George Terry

Though the red pigments are an important class, they are not numerous, and, with the exception of a few lakes, they are drawn from the mineral kingdom. The most useful are compounds of the several metals, iron, lead, and mercury.

Antimony Vermilion.—This useful pigment is prepared by several methods, as follows:—

(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:—

(1) Pale.—Add a thin cream made from 10 lb. of quicklime to a chrome yellow made from 100 lb. of lead acetate, 30 lb. of soda or potash bichromate, and 21 lb. of soda sulphate. Boil.

(2) Pale.—Add a thin cream of 10 lb. of quicklime to a chrome yellow made from 200 lb. of baryta sulphate, 100 lb. of lead acetate, and 35 lb. of potash bichromate. Boil.

(3) Deep.—Precipitate a chrome yellow by adding 35 lb. of soda or potash bichromate to 100 lb. of lead acetate; settle. Draw off supernatant liquor and admit solution of 9 lb. of caustic soda at 77 per cent.

(4) Deep.—Add a cream of 10 lb. of quicklime to a chrome yellow made from 100 lb. of lead acetate, 75 lb. of baryta sulphate, and 35 lb. of potash bichromate. Boil.

In characters the lead oranges resemble Derby red (see p. 145.)

Minium.—The important red pigment known as minium or red lead is composed of two oxides of lead in combination, viz. about 65 per cent. of protoxide and 35 per cent. of binoxide. In its preparation, metallic lead is first converted by roasting into protoxide (termed “massicot,” “dross,” or “casing”) and this protoxide is further subjected to heat in a reverberatory furnace whereby a portion of it is changed into binoxide. It is also possible to produce red lead by the decomposition of the carbonate of lead (white lead) at a high temperature, but this does not seem to be an industrial process. The following methods are recognised:—

(1) The practice in France, as carried on near Tours, at the white lead works using the ThÉnard process, is to calcine the best metallic lead in reverberatory furnaces built in the rock. These furnaces are five in number, with double fireplaces, four being constantly in operation, dealing with about 4000 lb. at a charge, and using bituminous coal as fuel. Each furnace is nearly circular in shape and about 11 feet in diameter, with a fire-place on each side of the hearth. The latter is constructed of fire-brick containing as little silica as possible, and is made hollow so as to retain the metallic lead when the heat has rendered it fluid.

The products of combustion from the side fire-places, having heated the hearth and its contents, pass through an aperture in front of the charging door of the hearth, and thence go to furnish heat to an upper hearth where the conversion of the oxide into red lead takes place.

A period of about 12 hours is occupied in the oxidation of a charge, which is repeatedly “rabbled.” Even then a considerable amount of the metallic lead remains unoxidised and is returned to the calciner with the next charge. Half the oxide is utilised for making white lead, as described in a later chapter, and the other half is converted into red lead by the method detailed hereunder.

The crude oxide is pulverised in a small mill and separated from the unconverted metal. The mill takes the form of a flat circular cast-iron plate on which rotates a cast-iron muller. Water and an agitating arrangement are also provided.

As the muller revolves the material undergoes comminution, and the small particles of oxide as formed are disturbed by the agitator and kept in suspension in the water, by the overflow of which they are continuously carried away into settling pits. The residual metallic lead is not pulverised, and of course never becomes suspended in the water, consequently it accumulates at the bottom of the mill, whence it is occasionally withdrawn for re-calcination.

Sufficient oxide having collected in the settling pits, it is transferred to a shallow pan heated by the waste heat from the furnaces and is there rendered almost dry. In this state it is put into small square dishes made of sheet iron, and adapted to hold about 30 lb. each.

A charge consists of a hundred of these dishes, which are placed in the heated furnaces at the end of each day. The roasting is repeated several times, and the product is accordingly known as “two fires,” “three fires,” &c. The material at this stage is lumpy and coarse, and has to undergo dry pulverisation, the fine particles as they are produced being drawn off by means of a pneumatic fan, and collected.

(2) What may in contradistinction be called the English method of making minium does not differ materially from the preceding. The “drossing” furnace, where the metallic lead is first oxidised, receives a smaller charge as a rule, and perhaps greater care is given to the rabbling, and to the regulation of the temperature so that it is only just above the melting point of the metallic lead, and not sufficient to fuse the massicot.

Minium or red lead is one of the most important and useful red pigments, as it mixes well with oil, has good covering power, dries quickly, and is permanent except in presence of sulphur or sulphides.

