Continuous systems, involving a battery of digestors connected by pipes, have also been devised. Arrangements are made of course for admitting water and steam as required.

REFERENCES.

"Glue and Glue Testing," by S. Rideal, D.Sc., 2nd ed., pp. 47-56 and 61.

"Gelatine, Glue and their Allied Products," by T. Lambert, pp.

21-24, 40, 42-44, 49 and 51.

"Encyclopedie chimique," Fremy, tome x., p. 83.

PATENTS.

Edison: U.S.A. patent, 1902, 703204.

Bertram: English patent, 1892, 951.

Dorenburg: German patent, 1911, 239676.

Lehmann: French patent, 1912, 441548.

SECTION IV.--CLARIFICATION AND DECOLORIZATION

After the raw material has been appropriately prepared and an aqueous extract or gelatine sol obtained therefrom, there are certain refinements necessary before the weak sol is evaporated. These purifying processes include (1) clarification, (2) decolorization, and (3) bleaching. Whilst most manufacturers have more or less successfully solved the problems involved in these processes, the practical methods that are in common use have been evolved and elaborated in a purely empirical way, and the underlying principles have been very imperfectly recognized, and indeed often confused and misunderstood. Hence it is even yet not uncommon to find these terms rather loosely used, and it is one aim of this section to define and distinguish these various operations in principle as well as in practice.

Clarification consists essentially in the removal of suspended matters, with the consequent production of a sol or gel which is bright, clear, and apparently h.o.m.ogeneous. Bleaching consists essentially in destroying the colouring matters of the sol by chemical action, such as oxidation or reduction. Decolorization involves the removal rather than the destruction of colouring matters, and does not therefore imply a chemical action in the ordinary sense.

Clarification may be now considered more particularly. It is necessary in this connection to consider what is meant by "suspended matter." The modern view is that the difference between a true solution and a muddy liquor or an emulsion is one chiefly of degree. If the particles of matter in suspension or emulsion (the disperse phase) be reduced in size they eventually merge into colloidal sols which are sometimes a.n.a.logously named "suspensoids" and "emulsoids," if further reduced in size into "suspensides" and "emulsides," and with further reduction into true solutions. On this view not only suspensions and emulsions, but also sols, solutides and solutions are all heterogeneous. Now in practice the clarifying of a gelatine sol involves only the removal of the particles which are evident to sight. What is needed is that the product should make a sol or gel which to the naked eye appears to be optically clear both to reflected and to transmitted light. If desired, the limit could be expressed in terms of dispersity or specific surface.

Now it is a comparatively easy matter to remove the coa.r.s.er substances which often pa.s.s into the sol, _e.g._ undissolved portions of raw material or the insoluble portions, such as the hair, the grain (hyaline layer), and the elastic fibres of skin gelatine material, and the fibres which even remain in extracting acidulated bones. A more difficult proposition is the removal of still finer particles which may be almost said to be in colloidal solution, but which at any rate are so large that they cause a visible opalescence or even a turbidity of the gelatine sol. A more difficult task also is the removal of minute particles of grease, which are an exceedingly common cause of turbidity and which are often very effectively emulsified in the sol.

