_Pix sicca vel solida vel navalis_; _Pitch_, _Black Pitch_; F. _Poix
noire_; G. _Schiffspech_, _Schusterpech_, _Schwarzes Pech_.
=Botanical Origin=—see _Pix liquida_.
=Production=—When the crude products of the dry distillation of
pine wood, as described in the previous article, are submitted to
re-distillation, the following results are obtained. The first 10 to
15 per cent. of volatile matter consists chiefly of methylic alcohol
and acetone. A higher temperature causes the vaporization of the
acetic acid, while the still retains the tar. This last, subjected
to a further distillation, may be separated into a liquid portion
called _Oil of Tar_ (_Oleum Picis liquidæ_), and a residuum which,
on cooling, hardens and forms the product under notice, namely _Black
Pitch_. Again heated to a very elevated temperature, it is capable of
yielding paraffin, anthracene and naphthalene.
=Description=—Pitch is an opaque-looking, black substance, breaking
with a shining conchoidal fracture, the fragments showing at the
thin translucent edges a brownish colour. No trace of distinct
crystallization is observable when very thin fragments are examined,
even by polarized light. Pitch has a peculiar disagreeable odour,
rather different from that of tar. Its alcoholic solution has a feeble
taste somewhat like that of tar, but pitch itself when masticated is
almost tasteless. It softens by the warmth of the hand, and may then
be kneaded. It readily dissolves in those liquids which are solvents
of tar. Alcohol of 75 per cent. acts freely on it, leaving behind in
small proportion a dark viscid residue. The brown solution reddens
litmus paper, and yields a dingy brownish precipitate with perchloride
of iron, and whitish precipitates with alcoholic solution of neutral
acetate of lead, or with pure water. Pitch dissolves in solution of
caustic potash, evolving an offensive odour.
=Chemical Composition=—From the method in which pitch is prepared,
we may infer that it contains some of the less volatile and less
crystallizable compounds found in tar. Ekstrand (1875) extracted from
it _Retene_, C₁₈H₁₈, a colourless, inodorous crystalline substance,
melting at 90° C.
The pitch of beech-wood boiled with a caustic alkali, yields a fœtid
volatile oil; when this solution is acidulated, fatty volatile acids
are evolved. These principles however have not yet been isolated either
from the pitch of pine or beech. The whitish compound formed by acetate
of lead in an alcoholic solution of pitch deserves investigation, and
perhaps might be the starting point for acquiring a better knowledge of
the chemistry of this substance.
=Commerce=—The same countries that produce tar produce also pitch. The
quantity of the latter imported into the United Kingdom during 1872 was
35,482 cwt., four-fifths of which were supplied by Russia. Pitch is
also manufactured from tar in Great Britain.
=Uses=—Pitch is occasionally administered in the form of pills, or
externally as an ointment; but its medicinal properties are, to say the
least, very questionable.