However the oil may have been prepared, it will, if kept a long time, deposit a sediment. At first this contains mucilage; but the sediment from old oil consists chiefly of the products of decomposition of the oil itself. The presence of oxygen is not necessary for this decomposition; but it is increased by the action of light. Raw linseed-oil dries more slowly than boiled; but the resulting film is more brilliant and durable. Raw and boiled oil are therefore usually mixed in proportions varying according to the time which can be allowed for the paint to dry, or to the properties required of the film. For the ordinary kinds of paint, equal parts of boiled and raw oils are customary. Linseed-oil heated to a temperature of 350°-400° F. dries much more rapidly than in its raw state.
Menhaden Oil.—A fish eagerly sought for its oil on the Atlantic coast of America’ is the “Menhaden” or “porgie” (Alosa [Brevoordia] Menhaden), a member of the herring family, about 8-14 in. long. The fishery is carried on all along the coast from Maine to Maryland. The fish leave the Gulf Stream and strike the coast of New Jersey in April, reaching the coast of Maine in May-June, and remaining till October-November. They migrate in enormous schools, and are caught in seines, carried by the fastest and smartest yachts. Very few of the fish are sent to the table; nearly all are boiled down for their oil.
This is performed in the following manner:—The fish are shot into receiving-tanks situated outside the building; thence a sliding door opens into the boiling-tanks, which are long, watertight, uncovered boxes, of varying capacity, provided with a coil of perforated pipe for the admission of steam, and a plug-hole for the exit of the liquid after boiling. Some water is put into the tanks ready for the fish, and as soon as the latter have been introduced, steam is turned on, and the whole mass is boiled for 20-40 minutes. When the cooking is completed, the liquor, containing a portion of the oil of the fish, is drawn off into settling tanks, for the recovery of the oil. The “pomace” or cooked fish is raked into “curbs,” perforated cylinders fitted with hinged bottoms, and these, when full, are placed under hydraulic presses. Pressure is applied so long as water and oil continue to escape from the mass. The remaining solid matters, called “scrap,” are treated for the preparation of a fertilising compost. The oil and water pass by gutters into settling tanks, where the oil soon rises to the surface, and is skimmed off, or allowed to escape over a separating partition.
The oil is still crude, and requires clarifying and bleaching before it becomes a saleable commodity. This is effected in several ways. It is first boiled, to free it completely from water. It is purified from solid matters by running it into filter-bags suspended over casks, and then subjecting it to pressure in bags, the oil escaping while the sediment remains in the bags. This refuse, termed “foots,” is bleached and used for soap-making. The oil thus refined is termed “traits,” and is ready for barrelling. “Bank” is an inferior grade. Bleaching is sometimes performed by exposure to the sun in shallow tanks, having glass covers to exclude dust when a superior quality is desired.
Its principal application in America is for tanning and currying purposes. In France, it is largely employed as a substitute for cod-liver oil. In this country, it is often passed off as olive-oil, and considerable quantities of it are mixed with linseed-oil for painters’ use. The rapidity with which it oxidises, and its good body, render it not unsuitable as a vehicle for paint.
Poppy-seed Oils.—Oil is yielded by the seeds of three kinds of poppy—the opium-poppy (Papaver somniferum), the spiny-poppy (Argemene mexicana), and the yellow-horn poppy (Glaucium luteum).
In Asia Minor and Persia, after the collection of the opium from the poppy-heads, the latter are gathered, and the seed is shaken out and preserved. It is black, brown, yellow, or white; some districts produce more white seed than others. The seed is pressed in wooden lever presses to extract the oil, which is used by the peasants for culinary and illuminating purposes. Some of the seed is also sold to Smyrna merchants, who ship it to Marseilles, where it is employed in soap-making, and as a substitute for linseed-oil. The average yield of oil is 35-42 per cent., the white seed being considered the richest.
The same economy takes place in India, where the plant is also grown for the sake of its seed alone in some districts. In this latter case, the sowing takes place in March-April, about 2 lb. of seed being sown broadcast to one acre. The seed vessels ripen in August; the heads are then cut off, sun-dried, sorted, and trodden out in bags, or threshed. The seed is immediately crushed and pressed, the yield of oil being in proportion to the freshness of the seed, and amounting to 14 oz. from 4 lb. under favourable conditions. The oil readily bleaches by exposure to the sun in shallow vessels, and is then transparent and almost tasteless. The natives use it very largely for cooking purposes, and as a lamp-oil. The cake is consumed as food by the poorer classes. The unpressed seed is largely exported from India.
France grows a large quantity of poppy-seed at home, over 100,000 acres having been returned as under this crop some few years since. The French oil is of two kinds, a white cold-drawn oil, and a coarser oil obtained by a second expression and from inferior seed, the total yield being 40 per cent. The finer oil is fit for alimentary purposes, and is largely used to adulterate olive-oil; it is also employed as a lamp-oil, and very extensively by artists for grinding light pigments, as, though possessing less strength and tenacity than linseed-oil, it keeps its colour better. The pure oil has a golden-yellow tint and agreeable flavour; its sp. gr. is 0·924 at 59° F.; it solidifies at 0° F., and remains long in this state at 28½° F., is slow to become rancid, and saponifies readily; dissolves in 25 parts cold and 6 parts boiling alcohol, and dries in the air more rapidly than linseed oil.
Glaucium luteum is a common plant on the sandy shores of the Mediterranean, the western coast of Europe as far as Scandinavia, and some parts of North America. It is very hardy and cultivated with little trouble. It prefers stony and chalky soils, where few other plants will thrive, and has therefore been recommended for culture on such otherwise waste land. Under cultivation, it affords about 10 bush. of seed per acre. The seed contains 42½ per cent. of oil, and yields about 32 per cent. by pressure. The oil obtained by cold expression is devoid of odour and flavour, and has a sp. gr. of 0·913. It is applicable to culinary and illuminating purposes, as well as for soap-making and paint. The cake is a good phosphatic manure. It seems to have been very little utilised, probably on account of the comparatively small yield of seed.