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

The Brunswick greens are in the front rank of green pigments so far as covering power is concerned, and, when made from reliable materials, are reasonably durable under the influence of air and light, in which respect, however, they vary considerably. They can be used as water colours, but are superior in oil paints. Precautions are necessary in mixing them with other pigments. By the action of sulphuretted hydrogen, or sulphur in any form, the colour is darkened to a notable degree; by the action of acids, the chrome is destroyed and the green becomes blue; by the action of alkalies, both the blue and the yellow constituents are affected, and the green gives place to a reddish hue. The pale and medium shades are yellow greens; the deep and extra deep are blue greens.

These colours can be distinguished by heating them with caustic soda, which turns them brownish in tone, owing to the destruction of the Prussian blue. If the residue be filtered, and to the filtrate some acid and ferric chloride be added, a blue precipitate will be obtained, indicative of the presence of Prussian blue. On washing the residue with water and treating with hydrochloric acid, the brown colour disappears, and, in most cases, only a white residue of barytes is left; sometimes the residue may have a faint yellowish colour. The solution in hydrochloric acid will give the characteristic tests for iron. The yellow element can be recognised by boiling in hydrochloric acid, filtering, and allowing the filtrate to cool, when crystals of lead chloride will deposit; these, separated out and dissolved in boiling water, will give the characteristic tests for lead, such as a white precipitate with sulphuric acid, and yellow precipitate with bichromate of potash. The filtrate will have a green colour, indicative of chromium.

Chinese Green.—Another name for the vegetable pigment known in China as Lokao (q.v.)

Chrome Green.—This name is often applied to any green in which chrome enters as an element, but more particularly to the modern Brunswick greens described on pp. 114-118; and to the green which bears the name of its first maker, Guignet, and described under the title of Guignet’s Green, see p. 125.

Cobalt Green.—This remarkably stable, but somewhat costly, pigment is also known by the names of Rinmann green and zinc green, the former after the name of the chemist who first prepared it, and the latter because it contains a large proportion of zinc. It is in fact a combination of the oxides of cobalt and zinc, and was originally produced in the following manner:-½ lb. pure cobalt ore was dissolved in 4 lb. concentrated nitric acid, and added to a solution of 1 lb. zinc in 5 lb. nitric acid; the mixture was diluted with water, and a solution of potash carbonate was added, throwing down a pinkish precipitate, which was washed on a filter, dried, and calcined at a high temperature.

Wagner found that an indispensable condition was to have a protoxide of cobalt as free as possible from foreign metals, with which object he practised the following method:—Cobalt oxide is dissolved in three equivalents of hydrochloric acid, and the solution is evaporated to dryness; the residue is dissolved in six equivalents of water, and through the solution is passed a current of sulphuretted hydrogen gas, so long as any precipitate is formed. This precipitate consists of sulphides of the foreign metals. The clear solution is siphoned off, evaporated to dryness, and the residue is dissolved in water. As required, this solution is treated with carbonate of soda, and the precipitate, washed, and while still wet, is mixed with zinc white. The reddish mass produced in this way is dried and calcined. The best tone is attained by combining 9 to 10 parts of zinc oxide with 1 to 1½ parts of cobalt protoxide.

Louyet has shown that if the cobaltic solution be precipitated by the phosphate or the arseniate of potash, the corresponding salt of cobalt thus produced possesses the property of imparting a green colour to zinc white at a much lower temperature than is required in the case of ordinary protoxide of cobalt: moreover, the pigment gains in body, and the colour gains in purity and brightness. If a small quantity of arsenious acid is added to the ordinary mixture before calcination, the calcined mass will assume a remarkably bright green colour; and its structure being loosened by the disengagement of fumes of arsenious acid, it will be easy to grind.

According to Barruel and Leclaire’s method, 1 lb. of pure dry sulphate of cobalt, dissolved in hot water, is mixed with 5 lb. of zinc oxide. The mixture is dried, and calcined for three hours at a clear red heat in a muffle; when cooled, it is thrown into water, washed, and dried.

The composition of cobalt green has been shown by Wagner to vary considerably, as is to be expected from the methods of its preparation. The proportion of zinc oxide ranges from 71½ to 88 per cent., and the cobalt protoxide from 11½ to 19 per cent.; in addition, there will be fluctuating percentages of phosphoric acid, soda, oxide of iron, &c., according to the process followed.

With the single exception of its costliness, cobalt green possesses advantages over most other green pigments. It has a bright colour, sometimes inclining to a yellowish tint, or, when phosphates are used in its preparation, leaning to a blue shade. But it is always permanent, not only under the influence of air and light, but also in the presence of alkalies and any but concentrated acids; thus it may safely be compounded with other pigments.

Douglas Green.—This pigment, which is fairly permanent, and possessed of considerable covering power, owes its name to the chemist who proposes its use, and its colour to the oxide of chromium. The method by which it is prepared is as follows:—Solutions of barium chloride and potassium chromate are mixed together. To the barium chromate thus produced is added one-fifth of its weight of concentrated sulphuric acid, whereby partial decomposition is brought about, resulting in a mixture of barium chromate, barium sulphate, and chromic acid. This mixture is dried, and calcined in a crucible at bright red heat, the effect of which is that the chromic acid is converted into green oxide of chromium, and, being scattered throughout the mass, imbues it with a green colour.

Emerald Green.—This is quite an old-fashioned pigment, having been in use some 80 years. It is a combination of acetate and arsenite of copper, and varies in tint from a dark to a pale green, always with a bluish cast. It possesses good covering power, and can be used either as an oil-or as a water-colour, but particularly as the latter, and is much used in paper staining. In composition it varies considerably, as there are some half-dozen industrial methods of making it; but in general terms it usually contains over 50 per cent. of arsenious acid, and about 30 per cent. of oxide of copper, together with various impurities. Following are some of the processes by which it is manufactured.

