Pigments, Paint and Painting: A practical book for practical men

so that the ore is not over burned. When this happens, the product has a black scoriaceous appearance, and is unfit for the manufacture of metallic paint, as it is extremely hard to grind.

The calcined ore is carried from the kiln in wagons to the mill, where it is broken to the size of grains of corn in a rotating crusher. The broken ore is carried by elevators to the stock-bins at the top of the building, and thence by shutes to the hoppers of the mills, which grind it to the necessary degree of fineness. Elevators again carry it to the packing-machine by a spout, and it is packed into barrels holding 500, 300, or 100 lb. each.

Characters.—Ochres owe their colour to hydrated oxide of iron, besides which body they contain clayey matter (silicate of alumina), earthy matters, barytes, carbonate and sulphate of calcium, &c., dependent upon the locality from whence they are obtained; thus Derbyshire ochres contain mostly calcareous earthy matters, barytes, gypsum, &c., while Oxford ochres and French ochres contain clayey matter; Welsh ochres are variable, and usually contain a good deal of silicious matter.

Crude ochres should first be assayed for actual colouring matter and grit or refuse. This can be done by a kind of levigation method: 200 grains of the crude ochre are crushed in a mortar; the grinding must not be too well done, or otherwise faulty results will be obtained. The crushed ochre is put into a tall conical glass; a long glass funnel passes to the bottom of the glass, and the whole is arranged in a large glass basin or dish. A current of water is now caused to flow down the glass funnel; this washes the fine particles of ochre away from the grit, and they are carried over the sides of the glass into the dish. Here they are allowed to settle, and are collected and weighed after drying, an operation which gives the amount of ochre in the crude material.

Levigated and prepared ochres can be tested for colour and covering power, by the usual methods. These are the most important points about ochres to which attention should be paid.

Orpiment.—Orpiment, king’s yellow, or trisulphide of arsenic, is a lemon or orange-yellow coloured substance, found native in Hungary, the Hartz, and other places. The finest samples used by artists (golden orpiment) come from Persia. The commercial article is artificially prepared for use as a pigment in the following way:—

A mixture of arsenious acid and sulphur is placed in an iron subliming-pot, similar to those used in the preparation of crude white arsenic. The mixture is then heated until the sublimate which immediately forms upon the rings fixed above the pot begins to melt. The proportions of the two ingredients used vary largely, the best colours being probably produced when the mixture contains from one-third to one-fifth of sulphur; for the lighter colours, a smaller proportion of sulphur is employed. Orpiment made in this manner consists of a mechanical mixture of sulphide and oxide of arsenic.

Orpiment is also employed as a dye, in the preparation of fireworks, and in some depilatories. The native sulphide is preferred to the artificial variety by artists and dyers, by reason of its richer colour; but it is a colour which in reality is hardly ever used now. Sometimes it is employed in water-colours, but as a pigment it is worthless. If it comes in contact with white lead it is decomposed in time, and a brown or black sulphide of lead is formed. While it endures it is a very brilliant colour.

Realgar.—Realgar or disulphide of arsenic, is a deep orange-red substance, soluble in water, and highly volatile and poisonous. It is found native in some volcanic districts, especially in the neighbourhood of Naples; but the commercial article is made by distilling, in earthenware retorts, arsenical pyrites, or a mixture of sulphur and arsenic, or of orpiment and sulphur, or of arsenious acid, sulphur, and charcoal, in the proper proportions; it has not the brilliant colour of the native mineral, and is much more poisonous.

On a large scale, the manufacture is carried on in the following way:—The ingredients are mixed together in such proportions that the mixture shall contain 15 per cent. of arsenic, and from 26 to 28 per cent. of sulphur, in order to make allowance for the volatilisation of a portion of the latter substance. The mixture is then placed in a series of earthenware retorts, which are charged every twelve hours with about 60 lb.; this quantity should fill them three parts full. These are then gradually heated to redness for from eight to twelve hours, during which time the realgar distils off, and is collected in earthen receivers, similar to the retorts, but perforated with small holes to permit the escape of these gases. After the operation, the receivers are emptied, and the crude product is remelted. This is performed in cast-iron pots, the contents being well agitated, and the slag carefully removed. The requisite amount of sulphur or arsenic is then added, according to the colour of the mixture, or else a proper quantity of realgar containing an excess of the required constituent, and the mass is again stirred. When, on cooling, it exhibits the correct colour and compactness, it is run off into conical moulds of sheet iron, cooled and broken up; it is sometimes refined by re-sublimation. The chief use of realgar is as a pigment; and in pyrotechny in the preparation of white fires.

