[163] Kathreiner states that this method is a useful test for fish and liver oils, those which dry most rapidly being specially liable to “spue.”
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.).
[164] Cp. F. H. Thorpe, Abst. Jour. Soc. Chem. Ind., 1890, 628, from Technology, Quart., iii. pp. 9-16.
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.