(1) According to the method introduced by Liebig, 1 part of verdigris is heated in a copper kettle with sufficient distilled vinegar to effect its solution, and to this is added a solution of 1 part of arsenious acid in water. The result is a precipitate of a dirty green colour, which is dissolved in a new quantity of vinegar and boiled for some time. In this way is obtained a new precipitate, granular and crystalline, and exhibiting a splendid green colour. When this has been filtered off, washed, and drained, it is boiled with one-tenth of its weight of commercial potash, in order to deepen and brighten the colour and destroy the bluish tint. Should the waste liquor obtained after the filtration of the pigment from the second boiling in vinegar contain any remaining copper, arsenious acid is added; and if arsenious acid be present, copper acetate is added; while if acetic acid survives it may be used again for dissolving another lot of verdigris.

(2) Form a paste with 1 part verdigris in sufficient boiling water, pass it through, a sieve to remove lumps, and gradually add it to a boiling solution of 1 part arsenious acid in 10 parts water, the mixture being constantly stirred until the precipitate becomes a heavy granular powder, when it is filtered through calico, and dried very carefully.

(3) Acetate of copper is mixed with a sufficient quantity of water heated to 122° F., to make a homogeneous and liquid paste. To 10 parts of acetate of copper in this condition is added a solution of 8 parts of arsenious acid in 100 parts of boiling water, the whole being then kept in a state of ebullition. The addition of a little acetic acid helps to develop the beauty of the colour. When precipitation is complete, the clear liquor is drawn off, and forms a convenient solvent for the next charge of arsenic, the operation being facilitated by adding a little carbonate of potash, forming an arsenite of potash. The precipitate constituting the desired green pigment is filtered off and dried at the lowest effective temperature.

(4) Dissolve 5 lb. of sulphate of copper in water, and add to it a solution of 1 lb. of lime in 2 gallons of vinegar. Mix 5 lb. of white arsenic with sufficient water to form a paste. Add the arsenic paste to the copper and lime mixture, and leave the whole at rest in a moderate degree of heat. Mutual decomposition slowly ensues, with consequent formation of the green pigment, which is filtered off, washed, and dried with the same precaution as before.

When sulphate of copper is used in the production of emerald green, it is very desirable that it shall be free from sulphate of iron, which is a common impurity in the commercial article, and greatly detracts from the purity and brilliance of the pigment. A good method of eliminating this iron is to add to the sulphate of copper solution a small quantity of a gelatinous precipitate of carbonate of copper, produced by decomposing a copper sulphate solution by a soda carbonate solution, and washing. On adding the gelatinous carbonate of copper, with agitation, the iron is soon thrown down in flakes of oxide, and pure sulphate of copper may be filtered off.

(5) Braconnot proceeds as follows:—A solution of 3 lb. of sulphate of copper is made in a small quantity of hot water; and a second solution of 3 lb. of arsenious acid and 4 lb. of commercial carbonate of potash in boiling water. When the evolution of carbonic acid gas has ceased, the two liquors are mixed together while being kept continuously stirred; the result is an abundant precipitate of a dirty yellowish-green colour. On adding a slight excess of acetic acid, a fine crystalline green is developed; this is washed with boiling water on a filter, and dried very slowly and carefully.

(6) A rough and ready process is to mix white arsenic with water, and then stir in an equal weight of verdigris, allowing the mixture to be at rest for a time in a moderately warm temperature till the pigment is completely precipitated, when it is washed on a filter, and dried very gradually.

(7) A method due to Köchlin is described in the following terms:—An aqueous solution of sulphate of copper is made by adding 100 grammes of the salt to 500 cc. of water. To this, when solution is complete, is added 187½ cc. of a solution of arsenite of soda, which is of the strength represented by 500 grammes of arsenite in 1 litre of water. The result is that a precipitate of arsenite of copper is thrown down. This precipitate is treated with 62 cc. of acetic acid at 11° to 12° Tw., or half that quantity of pure formic acid, for one hour, at a temperature ranging from 104° to 122° F. The pigment thus produced is of good colour, but its superiority would not seem to justify the use of such an expensive article as pure formic acid, nor the minute adjustment of the proportions of the ingredients, in an operation to be conducted on a commercial scale.

(8) Another complicated process has been invented by Prof. Galloway, which, under skilled supervision, and when the correct proportions of the several ingredients have been ascertained by careful experiment, may give good results, but several precautions have to be observed which cannot be entrusted to ordinary factory hands. The principle of the process is that when a quantity of sulphate of copper is dissolved in water, sufficient carbonate of soda is added to throw down one-fourth of that copper sulphate as carbonate of copper, and then so much acetic acid is introduced as will convert that copper carbonate into acetate. In order to convert the balance of the copper sulphate into arsenite, a solution of arsenic in boiling carbonate of soda is made and added to the copper acetate solution, both solutions being at a boiling temperature.

Emerald green is a pigment which possesses considerable stability in dry pure air, but in damp atmospheres it becomes brown; in the presence of acid or ammoniacal vapours it turns blue, and under the influence of sulphuretted hydrogen it blackens; moreover, strong alkalies destroy it. Consequently it cannot be used in many situations, nor in association with such pigments as contain sulphur compounds. In decorative painting it is difficult to apply on large flat surfaces, and necessitates stippling in order to get it to lie well; but when stippled on a ground of proper green it develops an exceedingly beautiful bloom-like appearance.

Its peculiar shade distinguishes it from all other green pigments, none of which approaches it in the paleness and brightness of its colour. It can be distinguished by the fact that it is soluble in acids and ammonia, to a blue solution which does not change on boiling. In caustic soda it also dissolves with a blue colour: on boiling, a red precipitate of cuprous oxide falls down. No other green pigment answers to all these tests.

