The term "heavy chrome leather" is taken to include chrome sole leather, chrome strap and harness butts, waterproof chrome upper leathers, motor butts and picking band butts. These will be discussed in turn.
Chrome sole leather, as stated in Section I., has made headway in Britain during the European War, the Army authorities having recognized its great advantages in durability and waterproofness. At the time of writing, however, its manufacture has received a set back, and many factories are reducing their output. The primary cause of this is that the Army purchases have largely ceased, whilst the general public have not yet been educated to its value. Men who take chrome uppers for granted talk of chrome sole as a "leather substitute" with an implication that it is of inferior value. It must be recognized, too, that there is some interested opposition to its development. Cobblers and bootmakers complain that it ruins their tools, being so hard to cut. Now, it is manifestly impossible for it to be soft to cut and hard to wear out; the complaint is therefore an excellent testimonial. There is also a stupid fear that an article which lasts twice as long will reduce repairs and retail sales by 50 per cent. Even the manufacturer has sometimes a suspicion that a demand reduced in proportion to durability will not be balanced by an extended export trade. These points of view will become minor considerations when the public realize its relative economy, and when the community as a whole grasp that a durable article is a natural asset. Meanwhile credit is due to those firms who persevere in their pioneering work of educating the public.
The manufacture of chrome sole leather presents many analogies with the vegetable tannages. The soaking and liming should be about identical,but the hides for chrome are generally given more sulphide and the depilation is reduced to about a week. The methods used for deliming differ widely in different factories. Some delime completely with mineral acids, some even pickle in acid and salt, whilst others merely delime the grain with boric acid. The last is really quite sufficient. Again, in tanning one finds similar divergences of method. Drum tanning is practised, but tannage in pits by suspension is more usual, though, as this last involves more dilute liquors, it involves also greater time to tan. In drum tannages a few days only are sufficient. In pit tanning at least a week is given, but sometimes up to a month, according to the strength of the final liquor and the rate of progress of the goods into stronger liquors. Liquors containing over 1 per cent. of chromium may easily be spent out so as to contain only 0.01 per cent. Labour and time are saved in pit tanning by the use of rockers. The press system of avoiding handling, however, so complicates the analytical control that its advantage is doubtful, a better way being to shift the liquors by an air ejector, which may also be used as an agitator of the liquor and thus abolish the need for rockers ("Forsare" patent). Chrome butts are tanned out in suspension. No floats or layers are used. The neutralization need not be so thorough as for light chrome uppers, as dyeing is not practised and trouble does not arise with emulsions made from sulphonated oils. Thorough washing is advisable, and the butts are usually then cut into bends and may be oiled before drying if desired. The bends are dried strained, to obtain flatness and smooth grain, for no machines, such as strikers and rollers, are usually employed. It is necessary to dry very thoroughly, for the bends are waterproofed by dipping the dry leather into molten waxes. The most commonly used wax and the cheapest is paraffin wax with a m.p. of about 127° F. It is rather a brittle wax, however, and as the finished leather consists of up to one-third of the wax, it is better to use at least some proportion of hard fat, Japan wax or ceresin wax, to obtain a stuffing materialwith less crystalline texture. The use of 10-30 per cent. rosin in the stuffing grease is also usual. This prevents the leather from being so slippery when in wear. The stuffing should take place at temperatures from 150°-195° F., according to the melting-point of the grease employed. The bends are taken out and laid in pile to cool and set in a flat condition, and are then finished.
The chrome tannage of butts for strapping and harness backs, and for motor butts and picking bands may be similar to that for chrome sole, but drum tannages are more common and the two-bath process is often used. In the latter case the acid chroming bath is preferred, using 6 per cent. of dichromate and of acid, with up to 15 per cent. of salt, and reducing with 15 per cent. thiosulphate and acid as needed. This process assists in the production of the light colour which is preferred in the case of some of these leathers.
Strap butts after tanning are very thoroughly washed with cold water in pits, and repeatedly struck out by machine between the washings. They are then oiled with heavy mineral oil, and stretched by powerful machines. They are dried and curried during the stretching. Degras, wool fat and vaseline are greases used, and the drying and stretching finished off at 120° F. They are then fluffed on the flesh, French-chalked and heavily rolled.
Harness backs are neutralized, machine sammed, and lightly fat liquored with 4½ per cent. soap. They are then struck and oiled with heavy mineral oil and dried for stuffing. Hand stuffing, drum stuffing, and "burning in" are all used (see Part I., Section IV.). Stearin, paraffin wax, ceresin wax, wool fat, sod oil and mineral oil are the greases employed. The butts are blacked after stuffing with lamp black and oil, glassed well and buck-tallowed on the grain.
Motor butts are fat liquored lightly, using soap only. They have to be softened, therefore, during the drying by being mechanically worked. A boarding machine is repeatedly used during the drying. They arefinished off with French chalk on flesh and grain.
Picking band butts are neutralized by using warm water and then borax solution, and are then sammed by machine and very heavily fat liquored with cod oil and tallow and hard soap, to which degras may also be added. Up to 20 per cent. of greases (on the pelt weight) may be used. They are well drummed in this, struck out, French chalked, and dried out. They are softened finally by machine.
Waterproof chrome upper leathers are manufactured usually from hides tanned by the two-bath process, which is said to give a mellower leather. The neutral type of chroming bath is common. The butts are neutralized, machine sammed and struck, and then fat liquored with 2 per cent. each of neatsfoot oil and soft soap. They are then sammed, shaved and blacked on the grain with logwood and iron, and dried further. They are stuffed then by brushing with an abundant amount of concentrated fat liquor. This gives the waterproofness. They are staked after drying further, and often grained three ways. A further waterproof finish is given consisting of a fat liquor containing beeswax. They are finally brushed and re-oiled with linseed oil, to which some mineral oil may be added. This leather is much the most durable type for a shooting boot, or where waterproof uppers are desirable.
REFERENCES.Procter, "Principles of Leather Manufacture," p. 198.Bennett, "Manufacture of Leather," pp. 234, 368.
The use of alum for making pelt into leather is several centuries old. It was the first case of what are called "mineral tannages." The tannage is closely analogous in theory to the chrome tannages discussed in Part III., and if soda be added to ordinary potash alum in solution, a basic alum liquor is obtained which is quite capable of yielding a satisfactory leather, and which is thus a strict analogy of the basic chrome alum liquor described in Part III., Section II. The range of basicity which is practicable is very limited, however, and it is much more usual to use common salt with the alum instead of soda. The alum is, of course, hydrolyzed and free sulphuric acid is quickly adsorbed, whilst the colloidal solution of alumina is adsorbed also but more slowly. The adsorbed acid tends to swell the pelt and to cause it to take up the alumina less readily. The function of the salt is to repress the swelling by a pickling action. The actual result is thus partly due to the alum tannage and partly due to the temporary tannage given by the pickle. Hence such tannages are not firmly "fixed," nor is the result water-resisting, for much of the tanning material may be washed out. If, however, such leathers be stored for a time in a dry condition, the alumina becomes much more firmly fixed, owing probably to a further dehydration of the alumina gel deposited upon the fibres. The tannage is thus relatively more "irreversible," and such storage is practised in commerce for this purpose, being known as the "ageing" of the leather. It will be understood that it is possible to use too great a proportionof salt, the hygroscopic nature of which would keep the leather moist and thus interfere with a glossy finish. About one-third the weight of the alum used is usually sufficient.
