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Raw and Boiled Oil
So far as the ordinary decorator is concerned, the choice lies between raw and boiled oil and a great deal has been written on their comparative merits.Probably the majority of painters prefer raw oil for general purposes.
It works more freely under the brush, dries reasonably quickly and is capable of satisfying the suction of surfaces of average absorption.
It is at its best when used with white lead for which it has a natural affinity; this can be seen in the ability of the pigment to absorb the oil even in the presence of water.
With zinc oxide, raw oil is not so suitable; unless stand oil or a similar" bodied" oil is used, or a suitable varnish added to raw oil, zinc oxide tends to produce rather a brittle film.
Boiled oil gives a film which has a better initial gloss and which dries more quickly than when raw oil is used, and these are unquestionably advantages.
Against this must be put the fact that a boiled-oil paint does not brush out so readily, and probably is not quite so durable.
The film remains softer and consequently boiled oil is not usually satisfactory for use in undercoats.
Where the surface is unduly porous, boiled oil is to be preferred, since there is less tendency than in raw oil for an undue proportion of the vehicle to be absorbed, leaving the pigment underbound on the surface.
For pigments which are poor driers, such as Vandyke brown or the carbon blacks, boiled oil has some advantages.
Other Oils
Tung Oil, or China Wood Oil, is derived from the nut of Aleurites cordata, a tree which flourishes in China.The oil is of pale-yellow to brownish tinge and is slightly viscous.
It is used almost entirely by manufacturers for the production of high-grade enamels, varnishes, and other finishes.
When dry, a film of tung oil is duller and more opaque than that of linseed oil and has a curious crinkled appearance.
A peculiar characteristic is that it slowly gelatinises on heating.
It is regarded as one of the most valuable drying oils for paints, since it has great powers of moisture resistance and dries rapidly.
It is largely used for making high-grade quick-drying finishes. The main source of supply of tung oil is.
China and during the Sino Japanese and the recent World War(WW1) exports from that country practIcally ceased.
The irregularity of the supply in previous years, the high price, and the rather crude methods of collection by the Chinese, led to experiments in cultivating the trees being carried out in various countries, notably in the USA. and in certain parts of the British Empire.
The results, especially in America, are promising, but it must be a long time before the trees yield on a large-enough scale to make much appreciable difference.
Oiticica Oil.-This oil, derived from the nuts of a tree which grows in Brazil, has long been known to have possibilities for we in paints, but it was not until the Sino-Japanese War, when supplies of tung oil began to be shut off, that any serious attempt to make use of it was begun.
Since it possesses properties not unlike those of tung oil, it seems probable that it will play an important part in the finishes of the near future.
Hitherto, although the trees usually yield heavy crops of nuts, the immense distances to be covered in collecting them, and transport difficulties in general, have restricted the amount of oil which reaches the paint manufacturer, and most of it is used in America.
Castor OiL-The inclusion in this list of castor, which has long been regarded as a typical non-drying oil, needs a brief explanation.
The drying of oils depends on their taking up oxygen from the air and their ability to do so is determined by what is usually known as their" degree of unsaturation."
All natural oils are mixtures of various fatty acids combined with glycerine and there are found in all of them mixtures of drying and non-drying glycerides, the relative proportion of which determines whether or not the oil will dry.
In castor oil, the low degree of unsaturation is due to the fact that the glyceride has been chemically combined with water, which prevents the oil from taking up oxygen.
Attempts had been made from time to time prior to the last war to remove this water and, when the shortage of tung oil became acute, efforts were intensified, with the result that it was found possible to produce a " dehydrated" castor oil which would dry quickly, have good resistance to alkalis and good durability.
This was used on a considerable scale in the war, notably in the production of synthetic-resin finishes.
Soya-bean Oil-This oil, which up to the present has been used only on a relatively small scale as a paint vehicle, is produced from the seeds of a plant grown in China and the Far East.
Its properties are akin to those of linseed oil.
Its drying properties are not particularly good but can be improved by the addition of suitable drying agents.
Poppy Oil; Walnut OiL-These oils, the first derived from the seeds of the opium poppy and the second from the ordinary walnut, are almost exclusively used in the preparation of artists' colours.
Fish Oil, prepared from the menhaden fish, is being largely employed in the United States in the preparation of paints, replacing linseed oil, which it closely resembles.
It has a slightly brownish colour, but when refined is of a pale straw tint; it is not used to any great extent for paints in this country, mainly due to its odour, which is somewhat objectionable.