Now at this stage it is necessary to point out that besides the difference in the size of the particles of the disperse phase, there is another important difference involved, viz. that the particles of a colloid sol carry an electric charge owing to the adsorption of electrically charged ions of the electrolytes (salts, acids or alkalies) present. If this charge be removed the colloid is precipitated (coagulated, flocculated) and is then filtered off with comparative ease. This precipitation can be brought about by a reduction or elimination of the potential difference between the disperse phase and the continuous phase. The electric charge given by the adsorbed ions may be reduced by dilution, for dilution causes a lessened adsorption of the charging ions. Hence the well-known practical fact that it is more satisfactory to filter a dilute gelatine sol. Further, the electric charge may be reduced also by causing the adsorption of an ion of opposite charge. This is the principle underlying the precipitation (of any colloid) by adding electrolytes. It is essential here to consider which ions are most likely to be adsorbed, and also to bear in mind what charge they carry. Now the hydrion (H+) of acids and the hydroxyl ion (OH-) of alkalies are most strongly adsorbed, so that to precipitate a negative sol, acid is very effective, whilst with a positive sol an alkali is an appropriate precipitant. Further, it is known that organic ions are usually more strongly adsorbed, hence when precipitating from an alkaline sol (negative sol), one should preferably select an inorganic or mineral acid rather than an organic acid. Thus in clarifying an alkaline gelatine sol, hydrochloric or sulphuric acid is to be preferred to acetic or lactic acid. Again, it is necessary to remember that a divalent ion carries twice the charge of a univalent ion, hence the precipitating power of an electrolyte depends upon the valency of the ion whose electric charge is opposite to that on the sol (Hardy's valency rule). Thus a negative sol is most easily precipitated by a mon.o.basic acid. Thus hydrochloric acid is better than sulphuric, on account of the stabilizing effect of the divalent SO{4}-- ion on a negative sol. In such a sol, also, the valency rule indicates that the multivalent kations, _e.g._ iron, Fe+++; chromium, Cr+++; and aluminium, Al+++, should have great precipitating and clarifying effect. This of course is known to be the case, aluminium salts having long been used.

The rule indicates, also, that aluminium chloride would be better than the sulphate or than potash alum. Another feature of precipitation worthy of mention is the phenomenon of "acclimatization." This describes the fact that when the precipitating reagent is added very slowly, or a little at a time, a larger amount must be used, and the slower the addition the greater the excess required. Hence in precipitating matters from an alkaline gelatine sol the acid, if practicable, should be added all at once. In any case it is clear that one should aim at filtering a gelatine sol when it is near the iso-electric point, which is stable enough for gelatine itself, but a point of instability for many undesired impurities. Yet another phenomenon of colloid chemistry is concerned, viz. "protection." The particles it is desired to precipitate not only adsorb ions of electrolytes, but also the gelatine sol itself, and the particles, thus covered by a layer of a stable emulsoid sol, attain much of the stability of this gelatine sol. Unfortunately for gelatine manufacturers, gelatine possesses very great powers as "protective colloid," and this no doubt greatly enhances the practical difficulty of obtaining a clear and bright sol or gel. Here again dilution of the sol reduces the adsorption and correspondingly reduces, to some extent, the difficulty.

With regard to the turbidity or opalescence in a gelatine sol due to minute globules of grease, the case presents some a.n.a.logy to the coa.r.s.er colloid solutions, but the a.n.a.logy has its limits, for an emulsion of grease is not an emulsoid sol. Doubtless the grease globules exhibit adsorptive phenomena, in which case the valency rule comes into force; the gelatine, also, by lowering interfacial tension, a.s.sists in protecting the emulsion; but grease emulsions are certainly stabilized in alkaline media (hence the detergent effect of soap, soda, borax, etc.), and it is undoubtedly easier to separate the emulsion by making the medium acid. Hence the practical fact that an acid sol is more easily clarified from grease than an alkaline or even than a neutral one.

The next stage in clarification is the separation of precipitated matters and of the coalesced particles of grease. This may be attained by the two processes usual in such a problem of chemical engineering, viz. sedimentation and filtration. After precipitation, therefore, the sol should be allowed to stand for some hours, during which time the precipitate not only flocculates but also settles to the bottom, and the globules of grease coalesce further and rise to the top, from which they may be skimmed off. Sedimentation alone is both too slow and too incomplete to be sufficient for proper clarification, and in these days it is always supplemented by the use of the filter-press. This well-known appliance can easily be adapted to the local requirements of the manufacturer. As speed of working is an essential requirement it is necessary to have a large filtering surface, and this may be done either by increasing the number of plates in the press or by increasing the area of the plates used. The large plates, however, are often c.u.mbrous and inconvenient, and if of metal are very heavy. The plates may be constructed of well-seasoned wood, or in the case of alkaline gelatine and glues, even of iron. The framework is in any case usually iron. Acid gelatines and glues may have wooden plates, but "acid-proof" alloys are sometimes used to make them. Where it is essential to filter quickly two presses may be arranged _in parallel_, thus doubling the active filtering surface. When it is essential to obtain the highest possible clarity, two presses may be worked _in series_, which, in effect, means that the sol is filtered twice. In using the filter press for gelatine and glue it is most necessary to observe the most scrupulous cleanliness, and the plates must be frequently washed and sterilized.