All that has been said in Part III. as to the empty nature of the chrome tannage is equally applicable to the alum tannages. It is as necessary therefore to employ filling agents. A fat liquor is quite satisfactory for many purposes, but is too dark coloured and greasy for glove leather. Egg yolk is the favourite emulsion in these cases. It contains about 30 per cent. of an oil very similar to olein and in very perfect emulsion. Olive oil is also largely olein and is also used, being emulsified by the egg yolk and effectively reducing the proportion required of this expensive material. Flour is also used as a filling agent. It acts also as a whitening agent and as an emulsifier. Its use enables the tanner to obtain the required fullness without so much greasiness. Thus softness and fullness may be obtained, and yet a glossy finish be possible. It will be clear that the more flour is used, the more oil may also be used.
The materials mentioned, viz. alum, salt, flour, egg yolk and olive oil, are all mixed together into a paste with some amount of water. The goods are drummed in this paste and then dried out. This operation is known as "tawing." The goods are then "aged" for several weeks and finished as required.
The manufacture of "glove kid" from lambskins and kid skins is the most typical example of alum tannage. Lambskins are unwoolled very usually by painting the flesh with a mixture of lime and sodium sulphide. There must not be too much of the latter on account of its tendency to give harshness, a fatal defect in glove kid. The addition of calcium chloride is desirable, and the skins, which should be pulled as soon as possible, should be quickly placed in soft water or weak lime. For kidskins a set of lime liquors may be used, and in preference to sodium sulphide red arsenic is employed. About one per cent. realgar on the weight of the lime is used, but more often larger quantities are preferred, even up to 6 per cent. The liming is thus shortened to 4 or 5 days. Fresh limeliquors are sometimes used without any sulphides. Another method is to place the skins in a paste of lime to which realgar has been added in slaking. In any method it is necessary to saponify or emulsify the grease on the grain, or difficulties occur in dyeing and finishing.
Skins which are to be tawed for glove kid are both puered and drenched. They are heavily puered at 70° F. for 3 hours, or even longer for the heavier skins. After scudding they are drenched with 10 per cent. bran and some pea meal at 95° F. for a few hours only.
In preparing the tawing paste, the flour should be mixed with tepid water; the egg yolk should also be diluted with tepid water slightly, and strained if necessary, and then added to the flour. The oil is then carefully mixed in. The alum and salt are dissolved separately at 110° F. and added to the flour and oil. The tawing paste should be used at about 105° F. For every hundred medium-sized lambskins there will be required: 10 lbs. flour in 2½ gallons water, 1 quart preserved egg yolk, 3¾ lbs. alum and 1¼ lbs. salt. The skins are drummed in this for an hour or so and dried out on poles rapidly, but not with great heat. This is essential to get "stretch." They are next wet back, staked, dried and staked again. They are then "aged."
To wet back for dyeing and finishing the skins are drawn through warm water and then drummed in water at 95° F. for 15 minutes to wet evenly and thoroughly. This liquor, which contains much of the tawing material, is run off and replaced by the dye solution,e.g.fustic or turmeric, with which the goods are drummed for half an hour. Iron, chrome or copper salts may be used for saddening. After this "bottom" colour is obtained, a coal tar colour is added for "topping" and the drumming continued until the required shade is obtained. The excess liquor is now run off, and the materials lost in soaking are replaced by drumming further with egg yolk and salt for 15 minutes. This is known as "re-egging." Blacks are obtained with logwood and iron. Afterre-egging, the skins are dried out and staked. They are "seasoned" with a weak emulsion of soap and oil, dried, oiled lightly with linseed oil, ironed, re-oiled and finally brushed. Whites are undyed, and 10 lbs. French chalk per 100 skins is used in re-egging.
"Calf kid" is a once popular but now obsolete upper leather made by tawing calfskins. The skins were well plumped in limes, delimed by washing and drenching, tawed much as for glove kid, split, dried out rapidly, staked and aged. They were finished dull and black with soap and wax.
The various white leathers used for belts, laces, whip lashes, aprons, covers for stoppered bottles, etc., are very usually made with an alum tannage. Alum, salt and flour only are used. Whitening is also mixed in and acts as neutralizing agent as well as pigment dye.
Wool rugs are manufactured from suitable sheepskins by an alum tannage. They are first well cleaned, using soap on wool and flesh. They are next degreased by painting with fuller's earth paste and drying. They are tawed by painting the flesh with a strong solution of alum and salt, or even by rubbing on the solid salts. They are dried out, aged and sorted for suitable colours. The dyeing is rather difficult, as many artificial dyestuffs are of no use. It is usual to bleach the skins first in a weak solution of bleaching powder, and afterwards to dye with infusions of the dyewoods,e.g.logwood, fustic, sandalwood, terra japonica, quercitron bark, turmeric, indigo, etc. Vat dyeing is usual. After dyeing, retanning with alum and salt is necessary, on account of the loss of these in bleaching and dyeing. Rugs are usually finished black, white, grey, brown, walnut, crimson, blue or green.
REFERENCES.Procter, "Principles of Leather Manufacture," pp. 184, 236.Bennett, "Manufacture of Leather," pp. 239, 371.
For the manufacture of a permanent leather the essential requirements are that the fibres of the hide or skins gel should be dried in a separate condition, and that they should be coated by some waterproof or insoluble material. Many substances fulfil the first but not the second of these conditions. For example, the dehydration only may be accomplished more or less by salt (as in curing hides), still better by salt if a little mineral acid be used (as in pickling), and by other salts such as potassium carbonate and ammonium sulphate, and dehydrating agents such as alcohol. Such "temporary leathers," however, are not water-resisting, as the second requirement has not been fulfilled, viz. the coating of the fibres with some more or less waterproof material. Thus if pelts dehydrated with alcohol be treated with an alcoholic solution of stearic acid, the second condition is fulfilled and a permanent leather is obtained.
Now, many tanning agents accomplish these two requirements only imperfectly. As we have noted in the preceding section, the alum-tanned leathers are not very water resisting, and much of the tannage will wash out. Leathers made by the vegetable tannages usually contain some excess of vegetable tanning matters which are soluble in and removed by water, though much tannin can no longer be thus removed, owing to the mutual precipitation of the oppositely charged tannin sol and hide gel. The necessity for fulfilling the second requirement mentioned is one reason for the practice of following these tannages by applications of oil, fat or of both. In this way the isolated fibres are not only dried separately, but are coated with a typical water-resisting material.