Orange Mineral.—The pigment known as orange mineral or orange lead is simply minium which has been imperfectly calcined. Consequently it is almost identical with red lead in composition, qualities, and method of manufacture, the only exception being that, as the calcination is not carried quite so far, therefore the colour is not so fully developed, and is an orange rather than a red. As with minium, practically the only adulterant is iron oxide red, which may be detected by boiling the pigment to a colourless solution with nitric acid, when addition of prussiate of potash will give a blue precipitate.

Oxide Reds.—Under various names—such as Persian red, light red, Indian red, scarlet red, rouge, colcothar, red oxide, purple oxide, &c.—many pigments, of which the base is the ferric oxide Fe₂O₃, are now made. These vary in shade from a deep scarlet red to a dark violet. They are obtained both from natural and artificial sources. Oxide of iron occurs naturally as the mineral hematite, and some varieties of this are bright enough and soft enough to be used as pigment when ground up. These are usually nearly pure oxide of iron. Then the ochres, when calcined, yield red pigments known as light red, Indian red, &c., and a good many reds are obtained from this source. The composition of these is variable, being dependent upon that of the ochres from which they are made, and these, as has already been pointed out, vary very much. Then, in preparing fuming sulphuric acid from copperas, oxide of iron which is specially sold as rouge, is obtained. Colcothar is produced as a residue; this is nearly pure oxide of iron, and usually has a red colour. In the manufacture of sulphuric acid from pyrites, a dark violet oxide of iron is left as a residue, and much of this is used as a pigment under the name of purple oxide. Then a large quantity of oxide of iron reds are made artificially from waste liquors obtained in copper refining, galvanising iron, &c. The composition of the oxide of iron reds, therefore, is very variable.

The whole group of oxide reds is of foremost importance, by reason of their good colour, covering power, and durability, besides which, being mostly bye-products of much more important manufactures, their cost is reasonable.

The methods of preparation of oxide reds vary slightly in detail according to the material from which they are made, but the general features of the processes are almost identical and eminently simple. The principal sources are impure native oxides of iron, such as the ochres, various waste liquors containing iron salts in solution, and copperas (protosulphate of iron).

(1) Native oxides. The iron present in the ochres and similar native earths exists in the form of hydrated oxide, and has a brown red colour. For many purposes this hue is satisfactory, and the preparation of such a pigment consists simply in grinding the mineral in a wet mill, subjecting it to levigation till all grit is removed, and drying.

In order to obtain a brighter red from the native oxides they must be calcined to effect dehydration. This can be accomplished in the most rudimentary forms of furnace, and many kinds are in use. The colour produced depends on the degree and duration of the heat to which the material is exposed, the shade becoming deeper as the roasting is prolonged or the temperature increased. As no two samples of ochre are just alike it is impossible to fix a precise time for the length of the operation, and therefore it is necessary to repeatedly draw samples in order to judge of the progress of the dehydration and development of the colour desired. When the requisite shade is attained, the charge is drawn and allowed to cool.

(2) Waste Products. The pyrites cinders from sulphuric acid works afford an abundance of oxide of iron. When the pyrites has contained no copper, the cinders merely require grinding and levigating, the iron being present as oxide. But when the pyriteshas been treated for the recovery of the copper, by a second roasting with salt, the liquors contain the iron as chloride and sulphate, and lime has to be added to precipitate the oxide. This last is dried and calcined in the same manner as the native oxides, and grinding and levigation can be dispensed with.

The liquors from galvanising works contain acid sulphate of iron (green copperas) in solution. To correct the acidity, more iron is added in the form of scrap. Then lime or other alkaline substance is introduced to throw down the iron as oxide, and this last is filtered out, dried, and calcined in the usual way.

(3) Copperas. Where beds of common iron pyrites occur, the iron sulphide is converted into sulphate by exposure to the oxidising influence of the air. The result is an acid sulphate of iron, which is leached out and neutralised by addition of more iron in the form of scrap. The neutral sulphate is crystallised out of the liquor, and calcined in a muffle furnace, the shade of the ultimate product being governed by the degree or duration of the roasting. The sulphurous acid liberated in the roasting is sometimes utilised for making sulphuric acid, but is more often wasted, because, to be commercially successful, the sulphuric acid manufacture must be conducted on a large scale, demanding 100l. of capital for every 1l. necessary for the copper and red oxide fabrication.

