“Antiseptics” are often defined as substances which check putrefaction without necessarily destroying bacteria and their spores, while “disinfectants” are poisonous to ferment-organisms, and actually destroy them; great differences exist in the extent of their sterilising power, and the whole distinction is one rather of degree than of kind, and has little practical value. Thus common salt is incapable of killing most bacteria, even in concentrated solution, though it holds putrefaction in check both by withdrawing water from the hide and by directly preventing the multiplication of bacteria. If the salt be washed out of the hide, putrefaction is at once resumed by the organisms present. Hides, on the other hand, which have once been sterilised by powerful disinfectants, such as phenol (“carbolic acid”) or mercuric chloride, do not again putrefy till the organisms which are killed are replaced by fresh ones from outside. The action of sodium sulphate, and many other salts, is similar to common salt in this respect, while a large proportion of the aromatic compounds are permanently disinfectant, though their efficiency varies with the species of bacteria involved.

Biernacki and others have shown that some disinfectants when extremely diluted actually stimulate alcoholic fermentation, and probably the growth of other ferments, e.g. mercuric chloride 1 in 300,000, salicylic acid 1 in 6000, and boric acid 1 in 8000, and in many cases organisms become habituated to antiseptics in doses which would at first have proved fatal.

The number of antiseptics available is now so great that it is impossible to give a detailed account of all, but the following are among those which are best known and have been practically employed.

Lime possesses some antiseptic properties, and is largely used in the preservation of fleshings before they are sent off to the glue factory. They are most conveniently stored in a large vat filled with a strong milk of lime. Dilute solutions of caustic alkalies have an effect similar to that of lime.

Common salt, sodium chloride, NaCl, acts to a certain extent by its solubility and a dehydrating effect on animal tissues common to chlorides, which removes water from hides and other materials which it is used to preserve. Probably the latter characteristic has a good deal to do with its effect in checking the development of bacteria, since many species thrive quite well in weak salt solutions, and some even in brine, and the dehydrating effect of the salt enables it to harden many animal tissues if used in sufficient quantity, the water they contain running away in the form of brine.

Ordinary rock salt frequently contains ferric chloride, and this, either originally present in the salt, or in some cases derived from the action of the latter upon the iron contained in the blood, is the cause of what is known as “salt-stains.” These show but little during the liming of the hides, unless sulphides are used, when stains appear of a greenish black, from the formation of sulphide of iron; when, however, the hides come into the tanning liquors, black or blue stains are produced by the action of the tannin, which are partially removed by the acids of the liquors during the tanning process, but generally show to some extent in the finished hide. There is another species of salt-stain, not apparently due to iron, but to the colouring matter produced by some fungoid or bacterial growth, which it is practically impossible to remove, and which is stated to be sometimes caused by the use of old salt with which hides have been previously salted. Iron stains are most readily recognised by the use of a solution of potassium ferrocyanide or thiocyanate slightly acidified by hydrochloric acid. If this be applied to the leather, the stains will be changed from a blackish to a blue, if the former, or a red colour if the latter salt has been used. A more absolutely conclusive proof is to lay a piece of filter paper soaked in dilute hydrochloric acid upon the stain, and then to test for iron upon the paper with ferrocyanide or thiocyanate. The freedom of the paper itself from iron must be ascertained before use. Iron-stains produced in the salted state are more difficult to discharge than those which are caused later in the tanning process, since iron salts have distinct tanning power, and attach themselves firmly to the untanned fibre. On the Continent, where common salt is heavily taxed, alum, carbolic acid, naphthalene and other materials are frequently added to it to “denaturise,” or render it incapable of being used as food, and these additions are often the cause of trouble to the tanner.

Sodium sulphate, Na₂SO₄, has little if any disinfectant power in dilute solution, but if used in the calcined form (anhydrous sodium sulphate) as proposed by Eitner[7] as a substitute for common salt in preserving hides, it withdraws water from the hide and crystallises with 10 Aq (about 56 per cent.). This does not run away like brine, but remains in the hide, which retains its weight, and remains plump and swells well in the limes and liquors, which chlorides have a great tendency to prevent; 10-15 per cent. on the weight of the hide is sufficient, while salt must be used in nearly double this quantity. Care must be taken that the sulphate used is free from bisulphate, NaHSO₄, which has a powerful swelling effect upon the hide-fibre, like sulphuric acid. The neutral sulphate does not redden methyl orange or litmus. Pickled skivers may be in part preserved by the sodium sulphate formed by the action of sulphuric acid upon the salt employed in the pickling bath (see p. 90).