In the fat tannages an attempt is made to fulfil this second requirement without the use of any specific "tanning agent" for producing the first requirements;i.e.anattempt is made to dry the fibres separately in an "untanned" condition, and to coat them simultaneously with fat so that a permanent leather is obtained. It is only possible to do this, if the pelt is constantly during drying subjected to mechanical working,e.g.by twisting, folding, bending, drumming, staking, etc. The resulting leather is often called "rawhide leather," and presents a real advantage over other leathers in its great tensile strength. Where toughness is an essential quality, there is much to be said for the fat tannages. It is also possible, of course, to effect compromises between ordinary tannages and the straight fat tannages; thus picking band butts, which must be tough, are often very lightly tanned with oak bark or chrome, and then given what is practically a heavy fat tannage. In the most typical of fat tannages, moreover, it is often common to "colour" the goods by a brief immersion in a weak vegetable tan liquor. Further, the employment of fats in the currying of dressing leather is in effect a fat tannage superimposed upon the vegetable tannage. (See Combination Tannages, Section VI.)
The fat tannage is undoubtedly one of the earliest methods for making leather. Prehistoric man discovered that the skins of animals killed in hunting could, by alternately rubbing with fats and then drying slightly, be eventually converted into a useful leather, whereas without the fat it was stiff and horny. Even yet similar methods are in use, thongs of raw hide being continually twisted during drying, with intermittent application of fats.
In the modern fat tannages drums are used to give the necessary mechanical working to the goods. The raw hide leather produced in the U.S.A. is made by drumming the nearly delimed goods with tallow and neatsfoot oil. In this country the fat tannages have been typified by the "Crown" and "Helvetia" leathers. The hides are thoroughly limed in mellow limes, and after the beam work are delimed by drenching, scudded, and sometimes fleshed again, and then coloured off in tan liquor. After partial drying, they are drummed warm for some hours to ensure isolation of the fibres. After further drying they are coated with the tanningpaste, which consists essentially of soft fats and flour to produce partial emulsification. Equal parts of soft fats and of flour may be used, to which may be added smaller proportions of degras, cod oil, mutton tallow, salt, together with about 25 per cent. water. The goods are coated with this mixture, drummed, and dried further, and this routine repeated as often as necessary to fill the interstices thoroughly with fat. The temperature in the drum may reach 95° F. In finishing an attempt is made to stuff further with grease. The goods are thoroughly set out, dried a little, and coated again, flesh and grain, with a mixture of tallow, cod oil, glycerine and degras, and dried further. The excess grease is slicked off and the goods again set out and grained. They are then dried out.
REFERENCES.Bennett, "Manufacture of Leather," pp. 245, 246 and 376.Procter, "Principles of Leather Manufacture," p. 378.
There are very obvious analogies between the fat tannages discussed in Section II. and the oil tannages now to be dealt with, but there is nevertheless a distinct departure in principle involved. In the oil tannages the mechanical treatment is generally more vigorous, and the "drying" process is conducted at a much higher temperature, with the result that there is a vigorous oxidation of the oil. This results in the formation of insoluble oxidation products which coat the fibre and play an essential part in the production of a permanent leather. Pungent vapours are evolved in the drying operations, amongst which is acrolein and probably also other aldehydes, and it is thought by Procter that these aldehydes also are essential tanning agents and typical of the process (cf. Section IV.). Fahrion considers that the tanning action is due solely to unsaturated fatty acids with more than one double linkage. Garelli and Apostolo, however, believe that the tannage is due to a coating of fatty acid whether saturated or not. These observers made leather with stearic and palmatic acids in colloidal aqueous solution.
The manufacture of chamois leather from the flesh splits of sheepskins comprises the largest and most typical branch of the oil tannages. The sheep pelts are split in the limed state, and the fleshes are given another sharp liming which may last up to a fortnight. They are next "frized,"i.e.scraped over the beam with a sharp two-handled knife, to remove roughness and loose fat. The goods are next thoroughly washed in running water and drenched. A paddle drench is often preferred, and if not used the handling should be frequent. Paddling drenching reduces the time required from about 16 hours to about 6 hours. An hour or morein a hydraulic press removes superfluous liquor and some more grease. The fleshes are separated, cooled and then stocked for 30 minutes to equalize the moisture in them. After removing from the stocks they are sprinkled on both sides with cod oil and thrown back into the stocks for a few hours. They are then dried cold for a day or two. The stocks used are similar to those once popular for softening dried hides during soaking, and consist of two heavy hammers which fall alternately upon the goods which are contained in a curved box below. The result is a mechanical kneading action. The fleshes are again sprinkled with cod oil, restocked for a few hours and dried again, this time at 100° F. They are then repeatedly sprinkled, stocked and dried, the last operation being conducted always at an increasing temperature until finally the final "heater" is even up to 160° F. As the operation proceeds it is advantageous to hang the splits also nearer one another, and in the final "heater" they are quite close. The next stage is to pack the goods quickly into suitable boxes and allow them to "heat,"i.e.to oxidize further. This is a rather critical stage in the process, and to prevent overheating ("burns") it is often necessary to open out and repack into another box, with possibly some little intermediate cooling. They are turned over thus repeatedly until the oxidation is complete, and then spread out to cool.
The fleshes are now a dark brown colour, and are next treated to remove excess of oxidized oil products. The goods are dipped through water at 110° F. and then subjected to hydraulic pressure. The grease and water which exude are allowed to separate by settling, and the thick yellow oil so obtained, known as "degras," forms a valuable material for leather dressing, as it more readily emulsifies with water than many oils, and impart this quality to other greases mixed with it. A further quantity of a similar oil is obtained by paddling the goods with a weak soda solution. The liquor obtained is treated with sulphuric acid to neutralize the alkali, and the grease recovered is known as "sod oil." The fleshes are now well washed with hot water (140° F.), fat liquoredwith cod oil and soft soap, machine sammed, either by a wringer or a centrifuge, and then dried out.
Much chamois leather is also made in France by closely similar methods. The skins are usually oiled on tables and folded up before stocking. Other marine oils (seal, whale, etc.) replace cod oil. Generally speaking the oxidation is more moderate, and the grease from the hydraulic press (moellon) is mixed with other fish oils to form commercial degras. An inferior quality of degras is obtained by subsequent treatment with soda.
The crust chamois obtained as above has only to be thoroughly staked to soften, "grounded" and "fluffed" to raise the nap, and then trimmed, and the ordinary wash-leather is obtained.
If intended for glove leathers superior skins are selected. These are fluffed carefully upon emery wheels, using first a coarse surface and eventually a fine surface so that a fine velvet effect is attained. The skins are next bleached.
In the "sun bleach" or "grass bleach" the goods are soaked in a 1½ per cent. soft soap solution and exposed to sunlight after being wrung. They are bleached in about 3 days in summer, but nearly a fortnight may be necessary in winter.
