Tar may be extracted from many of our native trees, especially from the pines, the kauri, totara, kahikatea, rimu, miro, matai, tanekaha, etc., also from the tooth-leaved and other beeches, which form such vast forests in many districts, and in all probability from the large kinds of rata and tea-tree.
The waste tops and branches of trees felled for timber, crooked pieces, knots, roots, etc., can be utilized for this purpose, so that the manufacture of tar and allied products would not only afford a profitable outlet for labour, but would remove a great source of danger, and materially reduce the serious loss arising from forest fires.
In the forests of the White Sea and the Baltic, tar is extracted from the Scotch fir (Pinus sylvestris), and the Baltic spruce fir (Abies communis); the wood and roots being cut into short billets, and then subjected to a process of slow combustion.
A funnel-shaped cavity of any convenient size is excavated in the side of a sloping bank; an iron pan is fitted tightly into the bottom of the hole, and communicates with the exterior by a pipe or tube, which passes through the side of the bank, and allows the tar to be drawn off as fast as it is extracted.
The billets are now tightly packed in the cavity, ends downward, until it is completely filled, when the surface is covered with turf, which is compactly beaten down by two men, one of whom uses a wooden stamper, the other a wooden mallet—so that the outer surface is sufficiently firm to prevent the escape of the volatile products. It is absolutely necessary that this part of the process should be efficiently performed.
A small portion of the turf is now removed, and fire applied to the stack; as soon as it is kindled, the turf is replaced. The exuded tar is received into the pan at the bottom of the hole, and is discharged by the spout into casks, which are at once bunged and made ready for shipment.
The quantity of billets subjected to slow combustion at the same time is frequently enormous, amounting to 50,000 or 60,000 cubic feet. In this
case the pile rises considerably above the surface of the cavity, but the whole must be carefully covered with sods, etc., as already described. A pile containing 50,000 cubic feet requires a fortnight for the process of combustion. It need scarcely be remarked that the wood must be dry before the operation is commenced.
In the Highlands of Scotland tar is sometimes extracted by a somewhat rougher method. A hole is dug in the side of a hill, a gutter being formed at the bottom of the hole, and terminating in a small aperture on the outside. The hole is filled with wood cut in proper lengths, and the top is covered with tiles or sods. The tar gradually drains into the gutter, and is discharged by the external aperture, which must, of course be very small, or air will be admitted in such quantity as to burn the entire mass.