Rideal recommends weak chlorine water or bleaching powder solution for this purpose.

The process of _decolorization_, by which colouring matters are removed without being chemically altered or destroyed, usually precedes or takes place concurrently with the filtration. The underlying principle of this operation is adsorption. The colouring matters are usually in colloidal solution and most frequently are emulsoids, hence they are substances which are known to be exceedingly susceptible to positive adsorption. It is probable, also, that in a gelatine sol are particles which cause turbidity, though not coloured, and which are capable of being adsorbed.

Hence the adsorption of colouring matters not only makes the sol more colourless, but in all probability makes it brighter and clearer.

Further, decolorization by adsorption probably also involves the removal of the last traces of emulsified grease. It will be clear, therefore, that in the improvement in brightness and colour of a gelatine sol, adsorption fulfils a triple usefulness. The ordinary processes of dyeing fabrics or leather are adsorption processes, and the decolorization of gelatine sols consists essentially of the same process, except that the concentration of the dyestuff is much less, and the liquor remaining, instead of the adsorbent, is the primary consideration.

Decolorization of gelatine sols may be effected by any substance with a large specific surface. Indeed, a great variety of adsorbents are actually used in practice, and each factory has its favourite material or mixture, and its favourite mode, place, and time of application, determined partly by the nature of the adsorbent and partly by the precise form of apparatus used. Amongst the adsorbents which have received special favour are sand, kieselguhr, asbestos, animal charcoal, wood pulp fibre, alb.u.min and alumina. Sand is very effective, but a comparatively large weight is needed, and its cleansing for repeated use is troublesome. On the other hand, it may be completely renovated by ignition. Kieselguhr is a very powerful adsorbent, and only a little will do much good; it is, however, hardly sufficient alone. Animal charcoal has great specific surface, but its pores are very small for viscous liquors, and its use is less suitable in the case of gelatine than in the decolorization of liquors which may be boiled. Wood pulp fibre is a very popular decolorizing material, not only in gelatine but also in other trades. Its short, woolly fibres give a clarifying as well as a decolorizing effect. It may thus act as a mechanical filter for suspended matter and grease, as well as an adsorbent for colouring matters present as sols. Its two functions, however, are often confused.

It may be regenerated for repeated use by careful washing, and special pulp-washing machines are manufactured and sold for the purpose.

Detergents are usually employed in the wash waters. Asbestos is also a good adsorbent, and its long fibres make it much less liable to non-operating "channels" and "bursts." It also has the advantage that, if desired, it may be regenerated by ignition. It forms a very useful mixture with pulp fibre.

All the above decolorizing materials are insoluble and hydrophobe, and act in virtue of their finely divided conditions, which causes them to have a large specific surface; but there is another type or branch of substances, whose effect is due to surface action of rather a different type. These are the hydrophile gels. In a gelatine sol the colloid particles have largely adsorbed the colouring matters which it is desired to remove. This adsorption, which is after all only an equilibrium, is reduced by introducing another very strong adsorbent.

This latter, by adsorption from the continuous phase, reduces the adsorption of colouring matters by the gelatine particles. In the case under discussion another lyophile colloid is introduced, and after bringing about such an action is removed by appropriate means. The use of alb.u.min has long been known for such a purpose, its special advantage being that after its admixture and adsorptive action, it may easily be removed by raising the temperature above 70 C., when coagulation takes place, and by subsequent mechanical filtration. The coagulated alb.u.min takes down the adsorbed colouring matters. Alb.u.min has been used in this way not only for gelatine and glue liquors, but also for tanning extracts (Part I., Section III.) and other commercial preparations. Into this cla.s.s of decolorizing agents fall the insoluble inorganic gels which have been advocated by W. Gordon Bennett, _e.g._ alumina cream.

