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DRIERS are a class of bodies added to oil for the purpose of causing it to dry quicker than it would otherwise do. The bodies generally used for this purpose are salts of iron, lead, manganese, and zinc. The following list comprises all the compounds used as driers in paints and varnishes :—Red lead, litharge, lead acetate, lead borate, manganese oxide, manganese sulphate, manganese borate, manganese oxalate, zinc oxide, zinc sulphate, and ferrous sulphate.
Of these, the lead salts are most in use; the manganese compounds are largely used; the others but rarely.
Red Lead is fully described on p. 100, et seq.
Litharge is the monoxide of lead, and has the composition shown by the formula Pb O. It is prepared by oxidising lead in a current of air at a temperature sufficiently high to melt the oxide as it forms. On cooling, the litharge separates out in the form of flakes of a red-brown colour, which, on being ground up, forms a buff-coloured powder. Litharge is sold in the two forms here noted. It is soluble in dilute nitric acid and in acetic acid, forming the corresponding nitrate or acetate of lead. Hydrochloric acid dissolves it on boiling, forming the chloride; while sulphuric acid does not dissolve it, but forms the insoluble sulphate of lead. Mixed with oils, a slow action sets in, resulting in the formation of lead soaps, which are insoluble in water and many solvents. This action occurs with linseed oil; the lead linoleate so formed dissolves in the rest of the oil, forming a kind of varnish, which, on drying, leaves a lustrous coat. It is this feature of lead salts that makes them valuable in the production of paint. Litharge is a powerful drier, and should not be used too extravagantly in the boiling of oil; about } lb. to the cwt. of oil is quite sufficient.
Red lead is also a good drier, even better than litharge, from Ib. to 1 lb. being sufficient for 2 cwts. of oil. Its action on oil partakes more of the nature of an oxidising action than does
that of litharge, while it is dissolved in the oil in the nature of a lead soap, as is the case with litharge. Both litharge and red lead are largely used in the preparation of boiled oil. The oil so prepared has a dark red colour, but dries quickly, and leaves a coat which is elastic and yet firm to the touch, so that it is capable of resisting a great deal of rough wear and tear, as also exposure to considerable variations of temperature.
Lead acetate, Pb 2 C, H, O2, is a white crystalline solid prepared by dissolving lead or litharge in acetic acid, and evaporating the solution down to dryness, or until it crystallises. It is readily soluble in water, and to a small 'extent in alcohol. It is used as a drier, principally for mixing with paints, and then it gives good results. Paint to which lead acetate, or sugar of lead as it is called, is added dries better on greasy surfaces than · paint to which nothing has been added. As a drier it is not equal to either red lead or litharge, but it has the advantage of not causing the oil or paint to become dark or coloured. It is used in making what are called patent driers.
Lead borate is a white powder prepared by adding a solution of borax to one of lead acetate or nitrate. The precipitate is collected, washed, and dried. It is largely used as a drier both in boiling oil and in mixed paints. It does not lead to the discolouration of the oil so much as red lead, while its drying properties are nearly equal to those of litharge. From 1b. to i lb. is sufficient for 2 cwts. of oil or paint.
Manganese dioxide, the black oxide of manganese, Mn 0.2, is now very extensively used as a drier. It comes into the market from two sources, one natural, the other artificial. The natural manganese forms the mineral manganese or pyrolusite, and is found widely distributed in large quantities; for use, it is simply ground to a powder with water and then dried. It forms a greyish-black powder insoluble in water. Artificially, manganese is obtained from the still liquors of the bleachingpowder manufacturer, who, to prepare chlorine, treats manganese with hydrochloric acid, when he obtains a solution of manganese chloride, Mn Clą; this is treated by a process invented by Weldon, when all the manganese it contains is recovered in a usable form. While 'much of this recovered manganese is used over again in the preparation of chlorine, some of it is sold for other purposes. Manganese dioxide is soluble in hydrochloric acid with evolution of chlorine and the formation of manganese chloride, Mn Cl, ; in sulphuric acid it dissolves with evolution of oxygen and the formation of manganese sulphate, Mn S 04 Essentially it is a peroxide, a class of bodies which may be
described as containing more oxygen than is exactly equivalent to the metal present in them; this extra oxygen is often rather loosely combined, and ready to enter into combination with other bodies; it is this feature in the composition of manganese which makes it useful in oil boiling, for the oxygen, during the process, combines with the oil and oxidises it, while the manganese dissolves to some extent in the oil in the form of a manganese compound of the linoleic acid of the oil. Manganese is in consequence a powerful drier; in fact, the most powerful known. The proportion usually added in the process of boiling is ļ lb. to 1 cwt. of oil, and it is not desirable to increase this proportion much, as this would give rise to too much drying action, and cause the oil to form a hard and rather friable coat, not a firm elastic coat as it should do. Unfortunately, manganese has a tendency to make the oil dark. It is not a good drier for mixed paint, chiefly because the tendency in using it would be to add too much.
