THE PRESERVATION OF TIMBER
PUBLISHED BY COURTESY OF THE TIMBER DEVELOPMENT ASSOCIATION, LTD. LONDON
The principal causes of the deterio- ration of timber are attacks by wood- destroying fungi (decay or rot), insects and marine borers. Quite distinct from decay is the weathering of wood under the influence of the elements. It is the mechanical and chemical disintegra- tion of the surface of the wood caused by the alternate shrinking and swelling under different weather conditions, the action of dust and sand blown by the wind and also probably to the action. of sunlight. It is not normally checked by ordinary fungicidal and insecticidal preservatives. Mechanical wear and fire are other destructive agencies, but these do not normally come within the scope of wood preservation.
PRINCIPLES OF WOOD PRESERVATION.
In order to attack timber, both fungi and insects need favourable conditions of moisture, temperature, air, and also suitable food. Timber may be rendered immune from attack by controlling any one of these factors. Thus, if kept per- manently dry, timber lasts indefinitely (as was the case with wood buried in the tombs of ancient Egypt). But con- trol of most factors is often not prac- ticable, except in the case of the suit- able food, i.e., the wood itself, which can be treated with substances poison- ous to fungi and insects.
It is not normally necessary for the preservative to penetrate the wood completely, but only to form a continu- ous outer layer, through which the destructive agents cannot penetrate. This layer must, of course, be suffi- ciently thick so that any slight cracks or mechanical damage to the wood do not expose the core of untreated wood.
Paint cannot be regarded as a pre- servative in the true sense as it does not penetrate the wood; its action is purely a weather-proofing one. Whilst not being entirely waterproofing, paint prevents the wetting of the surface and so helps to maintain the timber at a safe moisture content. For this reason paint must not be allowed to crack or peel off.
One very old method of preserving timber is charring. This method is still used occasionally for fence posts, etc.; the wood must be thoroughly charred-not merely scorched—so that a layer of charcoal protects the inner core of timber. It is of doubtful value especially as the charcoal is so fragile. that it is easily broken off.
REQUIREMENTS OF GOOD
PRESERVATIVES
In the use of wood preservatives there are two main considerations: the
efficiency of the preservative itself and the effectiveness of the method of treatment. The best preservatives will give unsatisfactory results if they are badly applied. This is a most impor- tant point and should always be borne in mind.
The most important properties desirable in a preservative are that it should be:-
(a) poisonous to fungi and insects (but not to human being nor domestic animals);
(b)
(c)
permanent, not liable to leach out, and chemically stable for a long period;
cheap and plentiful:
(d)
safe to handle;
(e)
easy to apply;
(0)
(g)
(h)
(i)
(j)
not corrosive to metals; capable of good penetration fire-resistant; or al least not increasing the inflammability of wood;
capable of subsequent painting or finishing;
colourless and odourless.
No preservatives can claim all the above properties and the lack of cer- tain of them constitutes a guide to their special use. For example, if a preservative tends to wash out easily, it should only be used for interior work where there is no chance of water reaching the treated wood. In such circumstances it will be equally as effective as a solution having all the other properties. PROPRIETARY PRESERVATIVES There are a number of patent and proprietary preservatives and their effectiveness is very variable. Those made by reputable firms are effective if correctly used, and may be employed with confidence. It is important, how- to follow the manufacturers in- ever, structions and to realise that there is no magic in patented preservatives which will make them effective if casually applied with a half-dry brush. It is valuable to know into which of the three types described any particu lar brand of preservative falls as this will suggest its suitability for the pur- pose in hand. Many brands are made up in small tins, suitable for the house- holder, and frequently а range of colours is available. Both these factors necessarily increase the price and may discourage the liberal application which is vital for any surface treat- ment to be effective.
Or
The preservatives of the coal wood tar oil type are generally more refined than creosote. The use lighter type oils permits better pene-
of
tration and they may be more resistant to leaching and more toxic. Water solution types are also available and are generally sold in dry powder form to be dissolved in water; they are. therefore, somewhat cheaper-volume to volume than other types. The solvent types are often quick drying and may be useful for this reason, although they tend to be somewhat expensive.
CHOICE OF PRESERVATIVES
An examination of the characteris- tics of the different types of preserva- tive makes it apparent that their suit- ability for different purposes varies-- often considerably. Thus it is unwise tu use a far oil type on wood in con- tact with plaster. On the other hand a water-soluble type is not normally suitable for marine poling (unless it is of the type that is fixed in the wood) as it is liable to be washed out of the wood. More difficult, however, is the choice of a preservative for wood use in contact with foodstuffs. It must be realised that most wood preservatives are poisonous enough to human beings to make it necessary that treated wood is not placed in direct contact with food.
Wood to be used near growing plants, as in greenhouses, seed boxes, mushroom beds, etc.. should not be treated with tar oil types, as these are often poisonous to plants. Either of the other types would generally be suitable, and a subsequent application. of oil paint would prevent leaching and seal the wood surface.
Frequently protection from weather ing is required rather than preserva- tion in the usual meaning of the term, or in connection with it. Neither creo- sote nor other preservative is effective in preventing weathering; a protective skin is necessary, and its most usual form is oil paint or, where it is desired to retain the natural appearance of wood, linseed oil.
TYPES OF PRESERVATIVES
There are three main types of pre- servatives: the oily, the water soluble and the solvent. The last type is mostly used in the patent preservatives and is somewhat more expensive than the first two. It has the advantage of generally having greater penetration when applied by brush or spray. CREOSOTE. The various kinds of creosote form the main oil type. Most of the virtues of a good preservative (as given above) are found in creosote. except that the inflammability is in- creased especially in small-sized timber.
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