In the permanganate bleach, which is less tedious, the skins are first degreased by soaking in a warm ¾ per cent. solution of soda crystals and then drumming for 30 minutes in water at 95° F. They are then paddled in a ⅛ per cent. solution of commercial permanganate for an hour at the same temperature, rinsed through water, and the brown manganese dioxide is then removed by paddling or drumming the goods in a 3 per cent. solution of sodium bisulphite to which hydrochloric acid is added as required. The goods are well washed in warm water, and are then "tucked,"i.e.placed in a vat of boiling water containing a little soft soap, just for a few seconds. The goods shrink and curl up, and they are then dried out at 120°-140° F. to fix the tuck. They are then staked, fluffed, and dyed.
In dyeing with coal tar colours the alizarin colours may be used aftermordanting with chrome alum. Direct dyes, natural dyestuffs and pigment dyes are also used. The goods are struck out after dyeing, lightly fat liquored with commercial egg yolk, dried out at 110° to 120° F., staked and fluffed on the face side.
Buff leather is a similar leather made from hides. They are limed mellow for a fortnight, unhaired, fleshed, and then limed again for another week in sharp limes. The grain is then split off, and the goods rinsed and scudded, slightly delimed and hung up to dry. They are then treated in much the same way as fleshes for chamois, but lime is often added to the cod oil used in stocking.
Buck leather is a similar product obtained from deerskins, but much mock buck is made from cheaper raw material.
REFERENCES.Bennett, "Manufacture of Leather," pp. 247-250 and 376-379.Procter, "Principles of Leather Manufacture," p. 378.
The use of formalin for hardening gelatin has long been known, but it was left for Payne and Pullman to devise a commercial process for tanning pelt into leather by means of formaldehyde (H·CHO) solutions. Their process, which was patented, specified the use of alkalies in conjunction with formaldehyde or other aldehydes. The function of the alkalies is not very obvious, for it has been shown that formaldehyde will tan also in neutral and in acid solution. The precise action of the aldehydes is also as yet somewhat obscure, but it is noteworthy that very small proportions of formalin will give a complete tannage. It is probable that the action of formaldehyde is not perfectly analogous with that of its homologues, for it is a most reactive substance, and will certainly with proteids undergo reactions which are not analogous to those with other aldehydes. The leather obtained by tanning with formalin is quite white and resembles buff leather, but has advantages over the latter in that no bleaching is necessary.
According to the patent specifications the pelt should be drummed in water and the tanning liquor—a solution of formalin and sodium carbonate—added gradually at 15-minute intervals. Up to 6 hours for light skins, and up to 48 hours for heavy hides, are required for complete tannage. The temperature is raised during the process from 100° to 118° F. The tanning liquor may be made from 16 lbs. of commercial formalin (36 per cent. formaldehyde) and 32 lbs. soda (80 per cent. Na2CO3) in 10-15 gallons of water. This should be added, one gallon at a time, to 4 cwt. pelt in 100-120 gallons of water. After tannage is complete the goods should be paddled with a 1½ per cent. solution of ammonium sulphate to remove the soda, and "nourished" in a solution of soft soap and salt, about 2¼ per cent. of each on theweight of pelt. The goods are then dried out, and may be finished like chamois, buff, and buck leathers (Section III.).
REFERENCES.Payne and Pullman, English Patent 1898, 2872.Bennett, "Manufacture of Leather," pp. 250 and 379.
In spite of much valuable work on the constitution of the vegetable tannins and the compounds usually associated with them, such as that of E. Fischer, K. Freudenberg and their collaborators on gallo-tannic acid, and that of A.G. Perkin on ellagic acid and catechin, we are still in the dark with respect to the constitution of the tannins which are of commercial importance, and any synthetic production of these materials is thus out of the question as yet. Attempts, however, have been made to produce artificially substances which possess similar properties to the tannins and which may be used for converting pelt into leather. Into this category fall some of the earlier attempts to synthesize gallo-tannic acid by heating gallic acid with condensing reagents.
The first commercial success in this direction was attained by Stiasny, who produced condensation products of the phenolsulphonic acids, to which products he gave the general name of "syntans" (synthetic tannins). The Badische Co. placed one of these products on the market as "Neradol D," and later took out subsidiary patents for the manufacture of similar products by slightly differing methods of productions. Since the outbreak of the European War such patent rights have been suspended, and several British firms have been manufacturing synthetic tanning materials by similar methods, but doubtless with developments and improvements of their own discovery. These products (e.g.Cresyntan, Maxyntan, Paradol, Syntan, etc.) are now in use in many factories, and assist rather than substitute the vegetable tannins in producing leather of the desired colour and quality.
These synthetic tanning materials resemble the vegetable tannins in the following respects. They are organic acids containing phenolic groups.They are semi-colloidal, passing slowly through semipermeable membranes. They precipitate gelatin, basic dyestuffs and lead acetate, give a violet-blue colour with ferric salts, and convert hide into an undoubted leather. They differ from the vegetable tannins in that they contain sulphur and sulphonic acid groups, but they agree in that both are aromatic derivatives. In each case the tanning effect is diminished by alkalies, but the synthetic materials are the more sensitive.
Methods of Manufacture.—There are, broadly speaking, three types of method by which these condensation products are produced, viz., condensation by formaldehyde, condensation by phosphorus trichloride or similar reagents, and condensation by heat alone. Illustrative methods will now be given.
Condensation by formaldehyde was the first method used. The procedure is given by the Austrian patent 58,405. A phenol,e.g.crude cresylic acid, is heated with the equivalent amount of sulphuric acid for a few hours to 100°-210° C., cooled, and formaldehyde added slowly whilst cooling and stirring, in the proportion of one molecule of formaldehyde to 2 molecules of phenol. The free mineral acid is neutralized, and the resulting product is the syntan "Neradol." By this procedure only water-soluble products are obtained, but an alternative process is to heat the phenols in slightly acid solution, and then to render soluble the resinous products obtained by treating with sulphuric acid. The proportion of formaldehyde to phenol used led Steasny to conclude that the resulting products were diphenyl-methane derivatives which polymerize to form molecules of considerable size. The formaldehyde supplies the "carbon bridge." This view was criticized by A. G. Green as too simple, and he suggested the alternative theory that polymerization does not take place at all, but that more advanced or higher condensation products are formed; he thought that o-hydroxy-benzyl alcohols were first produced, that these condensed with another molecule, and afterwards the process was repeated. The result was a "colourless dyestuff." This view receives some support from the othertypes of method of manufacture.
With the use of other condensing reagents the procedure may be as in the process of the B.A.S.F. (Fr. pat. 451,875-6), thus: 225 parts of o-cresol-sulphonic acid are heated to 60° C. for 4 hours with 262.5 parts of phosphorus oxychloride. The excess of oxychloride is removed by distillation under reduced pressure and the residue washed with dilute hydrochloric acid.
Condensation by heat alone is illustrated by the method given in the same patents, thus: phenol-p-sulphonic acid is heated to 130° C. for 24 hours under a pressure of 20 mm. or in a current of dry air at atmospheric pressure. The product may be used direct or may be purified by dissolving in water, neutralizing with caustic soda, filtering and evaporating to dryness. A white powder is obtained which tans when its solution is acidified. An alternative is to mix phenol with sulphuric acid and heat the mixture to 140° C. for 72 hours under 20 mm. pressure and purify as before.
