CHAPTER XII.

VEHICLES AND DRYERS.

Paint consists essentially of two parts, the pigment (see Chapters I. to VIII.), and the vehicle or medium. In the case of oil paints, a third substance termed a dryer becomes necessary, to facilitate the “drying,” or solidification of the vehicle.

A perfect vehicle should mix readily with the pigment, forming a mass of about the consistency of treacle. It should itself be colourless, and have no chemical action upon the pigments with which it is mixed. When spread out in a thin layer upon a non-porous substance, it should solidify, and form a film not liable to subsequent disintegration or decay, and sufficiently elastic to resist a slight concussion.

Unfortunately, we possess no vehicle which complies with all these conditions; those which most nearly approach them are the drying oils. Oils are compound bodies containing acids and a base. Some oils oxidise very rapidly, while others do not oxidise at all. When oils oxidise they change their colour, and however white they may be at first, they gradually turn yellow and finally brown. The advantages of oils are that they mix kindly with most pigments, can be dissolved in turpentine, and can be used in almost any desired state of fluidity. Against these have to be set the disadvantage of the oxidation of the oil, to which oxidation the use of oil in paint is entirely due.

The use of oil in painting is said to have been invented in the 14th century, and, in a short time, it reached a considerable degree of perfection. We have only to compare a Van Eyck with a painting by a modern master--Turner, for instance--to see that even the best of recent painters have not succeeded in giving to their works that durability which the originators of the method attained. All organic substances are liable to a more or less rapid oxidation, especially if exposed to light and heat. Oil is no exception to this rule; but it seems that, in its pure state, it is much more durable than when mixed with other substances. Although ground-nut-and poppy-oils are sometimes employed by artists where freedom from colour is essential, yet linseed-oil is the vehicle of by far the larger proportion of paints used both for artistic and general purposes.

Oil-paint appears to have been unknown to the ancients, who used various vehicles, chiefly of animal origin. One of these, which was in high repute at Rome, was the white of eggs beaten with twigs of the fig-tree. No doubt the india-rubber contained in the milky juice exuding from the twigs contributed to the elasticity of the film resulting from the drying of this vehicle. Pliny was aware of the fact that when glue is dissolved in vinegar and allowed to dry, it is less soluble than in its original state. Many suggestions have been made in modern times for vehicles in which glue or size plays an important part. In order to render it insoluble, various chemicals have been added to its solution, such as tannin, alum, and a chromic salt. None of these vehicles, however useful for special purposes, has become sufficiently well known to warrant description here.

Substitutes which do claim attention are wax and dammar gum, or paraffin wax, dissolved in turpentine. The colours must then be ground in turpentine and not in oil. Such a vehicle is very pleasant to work with, and gives good results; moreover, it permits alterations or corrections to be made by rubbing out with turpentine. Nevertheless, both the wax and the turpentine undergo oxidation to some extent, and are therefore not altogether free from the same objections as oils. But benzol, especially when carefully prepared, answers all the purposes of turpentine without undergoing oxidation. The only drawback that can be urged against benzol is its odour, which some people have an aversion to; but it really has very little smell, and it evaporates away completely in a very short time. A mixture of wax, dammar, and benzol forms an excellent vehicle. The wax may be replaced by paraffin wax with advantage.

It is desirable to be able to ascertain whether the oil intended for use is, or is not, adulterated with non-drying oil. The distinction of non-drying oils is that they solidify when acted upon by peroxide of hydrogen, or by sub-nitrate of mercury--the oleic acid is concreted, and a substance called elaidin is formed. This does not take place with the drying oils.

The oils used in paint making are chiefly--

Ground-nut. Hempseed. Kukui or candle-nut. Linseed. Menhaden. Poppy-seed. Tobacco-seed. Walnut. Wood or Tung.

GROUND-NUT OIL.--The ground-nut or pea-nut (_Arachis hypogæa_) is very widely cultivated in the tropics for the sake of its oily seeds. In Java, the oil is extracted by drying the seeds in the sun, and then subjecting them to pressure. In European mills, the nuts are first cleaned, then decorticated and winnowed, by which the kernels are left perfectly clean. These are crushed like any other oil seed, and put into bags, which are introduced into cold presses; the expressed oil is refined by passing through filter-bags. The residual cake is ground very fine, and pressed under 3 tons to the inch, in the presence of steam-heat; this affords a second quantity of oil, inferior in quality to the cold pressed. The usual product is 1 gal. of oil from 1 bush. of nuts by the cold process, besides the extra yield by the hot pressing. In France, where the oil is most largely prepared, three expressions are adopted, as with some sorts of gingelly: the first gives about 18 per cent. of superfine oil, fit for alimentary purposes; the second, after moistening with cold water, affords 6 per cent. of a fine oil, suitable for lighting and for woollen-dressing; the third, after treating with hot water, yields 6 per cent. of _rabat_, or oil applicable only to soap-making. In India, the total mean yield is 37 per cent. at Pondicherry, and 43 in Madras.

The cold-pressed oil is almost colourless, of agreeable faint odour, and bland olive-like flavour. The best has a sp. gr. of about 0·918, or 0·9163 at 59° F.; it becomes turbid at 37½° F., concretes at 26½°-25°F., and hardens at 19½° F. By exposure it changes very slowly, but thickens with time, and assumes a rancid odour and flavour. It is not a good oil for paint.

HEMPSEED-OIL.--The seeds of the hemp plant, so well-known as a fibre-producer, are valued for their oil. It is from Russia and Lorraine that the seed for expressing mostly comes. When the fibrous stems are tied in bundles, the seed is rudely threshed out, and spread in thin layers under cover to dry. The extraction of the oil is performed in the same manner as with other seed oils, described on p. 308. The proportion of oil contained in the seed is about 34 per cent. on an average; the yield varies from 25 to 30 per cent. The oil is at first greenish or brownish-yellow, deepening with exposure to the air; the flavour is disagreeable, and the odour is mild. It has a sp. gr. of 0·9252 at 59° F.; it thickens at 5° F., and solidifies at-13° to-18° F.; it dissolves in 30 parts of cold alcohol and any proportion of boiling; it saponifies with difficulty, forming a soft soap, but less soft than that from linseed oil. It is inferior for the painter’s purposes.

KUKUI OR CANDLE-NUT OIL.--An oil bearing a multitude of names is obtained from the candle-nut (_Aleurites moluccana_). It is the most important product of the tree, and constitutes about two-thirds of the entire weight of the kernel of the nut. A great obstacle to its wider development is the difficulty encountered in extracting the kernels from the shells, both on account of the extreme hardness of the latter, and the obstinacy with which the two adhere. Boiling is out of the question, as the kernels are cooked long before the shells are affected; but there is every reason to suppose that a slight roasting would have the desired effect, inasmuch as this plan seems to be adopted successfully by the Samoans. The weight of the shells necessitates this treatment being performed on the spot, and, as the kernels quickly become rancid and dark-coloured after liberation, they must also be operated upon without removal. The local cheapness of labour is an additional argument in favour of preparing the oil at the places where the nut grows. The extraction of the oil is very simple. In Jamaica, Polynesia, and the East Indies, 50 per cent. is obtained by boiling the kernels in water; by reducing the kernels to meal, heating in a water-bath, and placing the mass in bags under hydraulic pressure, the yield is about 60-66 per cent. The shells are themselves excellent fuel. The oil is completely clarified by mere filtration. As ordinarily prepared, it is amber-coloured, tasteless and odourless; slightly viscid at the temperature of the air in England, congealing at 32° F.; its sp. gr. is 0·923; it is insoluble in alcohol, and saponifies readily, giving a very soft soda-soap. It dries less rapidly than linseed oil, and is used for mixing paints and making oil-varnishes. It is said to corrode tin plate and even platinum.

