substances, those rich in nitrogen being first added in small portions at a time. Their effect is to render the mass hard, dry, and difficult of fusion, whereupon the remainder of the animal charcoal should be promptly introduced. After thorough agitation, the working door is closed for a short time, and the contents of the furnace are rapidly discharged into a covered iron pan.

The character of the animal matters employed varies so much that it is impossible to lay down hard and fast rules for the proportions of the several ingredients, or the duration of the roasting. Nor is the value of a raw material always in proportion to its richness in nitrogen, because the poorer material may waste less potash, consume less fuel, and require less labour. The addition of iron filings or turnings is useful only in prolonging the life of the cast-iron crucibles.

Combination of the Cyanide and Iron Solutions.—A great number of recipes are in vogue for combining the two solutions of ferrocyanide and an iron salt, both with reference to their proportions, and to the addition of foreign matters of various kinds. These variations in the formulæ give rise to distinct names for some kinds of Prussian blue, which will be referred to below. The ordinary common Prussian blue has a greenish tendency, and is chiefly made according to one or the other of the following directions:—

(1) Mix a solution of 100 lb. yellow prussiate with a solution of 100 lb. green copperas (ferrous sulphate) and 18 lb. alum, to which 9 lb. sulphuric acid has been added, and let the mixture stand for 2-3 hours, or until the solid portion has completely settled out. Decant the clear supernatant liquor, and well wash the precipitate with clean waters. Finally throw it on a filter and subject it to repeated disturbance, so as to ensure the admission of air to every particle, in order that the requisite oxidation may take place. The proportion of alum used is subject to very great variation according to individual fancy; it renders the subsequent grinding of the pigment a very much easier matter, but it causes the shade of blue to be paler than it otherwise would be.

(2) The simple solutions of green copperas and yellow prussiate in equal proportions are mixed together without any other ingredient being added, and the precipitate produced is washed, filtered, and aërated as in (1). It is, however, inferior by reason of the oxide of iron formed in the pigment spoiling the purity of the colour, and necessitating the treatment of the wet mass with hydrochloric acid, at some expense, for removal of the iron oxide.

Antwerp Blue.—This pale variety of Prussian blue has but little importance now. It is prepared by adding a solution of 4 lb. yellow prussiate in 5-6 gallons of water to one of 2 lb. sulphate of iron, and 1 lb. each of alum and sulphate of zinc in an equal quantity of water. The resulting pigment consists of a mixture of the ferrocyanides of iron, alumina, and zinc; it is washed, filtered, aërated, and dried as other forms of Prussian blue.

Bong’s Blue.—When cyanide of potassium is added to an acid solution of a copper salt, a red colour is produced, which has already been mentioned by different observers. The substance formed is very changeable, at least in the liquid where it is formed. It is decomposed by acids, alkalies, cyanide of potassium, and even decomposes spontaneously, the colour changing to yellow. It is precipitated by insoluble cyanides; hence when a dilute acid is added to the red solution, the dye is at once thrown down along with the cyanide of copper. If the precipitate thus obtained is treated with sulphuretted hydrogen, it is decomposed and the substance is set free. This substance can combine with iron, like cyanogen, so as to conceal the properties of the iron. This compound is very permanent, and has lately been studied by Bong, who gives the following directions for its preparation:—

Cyanide of potassium is added in excess to an acid solution of a copper salt until the red colour at first formed has disappeared, when a ferric salt is at once added. On the addition of the iron salt, of course, a copious precipitation of Prussian blue takes place, and the liquid again turns to a dark purple-red. To separate the colouring substance from the alkaline salts in the liquid, a dilute acid is added, which precipitates it and the cyanide of copper. This precipitate is combined with the Prussian blue, which also contains a considerable quantity of the colouring substance, and then treated with a boiling solution of carbonate of ammonia, in which it dissolves. As the cyanide of copper also goes into solution, it is separated by again precipitating it with an acid, and treating the precipitate with sulphuretted hydrogen. The colouring substance thus liberated now contains a certain amount of hydroferrocyanic acid, which is removed after neutralisation by acetate of lead. It is now filtered, and the purification is completed by precipitating with a silver salt and treating the precipitate with sulphuretted hydrogen.

This purple-coloured compound crystallises very indistinctly. To determine its composition, Bong precipitated it with acetate of copper. When dried at 212° F., the rose-coloured precipitate had the following composition: Carbon 24·31, nitrogen 28·04, hydrogen 1·88, iron 13·66, copper 17·67, oxygen 14·44. Total 100·00. These numbers correspond to the formula Cu, Fe Cy₄ (HO)₄.