Methods of Use.—The synthetic tanning materials may be put to many uses. When well manufactured they make practically a white leather, and this fact makes a valuable opening for their use in connection with light leather tannages and the dressing of rugs. It is also claimed that they improve the colour usually obtained in the ordinary vegetable tannages. If used in the suspenders to the extent of 5-10 per cent. they are said to brighten the colour throughout the tannage. If used in bleaching and finishing they are said to lighten the colour of the finished leather. About 5 per cent. on the weight of the goods may be added to the bleach or vat liquors; they may be also mixed with sumac during finishing, and in effect act as a sumac substitute; solutions are also brushed over the grain before oiling, with a view to obtaining good colour. It is also claimed that their use prevents vegetable-tanned leather from becoming red under the action of sunlight. The syntans are also used to lighten the colour of chrome leather, even of chrome sole leather after it has been dipped.
It is claimed also that syntans produce a tough leather, and if used for heavy leather in the early stages they give a tough grain and assist in avoiding a cracky grain. On this account they are also recommended for re-tanning E.I. tanned kips. When used in heavy leather suspenders they are said to get rid of lime blast (CaCO3) and to quicken the tannage,i.e.to enable the same weight to be obtained in less time. Procter suggests that a tannage of commercial value might be obtained by blending them with wood pulp extract.
If used alone for tanning a series of pits containing liquors of 4° to 37° Bkr. may be used, but drum tannages may be given using liquors of 14°-29° Bkr., the goods being tanned in 6-8 hours. About 30 per cent. of syntans are said to be necessary for complete tannage.
REFERENCES.E. Stiasny, "A New Synthetic Tannin,"Collegium, 1913, 142-145. (See alsoJ.S.C.I., Abs. 1913, 500.)E. Stiasny, "Syntans—New Artificial Tanning Materials,"J.S.C.I., 1913, 775.Patents:—Austrian 58,405.German 262,558, Sept. 12, 1911.French 451,875, Dec. 13, 1912; 451,876, Dec. 13, 1912; 451,877, Dec. 13, 1912.
The formation of leather being due to the adsorption of colloidogenic substances at the interface of the tanning liquor and the hide gel, there is the obvious possibility that several such substances may be used simultaneously, and that the resulting leather may be due to the combined effect of these substances. Indeed, the average vegetable tannage consists of such a combination tannage, each tanning material contributing its own individual tannin and characteristic astringent non-tannins. There is evidently also the possibility that the differenttypesof tannage discussed above might be used either simultaneously or successively, and that a leather might be obtained which combines to some extent the qualities of each of the types in combination. It is such a case that is generally called a "combination tannage." There are many conceivable combinations, and in this section will be chiefly discussed a few which have demonstrated some commercial possibilities. Some of these have already received notice in the preceding sections. The manufacture of curried dressing leathers is a combination of vegetable and fat tannages. The manufacture of waterproof chrome uppers illustrates a combination of chrome and fat tannages. The use of "syntans" in conjunction with vegetable tanning materials is also a combination tannage. The case of chamois leather is possibly a combination of aldehyde tannage with fatty acid tannage. Two-bath chrome leather is a combination of chrome, sulphur and fat tannage. Formaldehyde and vegetable tannage is also a known possibility. It is clear that there are possibilities of endless complexity, and that what normally may appear as a simple tannage is in reality a very complex combination tannage. From this standpoint one might instructively consider the successive adsorptions involved in a goatskin tanned first with syntans, then with oak bark, "retanned" in sumac, mordanted withchrome, dyed with coal-tar dyestuffs and finally oiled with linseed oil. It will be easily seen that in a very strict sense nearly all tannages are combinations.
Usually, however, the term "combination tannage" is confined to those cases where the main tanning agents not only differ in type, but where none are in predominant quantity. A typical case is that of "semichrome leather," in which a vegetable tannage is succeeded by a chrome tannage. E.I. tanned sheep and goat skins are rather heavily "stripped" of their vegetable tannage and heavy oiling, by drumming with warm soda solutions, and after washing with water are chromed with the one-bath process; they are neutralized, dyed, fat liquored and finished for glacé upper leather.
In a precisely similar way kips and split hides which have received vegetable tannage are stripped and retanned in chrome and finished as for box calf, of which they are a good imitation. Such vegetable-chrome combination tannages possess many of the properties of chrome leather.
To chrome the pelt first and afterwards to subject it to vegetable tannage is also an obvious possibility, but has not yet been made a commercial success in this country, but has been increasingly used in the U.S.A. during the War.
Another typical case of combination tannage is the dongola leather produced by the use of gambier and of alum and salt. This is a vegetable-alum combination, and yields a good quality leather for light uppers, gloves, etc. Goatskins for "glazed dongola" are paddled tanned in gambier liquors, and alum and salt are subsequently added. They are tanned in 24 hours, well washed, and are fat liquored without ageing. The E.I. tanned skins may also be stripped with soda, and retanned in alum and salt, using flour also if desired. Dull dongola are first tawed and then retanned in gambier liquor. "Suède" and "velvet calf" are also tawed and retanned with gambier.
Yet another case of combination tannage is that of sheepskins for glacé uppers, which are first tawed thoroughly with alum, salt and flour anddried out for sorting, and are then retanned in chrome by the one-bath process, and finished as usual. Closely related to this is the method of "pickling" in alum and salt and then chrome tanning.
Another case is the combined one-bath, two-bath method of chrome tanning. The goods are chromed by a one-bath liquor containing dichromate (say 2 per cent.), and then pass into a reducing bath. There is not much advantage in such procedure, however.
From a strictly commercial point of view the "dongola" and "semichrome" leathers have proved the most successful combination tannages, but there seem to be possibilities in combinations of the vegetable tannins with synthetic tanning materials.
Many other substances are known to tan,e.g.iron salts, cerium salts, sulphur, quinones, fatty acids, the halogens, etc., etc.; hence there is always the possibility that new useful combination tannages may be discovered.
REFERENCES.Bennett, "Manufacture of Leather," pp. 243, 374-5.Procter, "Principles of Leather Manufacture," p. 236.
The leather trades are amongst the oldest of all industries, but their evolution has been much more rapid during the last two or three decades than at any other period of their history. The European War, moreover, has caused the commencement of another period of rapid development, and it is the aim of this section to point out some of the principal lines of change which have already become apparent.
Many of these lines of evolution in the methods of manufacture have been previously discussed in their appropriate sections. They may all be summarized as attempts at more economical production. Prominent amongst them is the persistent effort to attain quicker processes. During the last twenty-five years the time necessary to produce the heavy leathers has been reduced from 12 months to as many weeks. The tendency is to reduce the time further still, but this is of course increasingly difficult to accomplish. On the other hand, it is more urgent to strive in this direction than ever, because a needless week involves more capital lying idle than ever before. Moreover, as most leather factories are now large works, a saving even of 24 hours has become a serious item in economic production. Hence in liming, bating, tanning, drying and in warehousing there are increased efforts to make a quicker turnover.
