufacture (the acid process) the material is first degreased, and then the mineral matter is extracted or dissolved by acids, leaving the gelatinous matter behind for subsequent refinement and solution. The acid process has long been preferred for high-class bone gelatine, and hence needs further discussion.

The degreasing operation was once brought about by steaming only, but is now accomplished with the assistance of fat solvents.

The object of cleansing is not only to remove dirt, but also fleshy matter which often adheres to the bones. This may contain a little gelatine, but consists mainly of other proteins and insoluble fibre, neither of which are wanted in the water extraction. The mill consists of a large cylinder of stout wire gauze. This revolves round the axis of the cylinder, and the bones are fed in at one end by a hopper and are discharged at the other. The revolution of the mill causes the friction which polishes off the fleshy matter. The dirt and flesh fall through the gauze and are sent to the fertilizer factory. The polishings are sometimes further separated by a similar machine. Raw bones may thus yield nearly 60 per cent. of degreased bones, and about 56 per cent. cleansed bones ready for extraction, and 3 or 4 per cent. "bone meal."

The next stage is the extraction of the mineral matters by acid, for which purpose hydrochloric acid has proved very suitable, as both phosphate and carbonate of lime are dissolved by it. The usual counter-current system of extraction is used [cp. Leaching and extract manufacture, Part I., Section III., p. 35]. The process is methodical and regular, the acid liquor passing successively through a battery of six vats in such a manner that the liquor richest in lime salts comes into contact with the bones most recently charged; the fresh acid thus acts upon the nearly extracted bones. The hydrochloric acid used is of 8 to 10 per cent. strength (5° to 7° Bé.). Stronger acid is apt to hydrolyze ("rot") the gelatine, whilst weaker acid takes longer time. The process takes 8 to 10 days, though up to 14 days is sometimes given, and, on the other hand, the process has been occasionally reduced to 4 days. The gelatinous matter undissolved has the shape of the original bone, but is much swollen. When the acid liquor is saturated with lime salt, the liquor is drawn off from below the vats and sent to the phosphate precipitation tanks. The phosphate is usually precipitated by adding just sufficient milk of lime to neutralize the hydrochloric acid. The precipitated phosphate is then well washed by decantation to remove calcium chloride. It is then drained, and dried at a low temperature. As a large bulk of phosphate is obtained it is often filter-pressed and dried quickly in long revolving chambers through which a current of air is passed. The phosphate is sometimes also precipitated by ammonia. It is then more easily washed and dried, and the ammonium chloride is recovered and may be used to regenerate ammonia, or be sold as a valuable bye-product. Sometimes the acid liquor is not used for making precipitated phosphate, but is evaporated with animal charcoal and silica and then distilled to make phosphorus.

The next stage is the purification by washing of the gelatinous matter which remains. The vat is filled up with pure cold water and the material allowed to steep for six or seven hours. The acid and salts remaining diffuse outwards into the water. This is drained off and replaced by fresh water, and the procedure repeated half a dozen times or as often as necessary. The end is said to be determined by the absence of a precipitate on adding silver nitrate to the wash water, or by the absence of any action on blue litmus paper. It will be seen, however, that there are two actions involved, one being the removal of calcium chloride and the other the removal of excess acid. The former is the easier, and is almost necessarily brought about by the latter. Hence in some factories the neutralization is brought about, therefore, by the addition of a certain quantity of soda, or more usually by lime, and the material is sometimes submitted to a veritable liming by which it remains in milk of lime for about three weeks, the lime liquor being renewed several times. The product is finally washed again to remove excess lime. This is carried out in a rotating vessel through which passes a continuous stream of water. If a slightly acid gelatine is required, however, the lime and liming are both superfluous, and the procedure is simply to wash as thoroughly as possible and then to immerse the material in a 1 per cent. sulphurous acid solution for 3 hours to bleach, and then to proceed with the water extraction or solution of the gelatine. The hydrochloric acid used for these processes should be as pure as possible, and the degreasing as thorough as possible, for, if not, a gelatine with a bad odour is liable to be obtained.