Freshly precipitated alumina hydrate is a colloid gel with very considerable adsorptive powers. It has also the advantage that it is quite insoluble, easily removed in filtration, and has a powerful adsorptive action upon other objectionable impurities, especially the poisonous metals, a.r.s.enic, copper, zinc and lead. Its use is an undoubted advantage when in addition to the other clarifying agents and adsorbents. It is conceivable, in some cases, that when alum is employed as clarifying agent in an alkaline gelatine liquor, some alumina may be formed, and as such contribute to the total effect.

SECTION V.--BLEACHING

The adsorption law indicates that however much colouring matter is removed from the volume concentration (continuous phase) there must always be some left. After all that the decolorization processes can do, there still remains much colour that can only be removed by a chemical action of the ordinary sense. The amount of colouring matter of this kind is not large, but it is a deep red-brown, and when the gelatine sol has been evaporated and dried out the final product, if untreated, possesses this typical colour, and is known as glue. If, before gelation, a chemical bleaching action is applied to destroy this pigment, the product may be then dried out in a nearly colourless condition and is known as gelatine. Gelatine, therefore, is simply bleached glue. Many other definitions have been given, and many elaborate distinctions drawn, but the fact of bleaching is the essential difference. In these days when gelatine is so valuable, the higher-grade products are nearly always bleached, and the term "glue" is consequently more often applied to a lower-grade product, and is sometimes used in a sense implying this fact.

If it be desired to manufacture gelatine, it is fairly obvious that the task is lightened by observing the axiom that prevention is better than cure. If steps are taken to prevent the presence or development of such colouring matter, a great advantage is attained, for not only is the problem of bleaching easier, but also quicker and less expensive in chemicals. The nature of the colouring matters is but imperfectly investigated, but in the case of skin gelatine the pigment of the hair roots and epidermis is doubtless one factor. A long liming is said to a.s.sist in its destruction, possibly because this completes the loosening of epithelial structures and possibly because the alkali causes some hydrolysis of the pigment. In both skin and bone gelatine sols, however, there is a considerable tendency to develop the brown colouring matter typical of glue. This tendency is enhanced by an increase in temperature and also by the presence of acid or alkali. These facts seem to indicate that its development is a.s.sociated with a partial hydrolysis of the gelatine in some direction. Rideal says this colouring matter is allied to caramel. In harmony with this is the experience that its development is greatest in products which have been "burnt," _i.e._ subjected to unusually high temperature. The practical maxims which arise from these considerations are fairly obvious and widely known, viz. to conduct the extraction and evaporation at as low a temperature as possible and in as neutral a condition as practicable. The temperature is particularly important during evaporation (see Section VI.).

Fortunately for manufacturers of gelatine, the colouring matter to be attacked is very susceptible both to reduction and to oxidation, and both types of bleach are widely used in practice. It is somewhat curious that the same colouring matter should be destructible both by reduction and by oxidation, but there is no doubt that each type gives a perfectly satisfactory bleaching action and can result in a practically colourless gelatine. On the other hand, the reduction is the more unstable reaction, for the glue colour slowly develops again in the gelatine on keeping it, even in a dried condition. Gelatine bleached by oxidation, however, retains its colour quite well, and even tends to improve with keeping. It is quite possible that quite different reactions are involved in the two processes, but in the light of the above facts it is somewhat surprising to observe Rideal's statement that reduction followed by oxidation has been successful in practice.