Persian Red.—A name which is used somewhat indiscriminately both for Derby red (p. 145) and for oxide red (p. 150).

Realgar.—The native mineral realgar is a yellow-red bisulphide of arsenic, often called also ruby of arsenic, or arsenic orange. It occurs native in very limited quantities in some of the older rocks, and then only requires to be ground and levigated. But for painters’ purposes it is prepared artificially by heating a mixture of sulphur and arsenic in such a way that they are melted in company and react on each other to form the arsenic sulphide. The heating takes place in crucibles, and the proportions are two parts by weight of arsenious acid (white arsenic) to one of flowers of sulphur. When the reaction has ceased, the contents of the crucible are allowed to cool, and then reduced to very fine powder.

The pigment is exceedingly poisonous and not remarkably durable, besides which, it cannot be mixed with any other pigment which is affected by sulphur.

Red Lead.—A common name for minium, see p. 148.

Rouge.—One of the names for a particular shade of the oxide reds, see p. 150.

Venetian Red.—A fancy name for a special shade of oxide red, see p. 150.

Vermilion.—This old pigment is gradually going out of use; the newer reds, which are more brilliant in colour and cheaper, are gradually displacing it, although it is doubtful whether it will ever go completely out of use. It is the mercuric sulphide HgS. When pure, it is not attacked by acids or alkalies; only aqua regia, a mixture of hydrochloric and nitric acids, is capable of dissolving it, when it forms a clear solution. Heated in the flame of a Bunsen burner, it is completely volatile, a property possessed by no other pigment in common use, therefore any adulteration can be readily detected by simply heating a little vermilion in a crucible; if a known weight is taken and the residue is weighed, the amount of adulteration can be ascertained. Vermilion is chiefly adulterated with oxide of iron and orange lead. From the character of the residue left on heating in a crucible, the kind of adulteration can be readily ascertained.

(1) The following notes are taken from Christy’s translation of a brochure on the Imperial Quicksilver Works at Idria, Krain:—

In the oldest times of the existence of the present works, vermilion was manufactured. In the beginning it was merely pure pulverised cinnabar ore, then later it was a product made by sublimation from this substance; and there were formerly other works for vermilion manufacture than those for quicksilver production. When the Venetians and Dutch began to produce better wares, the production here sank steadily.

The researches of Christofoletti, 1681, and of Baron Richtenfels, 1726, for the improvement of Idrian vermilion, met with as little success as those of some Venetian women—1740-1741—who had lost their husbands in the Venetian works and had offered themselves to manufacture vermilion according to the Venetian method.

After Hacquet had strongly urged the manufacture of vermilion, OberhÜttenmeister Ignaz v. Passetzky succeeded, with the Dutchman Gussig assisting him, in making beautiful cake cinnabar in 1782, and in 1785 vermilion also, in the newly-built works on the right bank of the Idriza.

In 1796 OberhÜttenverwalter (manager of the works) Leopold v. Passetzky introduced the sublimate and precipitate manufacture, but it was abandoned as unprofitable in 1824.

The many foreign attempts to manufacture vermilion in the wet way caused similar ones here, as those of Fabriks-Controlor Rabitsch in 1838, and later of HÜttenverwalter M. Glowacki, which brought large amounts of the vermilion so manufactured into the market. Still this manufacture came to no full development, and became forgotten, until, finally, in the years 1877 and 1878, experiments led to its being discontinued on account of the costliness and uncertainty of the method. A new set of experiments in 1878 and 1879, by Assayer E. Teuber and Director of Works (HÜttenverwalter) H. Langer, under the direction of the Imperial Agricultural Ministry, led to favourable results. A new manufactory, set in operation in 1880, furnishes three sorts of vermilion manufactured in the wet way.

The arrangements of the works for the manufacture of vermilion in the dry way consist of:—One sulphur stamp battery. One amalgamating plant with eighteen small barrels; both pieces of apparatus being driven by a two horse-power water-wheel. Four sublimation furnaces, each with six retorts of cast iron. Four vermilion mills, each driven by a water-wheel of 2·5 horse-power. Kettles and vats for heating, digesting, and refining the ground cinnabar. One drying hearth. The preparation of vermilion as an article of commerce, falls into several separate operations, viz.:

1. Amalgamation; i. e. preparation of the raw mohr.

2. Sublimation; i. e. preparation of the cake cinnabar.

3. Grinding of the cake cinnabar, refining and drying of the vermilion.

For the preparation of the raw mohr, for each charge of eighteen kegs there are taken 80·64 kg. (117½ lb.) powdered and sifted sulphur, and 423·36 kg. (731½ lb.) of quicksilver.

