Pagina-afbeeldingen
PDF
ePub

oxidising it, which consists in bringing the hot oil (in the form of a spray) in contact with an air current. A special form of apparatus is provided; this consists of two boilers, set side by side in a suitably constructed furnace; from one rises a wide flue, which ultimately terminates in a chimney; round the base of this flue are apertures which admit air, a current of which is thereby ensured through the chimney. The other boiler is also provided with a flue or chimney which passes into the flue of the other boiler at about its middle point. The two boilers are connected together at the bottom. The apparatus is worked in the following manner. They are filled with oil which is then heated. From the second boiler the oil is pumped up to a perforated box in the top of the flue of the first boiler; it descends through the flue in the form of a spray, and meeting the air current it becomes oxidised, falling into the boiler, from which it flows into the other boiler to again be pumped up; this process is carried on as long as it may be considered necessary

Mr. E. Walton* has devised a process of preparing drying oils for use in the manufacture of floorcloth, in which he makes use of a special apparatus for oxidising the oil by means of air. The oil is mixed with a small quantity of driers, and is then poured through a column of air in a fine shower, by means of which it is oxidised. This oil is next placed in a cylindrical apparatus and mixed with whiting; here it is agitated while air is blown in by a blower. The oil rapidly oxidises, and when the action has proceeded far enough it is stopped and the cylinder emptied of its contents.

Prof. W. N. Hartley has investigated the boiling of the oil and has come to the following conclusionst:

1st. The chemical action of the manganese compound, employed as a drier, when dissolved in linseed oil is that of a carrier of oxygen from the atmosphere to the oil. Manganese oxide takes up oxygen from the air and transfers it to the oil, and in so doing it suffers alternately the opposite processes of oxidation and reduction.

2nd. To obtain the best result, the amount of manganese present must not exceed a certain small proportion of the oil.

3rd. Oil to which turpentine has been added dries more rapidly than oil without such addition, because the oil being diluted and rendered thinner it spreads over a larger surface,

[blocks in formation]

and is in contact, therefore, with a much larger quantity of oxygen.

4th. Turpentine does not act as a drier-that is, as a carrier of oxygen to the oil.

5th. Different white pigments behave differently when drying; the more powerfully basic the properties of the pigment, the more powerful is its action as a drier. Lead oxide and white lead (basic lead carbonate) combine more easily with the acids of linseed oil than zinc oxide does. But zinc oxide dries better than antimony oxide, because it is a stronger base, while arseniate of tin has no basic properties, therefore does not act as a drier.

6th. Indifferent substances—that is to say, those without chemical action on oil, such as lamp black, barytes, sulphate of lead-cannot act as “driers."

Properties of Boiled Oil.-Boiled oil is a slightly viscid oil of a reddish colour, varying a little in depth of colour according to the temperature, and the length of time it has been heated in the process of boiling. Its odour is peculiar, and its taste, which is rather acrid, somewhat characteristic. In specific gravity it varies a good deal, but the average is about 0.945 ; some samples will reach 0.950, while others may be as low as 0.940. Boiled oil is soluble in turpentine, petroleum spirit, shale spirit, benzene, carbon bisulphide and other similar solvents. When boiled with caustic soda or caustic potash it is saponified almost completely; there is usually a small trace of unsaponifiable hydrocarbon oil formed by the decomposition of the oil during the process of boiling.

When exposed to the air in thin layers it dries much more rapidly than raw linseed oil, and leaves behind a hard, lustrous coat; it is this property which makes boiled oil of so much use to the painter; yet it does not do to use boiled oil alone in the making of paints, because the coat which it leaves is too hard and rather liable to crack on exposure to the air; raw linseed oil is always added, as, by leaving a more elastic coat, it prevents this bad fault of boiled oil from showing itself.

Adulteration of Linseed Oil.—Both the raw and boiled linseed oils are frequently adulterated ; (substitutes for boiled oil will be described more fully later on); the principal adulterants used are mineral and rosin oils. Other fatty oils, such as cotton seed, niger seed, and whale oils, are sometimes used; but, as linseed oil is cheap, the small gain arising from their use does not compensate for the probable loss of custom

which must ensue if the adulteration be found out; while the great difference in the cost of linseed and mineral oils is a strong inducement for adulterating with the latter.

For the purpose of detecting adulteration the following tests may be applied :

1. Specific Gravity.--For raw linseed oil this should be about 0.932; if less than 0.930, adulteration with fish, seed, or mineral oils would be indicated; while if the gravity exceeds 0.937, then admixture with rosin oil is very likely. The specific gravity of boiled oil averages about 0.945; if much heavier than this it is quite probable that rosin oil has been mixed with the oil ; while if below 0.940, then other fatty and mineral oils may be looked for.

2. Flash Point.—Linseed oil, whether raw or boiled, flashes at about 470° F. Other fatty oils flash at about the same temperature. Rosin oil flashes at from 300° to 330° F., and during the process of testing a strong odour of rosin would be given off. Mineral oils, such as would be used to adulterate linseed oil, will flash at from 380° to 420° F., so that the flash point is one of the best tests for detecting the adulteration of linseed oil with mineral or rosin oils.