LINSEED-OIL.--The flax plant, so well known as yielding a textile fibre, affords a valuable oil-seed. The supplies of linseed for crushing are furnished chiefly by Russia and India. It is found that, as a general rule, the colder the climate in which the seed is grown, the greater are the drying properties of the oil, but the worse is its colour. In India, preference is given to white seed, as yielding 2 per cent. more oil, affording it more freely, and giving a softer and sweeter cake, than the red seed; the latter, moreover, always comes to market largely mixed with rape-seed, which is very difficult of separation, and greatly depreciates the market value. Oil from unripe seed is watery. The seed should always be kept for 3-4 months in a dry place, as the oil furnished after this lapse of time is much more abundant than when the expression takes place immediately after the harvest. The seed is crushed and pressed in the manner described on p. 308. The best and finest oil is that which is “cold-drawn”; it is paler, less odorous, and less flavoured, but the yield is only 21-22 per cent. of the seed. By the aid of a temperature not exceeding 200° F., and powerful and long-continued pressure, as much as 28 per cent. of very good oil can be obtained. The cake forms a valuable cattle food. The Italian variety is said to have a much more highly oleaginous seed than the Russian.

Linseed-oil has a faint colour, and mild odour and flavour when pure, but the commercial article is dark-yellow, with sharp repulsive flavour and odour. Its sp. gr. is 0·930; at 0° F., a little solid fat separates out; at-4° F., it solidifies. By exposure to the air, after heating with oxide of lead, it rapidly dries up to a transparent varnish. The fresh oil saponifies readily, giving a yellow and very soft soap with soda; by saponification, it yields 95 per cent. of fatty acids, chiefly linoleic, with a little oleic, palmitic, and myristic acids. It dissolves in 1·6 parts of ether, and in 32 parts of alcohol at 0·820 sp. gr. The oil is very extensively used in the manufacture of paint and oil-varnishes. For artists’ use it is purified by shaking up with whiting, and warming. Linseed-oil is never met with in commerce really pure, nor even the seed itself. Previous to the Crimean War, it was a recognised custom at the Black Sea ports to add one measure of hemp or other seed to every 39 of linseed. Since then the proportion has advanced to 1 in 19, in addition to which the Indian seed is grown mostly as a mixed crop with mustard and colza: pure linseed oil can only be obtained by picking out the seeds individually.

Linseed-oil, to be suitable for painting, must dry well. A reliable test is to cover a piece of glass with a film of the raw oil, and to expose it to a temperature of about 100° F. The time which the film requires to solidify is a measure of the quality of the oil. If the oil has been extracted from unripe or impure seed, the surface of the test-glass will remain “tacky” or sticky for some time, and the same will happen if the oil under examination has been adulterated with either an animal or vegetable non-drying oil.

Until recently, linseed oil was frequently adulterated with cotton-seed oil, extracted from the waste seeds of the cotton plant. Where the admixture was considerable, it could easily be detected by the sharp, acrid taste of the cotton-seed oil. Now, however, means have been found for removing this disagreeable taste, and the consequence has been that cotton-seed oil is so largely used for adulterating olive-oil, or as a substitute for it, that its price has risen above that of linseed oil.

Another adulterant which is rather difficult to detect is rosin. Oil containing this substance is thick, and darker in colour than pure oil. When the proportion of rosin is considerable, its presence may be ascertained by heating a film of the oil upon a metallic plate, when the characteristic smell of burning rosin will be perceptible. When the percentage of rosin is too small for detection in this manner, a film of the oil should be spread upon glass and allowed to dry. When quite hard, the film should be scraped off, and treated with cold turpentine, which will dissolve any rosin which may be present, without materially affecting the oxidised oil. The presence of rosin may also be detected by the following simple chemical test. The oil is boiled for a few minutes with a small quantity of alcohol (sp. gr. O·9), and is allowed to stand until the alcohol becomes clear. The supernatant liquid is then poured off, and treated with an alcoholic solution of acetate of lead. If the oil be pure, there will be but a very slight turbidity, while the presence of rosin causes a dense flocculent precipitate. Should linseed oil be adulterated with a non-drying oil, it will remain sticky for months when spread out in a thin film upon glass or any other non-absorbent substance.

The sp. gr. of linseed oil is, in some cases, of value in estimating its quality; but as the variations are slight, it would be difficult to detect them in so thick a liquid by means of an ordinary hydrometer. A simple method of obtaining an approximate result is to procure a sample of oil of known good quality, and to colour it with an aniline dye. A drop of this tinted oil will, when placed in the oil to be tested, indicate, by its sinking or swimming, the relative density of the liquid under examination. Freshly-extracted linseed oil is unfit for making paint. It contains water and organic impurities, respecting the composition of which little is known, and which are generally termed “mucilage.” By storing the oil in tanks for a long time, the water and the greater part of the impurities are precipitated, forming at the bottom of the cistern a pasty mass known as “foots.”

To accelerate the purification of the oil, and to remove at least a portion of the colouring matter, various methods are in use. The action of sulphuric acid upon linseed-oil is not so favourable as upon other oils. It is, however, sometimes employed, in the proportion of two parts of a mixture of equal volumes of commercial sulphuric acid and water to 100 parts of oil. The dilute acid is poured gradually into the oil, and the mixture is violently agitated for several hours. It is then run into tanks, and allowed to settle. A concentrated solution of chloride of zinc has been substituted for sulphuric acid in the proportion of about 1½ per cent. of the weight of the oil. When the reaction is complete, steam or warm water is admitted into the liquid, in order to clarify it. Oil treated in this way loses a considerable proportion of the colouring matter which it originally contained.

When the oil is to be used for white paint, it is sometimes bleached by exposing it to the action of light. On a large scale, this is done by placing it in shallow troughs, lined with lead and covered with glass. The lead itself appears to have some influence upon the bleaching of the oil, for the decoloration is not so rapid if the troughs be lined with zinc. For small quantities, a shallow tray of white porcelain or earthenware, similar to those in use for photographic purposes, gives very good results, the white surface increasing the photo-chemical action. It is not quite clear whether the presence of water accelerates the bleaching of oil by this method; some manufacturers consider its presence necessary, others omit it. Various salts are added to the water, the one most in use being copperas.

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.

TOBACCO-SEED OIL.--The seeds of the tobacco-plant contain about 30 per cent. of a fatty oil, which is extracted by powdering them, kneading them into a stiff paste with hot water, and pressing hot. The oil is clear, limpid, golden-yellow in colour, inodorous, and mild flavoured; its density is 0·923 at 59° F.; it remains liquid at 6° F., dissolves in 168 parts of alcohol at 0·811 sp. gr., and saponifies readily. One authority excludes it from the drying oils; another considers its drying quality to be unusually developed, and recommends it for paints and varnishes.

WALNUT OIL.--The common walnut (_Juglans regia_) is found native from Greece and Asia Minor, over Lebanon and Persia, along the Indu Kush to the Himalayas, and from the Caucasus almost throughout China, besides having been introduced generally throughout temperate Europe. In portions of the Alps and Apennines, it is very abundant, and is fairly plentiful in the forests of Lazistan, on the Black Sea, but is perhaps most common in Cashmere, whence come the walnuts imported into the plains of India.

