This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The object of seasoning timber is to expel or dry up the sap remaining in it, which otherwise putrefies and causes decay. One effect is to reduce the weight. Tredgold calls timber "seasoned" when it has lost 1/5, and considers it then fit for carpenters' work and common purposes; and "dry," fit for joiners' work and framing, when it has lost 1/3. The exact loss of weight depends, however, upon the nature of the timber and its state before seasoning. Timber should be well seasoned before being cut into scantlings; the scantlings should then be further seasoned, and, after conversion, left as long as possible to complete the process of seasoning before being painted or varnished. Logs season better and more quickly if a hole is bored through their centre; this also prevents splitting.
Natural seasoning is carried out by stacking the timber in such a way that the air can circulate freely round each piece, at the same time protecting it by a roof from the sun, rain, draughts, and high winds, and keeping it clear of the ground by bearers. The great object is to ensure regular drying; irregular drying causes the timber to split. Timber should be stacked in a yard, paved if possible, or covered with ashes, and free from vegetation. The bearers should be damp-proof, and keep the timber at least 12 in. off the ground; they should be laid perfectly level and out of winding, otherwise the timber will get a permanent twist. The timber should be turned frequently, so as to ensure equal drying all round the balks. When a permanent shed is not available, temporary roofs should be made over the timber stacks. Logs are stacked with the butts outwards, the inner ends being slightly raised so that the logs may be easily got out; packing pieces are inserted between the tiers of logs, so that by removing them any particular log may be withdrawn. That timber seasons better when stacked on end, seems doubtful, and the plan is practically difficult to carry out.
Boards may be laid flat and separated by pieces of dry wood 1 in. or so in thickness and 3-4 in. wide; any that are inclined to warp should be weighted or fixed down to prevent them from twisting; they are, however, frequently stacked vertically, or inclined at a high angle.
Laslett recommends that they should be seasoned in a dry cool shed, fitted with horizontal beams and vertical iron bars, to prevent the boards, which are placed on edge, from tilting over. The time required for natural seasoning differs with the size of the pieces, the nature of the timber, and its condition before seasoning. Laslett gives the following table of the approximate time required for seasoning timber under cover and protected from wind and weather: -
Pieces 24 in. and upward square require about.........
„ Under 24 in. to 20 " ...........
" 20 „ 16 " ..........
" " 16 " 12 " ..........
" " 12 " 8 " .........
" " 8 " 4 " .........
Planks 1/2-2/3 the above time, according to thickness. If the timber is kept longer than the periods above named, the fine shakes which show upon the surface in seasoning open deeper and wider, until they possibly render the logs unfit for conversion. The time required under cover is only 5/7 of that required in the open.
Water seasoning consists in totally immersing the timber, chaining it down under water, as soon as it is cut, for about a fortnight, by which a great part of the sap is washed out; it is then carefully dried, with free access of air, and turned daily. Timber thus seasoned is less liable to warp and crack, but is rendered brittle and unfit for purposes where strength and elasticity are required. Care must be taken that it is entirely submerged; partial immersion, such as is usual in timber ponds, injures the log along the water line. Timber that has been saturated should be thoroughly dried before use; when taken from a pond, cut up and used wet, dry-rot soon sets in. Salt water makes the wood harder, heavier, and more durable, but should not be applied to timber for use in ordinary buildings, because it gives a permanent tendency to attract moisture.
Boiling water quickens the operation of seasoning, and causes the timber to shrink less, but it is expensive to use, and reduces the strength and elasticity. The time required varies with the size and density of the timber, and according to circumstances; one rule is to allow 1 hour for every inch in thickness.
Steaming has much the same effect as boiling; but the timber is said to dry sooner, and it is by some considered that steaming prevents dry-rot. No doubt boiling and steaming partly remove the ferment spores.
Hot-air seasoning, or desiccation, is effected by exposing the timber in an oven to a current of hot air, which dries up the sap. This process takes only a few weeks, more or less, according to the size of the timber. When the wood is green, the heat should be applied gradually. Great care must be taken to prevent splitting; the heat must not be too high, and the ends should be clamped. Desiccation is useful only for small scantling; the expense of applying it to larger timber is very great; moreover, as wood is one of the worst conductors of heat, if this plan be applied to largo logs, the interior fibres still retain their original bulk, while those near the surface have a tendency to shrink, the consequence of which would be cracks and splits of more or less depth. Desiccated wood should not be exposed to damp before use. During this process ordinary woods lose their strength, and coloured woods become pale and wanting in lustre.