This substance is likewise precipitated by salts of zinc, mercury, and silver. All these precipitates are pink or purple, very beautiful, and of remarkable brilliancy. They are soluble in alkalies. Iron salts yield no precipitate, nor do lead salts, except in the presence of ammonia, when a blue-violet precipitate is formed. When treated with sulphuretted hydrogen, these precipitates yield purple-red and acid liquids, which undergo change in the air, especially if warm, forming Prussian blue. When these liquids are neutralised with alkali, purple compounds are formed, which are permanent in the air, soluble in water, slightly so in alcohol, and insoluble in ether. Their colouring power is exceptionally great. These pigments will unite with ferrocyanides, and in its preparation such a compound is produced in considerable quantity; it is likewise of a purple colour, and gives a rose-coloured precipitate with acetate of lead. Both alone and in this compound it is very permanent; it resists the action of sulphurous acid, concentrated and boiling alkalies, and dilute acids, but is rapidly destroyed by chlorine and nitric acid. If this pigment could be prepared cheaply enough, it would probably be used with advantage in the arts, on account of its resistance to chemical reagents and light, the variety of its shades, and its brilliancy. It does not colour fibres directly, but can readily be fixed on them from slightly acid solutions, if they are previously mordanted with metallic oxides.

Brunswick Blue.—This pigment is made in pale, medium, and deep shades, and is an extremely useful colour, being very fine, requiring no grinding, thoroughly permanent in light and air, hardly acted upon by acids, but turned brown by alkalies, and liable on standing to separate into two portions—a white and a blue—the latter coming to the surface while the former sinks, and necessitating a thorough stirring of the paint before use.

It generally consists simply of barytes, or gypsum, or china clay, coloured by a small percentage of Prussian blue, with or without the addition of a lesser proportion of ultramarine. The barytes or other base is very thoroughly agitated in water, while a solution of green copperas and a solution of yellow prussiate are gradually added without ceasing the agitation. When the incorporation of the ingredients has been completely accomplished, the precipitate is settled, washed, filtered, and dried. Following are a few recipes:—

In each case about 50-60 gallons of water are required.

To determine the amount of barytes present in a sample, boil about 50 gr. with caustic soda, filter, wash the residue free from soda, treat with sulphuric acid, well wash the insoluble residue, dry, and weigh.

Chinese Blue.—This well-known and favourite form of Prussian blue is prepared with great care, and is usually sold in fine powder or little cubes. Its composition is virtually identical with that of ordinary Prussian blue, but it is more free from impurities, and shows a fine bronze bloom or lustre on newly fractured surfaces. Being pure, it is entirely dissolved by oxalic acid; and its composition is about 52 per cent. oxide of iron, 43½ cyanogen, and 4½ water. In dyeing and calico-printing it is extensively employed. Its tint varies from greenish to violet, according to modifications in the method of manufacture, the chief difference being that yellow prussiate gives a greenish tone and red prussiate a violet.

The process of preparation is mainly as follows. In about 40 gallons of cold water dissolve 1 cwt. of green copperas selecting it carefully for freedom from insoluble oxide; add about 5 pints of sulphuric acid. This liquor very rapidly undergoes oxidation, by which oxide of iron is thrown down, and the solution is rendered unfit for making the best quality pigment. Therefore it should be prepared only immediately before it is used. In another vessel containing about 40 gallons of cold water, dissolve 1 cwt. of yellow prussiate (if a green shade is desired), or of red prussiate (if a violet tint is wished for). Even larger quantities of water may be used for the solutions, as the more dilute they are the finer is the colour precipitated and the greater the lustre on the surface of the finished pigment.

When the two solutions of yellow or red prussiate and acidified green copperas are brought together, a bluish-white precipitate is thrown down. This is allowed to completely separate itself, and then the clear supernatant liquid is drawn off.

The next step is to thoroughly oxidise the precipitate. This cannot be satisfactorily accomplished by utilising the oxygen of the atmosphere, as is done in other cases, because that method entails the production of a certain amount of oxide of iron, which prejudicially affects the purity of colour of the finished article. Of the chemical oxidising agents which are available, the most satisfactory in point of cost and efficiency is chloride of lime (bleaching powder). For each cwt. of green copperas, mix about 20 lb. of bleaching powder into a thin cream with water, and add it, in small quantities at a time, to the precipitate, constantly stirring so as to ensure the absorption of the whole of the chlorine by the blue. Without the addition be made gradually and under agitation, the chlorine will be generated more quickly than it can be absorbed, entailing a waste of gas and a noxious vapour to be breathed by the workmen. Sometimes the bleaching powder is added at an earlier stage, viz. to the green copperas solution, and in that case the blue assumes a violet tone.

