H. R. Procter

Fats and oils constitute a large class of substances, of animal or vegetable origin, which may be solid, pasty or more or less viscous liquids, but which in the latter case are commonly known as “fixed” or fatty oils, to distinguish them from the volatile, or essential oils, which may be distilled without decomposition, and which are the source of most of the odours of plants, and of quite different chemical constitution. The term “oil” is also applied to various products of mineral origin, and especially to those derived from petroleum, on account of their similarity in appearance and physical properties to the fixed oils, though, chemically, they form a very distinct class. The waxes are another group somewhat closely allied to the fats; and there are certain fixed oils, such as sperm oil, which though very similar in appearance and properties to the fatty oils, are chemically members of the group of waxes.

As it is obvious that there is no chemical distinction between the fats and fatty oils, except that of melting-point, it will be convenient to treat them together; especially as what is a solid fat in one climate may be an oil in another. Palm and cocoa-nut oils are cases in point, as the first is buttery, and the second a hard fat in this country, though they are both liquid in tropical climates.

For more detailed information on the chemistry of fats and oils, the reader must be referred to the ‘Leather Industries Laboratory Book,’ sect. xviii., or to the larger manuals devoted specially to the subject by Lewkowitsch, Jean, and others, or the very excellent section on oils in Allen’s ‘Commercial Organic Analysis,’ vol. ii.; but a few general facts must be recapitulated.

The true fats contain carbon, hydrogen and oxygen, but no nitrogen. They are all compounds of glycerin with organic acids which are generally termed “fatty acids,” and which resemble in many of their characteristics the fats themselves. Glycerin is a very weak base, of the nature of an alcohol, and consequently, when a fat is heated with a solution of one of the caustic alkalis, the fatty acid combines with the latter, and the glycerin is set free. The salts thus formed are denominated “soaps.” The reaction with stearin (glycerin stearate), the principal constituent of hard animal fats, is shown in the following equation.

Stearin		Sodium hydrate		Sodium stearate		Glycerin
(C17H35CO.O)3C3H5	+	3NaOH	=	3C17H35CO.ONa	+	C3H5(OH)3.

If a soap is treated with an acid stronger than its own, the latter is set free, while the new acid combines with the base. The following equation, for instance, shows the action of hydrochloric acid on the stearic soap.

Sodium Stearate		Hydro- chloric acid		Stearic acid		Sodium chloride
C17H35CO.ONa	+	HCl	=	C17H35CO.OH	+	NaCl.

If any soap be dissolved in hot water, and sufficient hydrochloric or sulphuric acid added to render the solution acid, the latter will turn first milky, and (if it be kept warm) the fatty acid will finally rise in an oily layer to the surface, which in many cases will harden, as it cools, to a solid mass. The amount of fatty acid in a soap may be roughly determined by weighing 25 grm., dissolving in 50 c.c. of boiling water, and adding excess of acid, and allowing the reaction to take place in a graduated cylinder, or a flask with a graduated neck, in a vessel of boiling water. When the fatty acid has risen to the top, its volume may be noted, and each c.c. may be roughly reckoned as 0·9 grm. (For more detailed methods cp. L.I.L.B., Sect. XVII.).

Soaps are insoluble in strong caustic alkaline solutions, and therefore saponification (as the decomposition of fats by alkalis is called), does not readily take place in them, and for this reason the soap-boiler generally dilutes his caustic soda solutions to a strength not exceeding 18° Tw. (sp. gr. 1·090) in gravity, and separates the soap at the end of the operation, by the addition of brine, in which it is insoluble. An easier method, and one which is often useful for the preparation of small quantities of special soaps for fat liquors and the like, is as follows.[160] 10 lb. of a good caustic soda, free from common salt, is dissolved in 4 gallons of water, and 75 lb. of oil or fat is warmed to about 25° C. or just sufficiently to render it liquid, and the soda solution is added in a thin stream, with constant stirring, which must be continued until the mass becomes too pasty. It is now set aside in a warm place for at least twenty-four hours, during which saponification gradually takes place. For leather purposes, a neutral soap, with a slight excess of fat, is generally advantageous, so that the fat may be increased to 80 lb.; or, in place of this, the operation will be facilitated by the addition of 5 lb. of commercial oleic acid. If soft soap is desired, 14 lb. of caustic potash may be used in place of the 10 lb. of caustic soda. The hardness or softness of soaps varies to some extent with the fat used, but potash soaps are always much softer than the corresponding soda soaps. It is obvious that with soaps made in this way, all the glycerin remains mixed with the soap. If, on testing, the soap does not prove to be free from caustic, it may be re-melted, which will generally complete the reaction. Before attempting to work with large quantities, a laboratory experiment is desirable, using 10 grm. of soda in 40 c.c. of water, and 75 to 80 grm. of oil or fat. The neutrality or freedom of the soap from caustic alkali may be tested by touching a freshly cut surface with an alcoholic solution of phenolphthalein, which the least trace of caustic soda or potash will render pink.

[160] Carpenter, ‘Soap, Candles and Lubricants,’ p. 144.

If solutions of soaps are mixed with those of salts of the heavy metals or of the alkaline earths, a mutual decomposition takes place, the acid of the salt combining with the alkali of the soap; and the fatty acid with the metallic base, to form a metallic soap. Most of these soaps are sticky masses, insoluble in water, but not unfrequently soluble in turpentine or petroleum spirit, if previously thoroughly dried, so that some of them have been applied to the production of varnish. Alumina soaps are occasionally used to thicken mineral oils, or render them more viscous. The general reaction of the stearin soap with calcium sulphate is shown in the following equation, though in practice it is sometimes more complex:

This is the reaction which causes the curdling of soap by hard water, page 93.

True fats cannot be distilled alone without decomposition. When distilled in a current of steam, some undecomposed fat passes over, but the greater part is broken up into free fatty acid and glycerin; and hydrocarbons practically identical with mineral oils are also formed.