The amalgamating kegs each contain 28 kg. (61½ lb.) of the charge, and are given intermittent rotating motion by a rack and pinion driven by a water-wheel. After an average of two and three-quarter hours, the amalgamation is complete, and the raw mohr is taken from the casks.

For the sublimation, four furnaces are used, each with six pear-shaped cast-iron retorts of considerable thickness. Each is charged with 58 kg. (127½ lb.) of mohr, the mouth covered with a loosely placed sheet-iron helmet, the furnace being slowly fired; the combination of the sulphur and the quicksilver then results in about fifteen minutes, with a detonation. As soon as this operation (das Abdampfen) is over, a clay helmet is placed over the retort, and the firing is increased, so that after two hours and twenty minutes the excess of sulphur evaporates from the tube. The condenser is now added (StÜckperiode—Cake-period) and luted, then the firing is still more urged, whereupon the cinnabar volatilises and deposits itself upon the glazed earthenware condensation apparatus (tube, helmet, &c.). After four hours, the sublimation is complete, and there is furnished by the helmet 69 per cent., by the tubes 26 per cent., by the condenser (Vorlage) 2 per cent., cinnabar.

The grinding of the cake cinnabar takes place in four mills driven by an undershot water-wheel. These mills have a fixed under and upper movable stone, and the grinding is done with water. The vermilion which leaves the spout and runs into glazed clay vessels has a temperature of about 100° F., that of the air being 59° F. The millstones make forty revolutions per minute, and after each passage of the charge are placed nearer together.

(2) A German chemist named Fleck has discovered that when a warm solution of hyposulphite of soda is added to a double salt of mercury, such as chloride of mercury and sodium, the solution becomes acid, and black sulphide of mercury is deposited. But if the hyposulphite solution is added in excess, and the temperature is not allowed to rise beyond 140° F., the solution remains neutral, and red sulphide of mercury, or vermilion, is deposited. The least quantity of acid causes the production of the black sulphide. The presence of a salt of zinc facilitates the production of the vermilion. The best method is as follows:—To four equivalents of hyposulphite of soda mixed with four equivalents of sulphate of zinc in diluted solution, is added, drop by drop, a solution containing one equivalent of corrosive sublimate. The whole is gently heated for 60 hours, at a temperature of 112° to 130° F.

(3) The following account of vermilion manufacture in China appeared over the initials T. I. B., in the Chemical News.

The Chinaman has no knowledge whatever of chemistry, and of the principles of natural philosophy and statics generally his notions are of the most rudimentary and primitive description. How, then, in the face of these obvious disadvantages have the Chinese contrived to place themselves in the front rank amongst nations in the matter of certain chemical manufactures, one of the most important of which is the subject of this article—Vermilion?

We have seen with what ingenuity and pertinacity in carrying out his ends the Chinaman has succeeded in making perhaps the most delicate and perfect iron castings in the world. He has succeeded in that instance, not by any deep researches into the hidden mysteries of Nature, by no process of thought involving an enquiry into the “reason why”; to this the Chinaman is averse, the whole tendency of his education, such as it is, tends to make him satisfied with observing effects; it is sufficient to him to know that things are so, without going into troublesome or elaborate investigations into those changeless laws of Nature into which his philosophy teaches him that, as he cannot alter or control, research is fruitless: but that he has in his own small, ingenious, patient way observed effects to very good purpose, the unrivalled excellence of some of his manufactures testifies.

We will now enter a vermilion manufactory and watch the process from the first stage of mixing its two ingredients—mercury and sulphur—to the final process of weighing and packing this costly and beautiful pigment for the market.

The first objects to attract the visitor’s attention on entering the yard attached to the works will probably be large piles or stacks of charcoal, crates or baskets of broken crockery ware, and numerous rusty old iron pans of somewhat similar shape to rice pans, but considerably thicker and heavier. There will also probably be a few broken and disused cast-iron mortars. All these articles are the cast off or worn out implements of the manufacture, and will be described in their proper order.