A good illustration of this "speeding up" in modern tanneries is the adoption by all large factories of much more rapid methods of extracting tannin. On the old press-leach system liquors may be percolating through the material for possibly a fortnight. The extract manufacturer reduces this operation to about two days. Steam generated from the spent bark is used to heat the extracting vats, and to work a vacuum pan or evaporator whereby more water can be used and a more complete as well as a more rapid extraction obtained. The evaporator also makes easy the preparationof the strong liquors used in modern tanning.
Hand-in-hand with quicker production and manipulation are the attempts to obtain a larger turnover. It is realized that the big business attains cheap production. Even before the war the smaller factories were disappearing. A small tannery must now either extend or close down. This has been better realized in the heavy than in the light leather trades. In the sole leather tanneries very often many thousand hides per week are put into work, but in the glacé kid factories there is nothing yet to correspond to the output of American glacé factories, which sometimes reaches three or four thousand dozen a day.
Another very prominent feature of factory evolution is the increased use of labour-saving machinery. This practice has been in operation for a considerable time, but with marked acceleration during the last few years owing to the labour shortage occasioned by military service. This development of machine work has largely dispensed with that labour which involved any skill or training. The journeyman currier is now practically extinct. In the beam house, too, fleshing, unhairing and scudding are rapidly becoming machine instead of hand operations. Many devices are now being adopted also which reduce the quantity of unskilled labour needed. Instead of "handling" the goods from pit to pit, modern tanneries aim at moving the liquors. Thus in the "Forsare" and "Tilston" systems of liming, hides are placed in a pit and lie undisturbed until ready for depilation, the soak liquors and lime liquors being supplied and run off just as required, whilst these liquors are agitated as often as desired by means of a current of compressed air. This agitation replaces the "handling" up and down once practised. In the tanyard proper the same tendency is at work, "rockers" are increasingly preferred to "handlers," and an inversion of the press leach system permits the exhaustion of tan liquors by a gravity flow, and so avoids the handling forward from pit to pit. There is also a tendency to install lifts, overhead runways, trucks on lines, motor lorries, etc., to replace carrying, barrowing, carting, etc., and so toarrange the tannery that the minimum transport is needed.
All these lines of evolution involve more intensive production, and necessitate much more careful supervision. It is not surprising, therefore, that the industry now feels that scientific oversight and administration are essential. A dozen years ago the trade chemists were largely unqualified men, whose work lay solely in the laboratory, and consisted mainly in the analysis of materials bought. To-day all large tanneries have qualified chemists, and it is realized that they are the practical tanners. Their function is so to control the manufacturing processes that all waste is avoided, and so to correlate and co-ordinate the manufacturing results with the analytical and experimental records of the laboratory, that constant improvements are made in the methods of production. The extended use of machinery, and the necessity for economy in coal and power, give the engineer also very large scope for useful work. Modern business conditions, moreover, have made necessary more skilful clerical work and accountancy in the large offices of a modern tannery.
In the creation of cordial relationships between capital and labour in the leather trades, there has been unfortunately little progress. The leather trade is not a sweated industry. Its workers have always enjoyed reasonable hours of work. In most factories an approximate 48-hour working week (involving no night work) has long been in operation. The industry, however, is not one in which high wages obtain. The average tannery worker receives a wage which is never much above the level of subsistence. This is mostly due to the fact that he is usually a quite unskilled labourer, and is therefore on the bottom rung of the labour ladder. In addition to this the work itself is often distressingly monotonous, and makes little demand upon the intelligence of the worker. The trade consequently offers little attraction to the intelligent labourer. The old system of apprenticeship is now quite obsolete, partly owing to the rapidity of the changes in the methods of manufacture, partly to the specialization of labour which results from the development of large factories, and partly also, because to understandmodern tanning involves a better general education than most workmen receive. It is indeed frequently difficult to find competent under-foremen for the different departments of the modern leather factory. Until recently leather workers have been either unorganized or badly organized, and their views and complaints have been confused and sporadic, but during the war period there has been a very rapid extension of trade union movements, and consequently a more articulate expression of the demands for "democratization" as well as "a greater share in the fruits" of the industry. In the leather trades, however, the gulf between the unskilled labourers and the wealthy employers is perhaps unusually wide, and there is little disposition on the part of capital to recognize the equity of either of the above demands of labour. Generally speaking, the leather trade firms are not public but private companies. There is absolutely no trace of "co-partnership" or "profit-sharing" schemes, or of co-operative production. There is little recognition that the trades' prosperity should be shared in any way by the workpeople, and still less recognition of any right to a voice in industrial conditions. This condition of affairs has an ominous reaction upon the attitude of labour, which believes that it is producing great wealth but not obtaining much more than subsistence. It is not the function of this volume to pronounce a verdict upon the wages question or upon the democratization of the leather trades, but one may be permitted earnestly to hope that if such be the future lines of development, there will be also, as an absolutely essential part of any such schemes, a much higher standard of education amongst the workers, for this is the only satisfactory guarantee that the voice of labour in council will have any practical value, or that higher wages will be at all wisely used by the recipients.
In his instructive and valuable volume on "The Evolution of Industry," Prof. MacGregor points out that modern industry has evolved three outstanding types, viz. the Co-operative Movement, the Trusts, and the methods of Public Trading. He also suggests that these types tend to blend. In the leather industry co-operative and municipal productionare unheard of, but the industry has certainly developed along the lines of the large trusts. Large businesses have replaced small, and later still have formed local federations, which in turn have combined to form the "United Tanners' Federation." War conditions have certainly stimulated evolution towards the trust type. The United Tanners' Federation has become possessed of powers which were not originally contemplated, such as the purchase and distribution to its members of hides, bark, extract, sulphide and other materials. How far some of these arrangements will be permanent is problematical, but one beneficial result is that the allied trades have certainly realized more thoroughly their unity of interests. This is shown by the much freer collaboration of the tanners, and by the encouragement now given to similar collaboration between their chemists. More evidence is found in the proposals for combined research.
There is also considerable reason to believe that there is some movement in the direction of partial State control. There is little doubt that evolution along trust lines will make this less difficult and possibly more desirable. The country cannot afford the spectacle of a Leather Trust permanently at war with a Labourers' Union. The public has realized that the well-being of the leather industry is vital to the national safety. It has realized that the leather trades are great producers of national wealth, and that increased production with the development of the export trade will materially assist to restore the country's financial position. It has realized also its own right to protection from bad leather and from exorbitant prices. On all these grounds it is probable, though there may be some reaction from the present position, that the State, which has already got its fingers in the pie, will refuse to draw them out altogether. The Imperial aspect of the question affords some further justification for this attitude. The leather trades operate very largely upon imported material, and it is clearly desirable that there should be close co-operation between the home industry and the colonial supplies of material. Here too the war has also given a great stimulus in this direction. Indian myrabolans has long been a staple tanning material. South African wattle bark hasduring the last few years replaced almost completely, and probably to a large extent permanently, Turkish valonia. There has also been great increase in the imports of Indian kips and of South African hides, and it is not at all an impossible proposition to maintain a self-contained Imperial Leather Trade, should this be necessary. French chestnut extract, and quebracho extract, however, are much too valuable tanning materials to exclude for merely sentimental reasons. These instances indicate possible advantages in Imperial co-operation, but also show the need for caution in the elaboration of such schemes.