Fats and oils are insoluble in water, and in most cases only sparingly soluble in alcohol, but freely soluble in ether, petroleum spirit, benzene, and most other hydrocarbons, as well as in chloroform, carbon tetrachloride, and carbon disulphide. Petroleum spirit, often called benzine, is largely used for their extraction, and for de-greasing leather, and removing grease from clothes. In the laboratory, carbon disulphide, or carbon tetrachloride is to be preferred. Castor oil is an exception to the rule, owing to the large proportion of oxygen which it contains, being readily soluble in alcohol, and very sparingly in petroleum-spirit; and other oils, when oxidised, usually become more soluble in alcohol, and less so in hydrocarbons.

Oils vary much in their tendency to “dry,” or become converted into solid or sticky resin-like substances. This tendency is greatest in some of the seed oils, and least in olive oil, and the oily part of animal fats (tallow oil, neatsfoot oil). Sperm oil, a “liquid wax,” is also very free from this tendency, but all fish oils possess it in a greater or less degree. It is not due to evaporation, but to the absorption of oxygen by the fatty acid. The tendency to oxygen-absorption, and consequently to drying (and, in the case of leather-oils, to “spueing”), is measured analytically by the “iodine-value,” the absorption of iodine being proportional to that of oxygen, while it is much more easily measured.

There are no simple tests by which the purity of oils can be determined, though in a few cases the presence of particular oils can be detected. The mixing and adulteration of oils is now a science, and those who practise it are well acquainted with the customary tests, and take care to adjust their mixtures so as to meet them. Taste and smell however, with practice, often furnish useful indications.

Natural oils and fats are invariably mixtures of the glycerides of several fatty acids, and their qualities depend simply on the character of these glycerides and the proportions in which they are mixed. The fatty acids form several groups, differing in their degree of “saturation,”[161] or, inversely, in their power of taking up oxygen, on which their tendency to drying depends. The members of any one of these groups resemble each other strongly, differing principally in melting points, density, and other physical characteristics.

Saturated Fatty Acids.—Stearic acid, C₁₈H₃₅O.OH, and palmitic acid, C₁₆H₃₁O.OH, are the most important. At ordinary temperatures they are hard, white, crystalline bodies, and melt at 69° and 62° C. respectively. They do not, under ordinary circumstances, absorb any oxygen, nor iodine, and are very little liable to chemical change. Together with oleic acid, they are the principal acids of tallow and other animal fats, while palmitic acid and some lower members of the same group are more common in vegetable oils. Free stearic acid is an important constituent of the “distilled stearines” used in currying; while “oleostearine” consists mainly of the neutral fats or glycerides of stearic and palmitic acids.

Liquid Fatty Acids, Non-drying.—Of these, oleic acid is much the most common and important; its glyceride, olein, forming the liquid part of animal fats, and being the principal constituent of vegetable non-drying oils. Olive oil consists almost entirely of olein, with a little palmitin. The formula of oleic acid is C₁₈H₃₃O.OH, thus differing from stearic acid in having two less atoms of hydrogen. The “bonds” or affinities corresponding to these two atoms are linked together, but can separate, and attach two atoms of iodine, bromine, or chlorine, or one of oxygen. The iodine-value of pure olein is 83·9 (that is, 100 grm. absorb 83·9 grm. iodine); and that of olive oil about 83. Any oil with a higher “iodine-value” than olein must contain drying oils, though a lower value does not necessarily indicate their absence, if palmitin or other saturated acids are also present.

Unsaturated Liquid Fatty Acids.—Of these there are several groups, differing in their degree of saturation, and also probably in their structure. Their glycerides, together with olein, and sometimes palmitin, are the constituents of the seed oils, the drying tendency of which depends on their proportion of unsaturated acids, and the particular group to which they belong. The fish oils contain a peculiar group of unsaturated acids, together with olein, and usually stearin and palmitin, like the other animal fats. Linolenic acid, C₁₈H₂₉O.OH, one of the acids of linseed oil, has six hydrogen atoms less than stearic acid, and therefore three double linkings, and will take up six atoms of iodine. Its theoretical iodine-value is 274, while linseed oil itself often has an iodine-value exceeding 180. The iodine-value of cod-liver oil is sometimes nearly as high. Both oils therefore contain other acids less unsaturated than linolenic.

The “spueing” of leather is due to the absorption of oxygen and consequent resinification of the oils, and therefore all drying oils, however pure, are capable of producing it, though some are more liable to do so than others (cp. pp. 363, 365, 366, 368, 390).

Linolenic acid, and probably other allied acids, become converted by absorption of oxygen into solid varnish-like substances, which are important to the tanner, as furnishing the principal constituents of japans for leather. The unsaturated acids of fish oils seldom give hard varnishes, though menhaden oil (page 367) is sometimes used as paint-oil for outside work.

Most fats are liable to become rancid by exposure to the air, acquiring a disagreeable taste and smell, and an acid reaction from the liberation of the fatty acids. The changes which take place are somewhat complex.

The fatty acid of castor oil is of peculiar constitution, being an oleic acid in which one of the hydrogen-atoms is replaced by a “hydroxyl” or OH group. The solubility of castor oil in alcohol has already been alluded to. It does not dry, and is an excellent oil for lubricating heavy machinery. It is sometimes adulterated with “blown” oils, which are made from non-drying, or slightly drying seed oils, like cotton-seed or rape, by blowing air through them in a warmed condition. Under this treatment they increase greatly in viscosity and density and in their solubility in alcohol, but do not acquire the other valuable properties of genuine castor oil.

The “foots” or sediments which oils deposit on standing, sometimes consist of animal or vegetable fibres, or mucilage combined with water, but often are simply the harder fats, stearin, palmitin, etc., which crystallise from the oil on cooling. In this case they are re-dissolved on warming the oil. Such oils, which like neatsfoot and tallow oils become turbid in cold weather, are styled “tender.”

Non-Drying Fats and Oils.

