The best means for preserving timber from decay are to have it thoroughly seasoned and well ventilated.

Several processes have, however, been introduced at different times with a view of preventing decay in timber by excluding moisture, or by drying up or expelling the sap within it.

A few of these processes will now be described.

Painting preserves timber if the wood is thoroughly seasoned before the paint is applied. Otherwise the filling up of the outer pores only confines the moisture and causes rot. The same may be said with regard to Tarring.

Sometimes before the paint is dry it is sprinkled with sand, which is said to make it more durable.

Tredgold says - "For timber that is not exposed to the weather, the utility of paint is somewhat doubtful. . . . Wood used in outdoor work should have those parts painted only where moisture is likely to find a lodgment, and all shakes or cracks and joints should be filled up with white lead ground in oil, or oil putty, previous to being painted over."

Charring Timber

The lower ends of posts put into the ground are generally charred with a view of preventing dry rot and the attacks of worms.

Care should be taken that the timber to which this process is applied is thoroughly seasoned, otherwise by confining the moisture it will induce decay and do more harm than good.

It may here be mentioned that posts should be put in upside down, with regard to the position in which they originally grew. The sap valves open upwards from the root, and when thus reversed they prevent the ascent of moisture in the wood.

Mr. Britton recommends that the charring process should be applied to the embedded portions of beams and joists, to joists of stables, wash-houses, etc., to wainscoting of ground-floors, to flooring beneath parquet work, to the joints of tongues and rebates, and to railway sleepers.

Mons. de Lapparent applied the method on a large scale by the use of a gas jet passed all over the surface of the timber, but Mr. Laslett, who experimented on timbers thus treated, says -

"I should not myself be inclined to use it on timber for works of construction, except as a possible means of preventing the generation of moisture or fungus where two unseasoned pieces of wood are placed in juxtaposition."

Creosoting, known also as Bethell's process, is effected by extracting the moisture and air from the tubes of the timber, and then forcing in kreasote (oil of tar), generally called creosote, at a high pressure.

The timber after being dried is placed in a closed wrought-iron cylinder. The air is then extracted from the cylinder and pores of the wood by a pump.

Creosote (see p. 454) at a temperature of about 120° is then forced into the cylinder, and penetrates the wood under a pressure of about 170 lbs. per square inch.

The creosote should be thick, rich in naphthaline, and free from ammonia.1

The amount of creosote pumped in depends upon the nature of the timber and the purpose for which it is intended. The sapwood absorbs it more readily than the heart.

Fir timber or other soft wood will take from 10 to 12 lbs. per cubic foot.

Mr. Bethell recommends 7 lbs. per cubic foot for railway works and 10 lbs. for marine works.

Somewhat larger quantities than these are now generally used.

Into oak and other hard woods it is difficult to force more than 2 or 3 lbs. per cubic foot.1

To soft woods an imperfect form of this process may be applied by drying the timber over fires, and placing it while warm in hot creosote.

Of all the preservative processes at present known, creosoting seems to be the most successful; it coagulates the albumen of the wood, fills its pores with an oily liquid, destroys insects and fungi, repels worms, excludes moisture, and prevents dry rot.

Experience seems to show that creosote will render timber proof against sea worms, and even against the white ant.

About twelve years ago a Commission was appointed by the Dutch Government to report upon the best method of protecting timber from the attacks of the sea-worm, known as the teredo (see p. 401).

This Commission tried every preservative means then known, including, among others - charring the surface, covering with paraffin, with sheet metals, nails (see p. 402), impregnation with all sorts of chemical substances, creosoting, and kyanising.

The conclusion they arrived at was that "the only process that could be relied upon for protecting wood from the attacks of the teredo was that of creosoting, and that this fails if not properly carried out." 2

Kyan's Process consists in injecting corrosive sublimate (bichloride of mercury) in the proportion of 1 pound of sublimate to 15 gallons of water.

The Dutch experiments showed that this process did to a certain extent, though not altogether, repel the sea-worm, and it is said that it has some effect in retarding dry rot. It is now, however, seldom if ever used.

Boucherie's Process consists in impregnating the timber with sulphate of copper by a very simple process.

A reservoir filled with the solution (about 1 lb. of sulphate copper to 121/2 gallons of water) is placed at a height of from 20 to 30 feet above the ground.

From this reservoir leads a pipe into a deep incision in the wood, so arranged that the liquid may reach the centre of the log. Thence it forces its way (under the pressure caused by the height of the tank) along the sap tubes, forces the sap out, and takes its place.

To see if the solution has passed right through the timber the far end is rubbed with prussiate of potash, which upon coming in contact with the sulphate of copper makes a brown stain.

1 Britton.

2 Dent.

Gardner's Process is one that has been lately introduced.

It is said1 to season timber more rapidly than any other process, to preserve it from decay and from the attacks of all kinds of worms and insects. It is also found 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 from 4 to 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.

The experiments showed that the sap was removed, that the resistance of the timber to crushing was augmented from 40 to 90 per cent, and its density was considerably increased.

Margary's Process was to soak the wood in acetate or sulphate of copper. It does not seem to have been successful.

Sir "William Burnet's System consists in steeping the timber in a solution composed of 1 lb. of chloride of zinc to 4 gallons of water.

Payne's Process involved two injections into the pores of the timber, the first being sulphate of iron, the other sulphate of zinc. It is said to make the timber incombustible but brittle.