After the addition of the bleaching powder solution to the bluish-white precipitate, it is acidified with hydrochloric acid, which develops the blue. When the whole has settled, the supernatant liquor is drawn off, and the blue powder is well washed and strained on a filter, then placed in pans and dried very gradually indeed in the dark, at a temperature never exceeding 130° F. The slower the drying the better is the gloss of the pigment. It is most essential that iron be excluded during the final grinding operation, or it may cause ignition of the mass, and its conversion into oxide of iron would speedily follow.

It has been proposed to treat the white precipitate (obtained in the usual manner from green copperas and yellow prussiate) by the chlorine contained in aqua regia (nitro-hydrochloric acid). The copperas, however, must be as free as possible from basic sulphate (oxide), which is ensured by keeping a little metallic iron in the acid solution of copperas. It is also desirable to effect the precipitation with crude prussiate, so as to avoid absorption of oxygen and premature development of the blue colour. Habich considers that the mistake is generally made of using too little copperas, and he has found that when 90 lb. of copperas have been added to 100 lb. of yellow prussiate, a drop of iron solution in the filtered liquor produces no precipitate, while the white precipitate has carried with it a certain proportion of prussiate, which can be washed out. He therefore proposes to avoid this waste by pouring the copperas solution into the prussiate solution, with constant agitation, till no further precipitate goes down, then adding one volume of the copperas solution equal to one-ninth of that already used. After fifteen minutes stirring, it is certain that all the prussiate carried down is decomposed.

The drained precipitate is blued (peroxidised) by adding aqua regia prepared several days previously, and in proportions depending on the strengths of the two acids. Generally, the aqua regia mixture will be 100 lb. of commercial nitric acid at 30° B. (containing 35·4 lb. of anhydrous acid) and 62·2 lb. commercial hydrochloric acid at 23° B. (containing 23·9 lb. of the anhydrous acid); and 40 lb. of this mixture will suffice for bluing the precipitate resulting from 100 lb. yellow prussiate. The addition of the aqua regia should take place in a wooden vessel with constant agitation.

According to another modification, the white precipitate obtained in the usual way is blued by adding a solution of perchloride of iron, which may be made from a hematite ore free from clay and carbonate of lime, or from rouge. The iron oxide, from whatever source, is ground to a very fine state, and treated with crude hydrochloric acid in a lead-lined tank, where the mixture remains for several days, and is constantly stirred. When saturated with iron the clear liquid is withdrawn for use. To receive it, the white precipitate is rapidly heated to boiling in a copper vessel, and is then transferred to a wooden vat, and the iron perchloride solution is stirred in till the desired tint is produced. The pigment is washed and dried in the ordinary way, while the supernatant liquor (essentially protochloride of iron) is poured over old scrap iron and used instead of copperas for a fresh batch of yellow prussiate.

A solution of perchloride of manganese may be used instead of perchloride of iron. Inferior qualities of manganese ore can be employed, and the residues left after treatment with hydrochloric acid may be washed and dried for sale as purified or peroxidised manganese.

Paris Blue.—(1) A synonym for the violet-tinted kind of Prussian blue.

(2) A series of compounds described below. [a] A thorough mixture of 2 parts sulphur and 1 part dry carbonate of soda is gradually heated in a covered crucible to redness or till fused; a mixture of silicate of soda and aluminate of soda is then sprinkled in, and the heat is continued for an hour; the little free sulphur present may be washed out by water. [b] An intimate mixture of 37 parts china-clay, 15 parts sulphate of soda, 22 parts carbonate of soda, 18 parts sulphur, and 8 parts charcoal, is heated in large crucibles for 24-30 hours; the mass is re-heated in cast-iron boxes at a moderate temperature till the desired tint appears, and is finally pulverised, washed, and dried. [c] Gently fuse 1075 oz. crystallised carbonate of soda in its water of crystallisation; shake in 5 oz. finely-pulverised orpiment, and, when partly decomposed, as much gelatinous alumina hydrate as contains 7 oz. anhydrous alumina; add 100 oz. finely-sifted clay, and 221 oz. flowers of sulphur; place the whole in a covered crucible, and heat gently till the water is driven off, then to redness, so that the ingredients sinter together without fusing; the mass is then cooled, finely pulverised, suspended in river-water, and filtered. The product is heated in a covered dish to dull redness for 1-2 hours, with occasional stirring. Colourless or brownish patches may occur, and must be removed.

