_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.
[2312] _Jahresbericht_ of Wiggers and Husemann for 1867. 37.
True vegetable tar has always a decidedly acid reaction. It is readily
miscible with alcohol, glacial acetic acid, ether, fixed and volatile
oils, chloroform, benzol, amylic alcohol or acetone. It is soluble in
caustic alkaline solutions, but not in pure water or watery liquids.
The sp. gr. of tar from the roots of conifers is about 1·06 (Hessel)
yet at a somewhat elevated temperature, it becomes lighter than warm
water.
Water agitated with tar acquires a light yellowish tint, and the taste
and odour of tar, as well as an acid reaction. On evaporation the
solution becomes brown, and at last microscopic crystals are obtained
with a brown residue like tar itself, which is no longer soluble in
water. A microscopical examination of tar which has been exhausted with
water, shows that all crystals have disappeared.
=Chemical Composition=—Dry wood may be heated to about 150° C. without
decomposition; but at a more elevated temperature, it commences to
undergo a change, yielding a large number of products, the nature and
comparative quantity of which depend upon circumstances. If the process
is carried on in a closed vessel, a residue will be got which has more
or less resemblance to coal. By heating fir-wood enclosed with some
water to 400° C., Daubrée (1857) obtained a coal-like substance, which
yielded by a subsequent increase of temperature scarcely any volatile
products.
The results are widely different if a process is followed which permits
the formation of volatile bodies; and these substances are formed in
largest proportion, if the heat acts quickly and intensely. At lower
degrees of heat, more charcoal results and more water is evolved.
Among the volatile products of destructive distillation, those alone
which are condensed at the ordinary temperature of the air are of
pharmaceutical interest; and of these, chiefly the portion not soluble
in water, or that which is called _Tar_ or _Liquid Pitch_. The aqueous
portion of the products consist principally of empyreumatic acetic
acid, to which tar owes its acid reaction.
The tissue of wood is chiefly formed of cellulose, intimately combined
with a saccharine substance, which may be separated if the wood is
boiled with dilute acids. The remaining cellulose is however not
yet pure, but is still united to a substance which, as shown by
Erdmann,[2313] is capable of yielding pyrocatechin.
[2313] Liebig, _Annalen der Chemie u. Pharmacie_, Suppl. v. (1867) 229.
It is well known that sugar subjected to an elevated temperature,
yields a series of pyrogenous products; and the same fact is observed
if purified cellulose is heated in similar manner. But for tar-making,
wood is preferred which is impregnated with resins and essential oils,
and these latter furnish another series of empyreumatic products.
From these circumstances, the components of wood-tar are of an
extremely complicated character, which is still more the case when
other woods than those of conifers form part of the material submitted
to distillation. In the case of beech-wood, _Creasote_ is formed,
which is obtained only in very small quantity from the _Coniferæ_.
Volatile alkaloids and carbolic acid, which are largely produced in the
destructive distillation of coal, appear not to be present in wood-tar.
The components of the latter may be considered under two heads:—first,
the _lighter aqueous portion_, which separates from the other products
of distillation, forming what is called _Impure Pyroligneous Acid_.
This contains chiefly acetic acid and _Methyl Alcohol_ or _Wood
Naphtha_, CH₄O; _Acetone_, C₃H₆O; besides other liquid products
abundantly soluble in water and acetic acid. In this portion, some
pyrocatechin also occurs.
The second class of pyrogenous products of wood consists of a
homologous series of liquid hydrocarbons, sparingly soluble in water,
and which therefore are chiefly retained in the heavy layer below the
pyroligneous acid, forming the proper wood-tar. The liquid in question
furnishes _Toluol_ or _Toluene_, C₇H₈ (boiling point 114° C.), _Xylole_
C₈H₁₀, and several other analogous substances.
If tar is redistilled, an elevated temperature being used towards the
end of the process, some crystallizable solid bodies are obtained, the
most important of which is that called _Paraffin_, having the formula
CₙH₍₂ₙ₊₂₎, _n_ varying from 20 to 24.
The crystals already mentioned as occurring in tar are _Pyrocatechin_.
They are easily sublimed at some degrees above their fusing point (104°
C.), or removed by acetic acid, in which as well as in water they are
readily soluble. Hence in some sorts of tar this substance does not
occur, it having probably been removed by water.
