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.
The mineral oils and waxes are largely capable of being distilled without decomposition, but if heated to high temperatures, are readily “cracked” or broken up into simpler and generally more volatile compounds—a fact which is employed in the production of gas, and the utilisation of some of the heavier products.