The stronger mineral acids have considerable antiseptic power, and are of course especially fatal to such ferments as thrive best in alkaline solutions. The use of sulphuric acid in pickling skivers has already been alluded to, and a very dilute solution applied without salt to raw hides prevents putrefaction, though the principal object in using it is to plump the hides and produce a fictitious weight and substance which disappear on tanning. Such hides of course have a powerful acid reaction to litmus. Sulphuric acid in small quantities has been used with advantage in soaking E.I. kips. A very small excess of hydrochloric acid will sterilise putrid effluents, and no doubt nitric or sulphuric acid would have the same effect. The powerful effect of mineral acids on animal fibre, and their solvent action on cements and iron, preclude however, their general use as antiseptics.

More important is the use of sulphurous acid and sulphur dioxide, which, from their mild acidity and great antiseptic powers, are capable of a variety of useful applications. Considerable doubt has been raised as to the germicide power of sulphur dioxide, and it is certain that the dry gas is less effective on dry objects than when applied in solution, or to moist materials, as is almost invariably the case in the tannery. It may possibly be more efficient in its action on some moulds and putrefaction-ferments than on the pathogenic bacteria which have been most frequently used to test the power of disinfectants; but in practice it is found extremely useful in the brewery and in gelatine manufacture, and there is no reason that it should be less so in the tannery.

The gas is most conveniently produced by burning sulphur, which produces double its weight of sulphur dioxide. If used for “stoving” drying rooms and other places infested with moulds, care must be taken to avoid risk of fire. A shallow cast-iron pot set on bricks or sand is generally the most suitable vessel, and the sulphur may be ignited by a piece of red-hot iron or a rag which has been previously dipped in melted sulphur. It is corrosive to metalwork, and bleaches many colours, but does not produce any marked injurious effect on leather, though the sulphuric acid formed by oxidation may, if not removed, ultimately make it tender.

For many purposes a solution of the gas is required, and this is most easily made by burning the sulphur in a small metal or firebrick stove from which the fumes are sucked through a “scrubber,” which, on a small scale, is conveniently made of large glazed sanitary pipes, packed with coke or broken earthenware, over which water is allowed to trickle. The lowest pipe has an opening for a branch pipe, which is connected with the stove and rests on three bricks in a tub, which collects the acid solution and forms a water-seal to prevent the escape of gas. Above the inlet for the gases is fixed a wooden grating on which the coke rests. The scrubber may be 10-15 feet in height and connected at the top with a chimney or steam ejector to produce the draught. The arrangement is illustrated in Fig. 5. Another method is to burn the sulphur in a closed cylinder and to force the products through water with an air-compressor or steam-jet injector.

In place of using a scrubber, the fumes may be blown by a steam ejector direct into a tank. This is a very good arrangement for washing and bleaching hair, etc., but where large quantities of solution are required is inferior to the scrubber. Ejectors of hard lead or regulus metal should be used, and are less acted on by the dry gases than by the very dilute moist exhaust from the scrubber (see p. 335).

Bisulphites have also strong antiseptic properties. “Bisulphite of soda” (hydric sodic sulphite) solution may be made by supplying the scrubber with solution of soda-ash or washing soda; bisulphite of lime, by using milk of lime or packing the scrubber with chalk or limestone (free from much iron) in place of the coke. In either case a much stronger solution is obtained than with water alone.

Boakes’ “metabisulphite of soda”[8] is a very convenient source of sulphurous acid when the latter is wanted in small quantities. It is an anhydrosulphite, Na₂O.2(SO₂), and contains 67·4 per cent. of its weight of SO₂. One molecule of the salt (= 190) requires one molecule of H₂SO₄ (= 98) to set free the whole of the sulphurous acid. For many purposes the sulphate of soda formed may be neglected and the acidified solution used direct.

For analysis of sulphites and sulphurous acid solution, see L.I.L.B., pp. 16 and 37.

Boric acid, borax and other borates are not very powerful disinfectants. They have no injurious action upon the skin, but to be effective require to be employed in pretty strong solutions, say 1 per cent., and their comparatively high cost unfits them for general use as antiseptics in the tannery, though boric (boracic) acid is very useful as a drenching and deliming agent (see pp. 156, 229, and L.I.L.B., p. 37).

Mercuric chloride, corrosive sublimate, HgCl₂, is an extremely powerful antiseptic, preventing the growth of some species of bacteria in solutions so dilute as 1 in 300,000 (Koch). 1 in 14,000 is disinfectant (Miquel), but its power varies very much upon different organisms (JÖrgensen states that 1 in 400 is required to kill Penicillium glaucum), and it is unsuited for most purposes in leather manufacture, both from its extremely poisonous character, and because it is rendered inactive by various substances present in the materials used.

