They differ greatly in their gravity and boiling-point, but not much in their ultimate composition, consisting largely of saturated or nearly saturated hydrocarbons (cp. [p. 354]), and hence are little liable to oxidation, and acted on by few chemical reagents. From their constitution they are of course unsaponifiable, and in this way can be separated from fats and oils with which they have been mixed. (For particulars of the method see L.I.L.B., [p. 178].)
The heavier mineral oils are a good deal used in mixture with other oils and fats, for stuffing leathers, those of a specific gravity of 0·880-0·900 being usually most suitable. They are quite incapable of “spueing,” and are useful in lessening that tendency in other oils with which they are mixed. They have not, however, the same affinity for the leather fibre as some of the true oils, and are to a certain slight extent volatile, and should generally be used in mixture, rather than alone.
Most mineral oils, when held so that a strong light (daylight or electric light rich in ultra-violet rays) falls upon them, show a green or violet fluorescence or “bloom.” This is very persistent, even when the oil is mixed with a large volume of other oils, and is often relied upon as a means of detecting them when used as adulterants. The test is, however, not infallible, since the effect is due to impurities which may be removed by purification, or masked by the addition of such substances as nitrobenzene or nitronaphthalene, and it also occurs in the hydrocarbon products produced in the distillation by steam of animal oils, and is occasionally seen to some extent even in oils which have not undergone distillation.
Vaseline and Vaseline Oil are the most viscous and densest of the petroleum oil products. They probably differ from the solid paraffins in chemical constitution, though their ultimate composition is almost the same. They are often useful constituents of stuffing greases.
Paraffin Wax consists of a mixture of hydrocarbons similar in chemical constitution to the paraffin and petroleum oils, but of higher boiling point, and solid at ordinary temperatures. Its hydrocarbons are mostly saturated, and hence very stable bodies, and little liable to oxidation. They are completely unsaponifiable, and unaffected by boiling with alcoholic potash, and in most cases by boiling with strong sulphuric acid, by which they may be separated from animal and vegetable waxes or fats with which they have been mixed. They are quite incapable of resinising by oxidation, or of causing “spueing” in leather. They are soluble in petroleum spirit, carbon disulphide and most of the ordinary solvents of fats, but insoluble in alcohol.
Paraffin wax separates from the liquid oils by crystallisation on cooling, and the remaining liquid which adheres is removed by hydraulic pressing, as in the case of tallow. The hardness and melting point vary according to the extent to which the pressing has been carried, and the temperature at which it has been done. The paraffins of higher melting point are as a rule the more costly.
Pure paraffin wax is a white, more or less hard and brittle substance which does not melt so easily as ordinary fats, and is on this account used in stuffing certain kinds of leather, hardening the stuffing grease, and making the leather feel less oily. When melted, paraffin wax forms a thin liquid, more resembling an ordinary petroleum lamp oil than the viscous vaselines and leather oils. On ignition it burns with a bright somewhat smoky flame, and leaves no ash behind. It is found on analysis when mixed with other waxes or oils in the “unsaponifiable matter” (see L.I.L.B., p. 178).
Ozokerit is a natural paraffin material used for the manufacture of cerasin candles, which sometimes occurs in the vicinity of petroleum springs, especially in Galicia. It is of pale yellow colour when pure, and has then a melting point of about 70° C. Its chief impurities are petroleum oils, water and clay. These are removed by melting the ozokerit, decanting off the clear oil, and filtering it through fine animal charcoal. If liquid oils are present the material is treated with alkali or with strong sulphuric acid, and is pressed before filtering through charcoal. The refined product is termed “cerasin,” and is of a more waxy and less crystalline texture than ordinary paraffin wax.
The Resin Oils are derived from resins, and mainly from colophony or common pine rosin, by destructive distillation. Their specific gravity ranges from 0·96 to 0·99, but their chemical composition is very imperfectly understood, and appears to be by no means constant. Like the mineral oils they are “unsaponifiable,” but often contain small amounts of soap-forming material (resin acids).
The detection and estimation of resin oils is often a matter of considerable difficulty, but further particulars on this point will be found in L.I.L.B., p. 180. From their cheapness, they are considerably employed as adulterants of other oils, and their high gravity makes them convenient to adjust the gravity of mineral oils when used for this purpose, as the latter are usually lighter than the fatty oils. As currying oils, they are not particularly suitable, though often employed in stuffing picker bands, and other heavily greased leathers. They have considerable antiseptic powers, and for this reason are useful in leather greases, preventing heating, and checking mildews.