Tallow (Fr. Suif; Ger. Talg) is the fat of various mammalia, principally of the ox and sheep, but occasionally also of the goat. The mixed fat obtained from all parts of the carcass is known as “rendered tallow,” while that obtained from the region of the kidneys (suet) is harder. A substance commonly referred to as “pressed tallow” or “oleo-stearine” is obtained by pressing ordinary tallow, in cloths, in the hydraulic press. The more liquid portion which is expressed is tallow-oil, the finer qualities of which are used in making margarine. Oleo-stearine must not be confounded with the “distilled stearine,” obtained from Yorkshire grease by distillation and pressure (page 359), nor with candlemakers’ “stearine,” which is a mixture of free stearic and palmitic acids.

Pure tallow is white and tasteless, but much of that sold is yellowish and of a disagreeable, slightly rancid flavour. Mutton tallow is usually harder and whiter than that of beef. Goat tallow has a characteristic odour, as have the recovered stearines and other waste greases from glue-works. Buck tallow, which is particularly hard, has now been largely replaced by oleo-stearine.

Beef tallow melts at about 40° C.; mutton tallow at 45°.

In chemical composition, tallow consists chiefly of a mixture of the tri-glycerides of palmitic, stearic and oleic acids; its hardness diminishing with the increase of the last.

Tallow should, when melted, be perfectly clear, turbidity indicating the presence of water or other foreign matters, due either to carelessness in the manufacture or, possibly, adulteration. Traces of phosphate of lime, or fragments of animal tissue, may be present as accidental impurities; lime, on the other hand, is sometimes added to thicken the tallow and enable it to retain more water; starch, china clay, whiting, heavy spar, etc., are also occasionally employed. Tallow has been not infrequently adulterated with the distilled fatty acids from wool grease. When this is the case, crystals of cholesterol (see L.I.L.B., p. 181) may be detected by examination of the unsaponifiable matter of the mixture under a microscope. It would also give the tallow an unusually high “acid-value.”

Methods for the proximate analysis of tallow are given in the ‘Laboratory Book,’ pp. 189 et seq.

The fats produced by the boiling of fleshings for glue, and by the pressing of sheep-skins, are of the nature of soft tallows. If the fleshings are delimed with acid, and boiled fresh, the grease is generally of good colour, and with little unpleasant odour, but contains traces of free fatty acids derived from the decomposition of the lime-soaps. If the fleshings have been dried and the lime carbonated, the grease will generally be brown, and more or less rancid; but the lime-soaps are not decomposed, unless the “scutch” or refuse be treated with acid, when a further yield of grease is obtained. The grease from sheep-skins is generally somewhat brown, and often smells of the volatile acids and other constituents of the tan-liquors, especially if larch bark has been used. These greases are usually much improved in appearance and odour, if well washed by boiling or steaming on water, or by blowing a mixture of air and steam through them, or sometimes even by mere heating to a sufficient temperature to evaporate the water and drive off the volatile matters. By allowing the grease to cool slowly, so as to favour crystallisation, till it is of a soupy consistency, and then pumping through a filter press with woollen cloths, the more liquid is separated easily from a more solid portion, and both may in many cases be used in leather manufacture, the tallow for currying, and the oil in place of neatsfoot oil.

Horse-fat, and especially that from the fatty portions of the neck (Ger. Kammfett), as well as various other animal greases, are used in the manufacture of leather. They differ from tallow chiefly in that they have a lower melting-point, and contain more olein in proportion to the stearin and palmitin than true tallow, and are consequently somewhat softer. Though often almost white, these greases are sometimes darkened in colour by the products of putrefying animal matter, but this does not, as a rule, interfere with the oil being used for leather dressing. They are usually so cheap that they are but little adulterated; means of determining their purity are, however, given in L.I.L.B., p. 191.

Neatsfoot oil is a yellowish, nearly odourless oil, of bland taste, which is largely employed in the dressing of calf-kid. It has a similar composition to tallow oil and the other oils obtained by subjecting the soft animal fats to great pressure at a low temperature. It is often adulterated with bone oil, lard oil and cotton-seed oil, and occasionally with mineral oil and recovered wool-grease.

As neatsfoot oil is somewhat costly, curriers may with advantage often use ordinary animal greases (horse-fat, etc.) after they have had the harder tallow extracted by cooling and pressure, the product thus obtained being, chemically, the same as neatsfoot oil, and in every respect as suitable, while it is much less liable to adulteration.

The true neatsfoot oil is prepared by boiling the feet of cattle, and sometimes of sheep and horses, with water, and skimming off and clarifying the oil which is thus obtained.

The physical and chemical characteristics of this oil are described in L.I.L.B., p. 192.

Wool-Fat (Fr. Suint, oesype; Ger. Wollschweissfett) is a grease of high specific gravity, exsuded from the sebaceous glands of the sheep, together with organic salts of potassium. It is obtained by extracting wool with solvents; or by washing with alkaline solutions, from which it is recovered by precipitation with acid, and subsequent hot-pressing of the “magma,” or, more recently, by evaporating the scouring liquor to small bulk, and centrifugating. Wool-fat is characterised by its low percentage of glycerides, the fatty acids which it contains being mainly combined with higher alcohols (bodies of alcoholic structure, but of a waxlike consistency), and chemically it is rather a wax than a true fat. Among the alcohols which it contains is included a marked percentage of cholesterol and isocholesterol. It is difficultly saponifiable, requiring to be heated to 105-110° C. with alcoholic potash under pressure; and even then about 44 per cent. of alcohols remain, which are incapable of further saponification. Care must therefore be taken not to assume that unsaponifiable matter in greases which may contain wool-fat is necessarily mineral oil. For details of analysis see L.I.L.B., p. 194.

Pure wool-fat is nearly white, of salve-like consistency and very slight smell, with a density of 0·973 at 15° C. Crude wool-fat is yellow or brown, with an unpleasant and very persistent characteristic smell. Both the pure and the crude wool-fat have an extraordinary power of emulsifying with water, which makes them very valuable as substitutes for dÉgras in stuffing greases. Lanoline (and several other preparations under different names) are mixtures of purified wool-fat and water, of which lanoline contains about 22 per cent.