Saxon Blue.—Following is a recipe for the preparation of this pigment, which possesses limited importance.

Dissolve 8 lb. alum and 1 lb. green copperas in 16 gallons of water. Add separate solutions of pearlash and yellow prussiate till precipitate ceases to go down. Collect the precipitate when it has completely settled; wash thoroughly, and dry.

Soluble Blue.—This term is applied to a variety of Prussian blue which, while possessing no difference in the matter of chemical composition, yet has the distinctive feature of being soluble in water, which the other varieties are not. It no longer enjoys the popularity it once had as a dye, on account of the severe competition of the coal-tar colours. Below are some of the most satisfactory formulæ for its preparation.

(1) Mix 10 lb. of Prussian blue thoroughly in about 10 gallons of cold water. Then add 5 lb. of yellow prussiate and let the whole mass boil steadily for several hours. Strain off the liquor and well wash the precipitate on a filter. Finally dry for use.

(2) Dissolve about 1 cwt. of red prussiate in water and make the solution hot. Prepare another solution of about 73 lb. of green copperas in hot water. Mix the two solutions together and boil them for about a couple of hours. Allow the solid matters to settle out, then put them on a filter and wash with clean water until a blue coloration manifests itself in the drainings. The blue residue is then dried as usual.

(3) Make one solution of 10 lb. of yellow prussiate, and another of 8 lb. of green copperas, water being the solvent in both cases. Mix these two solutions together and give them an hour’s boiling.

Add 3 lb. of a mixture of nitro-sulphuric acid, containing 2 parts of the former to 1 of the latter. Boil for another hour. Let the solid pigment precipitate itself thoroughly, and then filter, wash, and dry as in the other cases.

(4) Dissolve about 1 cwt. of perchloride of iron and 10 lb. of sulphate of soda in water. Also dissolve in another vessel 2 cwt. of yellow prussiate and 10 lb. of sulphate of soda. Pour the first solution into the second (never the contrary) and take care that the prussiate solution is always preponderant. The Glauber’s salt is useful in rendering the precipitation of the blue pigment more complete by reason of the insolubility of the latter in saline fluids. When the blue sediment is all thrown down it is drained off on a filter, and repeatedly washed till a blue tint appears in the wash-waters, when it is dried for use.

Turnbull’s Blue.—This is an old-fashioned name often applied, like the term Paris blue, to the violet shades of Prussian blue which have been prepared with red prussiate.

Ultramarine.—According to Rowland Williams, F.C.S., natural ultramarine is, perhaps, the most beautiful blue pigment known. It was formerly, and is now to a small extent, manufactured (chiefly for artists’ use) from lapis lazuli, a blue mineral which occurs, intermixed with limestone and iron pyrites, in Siberia, Thibet, and China. In order to obtain ultramarine from lapis lazuli, the roughly pulverised mineral is ignited, dipped into vinegar to remove carbonate of lime, and then reduced to the finest possible state of division. The powder is next mixed with a cement composed of rosin, linseed oil, white wax, and Burgundy pitch, and the resultant paste is worked under water until all the ultramarine is separated. The ultramarine is washed several times with water, and afterwards with alcohol, which removes any of the resinous compound which may have adhered. When treated in this manner, lapis lazuli yields from 2 to 3 per cent. of ultramarine. According to Clement and Desormes, lapis lazuli has the following composition:—

It will be seen, therefore, that ultramarine essentially consists of alumina, silica, soda, and sulphur, and may be regarded as a sodium aluminium sulphate, in combination either with polysulphide of sodium alone, or with a polysulphide and a polythionate of sodium. Clement and Desormes believe that the iron in lazulite (lapis lazuli) is an accidental impurity, and is neither essential to the mineral itself nor to the ultramarine derived from it. There is still some doubt on this point, however, many eminent chemists holding the opinion that iron is a necessary constituent of ultramarine blue.