The albuminous kernel of the walnut affords some 50 per cent. of oil. It is said that it furnishes one-third of all the oil made in France; it is extensively prepared in the central and southern departments, notably Charente, Charente-Inférieure, and Dordogne, where it is commonly met with in barrels of 50 _kilo_. In both Spain and Italy, outside the olive-region, walnut-oil is largely expressed. It is of considerable importance in the hill districts of India, but is seldom seen in the plains. Cashmere and Circassia also include it among their industrial products.

The oil should not be extracted from the nuts until 2-3 months after they have been gathered. This delay is absolutely necessary to secure an abundant yield, as the fresh kernel contains only a sort of emulsive milk, and the oil continues to form after the harvest has taken place; if too long a period elapse, the oil will be less sweet, and perhaps even rancid. The kernels are carefully freed from shell and skin, and crushed into a paste, which is put into bags and submitted to a press; the first oil which escapes is termed “virgin,” and is reserved for feeding purposes. The cake is then rubbed down in boiling water, and pressed anew; the second oil, called “fire-drawn,” is applied to industrial uses. The exhausted cake forms good cattle-food.

The virgin oil, recently extracted, is fluid, almost colourless, with a feeble odour, and not disagreeable flavour. Its sp. gr. is 0·926 at 59° F., and 0·871 at 201° F.; it thickens to a butter-like consistence at 5° F., and solidifies to a white mass at-17½° F. In the fresh state, it is largely used in Nassau, Switzerland, and other countries, as a substitute for olive-oil in salads, &c., but is scarcely to be considered as a first-class alimentary oil. The fire-drawn oil is greenish, caustic, and siccative, surpassing linseed-oil in the last respect and exhibiting the property more strongly as it becomes more rancid. On this account it is preferred by many artists before all other oils.

WOOD-OIL OR TUNG-OIL.--This fatty oil is a product of the so-called “oil tree” of China, Cochin China, and Japan (_Aleurites cordata_ [_Elæococca vernicia_, _Dryandra cordata_]), and must not be confounded with the Malayan article, which is an oleo-resin. The fruit capsules of the _t’ung_ are filled with rich oil-yielding kernels, from which 35 per cent. by weight of oil may be obtained by simple pressure in the cold. The sp. gr. of the oil is 0·9362 at 59° F. It possesses several remarkable properties: heated to 212°-392° F. out of contact with the air, it retains its original limpidity after cooling, but in contact with the air it solidifies almost instantaneously, melting again at 93° F, and exhibiting the same elementary composition; the cold expressed oil rapidly solidifies by light in the absence of air; and its drying qualities exceed those of any other known oil. It is devoid of colour, odour, and flavour. The oil is produced in immense quantities in China; in the provinces of Ichang and Szechuen, it is one of the principal articles of native manufacture.

In China the oil is universally employed for caulking and painting junks and boats, and for varnishing and preserving woodwork of all kinds. The oil is unknown to European commerce, but an attempt to naturalise the tree in Algeria has been projected. Its industrial value has been too long neglected.

EXTRACTION OF SEED-OILS.--The old-fashioned crude apparatus for extracting oil from seeds, which answered the purposes of our forefathers, has had to give way to modern improved machinery, such as that manufactured by Rose, Downs & Thompson, of Hull, and shown in the subjoined illustrations.

The arrangement of the mill is shown in plan in Fig. 33 and in elevation in Fig. 34. The seed or other material passes through the following course:--It runs from an upper floor through the roll frame A, by which it is crushed three or four times; it is then taken by the elevators B to the kettle C, where it is heated and damped. From beneath the kettle, it is drawn, in quantities sufficient to make a cake, by a box which conveys it to the moulding machine E. Here it undergoes preliminary compression, the objects of which are (1) to increase the number of cakes which may be inserted in the presses at one time, enabling 18 12-lb. cakes to be made where 4 8-lb. cakes were formerly made, and (2) to ensure uniform size and weight, and uniform density or consistence throughout.

The cakes are removed from the moulding machine, and put into the press F, 3 or 4 of which are required to each moulding machine. The pressure is applied either by means of hydraulic pumps, or by a high and low pressure accumulator; but unless extreme care is used with the latter, it gives too rapid a pressure, squirting out the seed at the side of the plates, and exercising a destructive effect upon the cloth employed. The pulsation caused by the pumps working directly to the press cylinder is more akin to the action of a wedge, and seems to extract the oil better than the dead pressure given by the accumulator. If the latter is used, a small cylinder may be applied to give the preliminary pressure in the moulding machine, in lieu of a cam. After remaining under pressure about 25 minutes, the cakes are withdrawn, and after being stripped of the cloth, are pared by the machine H, which completes the manufacture of the cakes. The parings fall under a very small pair of edge-running stones J, which automatically discharge them when sufficiently ground, into an elevator conducting to the kettle, where they are worked up with fresh seed. In a mill with 4 presses, 2 men and a boy in the press room can make 6 tons of cake in 11 hours, a rate of production requiring 6 men by the old process. The saving in steam power is about 30 per cent., chiefly due to the absence of the heavy edge runners, which also effects an economy of space. About 2 per cent. more oil is extracted, and the cakes are improved in appearance by not having the structureless texture caused by the trituration of the seed under edge-runners.

Having described the general routine of the process, some details may be added concerning the working of the several machines. The roll-frame, Fig. 35, consists of 4 or 5 chilled-iron rolls, each 3 ft. 6 in. long by 16 in. in diameter, placed one above the other. These rolls are used for crushing all the seed that passes through one set of presses, making 5½-6¼ tons linseed-cake per spell of 11 hours. The seed passes into the hopper in the usual manner, and is distributed to the crushing-rolls by a fluted feed roll the same length as the crushing-rolls, placed at the bottom of the hopper. When the seed passes the feed roll, it falls on a guide-plate that carries it between the 1st and 2nd roll. After passing between these rolls and being partly crushed, it falls on a guide-plate on the other side, which carries it back between the 2nd and 3rd rolls, where it is crushed more fully. It then falls on another guide-plate, which carries it between the 3rd and 4th rolls, where it is ground more fully. Then it falls on a 4th guide-plate, and is conveyed between the 4th and 5th rolls to receive the finishing touch. It is thus crushed four times.

The kettle is shown in Fig. 36, which represents one capable of heating sufficient seed to keep four 16-plate presses occupied, or to make 6 tons of cake per 11 hours. It is steam jacketed and furnished inside with a damping apparatus. The inside diameter is 5 ft., and the depth 2 ft. 6 in. The seed introduced is kept in motion by the stirring gear, and when sufficiently heated and damped, is withdrawn by the box A in quantities to form one cake, and transferred at once to the moulding machine, attached or separate.

This machine is illustrated in Figs. 37, 38. Its purpose is to measure the quantity of seed required to make each cake, to shape it as required, and to press it so much, without extracting any oil, as will enable the greatest number of cakes to be put into the press. The measure of seed is placed on a strip of woollen cloth, spread upon a thin iron tray, sliding on the guides B; the bottomless hinged mould C, having the exact shape of the intended cake, is closed upon it, and the measure A (Fig. 36), which is also bottomless, is drawn over guides in the upper surface of the mould C, thus accurately distributing the seed. The mould is next thrown upon its hinge (Fig. 37), and the ends of the strip of cloth are folded over the seed, the thickness of which is about 3½ in. The thin iron tray, with the mould of seed upon it, is then pushed along the guides B, beneath the die D. This action gives motion to a cam, shown above in the illustrations, but which may be placed beneath if necessary. This cam brings down the die and compresses the mould of seed to a thickness of 1¼ in.; its revolutions are so timed that the seed is under pressure long enough (about one-third of a minute) to let the workman have another cake ready.

