Excluding paint,the three main types of wood preservative are as follows:
This term was originally applied to a distillate of coal tar but now implies the oils produced from coal tar.
It varies in colour, according to the kind of coal from which it is produced and the way in which it 18 distilled, but the colour is not a reliable guide to its preservative properties.
These are due to its extreme toxicity to fungoid spores and to its permanence, as it is not leached out by moisture.
It has a characteristic smell.
One objection to its use is that it bleeds through and discolours paints subsequently applied directly to a surface on which it has been used.
Under this heading may also be included various other coal-tar derivatives, similar in their properties to creosote.
Water-soluble Metallic Salts.-
These include copper chromate, copper sulphate, zinc chloride, zinc sulphate, and mercuric chloride.
They are not so toxic to fungoid spores as coal-tar derivatives, but have the advantage of being colourless, odourless, and of not affecting any paint coating applied over them.
They are liable to be leached out by the weather and are not nearly so permanent as the creosote type.
Spirit-solvent Metallic Salts.-
These consist of various metallic compounds, such as copper naphthenate, in a volatile spirit solvent.
They penetrate deeply into the wood, after which the solvent evaporates, leaving the outer and more vulnerable portions of the wood impregnated.
They are relatively expensive and not so effective as the coal-tar derivatives; they can, however, be painted over.