Natural ultramarine has been almost entirely replaced by the artificial product, since methods have been devised for the manufacture of the latter on a large scale. The possibility of preparing artificial ultramarine suggested itself in a curious manner. About seventy years ago a French alkali maker noticed the occasional appearance of a blue coloured substance in his soda furnace. On analysis, Vauquelin found the substance to have the same chemical composition as lapis lazuli, and this incident led him to believe that ultramarine might be built up from its elements. Several years passed away before Guimet succeeded in manufacturing artificial ultramarine on anything like a large scale, but Gmelin is said to have prepared it in small quantity half a dozen years previously. There are four varieties of artificial ultramarine: (1) the pure deep blue, equal in colour to average native ultramarine; (2) pale blue; (3) violet or pink ultramarine; (4) green ultramarine. The latter is obtained in the first stage of the ultramarine manufacture, being the result of incomplete ignition of the materials employed. Ultramarine is generally manufactured by one of the following processes:—(a) “Sulphate”; (b) “Soda”; (c) “Silica.”

(a) “Sulphate” Ultramarine.—This may be prepared from sulphate of soda (Glauber’s salt), charcoal, and kaolin (china clay). The materials should be as free as possible from iron, and it has been found that clay having approximately the formula Al₂O₂ (SiO₂)₂ gives the best results. The clay and sulphate of soda must be thoroughly calcined. They are then intimately mixed with charcoal in the following proportions:—

Sometimes a portion of the sulphate of soda is omitted, and some carbonate of soda and sulphur added instead. The composition of the mixture then becomes:—

Caustic soda is also sometimes used instead of carbonate. These mixtures (whether sulphate alone or sulphate and carbonate) are made with a view to have the soda present in sufficient amount to combine with one-half the silica contained in the clay, and to leave sufficient soda to form polysulphide of sodium with a portion of the sulphur. There should then remain enough soda and sulphur to produce ordinary sulphide of sodium (Na₂S). If either of the two mixtures be ignited out of contact with air, a white compound is formed, which is sometimes termed white ultramarine. On leaving this exposed to the atmosphere for some time it becomes green, and on further ignition, with free access of air, it is converted into ultramarine blue. In actual working the carefully prepared mixture of the above mentioned materials is heated for several hours to a high temperature in fire-clay crucibles, only a limited supply of air being allowed to enter, and the temperature being eventually raised to a white heat. The product of this operation, when cool, has a grey or yellowish-green appearance. It is washed several times with water, dried, reduced to a fine powder, and then represents the green ultramarine of commerce. Stölzel found that green ultramarine had the following composition:—

Green ultramarine is transformed into blue by heating with about 4 per cent. of sulphur at a low temperature, with free access of air. Sulphur is afterwards added, if necessary, in small quantities at a time, and the heating is continued until the desired shade of blue is obtained. The mass is then powdered, the soluble matter (sulphide of soda, &c.) is removed by washing with water, and the blue is dried and assorted according to quality.

(b) “Soda” Ultramarine is sometimes made with soda alone (either carbonate or caustic), and at others with a mixture of soda and sulphate of soda. Rowland Williams found the following proportions of the respective ingredients to answer satisfactorily:—

The proportions for soda and sulphate of soda ultramarine have been previously given under “sulphate ultramarine.” The ignition is carried on in a manner similar to that already described. The resultant green product, owing to its avidity for oxygen, is partially changed into ultramarine blue by simple contact with the air. It is entirely converted into the blue variety by roasting with an additional quantity of sulphur. With care, ultramarine blue may be manufactured in one operation, by increasing the proportions of soda and sulphur.

(c) “Silica” Ultramarine is manufactured in the same way as soda ultramarine, except that, in addition to the other materials, silica to the extent of 5 or 10 per cent. of the weight of clay is employed. By this process, ultramarine blue of a slightly reddish tint is obtained in one operation. The method has, however, one decided drawback, viz. that the materials employed are rather liable to fuse during ignition. The faintly reddish hue of “silica” ultramarine becomes more intense according to the proportion of silica present. “Silica” ultramarine is said by some to be less readily attacked by acids and by strong alum solutions than ultramarine prepared by the “sulphate” and “soda” processes; but Rowland Williams’ experience does not confirm this statement. He mentions that good artificial ultramarine withstands the action of weak acids much better than is generally imagined. He had occasion to test many samples which resisted the action of dilute acids to a remarkable degree. Most strong acids, of course, decompose both artificial and native ultramarine, with evolution of sulphuretted hydrogen. Native ultramarine is, however, less susceptible to the influence of acids (both strong and dilute) than the artificial compound. This difference of behaviour is probably due to the fact that the former contains considerably less sulphur than the latter, and it is also possible that the constituents of natural ultramarine may be combined in a somewhat different manner from those of the artificial product.