“Yorkshire grease” differs from crude wool-fat, in being recovered from the waters employed in scouring woollen cloths, as well as wool, and hence contains the free fatty acids of soaps used in scouring, as well as the “oleines,” etc., used in oiling the cloth, and although it often contains much wool-fat, it is occasionally destitute of this substance.

Holden Fat consists of ordinary wool-grease mixed with fish oil, and is used either as a substitute for, or in admixture with dÉgras (q. v.).

Distilled Wool Grease is produced by distilling crude Yorkshire grease with steam. Most of the glycerides are broken up, but many of the free fatty acids, alcohols and waxes distil over unchanged, though a considerable part is decomposed into volatile hydrocarbons strongly resembling mineral oils. The distillate is separated by cooling and pressure into a liquid “oleine” and a solid “stearine.” The latter forms a very valuable stuffing-grease which, in England, largely takes the place of the “oleo-stearine” used in the United States—with which, however, it must not be confounded.

Distilled Stearine, prepared as above described, is a pale yellow-to-brown fat, which varies in hardness and in its melting point according to the conditions of its preparation. It has a characteristic odour which is very persistent, and it consists largely of free stearic and palmitic acids; most of the liquid hydrocarbons formed by distillation being removed with the “oleine.”

Olive Oil (Fr. Huile d’olive; Ger. Olivenoel, Baumoel) finds extensive use in leather dressing, and especially in the manufacture of “fat-liquors” (pp. 217, 240). It is extracted from the fruit of the olive tree by pressure, and of late years from the residues by extraction with carbon disulphide. Although it chemically resembles tallow and lard oils very strongly, its adulteration with these substances may usually be detected, at any rate roughly, by the taste and odour of the oil. It is principally characterised, from a chemical point of view, by containing the glyceride of palmitic but not that of stearic acid, and by having a much larger proportion of olein to solid glycerides than most of the non-drying animal oils. At low temperatures, olive oil solidifies to a product which can be separated by pressure into a solid tallow-like fat, and a fluid oil consisting essentially of tri-olein.

Olive oil is the type of a non-drying vegetable oil, but though it does not thicken materially on exposure, it becomes rancid somewhat rapidly, and is thus rendered unsuitable for lubrication. Unless the acidity is excessive it does not appear to spoil the oil for leather manufacture, and for some purposes is actually an advantage as aiding emulsification. Free acids in oils may be removed by shaking with sodium carbonate solution.

Olive oil always contains some free acid; which is of importance in the preparation of fat-liquors, as it facilitates the production of an emulsion. This quality may be increased by the addition, when necessary, of a little oleic acid.

Olive oil is frequently adulterated with other vegetable oils. Probably the most useful criterion is the iodine-value, which is raised by the addition of any seed oil. Examination in the refractometer also affords useful indications. Cotton-seed, sesame and arachis (earth-nut) oils are the most frequent adulterants of the better qualities, and in many cases may be recognised by special tests.

Castor Oil (Fr. Huile de ricin; Ger. Ricinusoel) is the oil expressed from the seeds of Ricinus communis, and is a transparent, colourless or pale yellowish liquid, having a faint odour and a disagreeable taste. At a low temperature it thickens and deposits slightly, and at -18° C. it solidifies to a pale yellow mass.

Castor oil is distinguished from all other natural fixed oils by its high density (0·960 to 0·964) and viscosity, and by its solubility in alcohol and its insolubility in petroleum ether. Genuine castor oil is completely soluble in an equal volume of absolute alcohol, or in four times its volume of “rectified spirit” at the ordinary temperature. It is practically insoluble in petroleum ether, but can dissolve an equal measure of that liquid.

For the purpose of the leather manufacturer, the ordinary hot-pressed oil, such as is used for lubricating machinery, is quite as good as the more costly cold-pressed oil which is used for medicinal purposes. It is generally imported in tins holding about 40 lb. of oil. Castor oil, and castor-oil soap made as described on p. 352, are very good for fat-liquors, seeming to interfere with dyeing and glazing less than most other oils. Boots oiled with castor oil may be blacked at once, and will take a good polish.

The only oils which are usually mixed with castor oil are “blown” or oxidised seed oils, or resin oil. Any other oils would so seriously lower the specific gravity as to render their use impracticable. For the detection and estimation of these the ‘Laboratory Book’ should be consulted, or if fuller details are required the reader is referred to Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s ‘Commercial Organic Analysis,’ vol. ii.

Sulphonated castor oil or Turkey-red oil is now largely used for “fat-liquoring,” for which it was probably first employed by the author, about 1890. This material—which must be carefully distinguished from the olive oil preparation which is also used for dyeing cotton a Turkey-red colour—is made by treating castor oil with one-quarter of its weight of strong sulphuric acid (specific gravity 1·8), adding the latter in very small quantities at a time, and taking care that the temperature of the mixture at no time exceeds 35° C. The mixture is then allowed to stand for twenty-four hours, with occasional stirring, and is washed with its own volume of water, allowed to stand until the water has all separated, and the oil is then syphoned off. If desired, the oil may be further washed once or twice with a solution of strong brine, but this is of doubtful advantage, and should in no case be excessive. The washed oil is finally neutralised by the cautious addition of one-hundredth of its volume of strong ammonia solution (sp. gr. 0·880).

If properly prepared, Turkey-red oil (sulphonated castor oil) will, when largely diluted with water, bear the addition of ammonia to alkaline reaction without showing any turbidity even on standing several hours. If a turbidity is produced, it indicates that the castor oil used was impure and contained some oil rich in stearin.

The alcohol test described on p. 360 may also be applied, as the oily layer will be entirely soluble if castor oil alone was used in the preparation of the red oil.

Turkey-red oil usually contains about 50 per cent. of fatty acids (Allen).

Linseed Oil (Fr. Huile de lin; Ger. Leinoel) is used by leather manufacturers in the preparation of the japan for making “patent leather,” and to some extent also in currying, for oiling off levants and moroccos, though for these purposes it has been largely superseded by mineral oils. It is obtained from the seeds of the flax plant, Linum usitatissimum, chiefly grown in Russia and India. The Russian oil is usually mixed with the oil from hemp to the extent of about 20 per cent., while that from India, being grown as a mixed crop with mustard and rape, is never perfectly pure. The Baltic oil is considered best for japans, and is improved by storing for a considerable time in tanks in a warm place.