Although there is a wide choice of reducing and of oxidizing agents, those which are suitable for application to gelatine cover a very limited field. This limitation arises not so much from the ineffectiveness of the bleach, as from the other effects of these substances upon the purity of the product and upon the elasticity of the gel which it can yield. Especially important is the lyotrope influence of the bleaching agent. Many reactive substances are ruled out simply because they either insolubilize the gelatine or weaken the gel it makes. Others are inadmissible on account of their poisonous nature. It must never be forgotten that whatever is used in bleaching is, like the gelatine itself, much concentrated during evaporation and drying. Its possible percentage in the finished product should be considered, and also the possibility that in these finishing operations what is present may not remain in solution, owing to supersaturation.

=Bleaching by Reduction.=--Of all the reducing agents suggested, sulphurous acid has proved to be much the most suitable and successful.

It has been used with equal success both for bone and for skin gelatine, but on the whole has proved more suitable for the former.

Sulphurous acid can fulfil in this instance a double function, viz. that of acid solvent for the bone phosphate, and that of bleaching agent also. As it penetrates the bone material, dissolving the phosphate, it also exercises its bleaching influence on the gelatinous part of the material. Changes of liquor tend to complete both actions, so that a counter-current system is found most convenient. The "acid process" for the manufacture of bone gelatine has been previously described (Section II.), and the use of sulphurous acid in this connection is typified in the Bergmann process. In this process bleaching is in effect merely a continued treatment.

In the case of skin gelatine, also, sulphurous acid may fulfil a double function, viz. that of deliming agent as well as of bleaching agent. In such instance it is necessary to use excess of bleaching acid, some acting as deliming material and the remainder as bleaching agent. As it is desirable to get rid of the lime and soda salts, several changes of liquor are given to the goods, possibly with intermediate washing. Here again approximation to a counter-current system is of advantage, as the employment of used bleach liquors for deliming purposes effects considerable economy of sulphurous acid. Indeed, there need be no waste acid at all.

Whether the material be for bone or skin gelatine, however, it will be seen that the extraction is conducted in an acid condition and the resulting sol is also acid. Most usually the decolorization and filtration processes are also conducted with such an acid sol. From what has been said (Section IV.) of the value of dibasic inorganic acids as clarifying agents, it will be understood that the presence of sulphurous acid at this stage is of great advantage in the production of a clear and bright gelatine. Indeed, it is well known in trade circles that sulphurous acid gelatines are usually of exceptional clarity and brightness.

The disadvantage of sulphurous acid processes is also found in the same fact that both sol, gel and cake are in an acid condition. To complete the bleach it is sometimes necessary to add sulphurous acid to the sol after extraction, or even after evaporation, but this is to be avoided if possible. Usually the ideal attempted is that the bleaching action should be as much as possible before extraction; the excess of sulphurous acid is then washed off just before the extraction, as far as practicable, and the rest is boiled off during extraction. The ideal is practically never attained, for the acid is strongly adsorbed, and the result is that the finished article is always an acid gelatine, and sometimes indeed very decidedly such. The acid condition is objectionable in the case of some forms of filter press on account of the solvent action on the metals, and is objectionable in evaporation for similar reasons. Acid gelatines are also objectionable for many purposes for which gelatine is usually sold, and this limits the commercial possibilities of the product thus obtained.

Sulphurous acid is itself, of course, a gas, and whilst the gas itself has been used for treating the material (_e.g._ bones), it has been found not only more convenient but also more effective to use an aqueous solution. This is mainly because it is possible to attain a greater adsorption in a liquor. Unfortunately, however, sulphurous acid is not a very soluble gas, and although 8-10 per cent. solutions may be, with great care, obtained, they are really supersaturated and readily yield the gas, even with slight mechanical agitation. Solutions even of 2 to 3 per cent. strength are also liable to this, and the general experience is that 1 to 2 per cent. solutions are most economical and convenient for practical purposes. As the freight on weak solutions is prohibitive, the manufacturer using sulphurous acid is faced with the necessity either of purchasing cylinders of sulphur dioxide liquefied by pressure or making the gas and solution himself. The former is the most convenient course when only small amounts are required, but the latter preferable for a gelatine factory of any size. Sulphurous acid is easily manufactured by burning sulphur and leading the fumes by induced draught up a scrubber down which water slowly trickles. Forced draught may also be used, as in the Sachsenburg plant.