There are a good many imitation emerald greens on the market, some of which are offered as genuine emerald greens, others as “emerald tint” green, which is much more honest. The composition of these greens necessarily varies greatly, some are prepared from coal tar greens, others by careful admixture of various green, blue, and yellow pigments. If the tint of these substitutes is right and they are sold for what they are, there is no reason why they should not be used in place of the real article, over which they have the advantage of not being poisonous, which is a great disadvantage of the genuine emerald green. Although one authority disputes this point, certainly the poisonous action of emerald green varies very considerably with different individuals. The genuine emerald green may be distinguished from the spurious by being perfectly soluble in acids and alkalies, which the imitations are not; the character of the latter must be inferred by the application of a few special tests, the nature of which will be readily deduced from what is said as to the properties of other green pigments. Emerald green should be assayed for purity and tint; this is important, as pure emerald green has a tint of its own, which is difficult to imitate, and sometimes really pure emerald green offered for sale is of a defective tint, due to some fault in the process of manufacture. Such samples should be rejected.

Guignet’s Green.—The greens of this class, which owe their colour to chromium oxide, are also known as “chrome greens,” a name which they share with a totally different group into whose composition chrome yellow enters as a constituent, and which have been already described under the synonym “Brunswick greens,” on pp. 114-118.

Though one of the simplest of chemical products, a great many ways of preparing chromium oxides have been proposed. One of the earliest for industrial application was that of Guignet, who has given his name to the pigment, and this may fitly commence the long list.

(1) The first method adopted by Guignet consisted in mixing bichromate of potash with three times its weight of boracic acid and moistening the mass with just sufficient water to form it into a thick paste. This paste is put on the hearth of a reverberatory furnace, which is carefully heated to a point never exceeding a dark red heat; if this precaution is neglected, the mass, instead of becoming porous, will fuse entirely, and the anhydrous oxide will be produced, which has a pale-green colour. The heated paste, while still red hot, is thrown into cold water and washed with boiling water, in order to remove borate of potash in solution; and this solution, when boiled down and treated with hydrochloric acid, can be made to yield up most of the boracic acid it contains. The filtered and washed residue is the hydrated oxide of chromium.

(2) A modification of (1), followed by Guignet, was to replace the bichromate of potash by chromate of soda, prepared by dissolving in boiling water 61 parts of neutral chromate of potash and 53 parts of nitrate of soda. For the neutral chromate of potash, also, may be substituted a mixture of 92 parts of bichromate of potash and 89 parts of crystallised carbonate of soda, the nitrate of soda remaining as before. On cooling, in either case, the solution deposits much nitrate of potash, which is commercially valuable. The chromate of soda present in the mother liquors is obtained by evaporating to dryness. The pigment produced by the chromate of soda process is lighter in colour than that obtained with bichromate of potash. It may be still further paled by adding a little alumina, baryta, or other white pigment to the bichromate and boracic acid mixture before calcining.

(3) Equal quantities of potash bichromate and potato starch are thoroughly mixed and then calcined in a crucible at a high temperature. The product is washed with boiling water, to remove the potash carbonate formed, and any remaining undecomposed bichromate. The precipitated chromium oxide is filtered, dried, and again calcined to drive off the water. The final result is a handsome pigment which flows well from the brush.

(4) On heating in a crucible a mixture of 3 parts of neutral chromate of potash with 2 parts of salammoniac, the two salts are decomposed, the result being formation of chromium oxide mixed with potassium chloride, which latter is removed by several washings with hot water. The brilliancy of the chromium oxide is enhanced by calcination at a dull red heat.

(5) Fuse together 3 parts of boracic acid and 1 part of potash bichromate at a dull red heat on the hearth of a reverberatory furnace. Thus is formed a borate of chromium and potash, with evolution of oxygen. The mass is repeatedly washed with boiling water, which causes decomposition, and consequent separation of hydrated oxide of chromium, and a soluble borate of potash. The chromium oxide is washed, and ground very fine.

(6) When a solution of potash bichromate is poured into a neutral solution of mercury proto-nitrate, it forms an orange-coloured precipitate, which is washed and gently dried, then powdered, and heated in a stoneware retort provided with an arm dipping into cold water, by which the mercury is distilled and condensed. The residue in the retort is a highly comminuted chromium oxide, of a fine dark-green colour.

(7) On calcining potash bichromate in a crucible at a very high temperature, it is decomposed, and results in chromium oxide and potash, the latter of which can be washed out. The chromium oxide thus obtained is very dense and of a dark-green colour resembling (6).

(8) Equal quantities of flowers of sulphur and bichromate of potash are thoroughly mixed, and heated to redness in a crucible, producing a mixture of oxide of chromium with sulphide and sulphate of potash. The latter are dissolved out by washing repeatedly with hot water, leaving the chromium oxide as a finely comminuted dense powder of an intense green colour.

(9) A modification of (8) consists in adding small successive quantities of flowers of sulphur to a boiling concentrated solution of potash bichromate. From this results a gelatinous oxide of chromium, which is washed with boiling water, dried, and calcined in a crucible at a red heat.

(10) Hydrochloric acid decomposes bichromate of potash, forming a soluble chloride of potash which can be removed by washing, and a residue of chromium oxide, which is washed on a filter and dried.

There remain for description two or three processes in which phosphoric acid plays a part, but the greens made by these methods do not possess the freshness of the others, and it is difficult to see what advantages can attend this modification.

(11) According to Arnaudon, 149 parts of bichromate of potash are thoroughly incorporated with 128 parts of crystallised neutral phosphate of ammonia, and the mixture is heated in thin layers to a temperature between 338° and 356° F., which brings about intumescence, change of colour, and disengagement of water and ammonia; the heating is continued for half an hour, but must not be allowed to exceed 392° F. When the development of the green colour is complete, the product is washed with hot water to remove soluble salts, and the residue constitutes an impalpable powder of chromium oxide, forming a leaf-green pigment.

(12) Dissolve 10 lb. of bichromate of potash and 18 lb. of phosphate of soda in boiling water, and add to the boiling mixture 10 lb. of thio-sulphate of soda solution and a little hydrochloric acid. A precipitate of phosphate of chromium is gradually thrown down as the boiling is maintained.