When the die of the moulding-machine rises, the cake and tray are removed and placed in the press (Fig. 39), the tray being withdrawn. The plates of the press are slightly thickened towards the edges, and bear the name of the

manufacturer in reverse. The press is suitable for extracting oil from linseed, rape-seed, cotton-seed, hemp-seed, niger-seed, sunflower-seed, gingelly-seed, castor-seed, ground-nuts, coco-nuts, olives, &c. It is made in various sizes. The No. 1

double press (not shown) is furnished with 4 cake boxes, suitable for making 4 tapered cakes at one pressing, each about 2 ft. 5 in. long, by 10½ in. wide at one end, and 7½ in. at the other, when using linseed, 48 lb. of Bombay seed being required to charge the press, and giving a cake weighing about 8 lb.; the maximum and minimum weights of its charges are 60 lb. and 40 lb., of the cakes, 13 lb. and 6½ lb. The charges vary from 3 to 6 an hour, being 4 for cotton-seed and 5 for linseed; most other seeds are worked the same as linseed, but rape and gingelly are worked twice. By using 2 presses for the first time and 3 for the second, 3 presses will crush as much seed as 5. These presses are made of a capacity to take 270-320 lb. of seed at a charge, giving cakes of 9-15 lb., and requiring 30-45 minutes for the operation. In all these presses, the hair wrappers, weighing some 26 lb., used in the old process, are dispensed with.

A very complete account of oils and fats will be found in Spon’s ‘Encyclopædia of the Industrial Arts,’ to which the reader is referred for further information.

DRYERS.--The maximum of drying power in oils is obtained by the addition of certain metallic oxides, which not only part with some of their own oxygen to the oil, but also act as carriers between the atmospheric oxygen and the heated liquid. This heating of the oil with oxides is known as boiling, although the liquid is not volatilised without decomposition, as is the case with water. At about 500° F., bubbles begin to rise in the oil, producing acrid, white fumes on coming into contact with the air. The gas thus given off consists chiefly of vapour of acrolein mingled with carbonic oxide. There is no advantage in heating the oil to a higher temperature than 350° F. Accurate experiments have shown that the drying properties of the oil are not increased by heating it beyond this point, while its colour is considerably darkened.

For the finer qualities of boiled oils, it is essential that the raw oil should have been stored for some time, so that it may be free from mucilage. This mucilage is the chief source of the dark colour of some boiled oils; when heated, it forms a brown substance, which is soluble in the oil itself, and extremely difficult to remove.

The oxides usually added to the oil during boiling are litharge or red-lead, the former being preferred on account of its lower price. About 2-5 per cent. by weight of the oxides or dryers is gradually stirred into the oil after it has been slowly raised to a temperature of about 300° F. The stirring should be continued until the litharge is dissolved, or it would cake on the bottom of the pan, and cause the oil to burn. Litharge may even be reduced to a cake of metallic lead when the fire is brisk. Some pans are furnished with stirrers and gearing by which the latter can be worked, either by hand or steam. The material of which the pans are made is either wrought or cast iron. Copper pans are sometimes used with the object of improving the colour of the oil.

Little is known respecting the chemical reactions which take place during the boiling of oil. Even when the air is excluded during the process, the drying properties are greatly increased, and, if boiled long enough, the oil is converted into a solid substance. The loss of weight which ensues is dependent upon the temperature and the time during which the operation continues. It is less when the air is freely admitted than if the pan is covered with a hood. The vapours given off by the oil are of an extremely irritating character, and should be destroyed by passing them through a furnace. As their mixture with air in certain proportions is explosive, this furnace should be situated at some distance, and the gases be conducted into it by means of an earthenware pipe.

Since zinc oxide has been introduced as a substitute for white lead in painting, researches have been made to replace litharge as a dryer, because it is not logical to discard the lead pigment and then use a lead dryer with a zinc pigment.

Several metallic oxides and salts, especially zinc sulphate, manganese oxide, and umber, have the property of combining with oils, which they render drying. To these may be added the protoxides of the metals of the third class, i. e. iron, cobalt, and tin. But these oxides are very unstable and difficult of preparation; hence it became desirable to discover some means by which they might be combined with bodies which would enable them to be prepared cheaply, and at the same time leave unimpaired their desiccating powers. Moreover, it is acknowledged that dryers in the dry state are preferable in many respects to drying oils. Following are some of the recently introduced dryers:--

_Cobalt and Manganese Benzoates._--Benzoic acid is dissolved in boiling water, the liquid being continually stirred, and neutralised with cobalt carbonate until effervescence ceases. Excess of carbonate is removed by filtration, and the liquor is evaporated to dryness. The salt thus prepared is an amorphous, hard, brownish material, which may be powdered like rosin, and kept in the pulverulent state in any climate, simply folded in paper. Painting executed with a paint composed of 3 parts of this dryer with 1000 of oil and 1200 of zinc-white, dries in 18 to 20 hours. Manganese benzoate is prepared in the same way, substituting manganese carbonate for that of cobalt. Applied under similar circumstances, it dries a little more rapidly, and a little less is required. Urobenzoic (hippuric) acid is equally efficacious.

_Cobalt and Manganese Borates._--These salts also, in the same proportions, are found to be of equal efficacy. The latter is extremely active, and requires to be used in much smaller proportions.

_Resinates._--If an alkaline resinate of potash or soda be dissolved in hot water, and this solution be precipitated by a solution of a proportionate quantity of a cobalt or manganese chloride or sulphate, an amorphous resinate is formed, which, after being collected on cloth filters, washed, and dried, forms an excellent drier.

_Zumatic (Transparent) Dryer._--Take zinc carbonate, 90 lb.; manganese borate, 10 lb.; linseed-oil, 90 lb. Grind thoroughly, and keep in bladders or tin tubes; the latter are preferable.

_Zumatic (Opaque) Dryer._--Manganese borate, as a dryer, is so energetic that it is proper to reduce its action in the following way:--Take zinc-white, 25 lb.; manganese borate, 1 lb. Mix thoroughly, first by hand, then in a revolving drum; 1 lb. of this mixed with 20 lb. paint ensures rapid drying.

_Manganese Oxide._--Purified linseed-oil is boiled for 6 or 8 hours, and to every 100 lb. boiled oil are added 5 lb. of powdered manganese peroxide, which may be kept suspended in a bag, like litharge. The liquid is boiled and stirred for 5 or 6 hours more, and then cooled and filtered. This drying oil is employed in the proportion of 5 to 10 per cent. of the zinc white.

_Guynemer’s._--Take pure manganese sulphate, 1 part; manganese acetate, 1 part; calcined zinc sulphate, 1 part; white zinc oxide, 97 parts. Grind the sulphates and acetate to impalpable powder, sift through a metallic sieve. Dust 3 parts of this powder over 97 of zinc oxide, spread out over a slab or board, thoroughly mix, and grind. The resulting white powder, mixed in the proportion of ½ or 1 per cent. with zinc-white, will enormously increase the drying property of this body, which will become dry in 10 or 12 hours.