Of the other reducing agents which have been used, sodium hydrosulphite (Na{2}SO{2}) deserves mention. It is a very powerful reducing agent, and has been found most useful when employed as an a.s.sistant to sulphurous acid. This reagent is usually added to the sol, after evaporation and before gelation. It is sold as a white powder, usually under trade names. Sometimes a mixture of bisulphite and powdered zinc replaces it, but this is objectionable for pure food gelatines. Its use also involves an impurity in the finished article, and a greater amount of "inorganic ash."

=Bleaching by Oxidation.=--Many oxidizing agents have been suggested for bleaching gelatine, but most of them have some practical disadvantage.

Most of them contradict the maxim (previously noted) that it is desirable to avoid adding any soluble substance, as this involves a permanent impurity, possibly concentrated to supersaturation in the finishing processes, and possibly involving a disadvantageous lyotrope influence. There is another objection to oxidizing agents also; whilst their bleaching action on the pigments is undoubted, some of them have also a special action upon the gelatine itself which is in reality akin to tanning, and may indeed involve an insolubilization of the gelatine.

Thus, chlorine gas (which Meunier patented for tanning) has been used for bleaching gelatine, but the conditions of success have not yet been thoroughly elucidated, and it is problematical indeed whether the process is consistent with best results. Hypochlorites and bleaching powder have also a similar action, which has been utilized with some success in practice. Rideal suggests that a suitable concentration for these reagents is 1:2000, and emphasizes the care necessary. An advantage of all these chlorinations is the formation of the strongly antiseptic chloramines, which preserve the gelatine from putrefaction.

Ozone has also been tried as an oxidation bleach for gelatine, but not successfully, partly on account of difficulties in controlling the quant.i.ty used. Peroxide of soda has also been used, but it is not only alkaline, but liable to contain sodium hydrate and carbonate as impurities, and this involves neutralization either before use or in the gelatine sol, and the consequent presence of sodium salts in the finished article. Peroxide of calcium is open to the same objections, except that calcium is more easily removed from the sol than sodium.

Rideal's suggestion for removing this lime, viz. precipitation by a current of carbonic acid, merits attention in this and in other directions also. Rideal also states that in the case of an acid bone gelatine, a good peroxide of lime is almost an ideal reagent for bleaching, inasmuch as "the lime carries down phosphate, several impurities and colouring matters." It thus acts as bleach, as neutralizing agent, and as precipitant, and the precipitate itself is a strong adsorbent. On account of its freedom from bases, and because its residue is simply water, peroxide of hydrogen has been found of great service in practice, and in most factories it has shown itself superior not only to the other peroxides, but also to all other oxidizing agents. Its application is simple, a concentrated solution being added to the gelatine sol before or after evaporation. It is the most "fool-proof" of all the oxidizing agents used in bleaching, and it yields the purest product. Its bleaching action is perfectly satisfactory, but only in a non-acid sol. Hydrogen peroxide is moderately stable in acid solution, and its bleaching action is best in slightly alkaline solution. An acid sol bleaches too slowly, or not at all; an alkaline sol induces evolution of oxygen and consequent waste.

The great disadvantage of peroxide of hydrogen is its great expense, which is enhanced by an increasing demand for it in other industries. A minor disadvantage is its instability, which leads to loss in transit and storage. It is sold usually in strengths indicated by the volume of oxygen obtained from unit volume of the solution, when treated with permanganate in a nitrometer (_e.g._ "15 vols. peroxide").

It is a fortunate feature of both the oxidizing and reducing agents usually employed in bleaching, that they have considerable antiseptic power. This a.s.sists materially in preserving the gelatine from putrefaction during the critical period between extraction and concentration.

REFERENCES.

"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed., pp. 61-66, 78-82.

"Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49, 51.