LINSEED OIL AND IRON RUST.
The oleaginous principle of linseed oil is said to be in the nature of neutral salts called linolein, consisting of linoleic acid combined with a glycerine base. Linolein is said by some writers to constitute three-fourths of the volume of linseed oil, and that the drying properties of the oil reside in the acid principle of the linolein; that is, linoleic acid has the property of attracting and combining with oxygen to form the substance known as dry linseed oil. This acid is said to be a compound of several different acid principles, combined in definite proportions. Writers seem to disagree as to what the acids are, and in what respect they differ from the acid properties of the non-drying fixed oils, but that is a question which need not be discussed here. The glycerine base of linolein seems to be common to all fixed oils, and is set down as an oxide consisting of one equivalent of water and five of oxygen; hence the affinity between the linoleic acid and its glycerine base.
Linoleic acid, like other acids, has an affinity for alkalies and the ordinary metallic oxides. It unites with them, forming neutral compounds. This affinity is said to be electrical; the alkalies and oxides electro-positive, and the acid electro-negative. The greater the contrast in this respect, the stronger the affinity; hence, some acids separate others from their bases and form new salts by precipitation. As an instance:
Drop sulphuric acid into a solution of acetate of lead. It will displace the acetic acid, form sulphate of lead and precipitate, leaving the liberated acetic acid in solution. In linolein, this acid is so constituted that the affinity, or attraction between it and its glycerine base, is too feeble to resist and keep back the oxygen of the air; hence, when linseed oil is exposed to the air in a thin layer, oxygen unites with its linoleic acid, and this process continues until the oil becomes dry to the touch. Beyond this point the process is slower, because the oil is now less penetrable; but the process goes on until the layer of oil becomes hard and brittle, no matter with what pigment it may be mixed, although the pigment may for a time retard the action of the destroying elements.
Linseed oil dries too slowly for general use by the painter, hence various ways have been devised to hasten the drying process. If the foregoing theory is correct, the process which will cause the oil to dry to a good wearing body in the time desired, and leave it in the best condition to resist the action of the elements and the absorption of oxygen, is the best. I regard the lead oxides as the best dryers for this purpose—at least according to my experience. When we add an oxide to linseed oil as a dryer in the small quantity which experience has taught us is best to use, it is evident that it is not sufficient in itself to oxidize the whole of the oil to an appreciable extent. Writers differ as to the peculiar action of the oxides upon the oil, but I think it safe to say that the dryer sets up some chemical reaction which increases the affinity between the linolein and the oxygen of the atmosphere; at any rate, there is no dispute upon the point that linseed oil in drying absorbs a large per cent. of oxygen.
A knowledge of this unanimously conceded point led me to believe that a coat of pure linseed oil might make the best possible priming coat for iron work which had commenced to rust. Why? Because iron rust is an oxide of iron, having an excess of oxygen. Spread on rusty iron, it penetrates the rust, absorbs its excess of oxygen and dries with the remaining neutral oxide held fast in its body. This is my theory; whether correct or not, numerous tests have proved to me that a coat of linseed oil will stop the rusting of iron if applied under proper conditions. When rust is thick or scaling there is no safety short of taking it off. Iron rust is more or less hydrated; to free it from moisture, give it the flame of the gasoline paint burner.