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The Thinner
So far as the decorator who makes up his own paints is concerned, the two main forms of thinner are turpentine and turpentine substitute, usually referred to as white spirit.Turpentine is the oldest and still the most important solvent or thinner for oil paints, although substitutes are extensively used for it, particularly by paint manufacturers and by many decorators.
The best of these substitutes are probably quite as satisfactory as genuine turpentine in most respects and, in one or two, may even be superior to it, but the average painter, especially if he be of the old school, still as a general rule prefers to use genuine turpentine whenever the cost of the work allows him to do so.
The source of turpentine is the resinous exudation from certain trees of the pine family in the USA, France, Spain, Greece, India, the USSR, Scandinavia, and a few other countries.
America is by far the greatest producer, turning out each year more than twice the amount made by the other countries put together.
Actually, so far as essential qualities are concerned, the properties of all true turpentines differ very little, no matter from what country they originate, and between the best grades of each there is not much to choose.
There are distinct differences in smell, Russian turpentine, in particular, having rather a smoky, disagreeable odour, and some differences in colour.
It is due mainly to prejudice, however, that in this country there is a strong preference for American turpentine.
Turpentine is produced both from the living trees and from pine stumps, fallen branches, and trimmings of trees which have been felled, the latter type being known as "wood" turpentine.
There is some slight difference in the chemical composition of the two types, but they are the same for all practical purposes.
In the USA, turpentine is classified according to the four methods by which it is produced as " gum spirits of turpentine" (otherwise known as " spirits of turpentine" or " oil of turpentine "), "steam-distilled wood turpentine," "destructively distilled wood turpentine," and" sulphate wood turpentine."
The crude gum is collected in most turpentine-producing countries in much the same way, by making a cut in the trunk of the pine so as to sever the resin ducts and affixing to the tree a cup or container into which the resin exudes.
A certain amount of the gum is distilled in crude stills but at all events, in America-the bulk of it is dealt with in industrial plants by modern methods.
The gum is loaded into copper pot stills with a little water.
When the still is heated, turpentine and water vapours are expelled and condensed in the copper coil, whence the two liquids flow into a separator; here they divide into two layers, with the turpentine on top, and are automatically drawn off.
In some plants, the turpentine layer is further dehydrated by being passed through a device containing rock salt which absorbs any water not previously accounted for.
Wood turpentine is produced by a steam distillation process, in which the wood is ground to powder and placed in the heated still, into which steam is forced.
The volatile components are freed by the heat and carried to the condensing coil by the steam.
The destructive distillation process involves placing pine branches and stumps in a retort, which is sealed and heated, the volatile components being carried off and condensed.
Sulphate wood turpentine is recovered as a by-product when paper pulp is produced from pine wood.
Properties.-Good-quality turpentine should be colourless and should evaporate almost completely on exposure to the air, though a slight gummy residue, hardly visible to the naked eye, may be left at times if the turpentine has been kept in a container which has not been completely sealed.
It is sometimes alleged that turpentine may have toxic effects on the human body and a number of diseases, including Bright's disease, painters' colic, and hardening of the arterial tissues, have been attributed to it.
Some years ago, a Government Commission investigated the matter but was unable to find any foundation for these suggestions.
On the other hand, it cannot be denied that some people are affected by turpentine vapour which is liable to cause them headaches and consequent nausea.
Properties as a Thinner.- The main reasons why turpentine is so excellent a thinner for oil paints can be summarised as follows:
It is a good solvent of linseed oil and other drying oils, but does not dissolve an old hard paint film, so that fresh paint can safely be applied on top of the old without seriously disturbing the latter.
It probably has some very slight superficial softening effect on an old coating (unless the film is very hard): this may help to make the new coating adhere more firmly.
It evaporates at a rate satisfactory for brush application in normal conditions and sufficiently slowly to keep the paint at a suitable working consistency.
It gives a good flow to the paint mixture and helps it to work easily under the brush and level out to a film of uniform thickness.
It has a comparatively high flash point and thus can be handled with safety.
Being itself of a resinous nature it has an affinity to wood and is therefore a desirable ingredient of paint for use on wood surfaces.
Its ability to carry oxygen into and through the paint film helps the latter to dry and harden.
Turpentine should not be kept in metal containers unless the metal is treated with shellac or other impervious material to prevent actual contact with the fluid.
It is best stored in glass or earthenware jars which must be kept carefully sealed; if it is exposed to the air for any length of time it is liable to take on a syrupy consistency.
Venice Turpentine.- The purified resin of the common larch. A yellowish, viscous body, soluble in volatile and fixed oils, ether, and alcohol.
Turpentine Substitute.-From the painter's point of view, the chief objection to pure turpentine is its price and consequently turpentine substitutes are widely used.
They are mainly obtained by distilling and refining petroleum; crude petroleum includes a number of spirits and oils which have the same proportionate chemical composition and similar properties, but which evaporate at different rates and boil at different temperatures.
The main divisions into which the original product is separated by repeated distillation are called" fractions," the lowest boiling fraction being light petrol and the heaviest, a range of lubricating oils.
In between, in order of heaviness, are heavy petrol, benzine, white spirit, and petroleum spirit.
When first the medium fractions of petroleum spirit began to be used as thinners for paints and varnishes, they were far from satisfactory for the 'purpose.
They had an unpleasant odour, due to the presence of traces of sulphur and other impurities, while there was no exact specification governing the rate of evaporation; there were often, too, small percentages of oily distillate.
Since then, great improvements in refining and fractionating have been made and good-quality white spirit is now available at a reasonable price: incidentally, although all kinds of turpentine substitute are commonly referred to in the paint and painting trades as " white spirit," this term should properly be applied to petroleum distillate which exactly conforms to the British Standard Specification for white spirit; other petroleum distillates are better referred to as turpentine substitutes.
When it was first put on the market many years ago, decorators, varnish manufacturers, and other users of turpentine were frankly opposed to it, and it must be admitted that, in its early days, there was some excuse for their prejudice against it. Since that time, however, great improvements have been made in its production, and although some people are still averse to its use, it is generally agreed that if it is free from heavy or greasy endfractions and of a highly refined quality, there can be no logical objection to its use, and true economy will be affected thereby.
Some enthusiasts claim that, for certain purposes, white spirit is better than pure turpentine, and in one or two cases there is probably some truth in this assertion.
In the preparation of the wax-type flat finishes and varnishes, for instance, turpentine is apt to prove too good a solvent for the particles of wax than is desirable, and any wax not in solution is swollen into rather large flat particles, whereas, if white spirit is used, they are maintained in finer form.
For lead paints and quick varnishes containing a high proportion of lead driers, however, pure turpentine, or, failing this, a mixture of pure turpentine and good-quality white spirit, is preferable to white spirit by itself, while in high-gloss enamels and good-quality varnishes turpentine is generally considered the correct thinner to use.
For any proprietary brand of paint, white spirit should not be employed as a thinner by the decorator unless its use is clearly indicated as permissible by the manufacturer.
The heavier fractions, such as kerosene or lamp oil, are not satisfactory for thinning paints and varnishes; they contain a substantial amount of heavy oily petroleum which does not evaporate, or only very slowly, in a normal room temperature.
The practice of adding paraffin to retard the drying of flat wall finishes or oil scumbles which set too rapidly under the brush in warm weather is consequently to be condemned.