As a pigment it possesses the same features and faults as its close ally orpiment.

Siennas.—Another name for ochres, described on p. 272.

CHAPTER IX.

LAKES.

Alumina in a state suitable for the preparation of the pigments known as “lakes” may be produced in the following manner:—Dissolve 1 lb. of alum in ½ gallon of water, and add 75 grains of sulphate of copper, and about ¼ lb. of zinc turnings; leave the mixture for three days in a warm place, renewing the water lost by evaporation. The copper is first deposited upon the zinc, the two metals thus forming a voltaic couple sufficiently strong. Hydrogen is disengaged, sulphate of zinc is formed, and the alumina gradually separates in the state of a very fine powder; the action is allowed to continue until there is no more alumina left in solution, or until ammonia ceases to give a precipitate. If the reaction is prolonged beyond this point, oxide of iron will precipitate if present. The alumina washes easily, and does not contract upon drying.

Brazil-wood Lake.—(a) Digest 1 lb. ground Brazil-wood in 4 gal. water for 24 hours, boil ½ hour, and add 1½ lb. alum dissolved in a little water; mix, decant, strain, add ½ lb. tin solution, again mix well, and filter; to the clear liquid cautiously add a solution of carbonate of soda while a precipitate forms, avoiding excess; collect, wash, and dry. The shade will vary according as the precipitate is collected.

(b) Add washed and recently-precipitated alumina to a strong filtered decoction of Brazil-wood.

Carminated Lake.—(a) The cochineal residue left in making carmine is boiled with repeated portions of water till exhausted; the liquor is mixed with that decanted off the carmine, and at once filtered; some recently-precipitated alumina is added, and the whole is gently heated, and well agitated for a short time. As soon as the alumina has absorbed enough colour, the mixture is allowed to settle, the clear portion is decanted, and the lake is collected on a filter, washed, and dried. The decanted liquor, if still coloured, is treated with fresh alumina till exhausted, and thus a lake of second quality is obtained.

(b) To the coloured liquor obtained from the carmine and cochineal as just stated, a solution of alum is added, the filtered liquor is precipitated with a solution of carbonate of potash, and the lake is collected and treated as before. The colour is brightened by addition of tin solution.

Carmine.—Boil 1 lb. cochineal and 4 dr. carbonate of potash in 7½ gal. water for ¼ hour. Remove from the fire, and stir in 8 dr. powdered alum, and allow to settle for 20 to 30 minutes. Pour the liquid into another vessel, and mix in a strained solution of 4 dr. isinglass in 1 pint water; when a skin has formed upon the surface, remove from the fire, stir rapidly, and allow to settle for ½ hour, when the deposited carmine is carefully collected, drained, and dried.

Cochineal Lake.—(a) Digest 1 oz. coarsely powdered cochineal in 2½ oz. each water and rectified alcohol for a week; filter, and precipitate by adding a few drops of tin solution every 2 hours, till the whole of the colouring matter is thrown down; wash the precipitate in distilled water, and dry.

(b) Digest powdered cochineal in ammonia water for a week; dilute with a little water, and add the liquid to a solution of alum as long as any precipitate (lake) falls.

(c) Boil 1 lb. coarsely powdered cochineal in 2 gal. water for 1 hour; decant, strain, add solution of 1 lb. cream of tartar, and precipitate with solution of alum. By adding the alum first and precipitating the lake with the tartar, the colour is slightly changed.

Madder Lake.—(a) Tie 2 oz. madder in a cloth, beat it well in a pint of water in a stone mortar, and repeat the process with about 5 pints of fresh water till it ceases to yield colour; boil the mixed liquor in an earthern vessel, pour into a large basin, and add 1 oz. alum dissolved in 1 pint boiling water; stir well, and gradually pour in 1½ oz. of strong solution of carbonate of potash; let stand until cold, pour off the yellow liquor from the top, drain, agitate the residue repeatedly in 1 qt. boiling water, decant, drain, and dry.