Although a partial, and indeed increasing, measure of State Control is probable, there has been as yet no serious proposal to nationalize the leather industry. Such a proposition, indeed, is hardly ripe even for discussion. Until the nationalization of transport and of mines is a proved success, and until the merely distributive undertakings of the municipalities (e.g.of coal and of milk and other foods) are past the experimental stage, any proposition to nationalize the leather trades seems premature. It is noteworthy, however, that in Queensland, Australia, the Government have the right to commence and to administer State Tanneries.
Any progress in the direction either of democratization or of nationalization, has been certainly postponed by the sudden and unprecedented trade slump which commenced in the earlier part of 1920. This depression, in spite of heavy falls in the prices of raw materials, has made economic production a much more difficult problem. It has undoubtedly given a further stimulus to evolution towards the trust type, and created a further tendency towards the closing of the smaller factories, and the employment of labour-saving devices. When the general fall in prices has made an appreciable fall in the cost of living, some reduction in the leather workers' wages, together with more efficient work, will also contribute to the solution of the difficulty. It is chiefly to be desired, however, that the export trade should be restored. The realization of this hope depends largely upon the establishment of peace and prosperity abroad, and the consequent stabilization of the various foreign exchanges.
Many of the chemical properties of gelatine, especially those which distinguish it from other proteins, have been described in the Introduction to this volume, and need no further comment. In this section its colloid nature and behaviour will chiefly be considered, for these points have greatest importance from the standpoint of industrial chemistry.
It is hoped, moreover, that this section will be of interest not only to the chemist concerned in the manufacture of gelatine and glue, but that it will be of value also to those concerned in leather manufacture. The difference between the "collagen" which composes the hide fibre and the high-grade gelatines is so small that for many practical purposes it may be considered negligible. Thus the description of the behaviour of a gelatine gel is very largely applicable to a hide gel also.
Gelatine has been crystallized by von Weimarn by evaporating a dilute solution in aqueous alcohol whilst in a desiccator containing potassium carbonate, the temperature being maintained at 60°-70° C. The carbonate takes up water only, and the concentration of the alcohol therefore slowly increases until the gelatine is no longer soluble. Gelatine is usually found and known in the colloid state, however, and its behaviour in this state only is of practical importance.
The fundamental idea of modern colloid chemistry is that colloids are heterogeneous systems, usually two-phased, in which one phase is liquid and the other phase either liquid or solid. The latter phase, which is divided into small separate volumes, is known as the "disperse phase,"whilst the other is the "continuous phase" or "dispersion medium." The "dispersity" is the degree to which the reduction of the dimensions of the disperse phase has been carried, and is best expressed numerically in terms of "specific surface,"i.e.surface area divided by volume, but it is also often expressed as the thickness or diameter of a film or particle. When the dispersity is not high, we have ordinary "suspensions" and "emulsions," which with increasing dispersity merge into the typical colloids. By analogy, colloids have been divided into "suspensoids" and "emulsoids," when the disperse phase is solid and liquid respectively. The classification, however, has not been found satisfactory, for some systems in which the disperse phase is undoubtedly liquid, exhibit characteristic properties of suspensoids, andvice versâ. A more satisfactory division, therefore, is found in the presence or absence of affinity between the two phases, the systems being termed "lyophile" and "lyophobe" respectively. If water be the continuous phase the terms "hydrophile" and "hydrophobe" are often used. Broadly speaking, the lyophile colloids correspond to the emulsoids, and the lyophobe colloids to the suspensoids. Gelatine is a typical hydrophile colloid.
Another fundamental idea of colloid chemistry is that the great extension of surface involved in a high dispersity causes the surface energy to be no longer a negligible fraction of the total energy of the system, and that the recent advances in knowledge respecting surface phenomena may be called in to assist in the explanation of the special properties of the colloid state. Particles which exhibit the Brownian movement, about 10-5cm. diameter, down to the limit of microscopic visibility (10-3cm.) are termedmicrons. Particles less than this, but just visible in the ultra-microscope (5×10-7cm.) are termedsubmicrons. Particles still less, approximately 10-7cm., have been shown to exist, and are termedamicrons. The dimensions of molecules such as may exist in true solutions are of the order of 10-8cm. A colloid sol may contain particles of various sizes. Thus a gelatine sol(like other lyophile systems) contains chiefly amicrons, but submicrons are also observable.
Owing to the contractile force of surface tension, it is concluded that the surface layer of a liquid is under very great pressure, much greater than the bulk of the liquid. Any extension of the surface of the liquid naturally causes a corresponding extension of the proportion of liquid which is thus compressed. If in a beaker of water there be placed a porous substance, such as animal charcoal, there is a great extension of the surface of the water, and a corresponding increase in the amount of compressed water. If instead there be substituted a large number of very small particles of a substance, a still further increase in the amount of compressed water is involved. As the specific surface of the substance inserted is increased, and its amount, the proportion of compressed and denser water increases also, until it is a practically appreciable percentage of the total volume. It is clear also that the extent of the zone of compression will be determined also by the nature of the substance with which the water is in contact at its surface,i.e.by the extent to which it is hydrophile, and this indeed may be the more important factor.
Now in a gelatine sol we have the necessary conditions for a system in which the compressed water bears an unusually large ratio to the total, owing to the enormous surface developed by the minute particles of the disperse phase (amicrons) and to the unusually wide zone of compression surrounding each particle caused by the strongly hydrophile nature of gelatine. It should be pointed out that these zones of compression do not involve any abrupt transition from the zone of non-compression, the layer nearest the particle is under the greatest pressure, and the concentric layers under less and less pressures, the actual compression being thus an inverse function of the distance from the particle. Now if there be a gradual increase in the concentration of the sol, the time will come when these zones of compression begin to come in contact, andthe system will then show a considerably increased viscosity. With further increase in concentration the zones of compression will overlap throughout the system, and when the layers under considerable pressure are thus continuous, the whole system will acquire a rigidity much greater than water and approaching that of a solid body. This is a gelatine gel, or "jelly." With increasing concentration the jelly becomes increasingly rigid, and if it be eventually dried out under suitable conditions it forms what is practically solid body—gelatine—which, however, still contains from 12 to 18 per cent. of water.