On entering the factory proper, scores of little stone mills, each being turned by one man, and other long rows of workmen weighing out and wrapping up the vermilion, will be seen. The furnaces are then arrived at: there may be a score or more in number, and may be ten or twelve in each furnace room, five or six on each side. After passing these, the stores of quicksilver, sulphur, alum, glue, new spare iron pans, serviceable crockery ware, and sieves and other utensils used in the factory are arrived at, and this completes the view of the works.

The iron pans in which the vermilion is sublimed are those referred to above; they are circular and hemispherical in shape; all are of the same size and weight; they are cast upside down, and in the casting, a runner or lump of iron, two and three-eighths inches in diameter by from six-eighths to one inch in depth, is purposely left on every pan in order to enable the workman the more readily to handle the pan when stirring up its contents. The size of the pans proved by actual measurement to be 29¼ inches in diameter, by 8? inches deep, and the weight 40 catties, or say about 53 lb.

These pans are set in rows of 5 or 6 on each side of a small rectangular room, in size some 12 feet by 15 feet; the door of this room is of wood and contains an aperture a few inches square in order to enable the workman to watch the progress of his operation, from time to time, without the necessity of lowering the temperature of the apartment by opening the door. The pans are set in brickwork, each pan having beneath it a grate to hold the charcoal used as fuel. There is no communication between the grates or furnaces under each pan, and no chimney, the flames and products of combustion finding exit from the front of the grate, which is left wholly open at all stages of the operation.

The process of manufacture is as follows:—Taking an iron pan which is of 4 inches smaller diameter than those described, and also in all other respects proportionally less, except the runner, which is of the same size, a skilled workman proceeds to weigh out 17? lb. of sulphur. This he places in the pan, and adds about half the contents of a bottle of quicksilver. The pan with its contents is then put upon a small earthen brazier or portable furnace, the fuel used in which is charcoal. When the sulphur is sufficiently melted, the workman, taking an iron spatula or stirrer, rapidly stirs up the quicksilver with the sulphur, and gradually adds the remaining contents of the bottle of quicksilver, stirring the two ingredients together meanwhile until the mercury has wholly disappeared, or “been killed,” as the Chinese put it.

When this takes place, the pan is removed from the fire, a small quantity of water is added, and rapidly stirred up with the contents of the pan, which have now assumed a dark blood-red appearance and semi-crystalline structure. This mass is then turned out of the pan into an iron mortar, and then broken up into a coarse powder. This forms a charge for one of the large pans previously described, and when sufficient material has been prepared to charge all the pans in one furnace chamber the sublimation is proceeded with as follows:—

All the pans having received their quantum of crude vermilion, this is covered with a number of crockery-or porcelain-ware plates, of tough, strong manufacture, each about 8 inches in diameter; some of these plates, however, are broken up, and are in a more or less fragmentary condition. When these plates have been piled up into a dome-shaped heap of the same shape as the bottom of the upper pan, to which they should extend, the whole is covered with one of the smaller pans previously described.

Now it will be remembered that the smaller pan was of 4 inches less diameter than the larger one; there will consequently be a circular space two inches all round between the circumferences of the pans. Consequently the rim of the upper or covering pan will be about 2 inches lower than the rim of the lower pan; there will also be some 4 inches space horizontally between the rim of the large lower pan and that portion of the smaller pan which is at the same height as the rim of the larger one. This space is carefully filled with a clay luting into which some holes, generally about four in number, are pierced, extending down to the rim of the smaller pan or cover; this is done in order to allow the heated air and other matters to escape.

All the pans in one furnace chamber being thus charged and covered, the fires are lighted. The flames from the charcoal should occasionally play several feet above the mouths of the furnaces. The door of the chamber is kept closed, except when it is open for a moment in order to enable the workmen to replenish the fires, which must be kept up at a fierce heat for eighteen hours. During this process a blue lambent flame is seen to play above each of the four holes which are pierced through the clay luting of the pans, so it is evident that a considerable quantity of either one or probably both the ingredients is wasted. After eighteen hours the fires are allowed to go out, and the contents of the pan cool down.

When this is accomplished, the greater portion of the vermilion will be found adhering to the lower surface of the broken-up porcelain plates with which the crude product is covered. The vermilion is then carefully removed from the porcelain by means of chisels, and is now ready for the elutriating mills. Another portion of vermilion of not so good quality is found adhering to the upper iron pan, and that obtained by washing the clay luting in a cradle, as diggers wash dirt for gold. This, together with the wipings and scrapings generally, is mixed up with alum and glue-water into cakes, and, after drying on a brick surface heated beneath by means of wood or charcoal, is powdered up on a mortar, and re-sublimed when a sufficient quantity has accumulated.

