Wood Preservatives Explained

Excluding paint, the three main types of wood preservative are as follows: Creosote

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 is 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.

Damp-proofing Preparations

These various preparations for stone, brick, and concrete or cement, may be either clear or coloured. Many of the older types were compounded on a paraffin-wax basis, and consequently had to be heated before use and put on while still warm. These have largely been superseded by a number of proprietary brands of liquids, either colourless or pigmented, which are easily applied by brush or by spray. They are, for the most part, quite effective but must be renewed after two or three years.

Petrifying Liquids

These are formulated in a variety of ways, but are usually emulsions of wax, rosin, etc., used as a medium for thinning oil-bound water paints, to give them greater resistance to the weather when used on exterior surfaces. They are sometimes employed, in addition, as a priming coat on interior walls which are very porous, before applying the water paint, in order to satisfy the suction, or to help ‘ bind ‘ plaster which is inclined to be friable.

Soda

The term soda is often loosely applied both to sodium hydroxide and sodium carbonate. Sodium hydroxide (caustic soda) is usually prepared by boiling sodium carbonate with milk of lime, boiling the clarified liquor and obtaining solid white crystals, which are soluble in water, producing considerable heat while dissolving. The solution is highly caustic, and is used for detergent purposes, but when employed for cleaning wood or iron it must be thoroughly well washed off before applying size or paint. Sodium carbonate (washing soda) is made by converting common salt into sodium sulphate by the addition of sulphuric acid. The resultant salt cake is heated and washed ; the solution containing caustic soda, and the residue of the liquor on further boiling leaves the well-known transparent crystals, which are soluble in water and used for washing. It is less active but much safer than caustic soda. However, even when soda carbonate is used for cleansing purposes, it should be neutralised by plentiful washing with tepid water, or with slightly acidulated water, before applying sizes, paints, stains, or varnishes.

Waxes and Fats-

The chief use of waxes in the painting trade is in the preparation of polishes, but they are also employed fairly extensively as matting agents in flat paints and varnishes and occasionally, by the decorator, as a megilp in graining work. The principal classes used are:

Beeswax

Wax to the decorator almost always implies beeswax which, dissolved in turpentine, is largely used for making polish. It takes on a good lustre, is resistant to moisture, and emulsifies well. Good-quality beeswax should not be too soft at ordinary temperatures and should be fairly tough and resistant to breaking. If adulterated, it loses most of its valuable properties; this can generally be detected by undue softness and stickiness.

Carnauba: This is chiefly employed by manufacturers of polishes, in admixture with one or more other types of wax. Used alone, it is too brittle, and also too expensive, for most purposes. Mixed with equal parts of beeswax, it gives an excellent polish, harder and more free from cling ‘ than beeswax alone. In conjunction with beeswax, it is also used in the making of flat or eggshell varnishes, though in recent years it has tended to be superseded by other forms of matting agents.

Paraffin Wax

This is sold in two or three degrees of hardness, but is not so suitable for polishes as harder waxes, although, tinted a brown or orange, it is often supplied for this purpose.

Montan Wax

This is an expensive wax which, though hard and tough, is not so brittle as carnauba. It has remarkable polishing properties but, owing to its price, is generally sold in admixture with beeswax or ceresine.

Ceresine

A modified paraffin wax, prepared by treating the latter with vitriol, which gives a somewhat harder product. It is the basis of a number of low-priced polishes.

Tallow

Though not a wax, this may conveniently be included here. It is obtained from animal fat and is yellowish in colour and fairly soft. It is often used in the preparation of limewash to act as a binder and impart moisture-resisting properties.

Stearin

This is also derived from animal fat, being the pure, crystalline acid obtained by purifying tallow. It is whiter, harder, and almost free from smell, and will combine with hot solutions of caustic or caustic lime in the presence of water. Like tallow, it is often employed in making up limewash.

Putty

Putty sold as ‘ Genuine Linseed-oil Putty ‘ should consist only of finely sifted whiting intimately mixed with linseed oil. Cheaper qualities of putty sometimes contain proportions of fish oil or mineral oil, or linseed oil ‘ foots,’ while occasionally a certain amount of barytes is used in place of the whiting.

Putty is widely used for stopping and for glazing work. Ordinary linseed-oil putty is not particularly satisfactory for metal sashes, for which it is generally too soft. There are a number of proprietary brands of putty for this purpose or ordinary linseed-oil putty can be adapted for it by adding approximately 1 oz. red lead to every 1 lb. of putty; the red lead should be of the ‘jointing,’ not the ‘non-setting,’ type.

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