Pyrocatechin, C₆H₄(OH)₂, can be obtained by the destructive
distillation of many other substances, as catechu, kino, the extracts
of rhatany and bearberry leaves, and other extracts rich in that form
of tannin which produces _greenish_ (not _blue-black_) precipitates in
salts of iron. It is extracted from the granular sorts of wood-tar,
by exposing them at a proper temperature to a current of heated dry
air, or by exhausting them with water. Ether when shaken with the
concentrated aqueous solution and left to evaporate, leaves colourless
crystals of pyrocatechin which after purification are devoid of acid
reaction. They have a peculiar burning persistent taste, and are
very pungent and irritating when allowed to evaporate. A solution of
pyrocatechin yields with perchloride of iron a dark green coloration
changing to black after a few moments, and becoming red on the addition
of potash. This mixture finally acquires a magnificent violet hue, like
a solution of alkaline permanganate. No alteration is produced in a
solution of pyrocatechin by protosalts of iron.
Among the few medicinal preparations of tar, is _Tar Water_, called
_Aqua vel Liquor Picis_, made by agitating wood-tar with water. The
presence in it of pyrocatechin is easily proved by the above-mentioned
reactions, or by a few drops of red chromate of potassium, which
produces a brownish black colouration. It may hence be inferred that
pyrocatechin is perhaps the active ingredient in tar-water, and that
for making this liquid the granular, crystalline sorts of tar should be
preferred.[2314]
=Commerce=—Tar as well as pitch is manufactured in Finland, and shipped
from various ports in the Gulf of Bothnia, as Uleaborg, Gamla Carleby,
Jacobstad, Ny Carleby and Christinestad; also from Archangel and Onega
on the White Sea. Some tar is also produced in Volhynia, and finds its
way by the Dnieper to the Black Sea.
The North of Sweden likewise produces tar, chiefly about Umea and
Lulea, the distillation being now performed in well-constructed
apparatus of iron.
The pine forests of North America afford tar and pitch. Wilmington
in North Carolina exported in 1871, 25,260 barrels of tar, and 3788
barrels of pitch.[2315]
[2314] We may suppose that the authors of the French _Codex_ were not
of this opinion, inasmuch as in making _Eau de Goudron_, they order
that the liquid obtained by the first maceration of the tar, shall be
thrown away.
[2315] Consul Walker, _Report on the Trade of North and South
Carolina—Consular Reports_ presented to Parliament, May, 1872.
The imports of tar into the United Kingdom in 1872, were 189,291
barrels, valued at £218,339. Of this quantity 145,483 barrels were
shipped from the northern ports of Russia.
The barrels in which tar arrives hold about 30 gallons. Smaller sized
vessels termed _half-barrels_ are also used, though less frequently.
=Uses=—In medicine of no great importance: an ointment of tar is
a common remedy in cutaneous diseases, and tar water is sometimes
taken internally. The consumption of tar in shipbuilding and for the
preservation of fences, sufficiently explains the large importations.
Other Varieties of Tar.
_Juniper Tar_, _Pyroleum Oxycedri_, _Oleum Juniperi empyreumaticum_,
_Oleum Cadinum_, _Huile de Cade_.—This is a tar originally obtained
by the destructive distillation of the wood of the _Cade_, _Juniperus
Oxycedrus_ L., a shrub or small tree, native of the countries bordering
the Mediterranean. It was for centuries used in the South of France as
an external remedy, chiefly for domestic animals, but had fallen into
complete oblivion until ten years ago, when it began to be prescribed
in skin complaints.
The _Huile de Cade_ now in use, is transparent and devoid of crystals.
It is somewhat thinner than Swedish tar, but closely agrees with it in
other respects. It is imported from the Continent, but where made and
from what wood we know not. _Huile de Cade_ is mentioned by Olivier de
Serres,[2316] a celebrated French writer on agriculture of the 16th
century; it is named by Parkinson[2317] in 1640; also by Pomet,[2318]
in whose time (1694) it was rarely genuine, common tar being sold in
its place.
[2316] _Théâtre d’Agriculture_, Paris, 1600. 941.
[2317] _Theatrum Botanicum_, 1033.
[2318] _Hist. des Drogues_, Paris, 1694. part i. chap. xii. xiv.
_Beech Tar_—Tar is also manufactured from the wood of the beech, _Fagus
silvatica_ L., and has a place in some pharmacopœias as the best source
of creasote.
_Birch Tar_—is made to a small extent in Russia, where it is called
_Dagget_, from the wood of _Betula alba_ L. It contains an abundance
of pyrocatechin, and is esteemed on account of its peculiar odour well
known in the Russia leather. A purified oil of birch tar is sold by the
Leipzig distillers.