_Manganese Oxalate._--A writer in the _Moniteur de Produits Chimiques_ draws attention to the properties possessed by manganese oxalate as a drier. This salt has hitherto not had any important industrial uses, but it can be readily prepared in a state of purity from the native carbonate by the action of oxalic acid; the author is of the opinion that it will be found of use for this purpose. If prepared from carbonate free from iron and lime, it can be obtained as a fine crystalline white powder, and two-fifths per cent. suffices to bring about the change. The oxalate is resolved by heat into manganese oxide, carbonic acid and carbon monoxide, and in the presence of fatty acids the manganese oxide formed combines with them, the decomposition taking place at about 130°. The operation is carried out by mixing in a mortar the oxalate with two or three times its weight of oil, and then adding the mixture to the main portion of the oil. The heat should be applied gradually, and the decomposition is known to be complete when there is no further evolution of gas. The boiled oil, under this treatment, preserves its limpidity and also remains colourless. Manganese oxalate has the advantage over oxide of lead, which is commonly employed for this purpose, in causing the oil to remain transparent when exposed to sulphur vapours. Manganese acetate has also been used, but it likewise causes a darkening in the colour of the oil, and the nitrate is dangerous owing to the possible action of nitric acid on the fats present in the oil. Manganese borate appears to be next in value to the oxalate as an oil drier.

In a paper recently read before the Society of Arts, Prof. Hartley remarked that paint, such as is used for ordinary purposes, is essentially composed of three materials, without taking into account the coloured pigments.

(1) White lead, or sublimed zinc-white.

(2) An oil, generally linseed or poppy oil, which is ground up with the white lead or zinc-white until it becomes a soft paste. This is mixed with variable preparations of linseed oil and spirit of turpentine.

(3) A substance called dryers, or siccative materials; it may be linseed oil in which litharge is dissolved, or it may be linseed oil containing a compound of manganese.

Paint owes to the dryers its property of drying more rapidly than it would do without it; and it is considered indispensable in buildings in all cases where paint applied to wood, stone, or metal, would not be quite dry in 48 hours, or at most in 72 hours, after the first application.

The first question which requires an answer is, what chemical process takes place when a paint dries.

BOILED OIL.--Linseed oil absorbs oxygen; and, when the oil contains manganese, it absorbs oxygen much more greedily; and when a manganese oil--that is to say, a boiled oil containing manganese--is mixed with linseed oil, the substance absorbs oxygen, from a limited supply of air contained in a closed space, until no trace of any other gas but nitrogen remains. The power of absorbing oxygen possessed by 100 volumes of linseed oil, compared with that of 100 volumes of a mixture of linseed oil and so-called manganese oil, is as 9·4 to 100. This may be termed the measure of its drying power. A mixture of linseed oil, with a little more than one-fourth of its volume of manganese oil, has a power of absorbing oxygen four and a half times greater than either of the components of the mixture taken separately. In this case Chevreul argues that linseed oil may be considered as a “dryer” to manganese oil.

Linseed oil, without any addition whatever, if boiled for three hours, becomes a better drying oil than it was previous to the action of heat.

Oil boiled with 10 per cent, of litharge for three hours, is a much better dryer than when heated without this oxide.

Oil boiled alone for five hours is an inferior drying oil to one heated for only three hours.

Oil previously boiled alone for five hours, and boiled alone again for three hours, is scarcely altered in drying power, but it becomes a better drying oil if it is boiled the second time with litharge. It is inferior to a drying oil which has been boiled only three hours with litharge, without being submitted to a previous boiling.

Oil boiled alone for five hours, boiled for a further period of three hours with manganese dioxide which has already been used for one operation, is very nearly as strong a dryer as that which has been boiled with litharge under the same conditions; but it is superior to an oil which has been boiled with manganese dioxide for eight hours. This no doubt arises from the longer boiling with manganese having caused a larger quantity of manganese to dissolve, and that the quantity dissolved is in excess of that which yields the best result.

Finally, oil boiled for five hours, and then boiled alone once more for eight hours, becomes viscous, and the first coat requires a considerable time to dry. We thus see that the oxides of lead and of manganese in certain proportions concur with heat in increasing the drying power of linseed oil. This drying of oils is a process of slow oxidation.

The following points of Chevreul’s appeared to be difficult of satisfactory explanation, and suggested to Prof. Hartley an examination _de novo_ of the facts, as well as an investigation of the chemistry of the subject generally:--

1. Linseed oil not boiled acted as a dryer to the same oil boiled with manganese dioxide.

2. Linseed oil, boiled with either litharge or manganese dried more rapidly when mixed with turpentine.

3. Oil, mixed with white lead, zinc white, antimony white, and arseniate of tin, acts differently, thus:--The white lead dries most rapidly, the zinc white next, but antimony white and arseniate of tin are incapable of acting as dryers, in fact, they retard the drying process.

4. Oil boiled alone for five hours, and boiled for a further period of three hours with manganese dioxide becomes a superior drying oil to one which has been boiled with manganese dioxide for eight hours.

From a series of experiments, which were continued for two years, on twenty-five weighed quantities of raw linseed oil, Prof. Hartley draws the following deductions:--

1. The chemical action of a manganese compound, 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.

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

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

4. Turpentine does not act as a dryer, that is, as a carrier of oxygen to linseed oil.

5. Different white pigments behave differently when drying, because the more powerfully basic the properties of the pigment, the more powerful is its action as a dryer. 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 dryer.

Different substances, that is to say, those without chemical action on oil, such as lamp-black, sulphate of baryta, and sulphate of lead, cannot act as “dryers.”

Linseed oil is a glyceride of a peculiar acid, called linoleic acid. Whatever the exact constitution of linoleic acid may be, linseed oil for the most part is composed of trilinolein. Raw linseed oil contains the following constituents:--

1. Glyceride of linoleic acid or trilinolein

{C_{18}H_{31}O}O_{3}. { C_{3}H_{5} }

2. Water.

3. Mucilage, with the composition _n_(C_{6}H_{10}O_{5}). On boiling with dilute acids this yields a gum and a sugar.

4. An essential oil, present in minute proportions, and of unknown composition.

5. A mixture of colouring matters of intense tinctorial power, viz. blue and yellow chlorophylls and erythrophyll.

The only useful and desirable substance is the trilinolein.

The effect of oxidation upon linseed oil is to destroy all the glycerine, and to produce therefrom carbonic, formic, and acetic acids, together with some acrolein. When boiled at a high temperature without the addition of any metallic oxide, the glyceride is decomposed, acrolein is formed, and linoleic acid is set free. In fact, whether oil is oxidised by air or by metallic oxides, or whether it be simply heated, the action in each case first leads to the destruction of the glycerine and the liberation of linoleic acid. But linoleic acid very readily absorbs oxygen, and the oxidised substance becomes a tough elastic solid, which is essentially a varnish.

In fact, the process which an oil undergoes in drying is not desiccation, or depriving it of moisture or of glycerine, but solidification, and the technical term “drying” is a misnomer. That, however, is of little consequence if we really know what is the chemical action of the “drying” process. When oxidised even at a low temperature, the glycerine is destroyed, and the oxidised products form a tough varnish.

There are various methods of converting linseed oil into a drying oil or varnish:--

1. Heating it to a high temperature with litharge.

2. Heating with red oxide of lead.

3. Heating with metallic lead.

4. Heating to a high temperature with manganic oxide.

5. Heating with manganese borate.

6. Heating with manganese oxalate.

7. By the joint action of air and heat upon the oil and manganous oxide, or a solution of manganese dioxide or manganous oxide in the oil.

