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68
HARBOUR ENGINEERING.
chafing of floating objecta against the surface of the wood soon wears away
its protective coating. The same objections apply to other substances, such
as tar, verdigris, and paraffin, which have either been used or proposed as a
substitute for paint.
The best and only really effective agency for increasing the longevity of
timber-work in contact with moisture is the process known as creosoting’.
It also acts as a deterrent to sea-worms, though not to the extent of rendering
the wood invulnérable. The process consists in coagulating the albtirnum in
the pores of the log, so that the latter become filled with an antiseptie,
bituminous substance, which excludes air and moisture, repels the lower forms
of vegetable and animal life, and prevents putrefaction and rot.
Creosote is an oily liquid contained in the second distillation of tar,
from which the ammonia has been expelled. Its composition is somewhat
variable; but in order to be effective, it should contain over 40 per cent, of
naphthalene, about 4 or 5 per cent, of carbolic acid, and as little pitch as
possible. It is essential to the efficacy of the treatment that, as a preliminary,
all moisture be abstracted from the interior of the timber.
Soft woods, such as fir and pine, may be simply immersed, direct from the
drying-house and while still warm, in an open tank of hot creosote. Logs
treated in this way will absorb from 8 to 9 Ibs. of creosote per cubic foot, and
this quantity is generally sufficient for inland purposes.
For marine work, however, and especially for piles and the timber-work of
jetties, imprégnation to the extent of 10 or 12 Ibs. per cubic foot is requisite.
In order to achieve this result, the timber, after being dried, is placed in a
vacuum. Creosote, at a temperature of about 120° Fahrenheit, is then intro-
duced into the containing cylinder, under a pressure of about 175 Ibs. per
square inch. By this means, with suitable woods, the amount of creosote
absorbed may reach a maximum of 16 Ibs. per cubic foot.
Hard, compact woods, such as oak, do not, under any degree of pressure,
absorb more than 3 Ibs. of creosote per cubic foot; but in their case this is
found to be sufficient.
Boucherie’s process consists in impregnating timber with a solution of
sulphate of copper (1 per cent, by weight) in water. The usual course of
procedure is to cap one end of a log in a water-tight manner and then to allow
the liquid to penetrate the pores from the other end, so displacing the sap,
under a head of 30 or 40 feet, which produces a pressure of 15 to 20 Ibs. per
square inch. The extent to which penetration takes place can be tested by
means of prussiate of potash : whenever this substance cornes in contact with
sulphate of copper, a brown stain is left.
Timber is kyanised by immersing it in a saturated solution of corrosive
sublimate (perchloride of mercury) contained in a wooden tank. The strength
of the solution varies from f to 1 per cent, by weight, according to the
porosity of the timber.
Burnettising ’ is the term applied to a process of treatment with a solu-
tion of chloride of zinc, containing 2| per cent, by weight of the chloride.