Drying oil is a kind of vegetable oil which drys in normal temperature. Linceed oil, Poppy oil, Walnut oil, Sunflower oil and Safflower oil are known as typical drying oil. Other kind of oils do not dry in normal temperature. Drying oil is the basic vehicle of oil paints. The more you incrude this oil into paints, the more the paints have transparensy and glossy. Drying oil can be increased its ablity by bleaching under the sun, heating to high temrelature or boiling. Bleached drying oil is called "Sun-Bleached oil" or "Sun-Thickened oil". Heated one is called "Stand oil".
Water or Turpentine drys by vaporizing, but Drying oil drys by the oxidization. Drying oil insede of paints on a canvas starts to harden by taking in oxygen from the air. Drying oil reamin on the canvas and never vaporize. Drying oil bonds pigments, and provides glossy and transparensy proper to oil colour.
Old Masters did not use raw vegetable oil. They must have used Sun-Thickened oil or Stand ooil. These oil have good ability of enduranse, drying, glossy more than raw oil. And they hardly change the color into dark or brown. Unfortunately, tube oil colors makers product use raw oils, because tube paints made of processing oil comparatively tend to change in quality when they are stocked at shops for long time. Therefore tube paints came to be made of raw oils after painting material makers appeared. It's needless to say that processing oils are better than raw oil concerning thier ability.
Linseed oil has been the most important drying oil whole of the oil painting history. It dries comparetly fast, and a layer on a canvas by this oil has good endurance. The only weak point is yellowing. The film tends to change its color into yellow or dark. Therefore almost painters avoid this oil for bright colors like white or yellow.
Linseed oil that was just expressed has a golden bright brown or yellow color. It become clear and transparent by refining and bleaching proccess. See How to bleach Drying oils ( under construction ). These oils are called Refined oil or Sun-Bleached oil. Linseed oil in commarece for art use is already refined and bleached. But sometimes Manufacturers use strong chemicals to bleach, and some technicalists say it may destroys good properties. Painters can buy untreated oil and refine and bleach it by them selves. Usually these untreated oils are named cold-pressed linseed oil.
The most important thing is that a layer by linseed oil is quite stronger than by poppy. You should use linseed oil for the ground colour and lower layers. Manufacturers use linseed oil as binder of tube oil paint for color whitch doesn't change into dark, and use poppy or safflawer oils for white or bright colors ( But only faundation white is frequently made of linseed oil ).
Poppy oil(Poppyseed oil) is as popular as linseed oil nowadays. The advantage of this oil is that a layer by poppy doesn't change its colour easily different from linseed oil. Therefore many painters used poppy since Impressionists(they painted thick layers of bright colour) instead of linseed. But Poppy oil dries very slow and a layer by poppy is inferior in endurance.
Do not use poppy for ground and lower layer. And shouldn't paint a linseed layer on a poppy layer. In that case upper linseed layer dries faster than lower poppy layer. It will be the cause of cracks and comming layers off. Also after layers dried perfectly, you shouldn't paint a haeder layer over a weaker layer such as poppy layer.
It was one of the most important drying oil around Renaissance. It seems that also Leonard da Vinci frequently used this oil. Pure Walnuts oil dries slow and tends to change its colour. But after bleaching under the sun and air, it dries fast and its colour doesn't change. And a layer by walnuts oil has better endurance. Therefore it seems that it was formerly used for bright colours.
The Why of that Drying oils dry and harden in nomal temperature is that these oils contain linolenic acid or linoleic acid. These acids combine with oxygen in the air. Linolenic acid is found in Linseed oil. This acid combine with oxygen rather than linoleic acid, and provides strong films and good drying speed. But it tends to be yellow and dark. On the other hand Linoleic acid is found in Poppy oil. The property of flims containing Linoleic acid is not as good as Linolenic acid's, but it's rather hard to increace yellowing. Linseed oil contains 60% of Linolenic acid and 10 to 20% of Linoleic acid. And Poppy oil contains 0% of Linolenic and 70% of Linoleic. On the other hand Walnuts oil contains 10 to 15% of Linolenic and 60% of Linoleic ( See the following table ) . This show us the characterristic of Walnuts oil is middle bitween Linseed and Poppy.
Walnuts oil comparatively tends to rot when it's stocked at shops for long time. Therefore this oil disappeared from painter's materials after baroque. But some makers still provide this oil, for example a Italian maker MAIMERI,etc.
But if you read some art technical books, you may notice that mentions about quarity of Walnuts oil are inconstant.
As same as poppy oil , the film by safflawer oil hardly change its color into dark. Recently this oil is frequently used as binder of tube oil paint instead of poppy. because its cost is cheaper than poppy oil. And safflawer can not be used as raw materials for drug different from poppy, so it's not difficult to produse or import it for some county. Therefore we find the indication of "safflawer oil" on a label of recent tube oil rather than Poppy oil.
Safflawer oil was started to use as madium for oil paint since 20th century. We haven't heared anything serious probrem about safflawer oil yet. Its velocity of drying is slow but faster than poppy.
Sun thickened oil
Painters expose raw linseed oil to the sun's rays for bleachig. It takes for 3 or 4 months. At that moment, the oil is just Sun-Refined or Sun-Bleached oil. After that if it also exposed to air, it will oxidize and bocome viscos little by little. The oil is called Sun-Thickened oil. See How to bleach Drying oils ( under construction ).
This oil produce a tough, enamel-like, none-yellowing film. And the drying speed is the fastest in all drying oils. It has been used since medieval times and one of the most recommendable medium.
Stand oil ( sometimes called Polymerized oil ) is made by heating linseed oil at high temperature ( 250 to 350 degree centigrade ), cutting off oxygen. Because the oil hasn't involved oxygen, the drying speed is very slow. The oil has a pale clear colour and extremely high viscosity. It produces a tough, enamel-like, none-yellowing film.
Though bodied heavy-boiled oil like this has been used since medieval times, some technicalists say the modern stand oil is very different from the traditoinal stand oil.
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