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).