For general utility no class of pigments can exceed the several forms of Guignet’s green. It is capable of affording a great variety of tints, all absolutely permanent under reasonable conditions. No ordinary agent will decompose them, and they will stand almost any test to which they may be subjected without losing colour. They are quite insoluble in acids and alkalies, and are not affected even by the extreme heat of the glass furnace. They possess good covering power, do not suffer in brightness or purity under artificial light, and are equally useful as oil or water colours, besides being admirably adapted for fresco and silicious painting, and employed in making green glass and in calico printing. They can be mixed with any other pigment. Adulteration with Brunswick or Prussian greens is often practised, but may be discovered by a portion being dissolved on boiling with caustic soda, the solution giving a precipitate of chrome yellow on adding acetic acid, and (a separate portion, of course) Prussian blue with hydrochloric acid and perchloride of iron.

Lokao.—This pigment, which is also known as “Chinese green,” was first met with as a sediment left after dyeing cotton cloths with the barks of one or more species of buckthorn, notably Rhamnus chlorophorus and R. utilis, and passing in China under the general name of Lo-Kao. This sediment is spread on blotting paper and thus dried, forming thin cakes. Latterly, the juice afforded by the berries of the same trees is extracted by pressure, absorbed by alum, and dried in the same form of little cakes. When first introduced into England it was highly valued as affording a pure green, even in artificial light. Its price on the London market in 1861 was 7s. 6d. an ounce. So long ago as 1853 it was imported into France and used for dyeing silk. The colouring principle appears to consist of a glucose (lokaose) and an acid (lokaonic acid). In 1864, Chauvin obtained an identical colouring matter from Rhamnus catharticus, or the common buckthorn, a shrub which grows wild in most parts of Europe, and found a ready market for the pigment at 37s. a pound. This was simply the article known as sap green (see p. 132.)

Malachite.—This is one of the names applied to mountain green (q.v.).

Manganese Green.—Several formulæ have been published for making a green pigment from manganese, as follows:—

(1) An intimate mixture of 80 parts of nitrate of barium, 14 parts of oxide of manganese and 6 parts of sulphate of barium, is placed in a crucible and heated to bright redness until the green colour is thoroughly developed. The fused green mass is poured out of the crucible, cooled, and ground wet to a fine condition.

(2) To 3 or 4 parts of caustic baryta moistened with water are added 2 parts of nitrate of barium and 2 parts of oxide of manganese; the whole mass is most intimately mixed, then put into a crucible in a furnace, and subjected to a dull red heat so long as may be necessary for securing complete decomposition. When the green colour is satisfactorily produced, the mass in a state of fusion is poured out, cooled, pulverised, digested in boiling water, then washed with cold water, and finally dried in an atmosphere which is free from carbonic acid.

(3) The oxide of manganese may be replaced by the nitrate, when the quantities are 46 parts nitrate of barium, 30 parts of sulphate of barium, and 24 parts of nitrate of manganese; the fusion, grinding and washing are repeated as before.

According to some recipes the powdery pigment, consisting essentially of manganate of barium, is mixed with a little dextrine to make sure of its stability, but it is not clear whether this is really essential.

Mineral Green.—This is only another name for the green made from copper carbonate, and described under mountain green, see p. 131.

Mitis Green.—This pigment is an arseniate of copper, and bears a very close relationship to the emerald green made according to Braconnot’s formula, and described in the fifth paragraph of that section, see p. 123. Mitis green is prepared by dissolving arseniate of potash in five times the quantity of hot water and adding a solution of an equal weight of sulphate of copper, keeping the whole in constant agitation. A pulverulent precipitate is formed, possessing a grass green colour. This is washed and dried. The tint can be varied by altering the proportions of the arseniate and sulphate. The arseniate of potash is made by boiling arsenious acid in concentrated nitric acid, filtering, and saturating with carbonate of potash. The arseniate is allowed to crystallise out of the liquor.

Mountain Green.—This pigment is also known by the names of malachite and mineral green.

(1) In its native form the mineral malachite or green carbonate of copper is very widely distributed in Europe, Asia, America, and Australia, but on a commercial scale it is chiefly produced in the Ural mountains of Siberia and in the Banat of Hungary. It only needs to be picked clean from adhering rock and to be ground to a very fine powder in order to render it ready for use. It is much superior to any of the artificial substitutes referred to below, but its cost confines its application to artistic work.

(2) Sometimes a little orpiment or chrome yellow is ground up with the malachite.

(3) A very simple formula for making the artificial pigment is to add solution of carbonate of soda or potash to a hot mixed solution of alum and bluestone (sulphate of copper).

(4) Other recipes for making mountain greens have been published which bear no relation to the composition of the original article, e. g. by mixing a solution containing potash and arsenic with a solution of bluestone; or, as a much more complicated example, treating a solution of bluestone first with slaked lime, then with a solution of arsenic and soda obtained by boiling in water, and finally with tartaric acid.

The advantages attendant on so much trouble in producing what is at best an unstable pigment are not very apparent.

Paris Green.—This is another name, used especially in America, for the emerald greens described on p. 121.

Prussian Green.—A name often applied to class b of the Brunswick greens (see p. 114), or in other words those which are prepared from Prussian blue.

Rinmann Green.—The first cobalt green (see p. 119), put on the market was made by Rinmann, and hence it is still often called by his name.

Sap Green.—This vegetable pigment or lake is closely allied to the Chinese green or lokao, described on p. 129.

It consists of the solidified juice extracted from the berries of the common buckthorn shrub (Rhamnus catharticus), which is obtained either by allowing the berries to undergo slight fermentation for about a week in wooden tubs, then pressing and straining; or by boiling the berries, and straining off the juice. In either case the clean juice is boiled down to a syrupy consistence, and a little alum (about ½ oz. to the pint of thickened juice) is added, the liquor being then evaporated to dryness, or very nearly to that point, the drying being left to complete itself after the pigment has been ran into bladders.