Mercuric iodide dissolved in iodide of potassium solution was patented by Messrs. Collin and Benoist as an antiseptic in tanning, but it is ineffective for the same reasons as mercuric chloride; although under favourable circumstances it is even more powerful than the latter.

Copper sulphate, zinc chloride and sulphate, and many other metallic salts are powerful antiseptics, but have only a limited application in leather industries, and do not usually actually sterilise. Arsenic (arsenious acid), which has been used in curing hides, is an excellent insecticide, but not particularly effective as an antiseptic; and sulphide of arsenic (realgar) when used in limes (see p. 139) seems to have but little antiseptic effect. Arsenious acid is easily soluble in alkaline solutions.

Fluorides have been suggested as antiseptics in the tannery, but do not seem of much practical value.

The most important antiseptics at present are those derived from coal tar, and belonging to the aromatic series. Of these, the phenols (carbolic acid, cresol, etc.) are the most used.

Pure phenol, “pure crystallised carbolic acid,” is hydroxybenzene C₆H₅(OH), but the crude forms which are generally employed contain cresols and higher members of the series in which one or more of the atoms of hydrogen are substituted by CH₃ groups. These are oily bodies scarcely soluble in water, and even pure phenol is only soluble in cold water to the extent of some 7 per cent. Crude carbolic acid should not be employed in the tannery, since the insoluble oily particles stain the hide, and render it unsusceptible of tanning. Suitable carbolic acid should be of a pale yellow colour when fresh (though it will darken on exposure to air and light), and it should be wholly soluble in a sufficient quantity of water. Its specific gravity should be 1·050 to 1·065. For methods of chemical examination, see L.I.L.B., p. 40. A saturated solution of carbolic acid sterilises hide completely against most putrefactive organisms, but has a sort of tanning effect, adhering obstinately to the fibre so that it cannot be removed by washing; and hides which have been cured with it cannot be unhaired by sweating, though they may be limed in the usual manner, if somewhat more slowly. Care should be taken in mixing with water or liquor, as undissolved drops will produce the same effects as those of the crude acid. Hides are occasionally stained, as has just been described, by salt which has been denaturised with common sorts of carbolic acid. Eitner recommends the use of a solution of carbolic acid in an equal weight of crude glycerine, which readily dissolves in water, and seems to prevent any injurious effect on the hide.

An aqueous solution containing 1 per cent. of carbolic acid is sufficient for mere sterilising of hides, but if it be desired to preserve them for a long period, stronger solutions (up to 4 per cent.) may be employed.[9]

Quantities so small as 1 part per 1000 control the fermentation of liquors, and prevent the formation of moulds on the surface, economising tannin, and preserving vegetable acids already present, but at the same time lessening their production by fermentation, and therefore sometimes leading to difficulties in the early stages of tanning. Carbolic acid is not, strictly speaking, an acid, but rather of the nature of an alcohol, although it forms weak combinations with bases. It is a powerful narcotic poison, and if dropped on the skin in a concentrated form it produces severe burns; these are best treated with oil, while in cases of poisoning, oil and chalk must be administered internally, but if the quantity of carbolic acid taken has been large, are not likely to be effective. From its cheapness and efficiency, carbolic acid is likely to be increasingly used, although for special uses some of the newer antiseptics have great advantages.

Eudermin is a tar-oil manufactured by Speyer and Grund, of Frankfort-on-Main, which is intended as an antiseptic addition to stuffing greases to prevent mould and spueing. It is recommended for the purpose by Eitner[10] and can be used in proportions such as 10 per cent. of the grease. Creasotes and cresols can be dissolved in oils and stuffing greases, and act as antiseptics, though less powerfully than in aqueous solution. Rosin oils and turpentine have also antiseptic properties.

Creasote, “heavy coal oil,” or “dead oil,” is a complex mixture of hydrocarbons, phenols and cresols, obtained by distillation of coal tar, heavier than water, and almost insoluble in it. It is largely used as a preservative for timber. Carbolineum is an oil of this class, boiling at over 300° C., and intended for application to wood. One or more coats are applied to the dry wood at a temperature of 80° C. The workman’s hands must be protected by gloves, as the hot creasote raises painful blisters. Eitner[11] recommends its use for preserving pits, posts and other woodwork in tanneries. Wood-creasote is a somewhat similar product obtained from wood-tar.

The heavier cresols are so little soluble in water as to be valueless in their ordinary form as antiseptics, but several preparations are made under the names of “Creolin,” “Jeye’s fluid,” “Lysol,” “Izal,” “Soluble phenyl,” etc., in which they are treated with additions of soap or alkalies, which cause them to emulsify or dissolve in water, generally as milky liquids. These are powerful germicides and have the advantage over phenol of being non-poisonous. 0·1 to 0·5 per cent. solution of creolin will sterilise hides after bating so that no putrefaction takes place in the liquors. Mr. J. T. Wood specially recommends creolin for the general purposes of the tannery, disinfecting pits and tubs, and for checking the action of puers and drenches on goods which have gone a little too far, by throwing them into a 0·2 per cent. solution.