It will be clear that, in the case of gelatine jellies (e.g.of 3-10 per cent. strength), an increase in temperature will cause an increase in the kinetic energy of the particles and effectively reduce the zones of compression. Indeed, they may be reduced to such an extent that they are no longer in contact, and the rigidity due to the continuous contact of the layers of great compression will then disappear; as we say usually, the jelly melts. On cooling, the decreased kinetic energy of the water molecules results in the return of the state of compression, with rapidly increasing viscosity and eventual gelation; as we say usually, the jelly sets. Neither of these changes takes place at a definite temperature (like a melting-point), and in "melting" (solation) or in "setting" (gelation) the temperature-viscosity curve is quite continuous. By various arbitrary devices, however, approximate melting and setting points may approximately be determined. The results also vary somewhat with the concentration of the gel or sol. Gels between 5 and 15 per cent. strong melt about 26°-30° C. and set at 18°-26° C.
On this view, we must regard a gelatine gel as a continuous network of water under great compression, and in this network are zones of still greater compression, which surround the particles of the disperse phase—the gelatine itself, and zones of less compression which in a weak gel, at any rate, have a compression equal to or much the same as the normal state of compression in water.
One consequence of this system is, that when a piece of gelatineswells, there is a considerable enlargement in the zones of compression; in other words, some, at least, of the imbibed water is compressed. Now the compression of water means that work is done, and when gelatine swells, therefore, we expect—and actually find—that heat is liberated (5.7 cal per g. gel). Hence also by the Le Chatelier theorem, we expect—and find—that gelatine swells best incoldwater. Further, the compression of water involves a decrease in volume, and we therefore expect—and actually find—that the volume of the swollen jelly is appreciably less than the volume of gelatine plus the volume of water imbibed.
Another consequence of such a compressed system is that a gelatine jelly, even in water, will have a surface tension towards water just as the water itself has such a tension to the water vapour above the liquid. This interfacial tension of the jelly will of course have a contractile effect, and will tend to resist swelling and to limit it as far as it possibly can. This force, tending to contract the jelly and resist imbibition is therefore one of the main influences at work in the swelling of gelatine, and is one of the two principal factors which determine the extent of the maximum swelling when equilibrium is established. The force tending to resist swelling is, in the ultimate, just surface tension. Its actual magnitude depends, of course, mainly upon the extent of compression in the dispersion medium of the gel, and will be a resultant which is a function of this compression. The magnitude will thus vary with the average compression in the continuous network of compressed water. It will be obvious that as the jelly swells the power of resisting the swelling will decrease, and the interfacial tension with the external water will tend to disappear. If the force tending to swell were great enough the swelling would continue until the zones of compression were no longer in contact and the gel would become sol.
As suggested above, it is probable that the extent of the zones of compression is determined by another factor in addition to the great development of surface. That factor is connected if not identical with that power which makes the system lyophile, and is evidently connectedalso with the solubility of the disperse phase, and may indeed be electrochemical forces tending to form a series of hydrates, or at least to cause an orientation or definite arrangements of the water molecules in the zone of compression. This idea receives some support from the hydrate theory of solution, and the zones of compression and orientation are the colloid analogue of the hydrates supposed to exist in solutions of electrolytes. The extension of such zones on cooling are then analogous with the series of hydrates formed, for instance, by manganese chloride with 2, 4, 6, 11, or 12 molecules of water when crystallized at temperatures of 20°, 15°,-21°,-30°, and-48° C. respectively, the idea being that the salts most hydrated in solution crystallize with most water.
As the compression is the result of two factors, one of which depends upon the nature of the disperse phase, we expect—and find—in other lyophile systems a considerable variation in their power of gelation. Some indeed, though very viscous,e.g.egg albumin, never quite set like gelatine, and others (e.g.agar-agar) set to a stiff gel from a much weaker sol than gelatine. When the zones of compression are large, as in gelatine, the magnitude of the compressing force on the outermost part of the zone is relatively small, and it is not surprising that time is necessary for the victory of this force over the kinetic energy of the water molecules. Hence we find a 5 per cent. jelly sets readily on cooling, but its elasticity increases steadily for many hours after it has set. This phenomenon, known as hysteresis, we should expect—and find—to be much more marked in a case where the zone of compression is unusually large (e.g.an agar gel). We should also expect—and find—that hysteresis is more marked in a high-grade gelatine than in a low-grade gelatine where both eventually form gels of equal elasticity. We should expect too—and we find—that hysteresis is more prominent in weak gels than in strong. These points are of obvious importance in testing gelatine by its elasticity,e.g.the well-known "finger test."
There are also other facts and considerations which have an important bearing upon the point under discussion. It is necessary ultimately to regard true solutions of electrolytes and other bodies as heterogeneous, though perhaps of a rather different order. From this point of view molecules and ions existing in an aqueous solution will present a surface and have associated zones of compression analogous with those suggested for the minute particles of gelatine.
Now recent investigations have shown that the essential physical properties of water are affected by dissolved substances in a definite manner and to a fixed extent, and that these substances exhibit a sequence in order of their effect. This sequence is also exhibited in the essential properties of water as solvent and as dispersion medium for colloid sols. The sequence is known as the "lyotrope series." Thus the numerical value of the compressibility of aqueous solutions is reduced below that of water by salts which, with the same kation, exhibit an effect in the following order:—
CO3> SO4> Cl > Br > NO3> I
This same order is observed, in the effect on the increased values for the surface tension, density and viscosity of these solutions. On the other hand, the kations have a similar sequence of effects,
Mg < NH4< Li < K < Na < Rb < Cs
which appears when salts of the same anion are chosen. It is not surprising to find that this lyotrope series exhibit an analogous influence on the chemical reactions of water,e.g.the hydrolysis of esters. In the hydrolysis by acids SO4retards the action, the other anions and the kations accelerate it, in the lyotrope order. In the hydrolysis by bases the series is reversed. Similarly the lyotrope series exert the same order of effect upon the inversion of cane sugar and other reactions.
This lyotrope influence has also been shown to exert considerable effect in the behaviour of lyophile sols. With the lyophobe sols the addition of foreign substances apparently affects the disperse phase only, but with the lyophile sols the effect on the continuous phase is also important,and may overshadow the other. Now, in gelatine and in hide gels and tanning sols we are dealing with lyophile systems, and there are many points of behaviour in which lyotrope influences become prominent. Similar effects are observed upon other lyophile sols (e.g.albumin, agar-agar, etc.) which differ widely in chemical nature. Thus the salting out of albumin (reversible precipitation) is influenced by sodium salts in lyotropic sequence as follows. The anions hinder precipitation; in order of precipitating power they are:
citrate > tartrate > SO4> acetate > Cl > NO3> ClO3> I > CNS
The sulphates illustrate the kation effect, which is independent and which favours precipitation:
Li > K > Na > NH4> Mg
If the experiments be carried out in faintly acid solution this order of effect is exactly reversed, iodide and thiocyanate having the greatest effect and citrates the least. The coagulation temperature of albumin and the coagulation by other organic substances are similarly influenced by the lyotrope series.
Lyotrope influence also exerts a powerful effect on the behaviour of gelatine sols and gels. The gelation temperature is influenced thus:—
raised by: SO4> citrate > tartrate > acetate
lowered by: Cl < ClO3< NO3< Br < I
The kation effect (small) is Na > K > NH4> Mg