The quality of this green is liable to serious fluctuation, owing to the neglect or ignorance of certain simple precautions. Thus, for a true green the berries should be selected before they have quite reached maturity. The more nearly ripe the berries are, the more yellow will be the tint of the green afforded by them. The boiling of the berries, if followed, and the evaporation of the juice, must be done at a low temperature, and the final stages of the evaporation cannot safely be done with direct fire heat, but should be effected in a water bath. The only substance incorporated with the juice should be potash alum. Sometimes it is replaced by carbonate of magnesia (which destroys the transparency of the pigment); or by carbonate of potash (which introduces a stickiness or viscosity).

Sap green possesses too little body and is too translucent for use as an oil paint; but being non-poisonous, and in fact perfectly harmless, it finds many useful applications outside of water colour and pastel painting, viz. in colouring alimentary substances such as drinks and sweets. Its true colour is a leaf green, glossy and translucent. In durability it is not remarkable.

Scheele’s Green.—For more than a century has Scheele’s green been a familiar pigment, but the reputation it enjoyed in its early days has long since departed, and it is now to be classed among the inferior green colouring matters. It consists essentially of a basic arsenite of copper, and contains from 8 to more than 40 per cent. of arsenic, according to the mode of preparation, of which there are several, as follows:—

(1) A mixture of 2 parts of commercial carbonate of potash and 1 part of powdered arsenious acid (white arsenic), are dissolved in 35 parts of boiling water; the solution is filtered clear, and then added gradually and while still warm to a filtered solution of 2 parts of sulphate of copper until no further precipitate goes down. This latter is collected, washed with warm water on a filter, and slowly dried without excess of heat.

(2) The preceding formula is modified by making one solution of the arsenic and the sulphate of copper, and precipitating by adding the carbonate of potash solution till the colour is fully developed, agitation being constantly maintained.

(3) Another variation is to mix the arsenic with soda crystals in boiling water, and to pour the arsenite of soda solution thus formed into the bluestone solution, the boiling being kept up for a few minutes.

Scheele’s green has a pale yellowish cast, and mixes well with either water or oil, but it lacks brightness, durability, and covering power, in addition to being highly poisonous, and though once much employed in staining wall papers, is now generally discarded.

Schweinfurth Green.—This is an old-fashioned name for emerald green, which has been described on pp. 121-125.

Terre Verte.—Rendered into English, the name terre verte means “green earth.” It is applied to a number of green-coloured earths found widely distributed in rocks of various ages, but especially in those of a basaltic or porphyritic character. In commercial quantity it occurs notably in Cyprus and near Verona in Italy; the latter locality is so important that the pigment is often known as “Verona earth.”

Notwithstanding minor points of dissimilarity in samples from different sources, there is a great family likeness among them, sufficient to indicate that the essential constituent is a silicate of iron and magnesia. The other ingredients vary with the locality producing the mineral. The same may be said of the physical characteristics, some specimens being soft and earthy, while others are hard and glassy. All possess the peculiar soapy touch of the magnesian earths, and a clay-like odour. Analysis of a Verona earth gave:—

While a Cyprus earth showed:—

The presence of copper would point suspiciously to adulteration, and in any case should suffice to condemn the sample for use.

Naturally there is considerable variety of tint among the many kinds of terre verte, but they all belong to the pale greyish class, and are more or less translucent, consequently their covering power is small. Their value lies in their durability, and the resistance they offer to the injurious effects of strong light and impure atmosphere. They can be employed either as oil or water colours. The only preparation to which the natural pigments are submitted is fine grinding and washing.

Titanium Green.—An excellent dark green pigment, though rather costly, can be prepared from rutile or any titaniferous iron ore by the following method:—

The ore is dressed clean, and fused with twelve times its weight of acid sulphate of potash in a crucible. When cool, it is reduced to fine powder, and digested at 120° F. in dilute hydrochloric acid (half water) until solution is complete. The hot solution is filtered off from the residue and carefully evaporated down to a syrupy consistence, when the nearly pure titanic acid is allowed to cool in the dish and thrown on a filter. When sufficiently drained, it is boiled in a large volume of water containing a little ammonia, and the precipitated titanic acid is filtered and washed.

If an ore is used containing carbonate of lime, it must first be treated with dilute hydrochloric acid before the sulphate of potash is applied.

The titanic acid on the filter is next mixed with a concentrated solution of sal ammoniac, and again filtered. Then it is digested in dilute hydrochloric acid at 120° to 140° F. till the solution is complete. On adding ferrocyanide of potassium to the acid liquor, and bringing quickly to a boil, a precipitate of ferro-cyanide of titanium is thrown down. This is very carefully and slowly dried, at a temperature never exceeding 200° F.

Verdigris.—The chemical examination of verdigris shows it to be a basic hydrated acetate of copper, containing variable proportions of the bibasic and tribasic acetates.

Commercially it is prepared in districts where acetic or pyroligneous acid can be had at small cost. Thin pieces of scrap copper are subjected to the action of fermenting grape skins in mass, or cider refuse, for a fortnight or three weeks; or to the influence of pyroligneous acid for four or five days. By this means the copper surfaces are attacked by the acetic acid being generated or liberated, and become coated with acetate of copper. At intervals the pieces are removed, and surfaces are cleaned of the accumulated acetate or verdigris and this is repeated till the metallic copper has thus been completely converted. The collected verdigris is washed, and carefully dried at a very low temperature.

Its composition is subject to many irregularities, and the colour varies from green to bluish green according to the proportion of sesquibasic acetate present. It is one of the least permanent pigments, especially in the presence of water, and is exceedingly poisonous. At one time it was largely used as a pigment, but is now gradually going, if indeed it has not already gone, out of use. It can be distinguished by its solubility in acids and ammonia, the latter giving a deep azure blue solution. On being heated, it turns black, owing to its parting with acetic acid and leaving the black oxide of copper behind. This should be entirely soluble in nitric acid, the solution giving the characteristic tests for copper. The solution should give no precipitate with chloride of barium or nitrate of silver, and the original pigment should be freely soluble in any acid and in ammonia without effervescence.