(b) Add a little solution of acetate of lead to a decoction of madder, to throw down the brown colouring matter; filter, add solution of tin or alum, precipitate with solution of carbonate of soda or potash, and proceed as before.

(c) Macerate 2 lb. ground madder in 1 gal. water for 10 minutes; strain and press quite dry; repeat a second and third time, and add to the mixed liquors ½ lb. alum dissolved in 3 qt. water; heat in water-bath for 3-4 hours, adding water as it evaporates; filter first through flannel, and when cold enough through paper; add solution of carbonate of soda as long as precipitate falls; wash the latter till the water comes off colourless, and dry.

Yellow Lakes.—(a) Boil 1 lb. Persian berries, quercitron-bark, or turmeric, and 1 oz. cream of tartar, in 1 gal. water till reduced to half; strain the decoction, and precipitate by solution of alum.

(b) Boil 1 lb. of the dyestuff with ½ lb. alum in 1 gal. water, and precipitate by solution of carbonate of potash.

(c) Boil 4 oz. annatto and 12 oz. pearlash in 1 gal. water for ½ hour; strain, precipitate by adding 1 lb. alum dissolved in 1 gal. water till it ceases to produce effervescence or a precipitate; strain and dry.

For information concerning the numerous coal-tar colours now largely manufactured into lakes, the reader is referred to “Spon’s Encyclopædia of Industrial Arts.”

CHAPTER X.

LUMINOUS PAINTS.

(2) A French compound.—100 lb. of a carbonate of lime and phosphate of lime produced by the calcination of sea-shells, and especially those of the genus Tridacna and the cuttle-fish bone, intimately mixed with 100 lb. of lime rendered chemically pure by calcination, 25 lb. of calcined sea-salt, 25-50 per cent. of the whole mass of sulphur, incorporated by the process of sublimation, and 3-7 per cent. of colouring matter in the form of powder composed of monosulphide of calcium, barium, strontium, uranium, magnesium, aluminium, or other mineral or substance producing the same physical appearances, i. e. which, after having been impregnated with light becomes luminous in the dark. After having mixed these five ingredients intimately, the composition obtained is ready for use. In certain cases, and more especially for augmenting the intensity and the duration of the luminous effect of the composition, a sixth ingredient is added, in the form of phosphorus reduced to powder, which is obtained from seaweed by the well-known process of calcination. As to proportion, it is found that the phosphorus contained in a quantity of seaweed, representing 25 per cent. of the weight of the composition formed by the five above-named ingredients, gives very good results.

The phosphorescent powder thus obtained and reduced to paste by the addition of a sufficient quantity of varnish, such as copal, may serve for illuminating a great number of objects, by arranging it in more or less thick coatings, or by the application of one or more coatings of the powder incorporated in the varnish, or by varnishing previously and sprinkling the dry powder upon the varnish. The amount of powder applied should not exceed the thickness of a thin sheet of cardboard.

The dry phosphorescent powders are also converted into translucent flexible sheets of unlimited length, thickness, and width, by mixing them with about 80 per cent. of their weight of ether and collodion in equal parts in a close vessel, and rolling the product into sheets, with which any objects may be covered which are intended to be luminous in the dark. The powders may also be intimately mixed with stearine, paraffin, rectified glue, isinglass, water glass, or other transparent solid matter, in the proportion of 20 to 30 per cent. of the former with 50 to 80 per cent. of either of these substances, and this mass is then reduced into sheets of variable length, width, and thickness, according to their intended applications. A luminous glass is also manufactured by means of the powders, by mixing them in glass in a fused state in the proportions of 5 to 20 per cent. of the mass of glass. After the composition has been puddled or mixed, it is converted into different articles, according to the ordinary processes; or after the manufacture of an object still warm and plastic, made of ordinary glass, it is sprinkled with the powders, which latter are then incorporated into the surface of the article by pressure exerted in the mould, or in any other suitable way.