The vermilion which was removed from the porcelain plates is of a blood-red colour and crystalline structure. This is then powdered up in a mortar and removed to the levigating mills. These are the ordinary little horizontal stone mills used by Chinese and other natives of the East to grind rice and other grain into flour or pulp, as the case may be. Each stone is about 2½ feet in diameter; the lower stone is stationary, the upper is turned by a direct-acting piece of wood having a hole in it which works a wooden peg affixed to the upper stone, which is made to revolve by a backward and forward movement of the piece of wood, or handle, some 3 or 4 feet long, previously mentioned. One man turns each mill. The upper stone has a small hole in it near its centre, down which the workman from time to time pours a little spoonful of the powdered vermilion, which he washes down into the mill with water; as he turns the mill, the workman keeps continually ladling little spoonfuls of water down the aperture or hole in the upper stone; the ground and thus elutriated vermilion, as it escapes from between the stones, is washed down by the water into a vessel placed beneath to receive it.

When work is suspended for the evening, the ground vermilion is carefully stirred up with a solution of glue and alum in water, in the proportion of about an ounce of each to the gallon. The glue has been made to mix with the water by previously heating it with a small quantity of water; the earthen pots in which this process is effected each hold about 6 gallons. The mixture is then left to settle. In the following morning the mixture of glue and alum is poured off the vermilion, and the upper portion of the cake of vermilion at the bottom of the vessel—that is, the portion which remained longest suspended in the liquid—will be found to be in a much finer state of subdivision than the lower portion, which requires to be again elutriated as on the previous day: this separation of the more finely divided vermilion from that which was coarser, by suspension in a dense medium, is a really most ingenious process, for which we should give the Chinaman every credit.

The process of grinding, elutriation, and separation of the coarsely ground from the fine vermilion, sometimes requires to be several times repeated, in order to fully bring out the colour. As a final process the damp cake of finely ground vermilion is stirred up with clean water, and allowed to settle down until the next morning, when the water is carefully poured off into large wooden vats to still further deposit a small quantity of vermilion yet remaining in suspension, and the vermilion is dried in the open air on the roof of the premises.

When quite dried, the cakes of now full-coloured pigment are carefully powdered, and sifted by means of square muslin-bottomed sieves, contained in a covered box some 2 feet high by 2½ wide, in which the sieves, which slide on a framework inside the box, are jerked backwards and forwards by means of a handle on the outside of the box or case containing them.

The now fully-prepared vermilion is removed to the packing house, where may be seen rows of workmen, men and boys, seated before a series of tables. Between every two workmen is a third, with a small pair of scales, which he holds in his left hand; and as the workmen on either side place before him the little pieces of paper in which the vermilion is to be wrapped up, he weighs into each paper one tael (about an ounce and a third avoirdupois) weight of vermilion; the papers are two in number, the inner a black or prepared paper, and the outer a piece of ordinary white paper. After being wrapped up, the packets are placed in rows before another workman, who stamps them with a seal containing in Chinese characters the name and address of the manufactory in which the article has been made, and the quantity and quality of vermilion contained in the packet.

The rapidity and deftness of the Chinese workmen at this employment is really surprising; the stamping, for instance, is effected at the average rate of sixty impressions per minute, and the wrapping up is carried on with proportionate rapidity. The mixture of alum, which is the ordinary aluminium potassium sulphate, with the vermilion, in one of its stages of manufacture as described above, is not added, as at first sight we thought it might be, merely to assist in clarifying or purifying the water by causing it to deposit its sediment, but seems to have some peculiar effect upon the colour, although what may be the rationale of the process, or how it acts, we cannot quite clearly see. The glue is added as described above merely to favour separation of the finely elutriated vermilion by holding it longer in suspension than the coarser particles, which sink first, and may therefore be separated in their order of stratification.