When obtained by cold pressure of the seeds, linseed oil is of a bright yellow colour; if a higher temperature be used in the extraction the oil is more or less brown, and tastes much more acrid. On exposure to air, linseed oil turns easily rancid, absorbs oxygen, and if spread out in a sufficiently thin film it dries to a neutral substance (linoxyn), which is insoluble in ether. This property is the one on which the chief value of linseed and other “drying oils” depends.

Linseed oil is chiefly adulterated with other seed oils, cottonseed being the most often used for this purpose, though menhaden and various other fish oils are occasionally employed. As the density of raw linseed oil varies between 0·932 and 0·936 at 15° C., the addition of other seed oils or of mineral oil would cause an appreciable lowering of this figure, whilst rosin or rosin oil would raise it. A judicious admixture of both mineral and rosin oils would give a product of normal density. Fish oils can be detected by their characteristic smell, especially on warming.

Various methods have been proposed for judging the quality of linseed oil, but none of them are perfectly satisfactory. The best oil is that which dries the most perfectly; but the rapidity of the drying, and the consistency of the dried product, are most important factors which must also be taken into account. The iodine-valve, which is a measure of the drying power, should not fall much below 180.

A satisfactory practical test, recommended by Allen,[162] consists in mixing the oil with three times its weight of genuine white lead, and covering a perfectly clean glass surface with the paint. An exactly similar experiment is made simultaneously with a standard sample of linseed oil, and the rates of drying and the characters of the coating of paint compared.

J. Muter has simplified this test by merely flooding a plate of glass with the oil and then exposing it to a temperature of 38° C. (100° F.) in a good current of air. The time required for drying, to such an extent that the coating will not come off when lightly touched, is noted, and compared with standard samples of oil. By applying the finger at intervals to different parts of the film surface the progress of the drying can be readily observed.[163]

Boiled Oils.—Its capacity for thus drying is much enhanced by heating, with addition of “driers,” to a temperature of 130° C. and upwards, while passing a current of air through the oil and then increasing the temperature until the oil begins to effervesce (“boil”). Large quantities of linseed oil are now treated in this way for use in the arts. The driers used are metallic salts, principally those of lead and manganese, which apparently act as oxygen-carriers. Litharge was formerly most commonly used, but its place has been taken to a considerable extent by acetate, borate and resinate of manganese. From 1 to 2 per cent. of either litharge or manganese borate may be used, though less quantities produce a marked effect. Apparently litharge gives the most rapid drying, and manganese a much paler colour.[164] Linseed oil is usually darkened by boiling, and increases both in actual weight and in specific gravity and viscosity. The chemical reactions which take place in boiling are not well understood, but it is in the main a process of oxidation and polymerisation, perhaps accompanied by the formation of anhydrides of the fatty acids, and a portion of the drier remains dissolved in the boiled oil. These driers may be detected by boiling an ounce or so of the oil with dilute hydrochloric acid, allowing the mixture to separate into two layers and then syphoning off the lower into another vessel, and testing for metals (lead, manganese, zinc) or acids (boric, oxalic, etc.).

Black japan for patent leathers is made by boiling linseed oil, without blowing air through it, for at least seven or eight hours, with Prussian blue, or with oxides of iron. The japan is brownish rather than blue in colour, and it is probable that the Prussian blue serves merely as a source of iron oxide, which acts both as a colouring matter and a drier. Other driers, such as litharge, are sometimes added, and for coloured enamels other pigments are substituted for the Prussian blue.

Cotton-seed Oil (Fr. Huile de coton; Ger. Cottonoel or Baumwollensamenoel) is now expressed in enormous quantities in the United States, on the continent of Europe and in Great Britain. The crude oil contains a very characteristic colouring matter which, though naturally ruby red, is sometimes so intense as to make the oil appear to be nearly black. This colouring matter causes the oil to produce stains, and is therefore removed by a process of refining, and a product of a straw- or golden-yellow colour is thus obtained. The refining is usually effected by shaking the crude oil with a cold 5 per cent. solution of caustic soda, using about ten times as much oil as soda solution.

Cotton-seed oil is, on account of its price, seldom or never adulterated, but is itself frequently employed as an adulterant of olive and neatsfoot oils. It is a semi-drying oil, and unsuitable for most purposes in leather manufacture. For a description of its characteristic properties, both chemical and physical, the reader is referred to Lewkowitsch’s ‘Oils, Fats and Waxes,’ or to Allen’s ‘Commercial Organic Analysis,’ vol. ii.

SesamÉ Oil (Fr. Huile de sÉsamÉ; Ger. Sesamoel; Teel oil, Gingeli oil) is another seed oil, usually of paler colour than cotton-seed oil, but resembling it in having scarcely any odour, and possessing a bland and agreeable, though not very characteristic taste. It is often used as an adulterant of olive oil.

SesamÉ oil is a non-drying oil, which does not easily turn rancid. When present in other oils, it may be detected by agitating 10 c.c. of the sample with 5 c.c. of concentrated hydrochloric acid in which 0·1 grm. of white sugar has previously been dissolved. After shaking together for at least ten minutes, the oil and acid are allowed to separate, when, if sesamÉ oil be present, the acid layer will have a marked rose colour, the intensity of which increases with the amount of sesamÉ oil in the sample (Baudouin’s test).

SesamÉ oil is largely used in India for oiling tanned sheep- and goat-skins (“Persians”), and has the characteristic property of being assimilable in large quantities by leather without the latter appearing oily. East India tanned skins often contain 25 and even 30 per cent. The oil is applied to them in the wet condition before they are dried. It is easily detected in the oils extracted from these skins by Baudouin’s test. The oil seems well adapted for many purposes in leather manufacture.