Salicylic acid, orthohydroxybenzoic acid, C₆H₄OH(COOH), is now artificially prepared from phenol. It is much less poisonous than the latter and has no smell, which makes it valuable for certain purposes, but is too dear for most technical applications. Many bacteria appear to become gradually habituated to its action, and the same is true of phenol to a less degree.

Salicylic acid is closely related to protocatechuic and gallic acids, and, like these, gives a blackish colour with iron salts. It is freely soluble in hot water, but very sparingly in cold. The addition of 1-2¹/₂ parts of sodium phosphate, sulphate, or potassium nitrate to each part of salicylic acid greatly increases its solubility. It seems much more powerful in preventing the development of bacteria than carbolic acid; a solution of 1 part of salicylic acid in 666 of water is said to be equal in this respect to 1 part of carbolic in 200.

Benzoic acid, C₆H₅COOH, though not much employed, except in medicine, is a still more powerful disinfectant, and has the advantage of being non-poisonous to human beings.

“Cresotinic acid,” which is derived from the cresols as salicylic acid is derived from phenol, is more soluble than salicylic acid. It is not very poisonous, and a powerful disinfectant. In a crude form it has been introduced by Hauff, of Feuerbach, for bating or removing lime from hides. This it does very well, though without the softening action of a true bate. It has a tendency to produce a pinkish stain, and in some degree a sort of tanning of the fibre. Its price, moreover, is rather high for extensive technical use. (See also p. 162.)

“Anticalcium” is a more recent preparation introduced as a bate by the same firm.[12] It is a solution of mixed sulphonic acids derived from cresols, and has considerable disinfectant powers. It removes lime very effectively, but from its acid character somewhat swells the skin. It is used very successfully as a drench for thin skins (p. 163).

“C.T.” (coal-tar) bate is a grey crystalline pasty mass, with a tarry smell, and is chemically very similar to anticalcium if not identical with it.

Naphthalene sulphonic acid has strong antiseptic properties. Its use in bating has been patented by Burns and Cross. (See p. 163.)

Naphthols, C₁₀H₇(OH).—These bodies, which have the same relation to naphthalene as the phenols to benzene, are powerful antiseptics; and naphthalene itself appears to have antiseptic power, and is occasionally used for denaturising salt. There are two naphthols, varying in the position of the OH group in the molecule, and denominated a and , of which a naphthol is the more powerful antiseptic and the less poisonous, though , being cheaper, is the common commercial article. It is said that quantities so small as 0·1-0·4 grams of a naphthol per liter are sufficient to prevent the development of microbes, while of naphthol about ten times that quantity is required.

Naphthols are not very expensive, but their value is diminished by the fact that they are insoluble in water. They are soluble in alkaline solutions, but their compounds with bases are of much lower antiseptic value, and the same is true of their alcoholic solutions; when an alcoholic solution is added to water the naphthol is precipitated, but if an addition of soap or camphor be made to the alcoholic solution, the naphthol remains in a very finely divided condition, if not dissolved.

Adopting Eitner’s suggestion with regard to oxynaphthoic acid (see below), hides may no doubt be sterilised by treatment first with an alkaline naphthol solution, and then with a very dilute acid to set the naphthol free.

“Hydronaphthol,” tetra-hydro-naphthol, C₁₀H₁₂O, is obtained by the reduction of naphthol by sodium (Rideal). It seems to be an excellent disinfectant.

Oxynaphthoic acid, a hydroxynaphthoic acid, C₁₀H₆(OH)COOH, which bears the same relation to naphthol as salicylic acid does to phenol, is cheaper than salicylic acid, and said to be a more powerful antiseptic. Its salts have no antiseptic power. In its commercial form it is a reddish crystalline powder, inodorous, but with a burning taste, and its dust causes violent sneezing. It is scarcely soluble in water, and is said to undergo some change on keeping which lessens its germicide power; it is readily soluble in alcohol, and the solution produces a milky fluid on mixture with water. Such a solution containing 15 grams of the acid in 4 liters of water, will sterilise a hide. Eitner recommends[13] that it should be dissolved in dilute soda solution, and the hides, after soaking in it, passed through water slightly acidified with hydrochloric acid, as has been suggested in the case of naphthol; the method is also applicable to creosotinic acid, the hides being permanently sterilised so that they cannot be unhaired by sweating, though they will lime in the usual manner.

[13] Gerber, 1888, p. 101; 1889, pp. 99 et seq. See also p. 163.