Verditer.—Green verditer is another of the copper greens which has practically disappeared from the modern painter’s list of pigments. It is a yellow tinted very fugitive colour, consisting of a basic carbonate of copper, and is manufactured by treating copper solutions with carbonate of soda, or of potash.

Verona Earth.—One kind of terre verte (see p. 134), is known by this name because it is produced in the neighbourhood of Verona.

Victoria Green.—This is a fancy name for the Brunswick greens compounded from Prussian blue, and already described on p. 114.

Vienna Green.—The aceto-arsenite of copper described under the heading of emerald green (see pp. 121-125), is sometimes called by this name.

Zinc Green.—The pigments described under cobalt green (see p. 119), as often pass by the name of zinc greens, and in fact they contain much more zinc than cobalt.

A handsome but not permanent green may be made by combining zinc with iron instead of cobalt, in the form of a double cyanide. The process is as follows:—Finely powdered Prussian blue is stirred into a concentrated solution of chloride of zinc, and put by to allow the decomposition to take place. After some time, the precipitated ferro-zinc cyanide is thoroughly washed, and dried out of reach of the light.

CHAPTER VI.

REDS.

(1) One of the earliest successful processes was that introduced by Mathieu Plessy, which gives a scarlet product. He obtains the pigment, a modified sulphide of antimony, by decomposition of hyposulphite of soda in the presence of chloride of antimony. The two solutions of hyposulphite of soda and chloride of antimony, each at 25° B., being prepared, the next step is to pour into a stoneware vessel 4 gals. of the antimony chloride solution, 6 gals. of water, and 10 gals. of soda hyposulphite solutions. The precipitate caused by the water is rapidly dissolved in the cold by the hyposulphite. The stoneware vessel is then placed in a hot water bath, and the temperature of the contents is thus gradually raised. At about 86° F. the precipitation of the sulphide commences, showing orange yellow at first, but becoming darker subsequently. When the temperature has reached 130° F., the vessel is removed from the water bath, and the deposition of the precipitate proceeds rapidly. The supernatant liquor is siphoned off, and the solid residue is washed first with water acidulated by adding to it one fifteenth of its bulk of hydrochloric acid, and then with clean water. Finally the residue is collected on a filter, and dried. It is exceedingly brilliant while wet, but loses a portion of its brightness when dried.

Provision must be made for disposing of the sulphurous oxide gas driven off during the process of manufacture.

(2) Kopp found certain disadvantages in working by the above method, and adopted instead the reaction of antimony chloride upon a dilute solution of hyposulphite of lime.

Experiencing much difficulty in the decomposition of antimony sulphide by hydrochloric acid on an industrial scale, he experimented on roasting the sulphide at a moderate temperature in contact with air and steam, whereby most of the antimony sulphide is converted into oxide, while the sulphurous acid driven off is utilised for making the hyposulphite of lime. This proved a most successful plan, and the resulting antimony oxide is readily dissolved by commercial hydrochloric acid.

During the oxidation of the antimony sulphide, a certain proportion of antimonious acid may be produced. This is but slightly soluble in hydrochloric acid. It may be collected, however, by saving the residues from the treatment by hydrochloric acid, and washing them with chloride or hyposulphite of lime, which will dissolve the adherent antimony chloride; they are then dried, and melted with a little antimony sulphide and quicklime, so as to transform the whole into antimony green, the quicklime having the effect of decomposing any small residue of antimony chloride.

The preparation of the hyposulphite of lime is cheaply effected by the action of sulphurous acid on sulphides of lime, the sulphurous acid being derived either from the roasting of the antimony sulphide, or from pyrites or brimstone in the usual way.

Calcium polysulphide is prepared by boiling finely powdered sulphur and newly slaked lime in water. Certain advantages arise from the addition to this solution of a little powdered calcium oxysulphide, or some quicklime.

In the reaction of sulphurous acid on calcium sulphide and oxysulphide, sulphur is set free and forms a sulphite of lime, which, in the presence of sulphur and undecomposed sulphide, is soon transformed into hyposulphite, the reaction being facilitated by the rise of temperature which takes place in the apparatus.

As soon as the liquor has become slightly acid, it is drawn off into a large settling tank. If, after agitating for some time, the liquor has not become neutralised by the undecomposed calcium oxysulphide contained in it, this is brought about by addition of a little calcium sulphide, and is recognisable by the appearance of a black precipitate of sulphide of iron. After due settlement, the clear liquor is decanted, and forms a solution of nearly pure hyposulphite of lime.

The production of antimony vermilion is effected from the foregoing solutions of antimony chloride and hyposulphite of lime, in apparatus consisting simply of a series of wooden tanks raised conveniently above the floor, holding about 500 gals. each, and provided with steam coils for heating their contents.

Sufficient hyposulphite of lime solution is run into the tanks to fill about seven-eighths of their depth; and then into the first tank is poured the chloride of antimony solution, in quantities of a few pints at a time. A white precipitate is formed, and rapidly dissolves at first; when it is slow in going into solution, even though stirred, the addition of antimony chloride should be stopped, as an excess of hyposulphite of lime is essential. The liquor in the tank must be perfectly clear and limpid, and should any white precipitate remain it must be dissolved by making small additions of hyposulphite.

At this stage steam is admitted into the coils, and thereby the temperature of the solutions is gradually raised to 120° or 140° F., or even to 160° F., while stirring is unceasingly carried on. The reaction is soon manifested by the successive colours of the liquor, passing from straw-yellow to lemon-yellow, orange-yellow, orange, orange-red, and lastly a very deep and brilliant red. The steam is shut off from the coil before the desired tint is arrived at, as the acquired heat and the agitation complete the development of the colour. If the heating is carried too far, the red gradually passes to a brown and later to nearly black. With experience, almost any desired shade of red can be produced.