Cod Oil (Fr. Huile de morue; Ger. Leberthran) is by far the most important oil used by leather manufacturers, and is obtained from the liver of the common cod-fish (Gadus Morrhua) and several other members of the genus Gadus. The chief seats of the cod fishery are the coasts and banks of Newfoundland, Nova Scotia, the Gulf of St. Lawrence, the coasts of Norway, Denmark and Germany, the Dogger Bank in the North Sea, and the shores of Alaska in the Pacific Ocean.

The oil was formerly obtained by keeping the livers of the fish in large wooden vats, stirring constantly until so much decomposition has taken place that the cells containing the oil burst, and the oil thus released rises to the surface and is skimmed off with wooden ladles. The crude oil is allowed to deposit any suspended matters by sedimentation in a tank, and is then poured into casks ready for sale. The “brown oil” so often used by tanners is obtained by boiling the solid matter left after extracting the oil as above in iron tanks until all the water has evaporated; the oil thus liberated is then strained off, clarified and put into barrels.

The purer qualities of cod-liver oil are now obtained by boiling the livers with water and skimming off the oil which rises to the surface. Three grades are on the market at the present time: medicinal, or ordinary bright; an inferior “light brown”; and “dark-brown,” or “tanners’ oil.” It is probable that these steam-extracted oils are much more liable to “spue” than those extracted by the old method at a higher temperature, since Eitner[165] has shown that seal oils extracted at a low temperature spue badly, but lose the tendency if heated for some time to 250-300° C.

Genuine cod oil, as suitable for use in leather manufacture, is always more or less brown in colour, of specific gravity about 0·928, and refractive index 1·482. At present prices it can only be adulterated with other fish oils, rosin, or mineral oil, or with water, gelatine or mucilage. Of these, rosin oil and petroleum are the most frequently employed in sophistication.

An inferior variety of oil, known as “coast cod,” made from the livers of various fish, such as ling, haddock and hake, is also sold, but, as it is frequently mixed with oils from other fish refuse, it has a very poor reputation.

Cod oil, together with most of the other oils obtained from fish livers, has the property of producing an intense reddish-violet colour when a drop of strong sulphuric acid is dropped upon ten or fifteen drops of the oil contained in a white porcelain tray or saucer. The reaction succeeds still better, if, instead of the oil itself, its solution in chloroform, carbon disulphide or tetrachloride is employed. This test, although very useful for the detection of liver oils when they are present in oils of a totally different character, such as rape or olive oils, does not in any way indicate whether a sample of fish oil is pure or otherwise. A very similar reaction is given by cholesterol which is present in wool-fat.

Shark-liver Oil (Fr. Huile de requin; Ger. Haifischthran) is obtained from the liver of the “basking shark,” or “ice-shark,” chiefly caught off the coast of Norway; but the livers of the dog-fish and several allied fish also are sometimes substituted.

Shark oil has been employed in tanneries as a substitute for cod-liver oil, but, according to Lewkowitsch, and to Allen, it is no longer employed in England. From its pale colour it is probably principally used to improve the appearance of darker oils. According to Eitner,[166] its use causes leather to “spue” badly if not previously heated.

Shark oil is characterised by the very notable proportion of unsaponifiable matter which it contains, which is of the same character as that of sperm oil, and not easily removed from its soap solution by petroleum ether. It gives a strong violet-blue coloration with concentrated sulphuric acid, the reaction being even more marked than with cod-liver oil itself, and of a bluer violet.

Whale Oil (Fr. Huile de baleine; Ger. Wallfischthran) is extracted from the blubber of various species of whale, and often contains traces of spermaceti, the substance which characterises the oil from the sperm whale. This yields on saponification higher alcohols, which are found in the unsaponifiable matter; but in ordinary whale oil the total unsaponifiable matter seldom exceeds 1¹/₂ to 2 per cent. Whale oil is largely used on the Continent for “chamoising” (q.v.), and is consequently a constituent of dÉgras. It is much less oxidisable than cod.

Seal oil (Fr. Huile de phoque; Ger. Robbenthran) is obtained from the common rough-coated seal, abundant in the Arctic regions. It bears a strong resemblance to both whale and fish oils, and cannot be detected in mixtures of these. The Swedish “Dreikronenthran” (Three Crown Oil) is a mixture of seal and fish oils. As genuine seal oil only contains about ¹/₂ per cent. of unsaponifiable matter, its adulteration by mineral or resin oils may be detected by a determination of the matter extracted by petroleum ether after saponification of the oil (see L.I.L.B., p. 178).

There is no simple test by which the purity or otherwise of a sample of oil can be determined, as the dealers know all the best tests which the users could try, and fake up their oils accordingly. For instance, if petroleum is to be added surreptitiously to a cod oil, the decrease in specific gravity of the oil caused by this addition would be corrected by the addition of a suitable quantity of soap or rosin oil, which would scarcely affect the colour, taste or odour of the sample. The only satisfactory method of detecting adulteration is to submit the oil to a complete chemical examination, and for this purpose L.I.L.B., pp. 156 et seq., or the larger text-books already named may be suitably consulted.

Menhaden Oil (Porgie oil, Straits oil) is largely used in certain districts as an adulterant or substitute for cod oil. It is obtained from the Alosa Brevoordia or menhaden, a member of the herring family, about a foot long. The fish is caught on the Atlantic coast of America, and is so plentiful that it is very doubtful whether cod oil can ever compete with it successfully in price. The fish are boiled in steam kettles, the oil squeezed by hydraulic presses, clarified, and bleached by exposing to the sun in shallow glass-covered tanks. An inferior grade is known as “Bank oil.” Menhaden oil is chiefly characterised by its very high “specific temperature reaction” (L.I.L.B., p. 169) which is about 306. It is not a good leather-oil, being very liable to “spue.”

Many other varieties of oil extracted from the bodies, and not from the livers only of fishes, are classed as fish oils. Menhaden oil is the principal of these; but Japanese oil, sardine and herring oils, and those obtained from the refuse of other fish are scarcely less important, though as they are derived from such different sources it is not possible to quote any definite characteristics by which they may be identified when mixed with more valuable oils. They are usually very liable to “spue.”