In the processes 1, 2, 3, there can be no doubt that a lead of linoleic acid is produced, and that this facilitates further oxidation in air, by forming salts with some of the acid products of such oxidation, while the oxidation of the linoleic acid continues. Heating with red lead favours oxidation, by the compound itself conveying oxygen to the oil. In the case of metallic lead, it must be noted that the metal is dissolved. Under certain circumstances, metals become dryers to oils; thus sheets of metallic lead are capable of acting as dryers to linseed oil.

Linseed oil is pre-eminent in its capacity for absorbing oxygen. This action of metallic lead as a dryer is due to the metal becoming oxidised at the expense of the glycerine of the oil, and so passing into solution by combining with the linoleic acid, or with acetic or formic acid, caused by the oxidation of the glycerine. It is the destruction of the glycerine with concurrent oxidation of the fatty acid which causes the drying or hardening of the oil.

When a drying oil which has been treated with metallic lead, or with litharge, is shaken up with a solution of zinc sulphate, all the lead is precipitated from the oil, and zinc passes into solution therein. By manganese sulphate or copper sulphate, the lead is removed by manganese or copper. Oil charged with lead dries in 24 hours when spread out in a thin layer on glass; it will dry completely in 5 or 6 hours if charged with manganese, in 30 or 36 hours with copper, zinc, or cobalt; and it requires more than 48 hours with nickel, iron, chromium, &c.

Although solidification of a drying oil charged with manganese takes place in from 5 to 6 hours when spread in thin films, the solidification of thicker films requires a longer time. A temperature of 122° to 140° F. accelerates the oxidation of the drying oils, partly because the oil becomes more fluid, and partly because the oxygen is more active at a higher temperature. Hence, oil which has been mixed with an equal volume of turpentine, or a light hydrocarbon, such as benzene, dries more rapidly than oil without such admixture.

When a boiled oil, prepared with manganese, is dissolved in an equal volume of benzene, and shaken up with air in a bottle, rapid absorption of oxygen occurs, especially about 120° F. If fresh air is repeatedly provided, the oxidation is sufficient to cause the liquid to become thick, and, on distilling off the sapient, a perfectly dry and elastic solid remains.

An oil containing manganese is a very superior drying oil to one which has been prepared with lead. This fact, however, is to be noted, that though a large proportion of manganese in an oil may hasten its drying, yet it is disadvantageous, because it does not form so tough a film. This arises from the film becoming hard upon the surface, and so protecting the oil underneath from absorbing oxygen from the air.

Though the oils containing large quantities of dryers dry, they afterwards lose weight, and become viscous under the same conditions.

Pure linoleates of lead and of zinc are not dryers; but if heated until it has turned brown, or begun to blacken, a lead dryer becomes effective, although it contains less of the lead compound.

In this case, some compound of lead is formed by absorption of oxygen, which either itself actually oxidises or causes the oxidation of ordinary linseed oil.

Having treated of the materials used for producing boiled oil, and of their action upon the oil, let us now consider how the operation is brought about.

_Process 1._--Oil is boiled at a high temperature, that is to say, it is heated until frothing and bubbles of gas escape, when litharge or a manganese compound is added.

_Process 2._--Oil is boiled at a steam heat, with litharge or a manganese compound, in conjunction with a blast of air.

_Process 1._--The chemical action in the first process is doubtless one which takes place in three stages. It commences by depriving the oil of water; in the second stage, it destroys the mucilage, by charring it; in the third, it destroys, in part, the glycerine, and sets free the fatty acids. After the litharge or manganese compound is added, there is formed in the oil a solution of lead salts of the fatty acids, or a manganese salt of the fatty acids.

The oil then, at the high temperature, loses glycerine by oxidation caused by the air, such oxidation being greatly facilitated by the presence of manganese compounds, which are repeatedly oxidised by the air and reduced by the oil, that is to say, they absorb oxygen and pass it over to the oil with great facility.

It matters little, so long as the ultimate action is oxidation, what salt of manganese or what oxide is used, if it be capable of undergoing processes of an alternate character called oxidations and reductions.

It is, however, certain that some manganese compounds are more suitable than others, owing to their more or less complete solubility in the oil, and their more readily undergoing the two different processes of oxidation and reduction in presence of air and of oil.

_Process 2._--The credit of being the first to boil oil without resorting to the dangerous expedient of using an open fire and a high, temperature in the manufacture, is due to Vincent. He used manganese compounds, or both manganese salts and litharge. His method of boiling oil for the manufacture of printing inks is, with some modifications in technical details, carried out on a large scale at the present time in the preparation of ordinary boiled oil. The essential parts of the plant are a steam-jacketed close boiler with agitating gear, and a pipe for conducting a current of air into the oil by means of a blowing engine. From the head of the boiler there passes a funnel under the back of the furnace fire, by which the disagreeable products of the chemical action are conducted to a place where they are destroyed. These products, as already mentioned, are volatile fatty acids and acrolein.

Oil boiling, as ordinarily carried out, is conducted by means of litharge along with compounds of manganese; in some processes these are mixed with salts of alumina and zinc. The oil so produced is brown and not clear, but it is clarified by keeping. Many samples of such boiled oil deposit insoluble matter when stored for some time, even although they may have become clear previously. This is not a desirable property. Sometimes rosin is added to hasten its drying.

The defects to be noticed, even in the best samples of boiled oil, are the following:--

1. The oil causes a brownish or yellow colour to be communicated to white lead or zinc white.

2. The oil darkens pigments containing brilliantly coloured metallic sulphides, such as vermilion, cadmium yellow, and ultramarine blue.

3. Delicate colours are darkened by the oil when exposed to ordinary town air, that is to say, air which, is not quite pure. This is the case even when the oils themselves may not injure the paints.

The causes of such alterations is, in nine cases out of ten, the use of lead dryers.

1. In the first place, boiled oil which contains litharge or other lead compounds takes a permanent brown colour, which affects the purity of white lead, zinc white, and delicate pale tints.

2. Lead forms, with extreme ease, lead sulphide, which, in very minute proportions, is yellow or brown; in larger quantity its colour is black. The lead sulphide is readily formed by contact with other sulphides, as, for instance, vermilion, cadmium yellow, and ultramarine.

3. Boiled oil, containing lead, is coloured brown by exposure to air, owing to the presence of minute quantities of sulphuretted hydrogen, which causes the formation of lead sulphide.

The remedy is obvious: no oil should be used which has been boiled with dryers containing lead. In other words, oil should be boiled with pure manganese compounds only.

In cases where it is desirable to have information of the presence or absence of lead in a boiled oil, the following test will be found most useful:--A mixture is made of 4 oz. of glycerine with 1 oz. of ammonium sulphide, the liquid being kept in a stoppered bottle. Or glycerine is mixed with an equal volume of water, and saturated with sulphuretted hydrogen. Half an ounce of the oil to be tested is placed in a white basin, with the addition of two or three drops of the glycerine solution. The two liquids are thoroughly incorporated, by stirring with a strip of glass. A brown or black colour, which gradually appears, indicates the presence of lead. A pure manganese oil simply becomes slightly yellow. It is true that, if iron is present, a black colour might appear, but iron is also an undesirable impurity. Should it be required to ascertain that the coloration is or is not caused by iron, two or three drops of glacial acetic acid may be stirred into the oil, when, if the black colour remains, it is certainly not caused by iron.