M'Neile's process consists in exposing the wood to a moderate heat in a moist atmosphere charged with various gases produced by the combustion of fuel. The wood is placed in a brick chamber, in which is a large surface of water to produce vapour. The timber is stacked in the usual way, with free air-space round every piece; about 1/3 of the whole content of the chamber should be air-space. Under the chamber is a fireplace. The fire having been lighted, the products of combustion (among which is carbonic acid gas) circulate freely in a moist state around the pieces of timber to be seasoned. The time required varies with the nature of the wood. Oak, ash, mahogany, and other hard wood planks 3 in. thick, take about 8 weeks; oak wainscot planks 2 in. thick take 5-6 weeks; deals 3 in. thick, something less than a month; flooring-boards and panelling, about 10 days or a fortnight. The greener the wood when first put into the stove the better. As a rule, if too great heat be not applied, not a piece of sound wood is split, warped, or opened in any way. The wood is rendered harder, denser, and tougher, and dry-rot is entirely prevented. The wood will not absorb by subsequent exposure to the atmosphere nearly so much moisture as does wood dried by exposure in the ordinary way.
The process seems to have no injurious effects upon the appearance or strength of the timber.
Gardner's process is said to season timber more rapidly than any other, to preserve it from decay and from the attacks of all kinds of worms and insects, to strengthen the timber, and render it uninflammable; and by it the timber may be permanently coloured to a variety of shades. The process takes 4-14 days, according to the bulk and density of the timber. It consists in dissolving the sap (by chemicals in open tanks), driving out the remaining moisture, leaving the fibre only. A further injection of chemical substances adds to the durability, or will make the timber uninflammable. The process has been satisfactorily tested in mine props, railway sleepers, logs of mahogany for cabinet-work, and in smaller scantlings of fir and pine. Experiments showed that the sap was removed, the resistance of the timber to crushing augmented 40-90 per cent., and its density considerably increased.
Rene, a pianoforte manufacturer, of Stettin, Germany, has devised a plan by which be utilizes the property of ozonized oxygen, to artificially season timber used for sounding-boards of musical instruments. It is a well-known fact that wood, which has been seasoned for years, is much more suitable for the manufacture of musical instruments than if used soon after it is thoroughly dried only. Rene claims that instruments made of wood which has been treated by his oxygen process possess a remarkably fine tone, which not only does not decrease with age, but as far as experience teaches, improves with age as does the tone of some famous old violins by Italian masters. Sounding-boards made of wood prepared in this manner have the quality of retaining the sound longer and more powerfully. "While other methods of impregnating woods with chemicals generally have a deteriorating influence on the fibre, timber prepared by this method, which is really an artificial ageing, becomes harder and stronger. The process is regularly carried on at Rene"s works, and the apparatus consists of a hermetically closed boiler or tank, in which the wood to be treated is placed on iron gratings; in a retort, by the side of the boiler and connected to it by a pipe with stop-valve, oxygen is developed and admitted into the boiler through the valve.
Provision is made in the boiler to ozonize the oxygen by means of an electric current, and the boiler is then gently fired and kept hot for 48-50 hours, after which time the process is complete.
Woods, of Cambridge, Mass., has introduced a method which is spoken of as leaving no room for improvement. The wood is placed in a tight chamber heated by steam, and having one side made into a condenser by means of coils of pipes with cold water continually circulating through them. The surface of these pipes is thus kept so much below the temperature of the chamber that the moisture drawn from the wood is condensed on them, and runs thence into a gutter for carrying it off. In the words of the United States Report on the Vienna Exhibition, "if the temperature of these condensing pipes can be kept at say 40° F., and that of the atmosphere be raised to 90° F., it will not require a long time to reach a degree of 20 per cent. of saturation, when the work of drying is thoroughly completed."
It is said that if timber be smoke-dried over a bonfire of furze, straw, or shavings, it will be rendered harder, more durable, and proof against attacks of worms; to prevent it from splitting, and to ensure the moisture drying out from the interior, the heat should be applied gradually.
Many woods require a second seasoning after they have been worked. Floor boards should, if possible, be laid and merely tacked down for several months before they are cramped up and regularly nailed. Doors, sashes, and other articles of joinery should be left as long as possible after being made, before they are wedged up and finished. Very often a board that seems thoroughly seasoned will commence to warp again if merely a shaving is planed off the surface.