Fish Tallow, which, according to Eitner, is a good and cheap substitute for dÉgras, is the solid grease obtained from different kinds of fish oil by subjecting them to a low temperature and separating the matter which is thus precipitated, or (as in China and Japan) the solid fat which is extracted at the same time as the oil from the body of the fish. Formerly fish tallow was only obtained from and with Japanese train oil, but it is now obtained from whale blubber. This latter yields a very pure form of the tallow, which does not need any rectification; but the Japanese variety, which is obtained from fish of the herring family, contains a sort of fish glue, which greatly deteriorates the quality of the product. By careful purification, however, this glutinous matter may be removed, and the refined product has none of the leather-staining properties so characteristic of the crude tallow. The refined tallow is sold in square flat cakes, melts at 42° and is not quite so stiff as ox tallow.

DÉgras and Sod Oil are products of chamois-leather dressing (p. 378) which are used in currying. Skins are treated with marine animal oils, and submitted to oxidation, and the surplus and partially altered oil is recovered. In the French method, whale and seal oils as well as liver oils are used, and the oxidation is slow and gradual, and the residual oil, being liquid, is recovered by pressure, and constitutes moellon, of which the first pressing (premiÈre torse) is the best. This is never sold for currying in its original purity; but, mixed with further quantities of fish oils, tallows, and sometimes wool-fat, it constitutes the ordinary dÉgras of commerce. The additions, though they lower the value, are not to be considered as simple adulterations, since the moellon alone would be less suitable for the purpose. After removal of as much oil as is possible by dipping in hot water and pressing, a further quantity is recovered by washing with solutions of potash or soda, from which it is separated by addition of acid, and constitutes a lower quality of degras. The moellon is of such value as a currying material, that factories are run in which chamoising is carried on solely for its production, the skins being oiled and oxidised repeatedly, till reduced to rags.

In the English method of chamoising, liver oils are almost exclusively used, and the oxidation is much more rapid and intense, the skins being packed in boxes or piled, and allowed to heat. The product obtained in this way is much more viscous, and can only be recovered by scouring with alkalis; and the product, recovered with acid, constitutes sod oil. In many English factories, a modified method is now adopted, and a product recovered by pressure, which scarcely differs from moellon.

An important peculiarity of dÉgras and sod oil is its ready emulsification with water, which from its mode of preparation, it always naturally contains, and which should be present in a good dÉgras to the extent of not less than 20 per cent. Such a mixture, containing water, is a sort of natural fat-liquor and is absorbed much more perfectly by the skins than an oil alone. Sod oils, however, are frequently “evaporated,” or deprived of water by heating above 100°, with the object not only of effecting a fancied improvement, but of getting rid more completely of the sulphuric acid which the water is apt to contain. This makes them more homogeneous, and consequently much darker in colour. It is not easy to neutralise the acid in an aqueous sod oil by direct addition of alkali; possibly ammonia is best adapted for the purpose; or a suggestion, I think due to Eitner, may be adopted, of incorporating a small quantity of a suitable soap. In any case, very complete mixture is required. If the sulphuric acid used in recovery has been insufficient for complete neutralisation of the alkali, the dÉgras or sod oil will naturally contain soaps, and sometimes also free alkali. Free acid and free alkali are both injurious to leather, the former if anything the more so, darkening the colour, and even rendering the leather tender. When dÉgras is used in mixture with other fats, care should be taken not to raise the temperature of the mixture so high as to drive off the water, to which a good deal of its special efficacy is due.

The chemical changes which take place during the chamoising process are as yet incompletely understood. A large proportion of the glycerine is dehydrated during the “heating,” forming acrolein (acrylic aldehyde), to the action of which it is very possible that the actual conversion of the skin into leather is due, while the fatty acids also undergo oxidation. DÉgras therefore always contains considerable quantities of oxidised fatty acids, which are sometimes associated with nitrogenous products from the skins, and which are soluble in alcohol, but insoluble in petroleum ether. To these products Simand gave the name of Degrasbildner (dÉgras-former, Fr. dÉgragÈne), and it has been considered a measure of the quality of the degras, but its exact value and function is rather doubtful. According to Simand, a genuine dÉgras should contain not less than 15 to 20 per cent. of the dÉgras-former as estimated by his method, calculated on the dry oil, and a smaller percentage is also present in the original fish oils. (For method of estimation see L.I.L.B., p. 182).

As the process of dÉgras manufacture is obviously mainly one of oxidation, many attempts have been made to produce it by direct oxidation of fish oils, without the agency of skins, both by blowing air through the oil, and by addition of oxidising agents such as nitric acid. Eitner states that such oxidised oils are more liable to “spue” than the original oils, as they already contain large quantities of resinised products; but this is certainly not true of all artificial dÉgras, some of which answers its purpose perfectly as a currying material, though it is very probably justified in other cases. Of course the methods of successful manufacturers are kept as profound secrets.

DÉgras and sod oil, when deprived of water, are dark and viscous oils, of high specific gravity (0·945-0·955), and therefore heavier than the oils which have been employed in their manufacture.

Waxes, as has already been stated, differ in their chemical character from true fats, in that their fatty acids, which are mostly of high molecular weight, are combined, not with glycerine, but with alcohols, also of high molecular weight and of wax-like consistency. Most waxes are solid bodies of high melting point, but some oils, especially sperm and bottlenose oils, are chemically liquid waxes; woolfat contains a considerable proportion of waxes; and many marine oils, such for instance as shark-liver oil (p. 366), contain waxes in smaller quantity in mixture with true fatty oils.

