Uses—Burgundy pitch is prescribed as an ingredient of plasters, and thus employed is useful as a mild stimulant. In Germany it has some economic applications, one of which is the lining of beer casks, for which purpose a composition is used called Brauerpech (brewers’ pitch), made by mixing it with colophony or galipot.
Adulteration—No drug is the subject of more adulteration than Burgundy pitch, so much so that the very name is understood by some pharmacologists to be that of a manufactured compound. The substance commonly sold in England is made by melting together colophony with palm oil or some other fat, water being stirred in to render the mixture opaque. In appearance it is very variable, different samples presenting different shades of bright or dull yellow or yellowish-brown. Many when broken exhibit numerous cavities containing air or water; all are more or less opaque, becoming in time transparent on the surface by the loss of water. Artificial Burgundy pitch is offered for sale in bladders; it has a weak terebinthinous odour, and is devoid of the peculiar fragrance of the genuine. The presence of a fatty oil is easily discovered by treatment with double its weight of glacial acetic acid, which forms a turbid mixture, separating by repose into two layers, the upper being oily.
PIX LIQUIDA.
Wood-Tar; F. Goudron végétal, Poix liquide; G. Holztheer, Fichtentheer.
Botanical Origin—Tar is obtained by submitting the wood of the stems and roots of coniferous trees to dry or destructive distillation. That found in commerce is produced in Northern Europe, chiefly from two species, namely Pinus silvestris L. and P. Ledebourii Endl. (Larix sibirica Ledeb.). These trees constitute the vast forests of Arctic Europe and Asia.
History—Theophrastus gives a circumstantial description of the preparation of tar, which applies with considerable accuracy to the processes still practised in those districts where no improved methods of manufacture have yet been introduced.
Production—The great bulk of the vegetable tar used in Europe, and known in commerce as Archangel or Stockholm Tar, is prepared in Finland, Central and Northern Russia, and Sweden.
The process is conducted in the following manner:—vast stacks of pine wood consisting chiefly of the roots and lower portions of the trunks (the more valuable parts of the trees being used as timber), and containing as much as 30,000 to 70,000 cubic feet, are carefully packed together, and then covered with a thick layer of turf, moss, and earth, beaten down with heavy stampers. The whole stack of billets is constructed over a conical or funnel-like cavity made in the ground, if possible on the side of a hill, this arrangement being adopted for the purpose of carrying on a downward distillation. Fire being applied the combustion of the mass of wood has to be carried on very slowly and without flame in order to obtain the due amount of tar and a charcoal of good quality. During its progress the products, chiefly tar, collect in the funnel-like cavity, from which they are discharged by a tube into a cast-iron pan placed beneath the stack, or simply into hollow tree trunks. The time required for combustion varies from one to four weeks, according to the size of the stack.
During the last few years this rude process has been improved and accelerated by the introduction of rationally constructed wrought-iron stills, furnished with refrigerating condensers, as proposed in Russia by Hessel in 1861. By this mode of manufacture the yield in tar of pine wood is about 14 per cent. from stems, dried by exposure to the open air; and 16 to 20 per cent. from roots. Large quantities of pyroligneous acid and oil of turpentine are at the same time secured. The wood of the beech and of other non-coniferous trees appears not to afford more than 10 per cent. of tar, while turf yields only from 3 to 9 per cent.
Description—The numerous empyreumatic products which result from the destructive distillation of pine wood, and which we call tar, constitute a dark brown or blackish semi-liquid substance, of peculiar odour and sharp taste. When deprived of water and seen in thin layers, tar is perfectly transparent. The magnifying glass shows some of the varieties to contain colourless crystals of Pyrocatechin, scattered throughout the dark viscid substance, and to these tar owes its occasionally granular, honey-like consistence.[2312] A gentle heat causes them to melt and mix with the other constituents.