Under the old process of oil-boiling at a high temperature, the brown colour of the oil was, to some extent, an indication that the oil had been sufficiently heated--that is to say, properly boiled; but in the modern processes, so largely used, in which oxidation is aided by a blast of air, this coloration is no indication whatever of the excellence of the oil; it may be, in fact, the very reverse.

This fact appears to be unknown, or, at any rate, is not a matter of common knowledge among practical men in this country, who, being uninformed as to the methods of preparing the oils, consider that a brown colour is desirable, if not essential.

When oil-boilers were compelled to adopt some expedient to give a reddish-brown colour to the oil, they added a small amount of litharge, the introduction of which actually spoils the oil, and makes it unsuitable for many purposes to which it is otherwise applicable. Of late years, pale boiled oils have been more largely manufactured for special purposes. It is obvious that, for decorative house painting, in which delicate tints are a leading feature, they may be advantageously employed.

Notwithstanding that some of the brown oils, when mixed with white lead, do not entirely retain the brownish tint, but, to some extent, lose it upon drying, yet they never preserve the whiteness of white lead. It follows, therefore, that a pale colour in the oil, provided it is not the yellow colour of raw oil, is greatly to be preferred. Moreover, when paints are mixed with zinc white, no trace of lead should be contained in the oil, otherwise, one of the valuable properties of zinc white pigments is destroyed, namely, its power to retain its whiteness in the atmosphere of a town, because its colour is not affected by sulphuretted hydrogen.

Very generally, zinc white and white lead paints are not mixed with drying oils, but with refined linseed or bleached oil. This, at any rate, is the practice on the Continent. That is to say, the pigments are mixed with an oil from which the impurities, and the natural yellow and red colouring matter, have been removed, so that the colour of the paint is white. If ordinary oil be used, the paint is more or less yellow. In order to render such paint quick drying, a certain amount of dryers, in a solid or liquid form, is added. These dryers almost invariably contain lead, so that zinc white paint is contaminated by lead in another way, which may not be suspected, or which is overlooked.

Now as to the chemical action of dryers on oils. Raw oil contains water and mucilage; the former can be absorbed by anhydrous zinc salts and by dried alum, and solutions of the salts and the salts themselves are capable of precipitating mucilage from the oil; hence these substances cause the impurities to become insoluble, so that they are carried down as “foots.” Heat greatly facilitates this action, particularly by causing the oil to become more fluid; and by the action of the anhydrous salts water is withdrawn from the oil. On the “drying” or oxidation of the oil, they exert no chemical action whatever.

Zinc linoleate and lead linoleate do not act as dryers when simply added to the oil. Though the former is soluble in hot oil it is insoluble in cold oil, and it therefore separates from the oil as it cools. The latter is very soluble in linseed oil, but only adds to its drying power when heated therewith.

In conjunction with a high temperature, lead dissolves in oil at the expense of the glycerine, which is decomposed into acrolein, while lead linoleate is formed.

When litharge is heated with linseed oil, the action is somewhat similar, the substances formed being acrolein, lead linoleate, and linoleic acid.

If we consider the action of red lead on trilinolein, we have not only the formation of these lead linoleates, but an excess of oxygen available for the oxidation of glycerine to acrolein and acrylic acid, or to acetic and formic acids.

These equations serve to show the effect of lead and lead oxides in what may be termed the initiation of the chemical action upon the oil. Subsequent changes, no doubt, depend upon the conditions which obtain at the time, notably upon the temperature and upon access of air to the oil. It is probable that acid linoleates are formed, and that compounds formed from the polymerisation of linoleic acid result eventually.

Whatever doubt there may be as to the action of lead salts, there can be none whatever as to that of manganese compounds. In the first place, manganous oxide is a powerful base, which readily dissolves in oil; manganic oxide is also readily soluble, yielding fatty acid salts of manganese, and causing oxidation of glycerine. Manganese borate and manganese oxalate are both soluble in oil, the former much more readily than the latter, but they are both salts of little stability at high temperatures in contact with oils. They both dissolve by the aid of heat, forming fatty acid salts of manganese. Borate liberates boric acid under these circumstances, but oxalate yields a mixture of carbon monoxide and carbon dioxide.

Of manganese oleate and linoleate nothing more may be said than that both are extremely soluble in oil, and both easily oxidised from colourless to brown compounds when submitted to the action of air.

The chief adulterants of linseed oil and of boiled oil are cotton-seed oil, rosin oil, and linoleic acid. Cotton-seed, which is to some extent a drying oil, can act as such when mixed with linseed, but when added to olive oil, it behaves as a non-drying oil. In fact, its behaviour is anomalous, and of such a character that it greatly facilitates its extensive use as an adulterating material for the more expensive oils.

Rosin oil is a deleterious adulterant, but one which may be more readily detected than cotton-seed oil. Rosin is added to boiled oil to hasten its drying; this also is an injurious substance. Of late years glycerine has become an article of greater value than formerly, and this may account for the manufacture of linoleic acid and its use as an adulterant of oleic acid and of linseed oil.

Lastly, it may be mentioned that certain samples of “pale boiled oil” have been found to contain what is practically a raw oil mixed with dryers. Although such oils will dry, their efficiency is nothing like so great as that of an oil “boiled” with a blast of air at a suitable temperature, and, moreover, such oils are deficient in body.

In bleaching vegetable oils, it is necessary to consider the nature of the colouring matters naturally contained in them. These consist of a mixture in varying proportions of the colouring matters known to exist in the leaves of plants, but which, in the case of oils, are derived from the fruit or seeds from which the oils are expressed. There can be no doubt that these substances are closely allied in chemical constitution; they all possess an intensely powerful colouring property, by which is meant that though the colour of some of them may not be dark, yet a very minute weight is capable of imparting a tint to a very large quantity of material.

The names of these substances are:--

Xanthophyll--yellow. Yellow chlorophyll--yellow. Blue chlorophyll--blue. Erythrophyll--red.

In some oils only the xanthophyll and yellow chlorophyll are present; in others, such as olive oil, the yellow and blue chlorophylls occur, and give the liquid a green tint; while in linseed erythrophyll is always present with more or less of the yellow and blue chlorophylls, and some xanthophyll. According to the different proportions of these colouring matters the oil varies in colour. For instance, linseed oil, when brown, contains a mixture of erythrophyll with yellow and blue chlorophylls; when greenish brown, the yellow and blue chlorophylls are present in somewhat larger proportion, but mixed with erythrophyll; while, generally speaking, a bright yellow or pale yellow oil contains xanthophyll only. These substances appear to be combined with the oils, or to be substances of a fatty nature. They are neither dissolved nor acted upon by water, nor by acids diluted with water, when naturally contained in the oils. They are freely soluble in alcohol, and an alcoholic solution is not only susceptible of being destroyed by the joint action of air and water, but by very dilute aqueous solutions of mineral acids, and by acetic acid. In aqueous and alcoholic solutions, light speedily modifies the blue, and eventually destroys all these colours. A solution in turpentine of the isolated colouring matters is also easily destroyed. But, on the other hand, a solution of the colours in melted paraffin wax is comparatively stable.