Sperm Oil (Fr. Huile de cachalot; Ger. Spermacetioel, Walratoel) is obtained from the sperm whale, an inhabitant of the Antarctic seas. “Arctic sperm” (Ger. Doeglingthran) is a very similar oil obtained from the “Bottlenose whale.” These oils are very fluid, do not dry, and are excellent lubricating oils for light machinery, and also good lamp oils. They contain little if any glycerides, and about 40 per cent. of unsaponifiable solid alcohols, which are soluble in ethyl-alcohol, and must not be confused with ordinary unsaponifiable mineral oils, which are frequently used as adulterants in mixture with fatty oils to adjust gravity and the “saponification value.” Mineral oils are liquid, and insoluble in alcohol. Sperm oil is the lightest of ordinary oils, its gravity being only about 0·880 at 15° C. From its price it is particularly liable to sophistication. It is used in leather manufacture in the finishing of some fine leathers, and sometimes as a constituent of fat-liquors. Spermaceti, a wax also obtained from the sperm whale, is an occasional constituent of leather polishes.

Beeswax (Fr. Cire des abeilles; Ger. Bienenwachs) is one of the most important waxes for the leather-dresser. As is well known, it is obtained from the honeycomb of the ordinary bee. It is a yellowish solid body, fairly plastic when fresh, and of “waxy” feel. At low temperatures it is brittle and of fine granular texture, and when pure is almost tasteless. It is often bleached by repeated melting and exposure to sunlight. As wax always contains a considerable amount of pollen it may be identified when in admixture with other substances by means of the microscope.

Beeswax is almost insoluble in cold alcohol, but boiling alcohol dissolves out the contained cerotic acid, which crystallises from it on cooling. Wax is saponified by alcoholic potash, but the resulting myricyl alcohol (about 54 per cent.) is not capable of further saponification.

Beeswax is frequently adulterated. Water and mineral matters (ochre, gypsum, etc.) also flour, starch, tallow, stearic acid, Japan wax, carnaÜba wax, resin and paraffin-wax are among the substances most commonly used in its sophistication.

The detection of these, and especially of the other waxes, is so difficult that it will not be described here. The reader is, however, referred to Benedikt and Lewkowitsch’s ‘Oils, Fats and Waxes,’ for further information.

CarnaÜba Wax (Fr. Cire de carnauba; Ger. Cearenwachs, Carnaubawachs) has come largely into use recently owing to the advent of the coloured leather shoe. As it is a very hard wax it has become very popular with boot polish makers, its low price being also in its favour. CarnaÜba wax is an exudation from the leaves of Copernica cerifera, a palm indigenous to Brazil, and is, on this account, often known as Brazilian wax. It is difficult to saponify, and with different experimenters has yielded very varied results on analysis; it is generally agreed, however, that it is a complicated mixture of several of the higher alcohols and acids.

Japan Wax is not a true wax, but a fat consisting of glycerides. It is a pale yellow, hard, waxy substance obtained from the berries of a sumach (Rhus succedanea, etc.). At ordinary temperatures its specific gravity is exactly that of water, and it melts at 56° C. Any admixture with other fats would lower the melting point, but japan wax is often adulterated with 15 to 30 per cent. of water. It is chiefly valuable to leather dressers as a substitute for beeswax on account of its lower price.

Volatile or Essential Oils.

These oils are distinguished from those described in the previous section in that they are capable of distillation without undergoing any serious amount of decomposition. They occur to some considerable extent in nature, but those of most importance to the leather trade are produced by the decomposition of more complicated materials.

Birch Oil is by far the most important of this class of oils so far as the leather-dresser is concerned, since it is the substance which gives to “Russian leather” its characteristic odour.

The oil is obtained by destructive distillation, and the process by which the peasants conduct this is one of the rudest that can be imagined. A cauldron is filled with dry birch-bark, closed, and heated over a fire. The vapours which are evolved are carried, by means of a pipe, to another vessel which is buried in the ground, and are there condensed. The dark-brown liquid (birch-tar) is allowed to cool, and the liquor which rises to the surface skimmed off. The tar is sometimes distilled, and an oil is thus obtained which does not give the true birch-oil scent very strongly though occasionally sold as a refined oil. The true odorous substance is evidently of very high boiling point and remains mainly in the tar.

The birch tar is almost entirely used for giving leathers a “Russian” odour, for although it smells somewhat strongly of tarry products, the oils causing this smell are far more volatile than the birch scent itself, and therefore disappear on storing the leather a short time. Tar obtained from various species of pine is sometimes substituted for birch tar, but it may readily be distinguished from the latter by the odour and the difference in the specific gravity. Birch tar has a specific gravity of 0·925 to 0·945, whilst fir tar has one of 1·02 to 1·05; thus the former floats on water while the latter sinks if it be entirely free from enclosed air. Fir tar, too, gives up a yellow colouring matter to water shaken up with it, while birch tar leaves the water colourless. Birch tar has a distinctly acid reaction, and must not be kept in iron vessels. (See p. 251).

The leaves and twigs of American black birch when distilled with water or steam, yield an oil which is practically identical with that of Gaultheria procumbens (wintergreen), and consists almost entirely of methyl salicylate. It is clarified, and to some extent decolorised, by filtration through woollen blankets and redistillation. A ton of brushwood is said to yield about four pounds of oil. This oil has quite a different odour to that of the real Russian oil, and cannot be used in the scenting of “Russia” leather. Sandalwood oil with a little black birch or wintergreen oil is sometimes employed for scenting small fancy articles and bears considerable resemblance to the true “Russia” leather odour. Black birch, aniseed, sassafras and various other essential oils are occasionally used in small quantities as preservatives, and to cover disagreeable odour in blood-seasonings, cements and other products used in the leather trade. The methods employed for their detection and estimation do not, however, come within the scope of a work such as the present one. Most essential oils have considerable power as antiseptics, and in preventing mildew and the attacks of insects.

Mineral Oils and Waxes.

This class of bodies is totally different in chemical constitution from the true oils and waxes, containing neither glycerides, fatty acids nor alcohols, but consisting of carbon and hydrogen only, approximately in the proportion of one atom of the former to two of the latter. They occur in underground lakes, from which they are obtained by springs or borings; or in shales, from which they are separated by distillation. It is commonly supposed that they have been formed, at some remote period of the earth’s history, by the decomposition of animal and vegetable matters, at a high temperature and under great pressure.[167]

[167] Oils from wells or springs are technically called “petroleum oils,” those from shale, “paraffin” oils, but chemically, there is no definite distinction.