It oxidises and becomes thick upon exposure to the air. This property is is very much increased by adding other substances to it and boiling them together (see Boiled Oil).
It is superior in drying powers, tenacity, and body to the other fixed oils.
The best oil comes from the Black Sea and the Baltic; that from East Indian seed is inferior, as the seed is less carefully cleaned, and contains too much stearine.
Raw linseed oil is clear and light in colour, works smoothly, and is used for internal work, for delicate tints, and for grinding up colours. Boiled oil is much thicker, darker, and more apt to clog. It is used for outside work, as its greater body and rapidity in drying make it a quicker and more efficient protection.
Raw Linseed Oil is obtained by allowing the oil, as first expressed from the seed, to settle until it can be drawn off clear.
When of good quality it should be pale in colour, perfectly transparent, almost free from smell, and sweet in taste.
When it is to be used for delicate tints, it is sometimes clarified by adding an acid (such as oil of vitriol), which is afterwards carefully washed out.
This clarification is stated to be of no permanent advantage, for the oil in drying recovers its original colour.
Darkness in colour and slowness in drying are defects in inferior linseed oil.
These, however, are greatly diminished, and the substance of the oil is improved by keeping.
The oil should never be used within six months after being expressed from the seed, and it is better if kept for several years.
Raw oil is more suited for delicate work than boiled oil, as it is thinner and lighter in colour.
The drying of raw linseed oil "may be improved by adding about 1 lb. of white lead, to every gallon of oil and allowing it to settle for at least a week; this also improves the colour of the oil, whilst the lead can be used afterwards for common work."1
The drying qualities of the raw oil can be greatly improved by boiling it alone, but other substances, such as those mentioned below, are generally added to it, which make it dry still more quickly.
When boiled it becomes much thicker, and not so suitable for indoor or delicate work, nor will it do for grinding colours, as it clogs and thickens too rapidly.
Boiled oil of a pale colour is necessary for use with light tints, but for deep colours a dark oil seems to be generally preferred, though apparently without much reason.
Dark Drying Oil may be made from the following ingredients : -
1 gallon linseed oil. 1 lb. red lead. 1 lb. umber. 1 lb. litharge.
The linseed oil is heated to about 200° Fahr.; when it looks brown and the scum is all burnt off the other substances are added; the whole is then raised to about 400° Fahr., and kept at that temperature for two or three hours. The oil is then drawn off, the albuminous matter being allowed to deposit, and is now clear and ready for use.
The umber is added simply to give the oil a dark colour.
Acetate of lead is sometimes used instead of the red lead and litharge, and tends to make the oil lighter in tint A little resin is sometimes added.
Chevreuil states that oil heated to 160° with 1/10 its weight of oxide of manganese has powerful drying properties.2
Good boiled oil spread in a film upon glass should be dry in from 12 to 24 hours,3 if raw it would take from 2 to 17 days, according to the atmosphere.4
Pale Drying Oil may consist of 1 gallon of linseed oil mixed with about 7 lbs. litharge or acetate of lead, and raised to a moderate warmth.
Boiled Oil to he used with zinc paint must be free from oxides of lead. About 5 per cent by weight of powdered peroxide of manganese is boiled in the oil for five or six hours. The mixture is then allowed to cool, and filtered.
Drying Oil for common work may be made by boiling l1/2 lb. red lead in a gallon of raw linseed oil, and allowing the mixture to settle.
Poppy Oil is extracted by pressure from the seeds of the common poppy. It should be colourless, or of a very light yellow tinge, sweet, and free from smell. Being very pale it is sometimes used for light tints, but though its colour stands longer than that of linseed oil, it eventually becomes of a brownish hue, and in drying and other qualities it is far inferior to linseed oil.
Nut Oil is expressed from walnuts. It should be nearly colourless, and therefore adapted for white and any light tints. It dries more rapidly than linseed oil, but is not durable, and is used only for common work, being cheap.