It has been observed, after various trials, that the passage of an electric current through the different compositions augments their luminous properties or brilliancy to a great extent; this peculiarity is intended to be utilised in various applications too numerous to describe, but of which buoys form a good example. The current of electricity is furnished by plates of zinc and copper mounted on the buoy itself, when the latter is used at sea; but in rivers and fresh-water inlets the battery will be carried in the interior of the buoy. To secure the full effect, 10 to 20 per cent. of fine zinc, copper, or antimony dust is added to the phosphorescent powder described.

(3) Take oyster-shells and clean them with warm water; put them into the fire for ½ hour; at the end of that time take them out and let them cool. When quite cool, pound them fine, and take away any grey parts, as they are of no use. Put the powder in a crucible with alternate layers of flowers of sulphur. Put on the lid, and cement with sand made into a stiff paste with beer. When dry, put over the fire and bake for an hour. Wait until quite cold before opening the lid. The product ought to be white. You must separate all grey parts, as they are not luminous. Make a sifter in the following manner:—Take a pot, put a piece of very fine muslin very loosely across it, tie around with a string, put the powder into the top, and rake about until only the coarse powder remains; open the pot, and you will find a very small powder. Mix it into a thin paint with gum water, as two thin applications are better than one thick one. This will give paint that will remain luminous far into the night, provided it is exposed to the light during the day.

(4) Sulphides of calcium, of barium, of strontium, &c., give phosphorescent powders when duly heated. Each sulphide has a predominant colour, but the temperature to which it is heated has a modifying effect on the colour. Calcine in a covered crucible, along with powdered charcoal, sulphate of lime, sulphate of baryta, or sulphate of strontia; there is produced in each case a greyish white powder, which, after exposure to strong light (either sun-light or magnesium light), will be phosphorescent, the colour depending on the sulphate used and the degree of heat employed.

(5) Five parts of a luminous sulphide of an alkaline earth, 10 of fluorspar, cryolite, or other similar fluoride, 1 of barium borate; powdered, mixed, made into a cream with water, painted on the glass or stone article, dried, and fired in the usual way for enamels. If the article contains an oxide of iron, lead, or other metal, it must be first glazed with ground felspar, silica, lime phosphate, or clay, to keep the sulphur of the sulphide from combining with the metal. The result is an enamelled luminous article. (Heaton and Bolas.)

(6) Boil for 1 hour 2¼ oz. caustic lime, recently prepared by calcining clean white shells at a strong red heat, with 1 oz. pure sulphur (flowers) and 1 qt. soft water. Set aside in a covered vessel for a few days; then pour off the liquid, collect the clear orange-coloured crystals which have deposited, and let them drain and dry on bibulous paper. Place the dried sulphide in a clean graphite crucible provided with a cover. Heat for ½ hour at a temperature just short of redness, then quickly for about 15 minutes at a white heat. Remove cover, and pack in clay until perfectly cold. A small quantity of pure calcium fluoride is added to the sulphide before heating it. It may be mixed with alcoholic copal varnish. (Boston Jl. Chem.)

The luminous calcic sulphide (also called sulphide of calcium), now obtainable in the market, has a yellowish white tint, which considerably limits its direct application as a paint. On the other hand, the calcic sulphide, or the luminous paint obtained therefrom, loses its luminous property, if it is directly mixed with the ordinary commercial paints.

Schatte, of Dresden, produces durable white or coloured paints, containing a luminous substance which causes them to shine in the dark, without changing or neutralising in daylight the tint of the colouring substance or substances contained in such paints.

For this purpose, Zanzibar or cowrie copal is melted over a charcoal fire, 15 parts of this melted mass are dissolved in 60 parts of French turpentine, and the resulting mixture is filtered, whereupon 25 parts of pure linseed oil are added, which linseed oil has been previously boiled and allowed to cool a little. The lake varnish thus obtained is carefully treated in a paint mill with granite rollers, and worked into a luminous paint by one of the processes hereinafter described.

Iron rollers capable of giving off under great pressure small particles of iron, which might affect the luminous power, should not be used. Lake varnish as obtained in commerce contains nearly always lead or manganese, which would destroy the luminous power of the calcic sulphide.