When the precipitate has attained the required colour, it is allowed to settle, and the tank is covered. The clear and limpid liquor, having a strong sulphurous odour, is let out through tap holes at various levels in the sides of the tanks, and run by wooden gutters or leaden pipes into a large reservoir holding a quantity of sulphide and oxysulphide of lime. Here the sulphurous liquor regenerates a certain amount of hyposulphite of lime.

The antimony chloride solution always contains a large proportion of chloride of iron, which provides an easy means of guiding the progress of this latter operation. All the iron remains soluble in the mother liquors of the antimony sulphide, and as soon as they are brought into contact with the calcium sulphide, an insoluble black precipitate of iron sulphide is formed. So long as this remains, the mother liquors charged with sulphurous acid have not been added in excess; but when it disappears by conversion into soluble hyposulphite of iron, that is a sign that the sulphurous solution is in excess. The contents of the reservoir are then well stirred, and calcium sulphide is introduced if necessary, until the precipitate of iron sulphide returns and remains. It is also needful to ensure that a certain proportion of hyposulphite of iron shall remain in solution. The clear liquor decanted off when all the precipitate has gone down is a neutral solution of hyposulphite of lime, containing some calcium chloride and hyposulphite of iron.

Another requisite precaution in this regeneration of hyposulphite of lime is that no excess of calcium sulphide be left, or it will give an orange-yellow tint to the vermilion; and if the hyposulphite of lime solution is alkaline and yellow, sulphurous acid liquor must be run in till all alkalinity is destroyed.

This regenerated solution of hyposulphite of lime is used like the first. The mother liquors charged with sulphurous acid are again neutralised in the large reservoir by new proportions of calcium sulphide and oxysulphide, until so much calcium chloride is present that they are useless for the purpose, say after 25 to 30 operations.

The antimony vermilion precipitated on the bottom of the first tank is received into a conical cloth filter, and the liquor drained off is passed to the reservoir. The first tank is then washed out with warm water, which also passes through the filter. The precipitate of red sulphide cannot be too carefully or completely washed, and finally is filtered and slowly dried below 140° F.

(3) Wagner’s method of making a scarlet pigment is to dissolve 6 lb. of tartaric acid and 8 lb. of tartar emetic in 4½ gallons of water at 140° F., adding a solution of hyposulphite of soda at 40° Tw., and heating the whole mixture to 180° F., whereby the red pigment is gradually precipitated. It is collected on a filter, well washed and dried.

(4) The process adopted by Murdoch, in which a solution of antimony chloride (prepared by dissolving black sulphide of antimony in hydrochloric acid) is acted on by a current of sulphuretted hydrogen gas, has disadvantages in the apparatus necessary, in the limited range of tints which can be produced, and in the almost certain presence of free sulphur in the finished pigment.

Antimony vermilion forms an exceedingly useful pigment, which can be prepared in every shade of red, from orange to red-brown. It is produced in the condition of a very fine powder, requiring no grinding, and mixes readily with water or oil, especially the latter, and moreover does not interfere with the drying of the oil. It possesses great covering powers, and can be made at a low price. It undergoes no change in strong light and impure air, and is insoluble in water, alcohol, essential oils, weak acids, ammonia, and alkaline carbonates; but it is destroyed by high temperatures, strong acids, and caustic alkalies. It cannot be mixed with other pigments which are intolerant of sulphur, nor with alkaline vehicles. When pure, it should consist of nothing but antimony sulphide and a little water; the presence of iron or lead indicates adulteration.

Baryta Red.—An orange red may be prepared, according to Wagner, in the form of a sulpho-antimonite of barium, by calcining in a clay or graphite crucible at red heat for several hours a mixture of 2 parts of finely powdered barytes, 1 part of native antimony sulphide, and 1 part of powdered charcoal. The calcined mass is not removed until the crucible is quite cold, as it is liable to undergo combustion. When cold, it is boiled in water and filtered. The residue, containing some undecomposed sulphate and sulphide of barium, is utilised in the next batch. The pale-yellow filtrate is treated with dilute sulphuric acid, by which sulphuretted hydrogen is driven off, and an orange precipitate is thrown down. This is collected, washed on a filter, and dried, constituting the pigment.

Cassius Purple.—This costly pigment is a stannate of protoxide of gold, much used in painting on porcelain and for miniatures. It is the precipitate which is thrown down when solutions of gold and tin chlorides are mixed under proper conditions, according to one of the following methods:—

(1) Buisson prepares three solutions: [a] a neutral solution of protochloride of tin by dissolving 1 part of tin in hydrochloric acid; [b] a solution (bichloride) of 2 parts of granulated tin in an aqua regia containing 3 parts of nitric to 1 of hydrochloric acid, removing the excess of acid; [c] a neutral solution of 7 parts of gold in an aqua regia composed of 1 part of nitric and 6 parts of hydrochloric acid. The gold chloride solution is largely diluted with water, and to it is added the solution b of bichloride, and finally the solution a of protochloride is introduced, a drop at a time, until the desired colour is produced in the precipitate. This last is rapidly washed by decantation, and finally dried away from the light.

(2) Figuier prepares a gold bichloride solution by dissolving 20 grammes of gold in 100 grammes of an aqua regia containing 4 parts of hydrochloric to 1 of nitric acid. The solution is evaporated to dryness in a water bath, and the residue is dissolved in 750 grammes of water. Into this solution, when duly filtered, pure granulated tin is introduced, and the whole is left for some days, at the end of which time all the gold will be in the state of stannate of protoxide; it is collected on a filter, carefully washed, and gently dried. The residues contain some gold, and should be preserved for subsequent operations.

Chinese Red.—One of the many names of the chromate of lead pigment, described under Derby red, see p. 145.