Zinc hydroxide, copper hydroxide, baryta, potash, and soda easily combine to form metallic salts with blue chlorophyll, less readily, though readily enough with yellow chlorophyll, but far less readily with xanthophyll and erythrophyll. The following facts will serve to show that this is the case. When a solution of the colouring matters contained in green leaves is made by extracting dry, but freshly-gathered, leaves with absolute alcohol, an addition of a saturated solution of baryta water, to the intensely green extract, precipitates first the compound of blue chlorophyll with baryta, then a further addition precipitates the yellow chlorophyll, also as a baryta salt; but xanthophyll and erythrophyll either remain in solution, or require a much larger addition of the base in order to be precipitated. A crystalline compound of blue chlorophyll with soda is comparatively stable. This substance is, no doubt, formed in green vegetables when they are boiled in water to which some carbonate of soda has been added to maintain their fresh appearance. The addition of a small trace of copper sulphate to peas and to pickles forms a very permanent copper compound with the colouring matter, which gives an attractive appearance to these articles. Such being an outline of the chief chemical properties of the natural colouring matters contained in oils, the facts mentioned will serve to render the processes for removing the colour from oils more intelligible than they otherwise would be.

Vegetable oils are decolorised, either partially or completely, by the application of one of the following processes:--

1. By the action of light, or by the joint action of light and air.

2. By acids.

3. By saponification.

4. By the action of chlorine.

1. By exposing raw linseed oil to the action of sunlight, it slowly becomes pale in colour, and finally colourless. It is in the highest degree probable that, as oxygen is absorbed by the oil and acid substances are thereby produced, these acids effect the destruction of the colouring matters. In such wise castor oil is bleached.

2. By treating linseed oil with moderately strong sulphuric acid. As the oil and sulphuric acid are of very different specific gravities, it is essential that they be very rapidly and thoroughly mixed by violent agitation. The impurities, such as mucilage and albuminous matters, are thus deprived of water, and more or less charred, and along with them the colouring matters are destroyed by the acid. It is essential for the success of the process that the oil and the acid be not long in contact without undergoing dilution, otherwise the oil itself may become charred. It is, however, possible to obtain oil by this process in a fairly colourless condition, after it has been thoroughly washed with water and allowed to settle.

3. Both rape oil and cotton oil may be rendered of a pale yellow, and even almost colourless, by a process of partial saponification with caustic alkali of a suitable strength. The colouring matters are saponified, and the resulting soap is of a dark yellow or brown colour, from the colouring matter having combined with the alkali.

4. By the action of chlorine produced in contact with the oil when, for instance, an aqueous solution of bleaching powder is acidified with a cheap mineral acid, such as dilute sulphuric. In this case rapid mixing and violent agitation are essential to the success of the process, otherwise chlorinised products are retained in the oil, which not only confer upon it a distinct flavour and odour, but also cause the oil to solidify with a very moderate lowering of the normal temperature. It is very questionable whether drying oils can with advantage be submitted to such treatment.

5. A variety of methods may be merely mentioned, such as treatment with sulphurous acid, with ferrous sulphate (green vitriol), and potassium dichromate and sulphuric acid.

6. Lastly, the method of Binks, to which reference will be made farther on.

Prof. Hartley next gives an account of certain improvements in the process of oil-boiling, designed with the object of producing a drying oil absolutely free from lead, and, as compared with ordinary oils, absolutely free from colour.

The operations have been carried out, on a manufacturing scale, by Mr. W. E. B. Blenkinsop and himself, and there is no doubt of the practicability of the process.

The process consists in, first, refining the oil, by the removal therefrom of water and mucilage; second, boiling and bleaching the oil at one operation.

It is a fact that water and mucilage can be removed from linseed oil by the action of certain dehydrating substances and solutions of metallic salts, as, for instance, by alum, by strong sulphuric acid, and by a solution of zinc chloride.

There are certain objections to each of these methods, which are of a practical nature: thus, in treating the oil with strong sulphuric acid, there is too frequently a charring of something, either the oil itself, or of some impurity therein, and this charring, though it may be very slight, has the effect of giving a pale brownish tinge to the oil, which cannot be completely removed by the bleaching process to which the natural colouring matters in the oil are amenable. It is quite true that this brown colour separates sometimes, but it is only after storage for a long period, when a finely divided flocculent matter separates by subsidence. Treatment with zinc chloride is satisfactory but expensive. Perfectly pure manganese sulphate, which is a neutral salt, has been used by Hartley and Blenkinsop in very strong solution, and where there is an objection to using an acid. For ordinary purposes, perfectly satisfactory results are obtained by the use of a dilute sulphuric acid containing about 30 per cent. of H_{2}SO_{4}, since, though it possesses the power of withdrawing water from the oil, it may remain in contact therewith without causing any charring, and at the same time it causes the precipitation in a complete and rapid manner of all the mucilage. A purified linseed oil is thus produced which is bright, clear, and slightly yellowish in colour, though somewhat paler than the ordinary oil. It is important that the strength of the oil should not exceed that degree of concentration which is sufficient for the purpose for which it is intended. The oil having been so treated, and the impurities separated by subsidence or otherwise, it is next submitted to the bleaching and oxidising treatment.

Binks bleached oils with oxides of manganese dissolved in the oil, but difficulty was experienced in carefully regulating the quantity of the manganese compounds which were to be introduced into the oil. For instance, he precipitated manganous hydroxide in contact with oil, and added the mixture to the bulk of the material, and he also modified the treatment by dissolving manganous hydroxide in ammonia, and added the solution to the oil.

Hartley and Blenkinsop prepare manganese linoleate, and dissolve this in a hydrocarbon, and add a sufficient quantity of the solution to the oil, whereby it dissolves easily and mixes completely. By this treatment, the colouring matter of the oil forms a compound with the manganese which, while it remains in solution, is very speedily oxidised in contact with air, especially when a current of air or oxygen is blown through. The oxidation destroys the colouring matter, and the manganese compound is deoxidised; subsequently it undergoes oxidation again, and the products of such oxidation taking place in the oil are acrolein, formic and acetic acids. After, or concurrently with, the oxidation of the colouring matters, the oil is oxidised, and, at a suitable temperature below 132° F., the oil is bleached, increased in density, and converted into a pale drying oil. By limiting the amount of the manganese linoleate to that which is capable of just oxidising the colouring matters, oils may be bleached with very little further oxidation.

Excellent drying oils have been produced by this process, of a very pale colour. The oil has been used for decorative house painting, for both indoor and outdoor work, on wood and on metal. It has also been used as a coating for iron work, without the addition of a pigment. The plant used in its production is the same as that employed in oil-boiling by the usual processes when a blast of air is used.

The advantages of a pale boiled oil, containing no lead, are the following:--

1. Zinc white retains its pure white colour.

2. Delicate tints, and colours containing sulphides, are not darkened in course of time.

It may be suggested that for indoor decoration, for the painting of ships, railway carriages, railway semaphores, signs, and stations, such oil is free from liability to alter the colours with which it is mixed, owing to its freedom from lead, which is darkened by traces of sulphuretted hydrogen in the air to which such paints are exposed.

Gasometers in gas-works may be painted an unalterable white with such oil and zinc white. But in this case also the zinc white must be free from lead carbonate or oxide.

In commenting on Prof. Hartley’s paper, Mr. Laurie said he had never used linoleate of manganese for boiling with oil, but by the use of borate one did get a boiled oil paler than the oil with which one started. If you take linseed oil which has been already bleached in the sun to a golden yellow, and convert it into boiled oil with manganese, a further bleaching process undoubtedly takes place. An oil prepared with manganese salts, spread on a glass plate, and allowed to dry in the dark, will remain almost colourless, whereas if it were boiled with a lead salt it quickly darkens, even if it is kept away from impure air. Even in a dark room, in pure air, a picture painted with oil boiled with lead will darken. That is another argument in favour of manganese, and he should say it ought always to be used in preparing oil for artistic purposes.