The actual composition of vermilion is 100 parts of mercury to 16 of sulphur, when both these ingredients are in a perfectly pure state; the excess of 5? lb. of sulphur added by the Chinese is probably volatilised and lost in the process of sublimation, or, as the sulphur used is generally not quite pure, a part may go for foreign matter contained in the sulphur; the balance being probably the raison d’Être of the blue lambent flame seen playing over the apertures in the luting during the sublimation process. For a people having, like the Chinese, no acquaintance with even the first rudiments of chemistry, the proportion of ingredients taken—56¼ catties to 13 catties, or say 75 lb. to 17? lb.—shows wonderfully accurate powers of observation and a knowledge of combining proportions only to be gained by much experience and a long extended series of careful observations highly creditable to the manufacturers. The entire process is one of the most ingenious and interesting to be seen in any part of the world.—(T. I. B.)

Another and briefer account of the Chinese vermilion manufacture is given by H. Maccallum, in the Proceedings of the Pharmaceutical Society.

He says there are three vermilion works in Hong Kong, the method of manufacture being exactly the same in each. The largest works consume about 6000 bottles of mercury annually, and it was in this one that the following operations were witnessed:—

First Step.—A large, very thin iron pan, containing a weighed quantity, about 14 lb., of sulphur, is placed over a slow fire, and two-thirds of a bottle of mercury added; as soon as the sulphur begins to melt, the mixture is vigorously stirred with an iron stirrer until it assumes a black pulverulent appearance with some melted sulphur floating on the surface; it is then removed from the fire and the remainder of the bottle of mercury is added, the whole being well stirred. A little water is now poured over the mass, which rapidly cools it; the pan is immediately emptied, when it is again ready for the next batch. The whole operation does not last more than ten minutes. The resulting black powder is not a definite sulphide, as uncombined mercury can be seen throughout the whole mass; besides, the quantity of sulphur used is much in excess of the amount required to form mercuric sulphide.

Second Step.—The black powder obtained in the first step is placed in a semi-hemispherical iron pan, built in with brick, and having a fireplace beneath, covered over with broken pieces of porcelain. These are built up in a loose porous manner, so as to fill another semi-hemispherical iron pan, which is then placed over the fixed one and securely luted with clay, a large stone being placed on the top of it to assist in keeping it in its place. The fire is then lighted and kept up for sixteen hours. The whole is then allowed to cool. When the top pan is removed, the vermilion, together with the greater part of the broken porcelain, is attached to it in a coherent mass, which is easily separated into its component parts. The surfaces of the vermilion which were attached to the porcelain have a brownish red and polished appearance, the broken surfaces being somewhat brighter and crystalline.

Third Step.—The sublimed mass obtained in the second step is pounded in a mortar to a coarse powder, and then ground with water between two stones, somewhat after the manner of grinding corn. The resulting semi-fluid mass is transferred to large vats of water and allowed to settle, the supernatant water is removed, and the sediment is dried at a gentle heat; when dried, it is again powdered, passed through a sieve, and is then fit for the market.—Proc. Pharm. Soc.

(4) Firmenich describes a process which he declares gives better results in the beauty of colour than any other. It consists in using sulphide of potassium, which must be in a state of great purity. Of the various methods for preparing potassium sulphide, Firmenich rejects those in which caustic potash lye is boiled with excess of flowers of sulphur, on account of the simultaneous formation of a hyposulphite or sulphate of potash, which interferes in the preparation of the vermilion.

The process adopted by Firmenich for making pure potassium sulphide is to reduce sulphate of potash by heating with charcoal, and subsequently saturating the lye with sulphur to the necessary degree.

Usually about 20 parts by weight of potassium sulphate and 6 parts by weight of charcoal are reduced to very fine powder and thoroughly incorporated. Placed in a Hessian crucible, the mass is covered, luted, and strongly heated. As considerable ebullition takes place the crucible should be of such a size that the charge only occupies two-thirds of its capacity. After fusion is complete, the mass, which is now potassium sulphide, is allowed to cool; it presents a reddish-brown crystalline appearance, and is very hygroscopic. It is put into a cast-iron pan, with addition of soft water in the proportion of 7 parts of water to every 2 parts of the potassium sulphide; after boiling, it is filtered and on cooling, the undecomposed sulphate of potash collects in crystals attached to the sides of the pan.

The thus purified lye is boiled a second time with flowers of sulphur, added in small doses until saturation is indicated by bubbling and effervescence. The simple (monosulphide) potassium sulphide in this manner takes up four additional atoms of sulphur, and becomes the pentasulphide.

The preparation of the vermilion then proceeds in the following manner:—Into a series of large flasks are put 11 lb. of mercury, 5 lb. of the potassium sulphide lye, and 2¼ lb. of sulphur. The contents are subjected to a moderate heat, and the flasks are then agitated in a curious manner by arranging them in pairs in baskets suspended from strings, over a straw mattress, on which the baskets bump each time they descend.

