Procter, "Principles of Leather Manufacture," p. 378.

SECTION III.--OIL TANNAGES

There are very obvious a.n.a.logies 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._ sc.r.a.ped 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 more in 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 quant.i.ty 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 liquored with 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 coa.r.s.e surface and eventually a fine surface so that a fine velvet effect is attained. The skins are next bleached.

In the "sun bleach" or "gra.s.s bleach" the goods are soaked in a 1-1/2 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 3/4 per cent. solution of soda crystals and then drumming for 30 minutes in water at 95 F. They are then paddled in a 1/8 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 after mordanting 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.

SECTION IV.--FORMALDEHYDE TANNAGE

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 (HCHO) 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 a.n.a.logous with that of its h.o.m.ologues, for it is a most reactive substance, and will certainly with proteids undergo reactions which are not a.n.a.logous 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.

Na{2}CO{3}) 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 1-1/2 per cent.

solution of ammonium sulphate to remove the soda, and "nourished" in a solution of soft soap and salt, about 2-1/4 per cent. of each on the weight 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.

SECTION V.--SYNTHETIC TANNING MATERIALS

In spite of much valuable work on the const.i.tution of the vegetable tannins and the compounds usually a.s.sociated 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 const.i.tution 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 a.s.sist rather than subst.i.tute 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, pa.s.sing 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. Ill.u.s.trative 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 other types 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 ill.u.s.trated 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 subst.i.tute; 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 a.s.sist in avoiding a cracky grain. On this account they are also recommended for retanning E.I. tanned kips. When used in heavy leather suspenders they are said to get rid of lime blast (CaCO{3}) 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 also _J.S.C.I._, Abs. 1913, 500.)

E. Stiasny, "Syntans--New Artificial Tanning Materials,"