The proportions given are as follows:—

Pure White: By mixing 40 parts of lake varnish obtained as described with 6 parts of prepared baric sulphate, 6 parts of prepared calcic carbonate, 12 parts of prepared zinc sulphide white, and 36 parts of calcic sulphide in a luminous condition, in an oil vessel, and worked into a coarse emulsion, which is then ground fine between the rollers.

Red: 50 parts of the said lake varnish are mixed with 8 parts of prepared baric sulphate, 2 parts of prepared madder lake, 6 parts of prepared realgar (diarsenious disulphide), and 34 parts of calcic sulphide in a luminous condition, and the mixture is worked in the same way as described for white.

Orange: 46 parts varnish are mixed with 17·5 parts prepared barium sulphate, 1 part prepared India yellow, 1·5 parts prepared madder lake, and 38 parts luminous calcium sulphide.

Yellow: 48 parts varnish are mixed with 10 parts prepared barium sulphate, 8 parts barium chromate, and 34 parts luminous calcium sulphide.

Green: 48 parts varnish are mixed with 10 parts prepared barium sulphate, 8 parts chromium oxide green, and 34 parts luminous calcium sulphide.

Blue: 42 parts varnish, 10·2 parts prepared barium sulphate, 6·4 parts ultramarine blue, 5·4 parts cobalt blue, and 46 parts luminous calcium sulphide.

Violet: 42 parts varnish, 10·2 parts prepared barium sulphate, 2·8 parts ultramarine violet, 9 parts cobalt arsenate, and 36 parts luminous calcium sulphide.

Grey: 45 parts of the varnish are mixed with 6 parts prepared barium sulphate, 6 parts prepared calcium carbonate, 0·5 part ultramarine blue, 6·5 parts grey zinc sulphide.

Yellowish-brown: 48 parts varnish, 10 parts precipitated barium sulphate, 8 parts auripigment, and 34 parts luminous calcium sulphide.

Luminous colours for artists’ use are prepared by using pure East India poppy oil, in the same quantity, instead of the varnish, and taking particular pains to grind the materials as fine as possible.

For luminous oil-colour paints, equal quantities of pure linseed oil are used in the place of the varnish. The linseed oil must be cold-pressed and thickened by heat.

All the above luminous paints can be used in the manufacture of coloured papers, &c., if the varnish is altogether omitted, and the dry mixtures are ground to a paste with water.

The luminous paints can also be used as wax colours for painting on glass and similar objects, by adding, instead of the varnish, 10 per cent. more of Japanese wax and one-fourth the quantity of the latter of olive oil. The wax colours prepared in this way may also be used for painting upon porcelain, and are then carefully burned without access of air. Paintings of this kind can also be treated with water glass.

CHAPTER XI.

EXAMINATION OF PIGMENTS.

Colour.—The colour or tint of a pigment can only be estimated by comparing it with a standard sample; this is done as follows:—A sheet of black paper, with, a dead surface, is spread out on a table in front of a window, a small heap of the standard is placed on the paper, and next to it a similar heap of the sample to be compared; by means of a palette knife the surface of the two heaps is flattened out; on now carefully looking at the two heaps, the one which has the purest colour can readily be picked out. The heaps should be looked at from several points of view before a final judgment is arrived at. If the pigment is dark-coloured, it should be spread on white paper, taking care that the same kind of paper is used for the two samples.

Durability.—Durability is not a difficult point to test, but it takes some time to make a complete test. The best method is to mix a small quantity of the pigment in question with raw linseed oil, cover a piece of glass with the mixture, and expose it outside to the action of the sun and air for some time, noting at intervals how it behaves. It is well for the sake of comparison to coat a second piece of glass with the mixture, and keep this in a dark place. The difference between the two from time to time will show how the pigment behaves under the influence of light and air. As the durability of pigments is decidedly different according as they are used in oil or water-colour painting, the oil in the former case acting as a protective agent, it is well to use a similar test, using a little gum water as a vehicle to mix the pigment with. It will take from two to three months at least to properly test the durability of a pigment in summer, while in winter the time will be increased considerably. Glass is the best substance to use, as it is quite neutral, and does not of itself introduce into the test any injurious element, as wood or paper might do, although these bodies may be used if thought desirable.

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 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.