SEASONING BY SMOKE DRYING.
Smoke drying in an open chamber, or the burning of furze, fern, shavings, or straw under the wood, is said to give it hardness and durability; and, by rendering it bitter, destroys and prevents worms. It also destroys the germ of any fungus which may have commenced. It is an old and well-founded observation that smoke drying contributes much to the hardness and durability of woods. Virgil appears to have been aware of its utility, when he wrote the passage which is thus translated by Dryden:
“Of beech, the plough-tail, and the bending yoke,
Or softer linden, hardened in the smoke.”—Georgics, i., 225.
Beckman, in his ‘History of Inventions,’ quotes a passage from Hesiod to the same effect; and adds, “as the houses of the ancients were so smoky, it may be easily comprehended how, by means of smoke, they could dry and harden pieces of timber.” In this manner were prepared the pieces of wood destined for ploughs, waggons, and the rudders of vessels:
“These long suspend, where smoke their strength explores,
And seasons into use, and binds their pores.”—Virgil.
The late Brigadier-General Sir Samuel Bentham bestowed much time and attention in endeavouring to ascertain the quickest and best means of drying oak. In his letter to the Navy Board, 6th March, 1812, he says: “By exposing block shells to the smoke of burning wood, they become in the course of two or three days well seasoned in every respect, hard, bright coloured, and, as it were, polished. But it was found in a very short time that the acid with which the shells were thus impregnated very rapidly corroded the iron pins which passed through them.
“In Russia many small articles, such as parts of wheels, wheel carriages, and sledges, are prepared in this manner; so are wheels, at least in some parts of America; and sabots and other small articles in France.”
In speaking of artificial heat, he says, in the same letter:
“From all the opportunities I have had of examining the state of timber so prepared by artificial heat, the due seasoning without cracking has appeared to depend on the ventilation happening to be constant, but very slow, joined to such a due regulation of the heat as that the interior of the timber should dry, and keep pace in its contraction with the outer circles.”
Mr. T. W. Silloway, in ‘American Carpentry,’ remarks: “If timber be dried by heat, the outside will become hardened, and the pores closed, so that moisture, instead of passing out, will be retained within.”
Bowden remarks, “that the timbers of a small ship underwent the process of charring, either by suspending them over a fire of chips, or by burning the exterior with red-hot irons, so as to char the external surface. Air trunks were also formed between the timbers, for the purpose of evaporating moisture. The state of this vessel was examined five years after she was launched, and it appeared that, although the timbers had been very strongly charred, fungi had grown to a considerable extent on both sides abaft the fore channels, and that the plank near the magazine was completely decayed.” The power of vegetation broke through the incrusted barrier against external affection.
A method is in operation at Tourlaville, near Cherbourg, for which the inventor, M. Guibert, has taken out a patent, and it is said to give at once more expeditious and sure results than those obtained from the use of dry and hot air. It consists in filling the drying-stove with smoke, produced by the distillation of certain combustible matters, such as sawdust, waste tan, and smiths’ coals, &c. By means of a ventilator, ingeniously arranged, a rotatory movement round the logs laid to season is given to the smoke, so as to obtain an average uniform temperature in every part. By this plan, as the distillation of combustibles is always attended with a considerable discharge of steam, all cracks and splits are said to be prevented.
There is much force in Sir Samuel Bentham’s observations respecting the drying of timber by artificial heat: it is certainly not well to attempt to dry it too quickly, for if it be subjected to great heat, a large portion of the carbon will pass off, and thereby weaken the timber. Timber too suddenly dried cracks badly, and is thus materially injured: planks of larch or beech are liable to warp and twist if their drying is hastened.