Occasionally the flasks are turned about, and after about two hours of this agitation they commence to grow hot, and the contents assume a greenish-brown colour. The lye is robbed of its sulphur by the mercury, and replenishes itself from the excess added.

Complete combination of the mercury and sulphur is accomplished in about three and a half hours, when the colour of the mass becomes dark brown. The next step is to cool the compound, an operation which must proceed very slowly, and should occupy about five hours.

Development of the colour is effected by heat, for which purpose the flasks are placed in a stove room or water bath, and subjected to a temperature which does not fluctuate beyond 113° and 122° F., under the influence of which the red colour appears. The greatest care is necessary in this heating process, as it determines the success or failure of the colour. It lasts several days, during which the flasks should be shaken three or four times daily.

In order to separate the vermilion from the excess of sulphur, water is added to the contents of each bottle, and, after thorough shaking, the whole is turned out into a filter. The clear liquor escapes, and the residual vermilion is mixed with caustic soda lye in stoneware jars, and thus the remaining free sulphur is dissolved out. Subsequently the lye is poured off as completely as possible, and the deposit is repeatedly washed, first by decantation and finally on a filter. The whole operation of filtering and washing cannot be completed in less than two or three days. When this is finished, the drying must be carried on at a very low temperature, till the vermilion can readily be broken and is dry to the touch, when it is put into iron basins and repeatedly stirred, while the temperature is allowed to reach 143° F., but never beyond that. The final desiccation occupies about five hours.

Vermilion made in this way is reputed more permanent and less costly than by the usual methods.

(5) Dutch vermilion has a good name, and one method adopted in Holland is as follows:—A mixture of 2 lb. of mercury and 1 lb. of sulphur is thoroughly ground, and to 100 lb. of the mixture are added 2½ lb. of minium or of granulated lead. About 2 cwt. of the compound is put into each sublimation pot, which is duly heated. When the operation is finished, the pots are allowed to cool for eighteen to twenty hours, when they are broken, and their contents are ground in a mill. The lead remains as a sulphide in the bottom of the pots.

(6) A modification of the Dutch method consists in making an intimate mixture of 54 lb. of mercury squeezed through chamois leather and 7½ lb. of flowers of sulphur, which is then moderately heated on a shallow iron dish, and the resulting black sulphide (“ethiops”) is coarsely broken, ground, and kept in pots. To convert the ethiops into vermilion, the former is put into large clay crucibles in a furnace, and heated to dark-redness, whereupon the mass takes fire. As soon as the flame has subsided, the crucibles are covered with a close-fitting iron plate, and the firing is continued for thirty-six hours. The mass is stirred every half-hour with an iron rod, and fresh additions of ethiops are made at four or five hours’ intervals. The vermilion is sublimed, and condenses on the cool portion of the interior of the crucibles, whence it is collected by breaking the crucibles when cold, and is finally ground and levigated.

(7) Kirchoff’s method requires special care, and consists in grinding 300 lb. of mercury with 68 lb. of flowers of sulphur in a mortar, the sulphur being first moistened with a few drops of caustic potash. The resulting black sulphide of mercury is added to 160 lb. of caustic potash dissolved in very little water, and the whole is heated for half an hour on a sand bath, with occasional addition of water to make up for loss by evaporation. Gradually the mass, under constant agitation, becomes brown and gelatinous, and finally red. Thereupon it is carried to the stove room and still agitated at intervals. After several washings it is drained, and dried very gently.

(8) Weshle mixes finely powdered cinnabar with 1 per cent. of antimony sulphide, and boils the mixture several times with three parts of potassium sulphide in a cast-iron pot. The precipitate is water-washed, digested with hydrochloric acid, washed again, and finally dried.

(9) Jacquelin takes 90 lb. of mercury, 30 of sulphur, 30 of water, and 20 of hydrated potash; the mercury and sulphur are put into a shallow cast-iron dish, dipping into cold water, and the potash solution is added by degrees while the mass is kept in agitation. Then the mixture is heated for an hour at 176° F., the evaporated water being replenished. The vermilion is washed in an excess of boiling water, and again several times in cold water, and finally filtered and dried.

Victoria Red.—One of the fancy names for Derby red. (See p. 145.)