Manganese sulphate, Mn S 0q, is prepared by dissolving manganese in sulphuric acid, and evaporating the solution down to dryness. It is a crystalline salt of a faint pink colour. Its drying action is, perhaps, rather more powerful than that of the lead compounds, but less than that of the last-named compound. Rather less than } lb. should be added to each cwt. of oil or paint. It possesses one advantage over manganese in not adding to the colour of the oil.
Some commercial samples of manganese sulphate contain a little manganese chloride; this imparts slight hygroscopic properties, which of course are undesirable in a drier. manganese sulphate is a fairly dry salt, and is largely used by oil boilers as a drier in preparing pale boiled oils.
Manganese borate is a powder of a faint pinkish hue prepared by adding a solution of borax to one of a manganese salt, such as the sulphate or acetate; the powder is collected, washed, and dried when it is ready for use. According to Prof. Hartley* manganese borate precipitated as above and dried in air has the formula Mn, H(BO3), H, 0; dried at 100° C. (212° F.), it has the formula Mn H, (BO3)2, having lost its water of hydration. When heated it loses more water of combination, and finally at a bright red heat it has the formula Mn (BO3)2Manganese borate is rather soluble in saline solutions. When manganese borate is made from borax and manganese sulphate a little free sulphuric acid is formed. If in place of using borax, a mixture of borax (20 parts) and caustic soda (4.2 parts) be
* Journ. Chem. Soc., vol. lxiii., p. 129, et seq.
used, no free acid is formed and the precipitate is not so soluble, while the borate of manganese is more completely precipitated.
Borate of manganese when moist readily oxidises and turns brown. As a drier it is one of the best and most powerful, being superior to the lead compounds, but inferior to manganese, although it has the advantage of not leading to any discolouration of the oil. Between and ž lb. is required for each cwt. of oil.
Manganese oxalate, Mn C, 0,, has lately been proposed to be used as a drier for oil, and is said to have some advantages over other manganese compounds. It is prepared by precipitating manganese salts with oxalate of potash or soda; or by treating manganese hydroxide or manganese carbonate with oxalic acid. One advantage is said to be that during the process of oil-boiling it is decomposed, and that, owing to the manganese dissolving in the oil in combination with the linoleic acid and to the oxalic acid being evolved in the form of carbonic acid, the metal is able to exert its greatest drying power. From
to į lb. may be used per cwt. of oil.
Zinc oxide, Zn 0, is used as a drier, but cannot act as such as it has no drying properties at all. It is often put in mixed driers (see below), where it acts as a diluent to decrease the drying power of the other ingredients. For a description of its properties see p. 58.
Zinc sulphate, Zn S 04, is also used as a drier, but its virtues in this respect are rather problematical. As the commercial product contains much water of crystallisation it is necessary to dry it before it is added to the oil.
Ferrous sulphate, copperas, Fe S , is frequently added as a drier, especially in the preparation of varnishes. It is in the form of pale bluish-green crystals, containing 5 molecules of water of crystallisation; hence, before being used, it must be dried to dehydrate it. It is soluble in water. It is prone to decomposition by oxidation, especially if the crystals be exposed to moist air; it is this property of being changed by oxidation from ferrous sulphate to ferric oxide that makes copperas useful as a drier. It should be used with care, as its tendency is to harden the coat of paint or varnish, and thus impart a tendency to crack. It is not by any means such a good drier as either lead or man ganese salts ; from 1 to 2 lbs. are required for 1 cwt. of oil.
Besides the simple driers described above, a variety of compound driers, usually composed of mixtures of the single driers in various proportions or with some linseed oil or boiled oil, are made; it is not intended to describe these in detail, but a few
recipes for the production of those principally in use will be given.
Patent Driers are mixtures of all or any of the above, and frequently contain large quantities of sulphate of barium, chalk, or sulphate of lime; such mixtures appear to be made without the slightest regard to the purpose for which they are used, and some of them, undoubtedly, have the opposite effect to that desired. Such, for instance, are those which contain 60 or 70 per cent. of sulphate of lime, sand, and chalk, the whole being ground up with oil or varnish “foots.” The following figures give the result of analyses of three samples of “patent driers ” :
Sample No. 1 is a very fair drier, the value of Nos. 2 and 3 is very doubtful. The presence of water in these samples is due to their having been ground in oil foots. The following are good mixtures :- Take 15 lbs. of dried zinc sulphate, 4 lbs. of lead acetate, and 7 lbs. of litharge; mix them with 4 lbs. of boiled oil, and grind well together. Mix 100 lbs. of Paris white, 50 lbs. of white lead, and 2 lbs. manganese acetate with 30 lbs. of boiled oil, grind, and then mix with the first mixture, adding sufficient boiled oil to give the mass the consistency of soft dough. 25 lbs. barytes, 50 lbs. whiting, 4 lbs. manganese acetate, and 10 lbs. white lead, ground into paste with about 10 lbs. boiled oil foots.
The composition of the commercial “patent driers varies with different makers, but the above are common forms.
Zumatic Drier.-25 lbs. of zinc white and 1 lb. of borate of manganese are ground together. The object of the zinc white is simply to dilute the manganese salt, and to form a powerful drier in a convenient form. The proportions generally used are 1 lb. of the drier to 25 lbs. of paint.