3. Proportion of Mineral or Rosin Oils in Linseed Oil.To determine the proportion of mineral or rosin oils, in adulterated linseed oils, place 20 grammes in a beaker with a little water and alcohol ; then add some caustic soda and boil for some time, stirring at intervals; the linseed oil becomes saponified, while the adulterants are not acted on; after about an hour's boil, the mass is allowed to cool a little; then it is poured into a separating funnel and some petroleum ether is added, which will take up the mineral oil and form a layer on the top of the aqueous layer; after allowing the two layers to separate completely, the bottom layer is run off, and the top layer is washed quite free from all traces of the soap formed by the action of the alkali on the linseed oil, by several treatments with warm water. The ethereal layer is then run into a weighed glass, the ether evaporated off, and the residue of mineral oil weighed. Whether the residue is mineral or rosin oil must be judged from the nature of the residual oil after evaporating off the ether; if this is heavy and viscid, and smells of rosin when heated, then rosin oil is present ; if the residual oil is light, then mineral oil is present.*

* A simple test for the presence of rosin or of mineral oils in either raw or boiled linseed oil, is to boil a small quantity with an alcoholic solution of caustic potash until it is completely saponified, then pour the solution into water; if the oil be pure, a clear mixture will be obtained ; if it contains either of the oils named it will be cloudy and turbid.

4. Cotton and other Fat Oils in Linseed Oil.- The detection of cotton seed, niger seed, or other fat oils in linseed oil is much more difficult, but much valuable information on this point will be gained by noticing the behaviour of the oil with strong sulphuric acid, the character of the mass formed, and the temperature which the mixture of acid and oil attains. The character of the soap formed on boiling the oil with caustic soda, the appearance, melting point, and combining equivalent of the fatty acids which may be obtained from the soap so formed are also valuable indications of the character of the fatty oil adulterants.

5. Driers in Boiled Oil.- About 25 grammes are boiled with a little dilute hydrochloric acid, with constant stirring, for about half an hour; the mass is allowed to stand to separate; the bottom acid layer contains the driers added during the boiling of the oil, this is run off and tested in the usual way; then the oil is boiled with caustic soda until it is saponified, then the mass is treated in the separating funnel, as described above, to separate the mineral or rosin oil used to adulterate the boiled oil. The aqueous layer which has been run off may be acidified, and the acids obtained tested for rosin by Gladding's test.

BOILED OIL SUBSTITUTES.- Many substitutes are offered for boiled oil, some of which have been patented. In composition they vary greatly, and it is not possible to do more than briefly indicate their general features. Some are mixtures of boiled oil, rosin, turpentine, rosin oil; others more closely approach an oil varnish in composition, being made by melting rosin, then mixing it with hot oil and thinning down with rosin spirit. Some are made by preparing a compound of lime or alkali with rosin or other resinous products, and dissolving this in oil and rosin spirit or turpentine.

The quality of these products varies very much. None of them are equal to good boiled oil, although one or two very nearly approach it; others are but inferior substitutes, and cannot be recommended even for inferior work. It is not possible to deal more particularly with these boiled oil substitutes in this book.

POPPY OIL.—This oil is obtained from the seeds of the poppy (Papaver somniferum) by pressure, or it may be extracted by means of solvents. This oil, although a very good drying oil, is not largely used, chiefly because its price does not allow it to compete with linseed oil; artists make use of it on account of its paleness in colour not interfering so much with pale tints as linseed oil does, its price not being so much an object with them as it is with house painters.

Poppy oil is usually of a pale straw colour, very limpid, has little or no odour when fresh, and a pleasant taste; the oil is free from the narcotic properties for which the plant itself is famous. In specific gravity it ranges from 0.924 to 0.927. It solidifies at - 18° C. It is soluble in about four times its volume of boiling alcohol and twenty-five times its volume of cold alcohol. Mixed with strong sulphuric acid (Maumene's test), the rise in temperature is about 88° to 90° C. It takes about 19

per

cent. of caustic potash (KOH) to saponify it, and it absorbs about 134 to 137 per cent. of iodine.

HEMPSEED OIL.—The hemp plant (Cannabis sativa) yields a roundish greenish-grey seed, very familiar to lovers of canaries, from which, on expression, an oil is obtained that is used for painting. The yield of oil varies from 15 to 25

per

cent. Hempseed oil when fresh has a greenish-yellow tint, but on keeping it slowly turns to a brownish-yellow; its odour and taste are rather unpleasant. Its specific gravity ranges from 0.925 to 0.931. It becomes turbid at a temperature of – 15° C., but does not set completely solid until a temperature of - 25° C. is attained. Strong sulphuric acid has a vigorous action on it, the increase in temperature being about 100°C. It absorbs from 143 to 144 per cent. of its weight of iodine, which indicates that it contains a large proportion of linoleic acid (linolic and linolenic acids), and shows that its drying properties must be good.

In this country hempseed oil is rarely used as a paint oil, its price being against it; still, it has been mixed with linseed oil, and it is difficult to obtain the latter free from it, owing to the Russian linseed growers mixing hempseed with the linseed. In Russia, and other places where hempseed is grown, the oil is used rather largely for painting.

WALNUT OIL.—The common walnut, the fruit of the walnut tree (Juglans regia) contains about 50 per cent. of its weight of an oil possessing drying properties. The process of extraction of this oil is as follows :

The nuts are collected and placed in heaps for a period of about three months, when they begin to decompose; they are then crushed and pressed; this gives “virgin nut oil,” often used as a food oil as well as a paint oil. The nuts still contain some oil, which is extracted by grinding the cake with hot water and again subjecting it to pressure; the oil so got is known as drawn nut oil.”

Walnut oil is usually of a pale yellowish-green tint, but it can be prepared almost colourless from fresh kernels.

The specific gravity varies from 0.925 to 0.927; it begins to be turbid at a

« fire

« VorigeDoorgaan »