Chrome Orange.—A popular name for the group of yellow-red pigments consisting essentially of lead chromate, and described under Lead orange, on p. 147.

Chrome Red.—Another of the synonyms for Derby red, see p. 145.

Cobalt Pink.—This costly and permanent artists’ colour is a combination of oxide of cobalt with magnesia. It is prepared by treating carbonate of magnesia with a concentrated solution of nitrate of cobalt; the resulting paste is dried in a stove, calcined in a porcelain crucible, and finally ground to a fine powder.

Cobalt Red.—A very deep-coloured and permanent red pigment used in oil painting is the arseniate of cobalt, which is found native in admixture with other substances in cobalt mines, or may be artificially produced.

The native mineral is treated with boiling nitric acid; the solution is filtered clear, and small portions of potash are added till all the iron has been thrown down as arseniate. After this is completed, the mass is allowed to settle, and the clear liquor is poured off. On adding further small portions of potash, the cobalt is also precipitated as arseniate.

To prepare artificial cobalt arseniate, grey cobalt ore (sulph-arsenide of cobalt), reduced to a powder, is mixed with a little sand and twice its weight of potash, and fused in a crucible. The slag of mixed sulphides which is formed is removed, and the remaining white arseniate of cobalt is pulverised and subjected to another fusion with potash. The slag is again removed, and the button of pure arsenide of cobalt remaining is finely powdered and again roasted to effect conversion into arseniate of cobalt. Lastly, it is ground very fine.

Colcothar.—A fancy name for a kind of iron oxide pigment, described under oxide reds (see p. 150).

Derby Red.—As a basic chromate of lead, often known as chrome red, Derby red is closely allied to chrome yellow, the preparation of which is described in a subsequent chapter.

It has been asserted that all the chrome reds, from the darkest cinnabar red to a lustreless minium red, are distinguishable simply by the size of the crystals composing the powder, as may be easily determined under the microscope, and that if various chrome reds of the same hue, but with different intensities of colour, are reduced by grinding to the same degree of comminution, the several powders will possess exactly the same degrees of intensity of coloration, though they lose in brightness. Therefore the conditions which give brilliancy and intensity of colour are those which favour crystallisation.

On this supposition it is recommended by Riffault to adopt a plan which dispenses with agitation, and he supports the following method:—

(1) Chrome yellow is precipitated in the usual manner, as described in a later chapter, without sulphuric acid, and is carefully washed. After draining, the mass is well stirred, and six or eight equal samples are drawn from it and put into glass vessels of equal size and thickness of structure. To each sample is added a different volume of caustic soda or potash lye, marking about 20° B. For instance, to 5 volumes of paste are added 2, 2½, 3, 3½, 4, 5, &c., volumes of lye. The different mixtures are rapidly and thoroughly agitated, but the chemical reaction is allowed to take place without any disturbance. After examination of the quality of the products, the relative proportions of pulp and lye are noted down for the best hues obtained. Too much lye will fail to deepen the red colour; in fact, Derby red is entirely soluble in an excess of lye, and forms needle-like crystals holding potash when the caustic solution has absorbed carbonic acid from the air.

On the industrial scale the operation is conducted in a large tub, which receives the mixture of pulp and caustic lye in precisely the proportions found by experiment to give the best results. The changes in colour soon manifest themselves, and the whole reaction is completed in about 12 hours. At the end of that time, the lye is drawn off, and carries with it much of the chromic acid. The precipitated pigment is carefully washed with pure water once in the tub, and the mass is gently stirred. The washing is continued in the filters by throwing water upon the pulp, and in this manner there is less friction between the crystals, which retain their deep colour. Of course a highly crystalline dark red cannot possess great covering power.

(2) Prinvalt mixes together 100 lb. of lead carbonate and 30½ lb. of potash bichromate neutralised with caustic potash, in 50 gallons of water, leaving them in contact for a couple of days under repeated agitation. About half an hour’s boiling then suffices to develop the red colour. After settling, the supernatant liquor is drawn off, and the precipitated pigment is washed twice with pure water and finally with acidulated water (1 lb. sulphuric acid in 10 gallons of water), and dried.

There are several other recipes published which differ in detail from (2), but they do not demand a lengthy description.

(3) 100 lb. of lead carbonate (white lead) made into a paste with water, then added to and boiled with a solution of 50 lb. of potash bichromate and 15 lb. of caustic soda of 77 per cent. Remainder of process as before.

(4) 4 cwt. of lead monoxide (litharge) and 60 lb. of salt dissolved in 50 gallons of water, and left with agitation for 4 or 5 days; then boiled for 2 hours with solution of 150 lb. of potash bichromate.

(5) 100 lb. of lead carbonate (white lead) made into a paste with water, then added to a solution of 30 lb. of potash bichromate and 12½ lb. of caustic soda at 77 per cent., and boiled.

Derby red possesses great covering power and considerable brilliancy; but if not very carefully washed it is liable to retain a little alkali, which renders it unstable. Otherwise, it well resists damp, strong light, and impure air so long as sulphuretted hydrogen is absent. Taken altogether it is not one of the best red pigments, and its consumption is declining.

Indian Red.—This is one of the names for the red pigments due to oxide of iron, and is described under oxide red, p. 150.

Lead Orange.—Equally well known as chrome orange, this pigment may be regarded as a Derby red in which the reactions have been curtailed. That is to say, the yellow normal lead chromate being in excess, the red chromate formed by the action of the alkali combines with that excess of the yellow salt and forms a yellow-red, i.e. orange. Obviously, therefore, a great variety of tints can be produced by altering the proportions of the alkali, and this is further regulated by the duration of the boiling, while the tint can also be weakened by admixture of barytes or gypsum. The better kinds of lead orange are prepared with the aid of caustic potash or soda as the alkali, while the cheaper sorts depend on lime. The operations are practically identical with those adopted in the case of Derby red (see p. 145), the chief differences lying in the proportions of the ingredients. Thus:—