Notwithstanding the large amount of research with reference to the chemical composition of ultramarine, the origin of its blue colour still remains in doubt. According to Wilkens (Ann. Ch. Pharm., xcix. 21), ultramarine consists of two portions, one of which is easily attacked by hydrochloric acid, and is regarded by him as the essential constituent, whilst the other portion is insoluble in hydrochloric acid, and contains variable proportions of clay, sand, oxide of iron, and sulphuric acid. From his analyses of the pure blue, Wilkens deduces the formula (2Al₂O₃ 3SiO₂) (Al₂O₃ 4SiO₂) Na₂S₂O₃ 3Na₂S:—

Wilkens regards the blue colouring principle of ultramarine as a compound of hyposulphite and sulphide of sodium. He considers the presence of iron is not necessary for the production of the blue; whilst Dr. Elsner, in a paper published in 1841, states that about 1 per cent. of iron (which he presumes to be in the state of sulphide) is essential. Rowland Williams asks whether it is not conceivable that the blue colour of ultramarine may be due to the presence of a small quantity of black sulphide of iron, most intimately combined with a colourless or comparatively colourless compound (such as white ultramarine), the whole mass (owing to the dilution of the black sulphide) showing a blue reflection.

Ultramarine is insoluble without decomposition in any known menstruum. According to P. Ebell (Ber. 16), ultramarine, when in the most finely divided state, will remain suspended in pure water for months. The liquid may be filtered unchanged through several layers of Swedish filter paper, and appears perfectly clear when examined in a ¾ in. layer, and on evaporation deposits the ultramarine as a lustrous coating on the sides of the vessel. Rowland Williams repeated the above experiment, and can confirm Ebell’s statement. This result shows the necessity of due precautions being taken during the washing of the ultramarine in the process of manufacture, otherwise a considerable amount of the finely divided blue may be lost. Ultramarine is largely used in calico printing for pigment styles, being fixed on the fibre by means of albumen. It is also employed for blueing linen and cotton, wax candles, lump sugar, &c. Ultramarine is not adulterated to a large extent, the chief sophistication being barium sulphate (barytes), and occasionally chalk and china clay.—(Rowland Williams, in Industries.)

Another writer in Industries says that the manufacture of ultramarine has perhaps hardly received the attention it deserves in England. The importance of the industry has been recognised in Germany, however, and though the palmy days of the trade, when the whole production was in the hands of a few firms, and the price was a matter of private friendly arrangement, are gone for ever, yet the business is in a flourishing state, and should prove lucrative if properly managed. It is a characteristically English failing to overlook branches of business not dealing with large quantities of staple commodities, and thus many of the smaller but remunerative industries have passed out of our hands. When one observes that almost every sheet of ordinary blue official paper is decolorised when accidentally brought into contact with an acid, betraying the fact at once that its colouring matter is ultramarine, one realises that a very considerable consumption for this and similar purposes must take place. Like most trades based upon chemical principles, the manufacture of ultramarine has recently made rapid strides, and some of the latest developments are recorded in a paper by J. Wunder, appearing in a recent number of the Chemiker Zeitung, which is worthy of some attention.

With most people not directly interested in it, the term ultramarine is taken to mean the blue pigment known under that name, the words being reckoned almost synonymous. Others, more erudite, recognise the existence of a green variety, but that the production of such colours as red and violet is possible is scarcely suspected. Of course the blue is the most important, but even that does not correspond to one specific substance, products of different shade being prepared by modifying the process of manufacture. As usually made, ultramarine is formed by heating together carbonate of soda, kaolin, sulphur, and charcoal, with limited access of air, the resulting pigment being green; this, on roasting with sulphur, becomes blue. If the operation be conducted with complete exclusion of air, so-called ultramarine white (in reality grey) is produced, which becomes green on further heating. Ultramarine blue capable of resisting the action of alum is sometimes required, and may be obtained by the use of a highly silicious charge and much sulphur, the burning being conducted in crucibles or in mass according to the purpose for which the pigment is required. The former process is costly, while the latter gives a product containing a good deal of free sulphur, which is objectionable for such purposes as calico printing. Removal of the excess of sulphur by heat or caustic soda is not feasible, as the colour suffers in either case, but a certain amount of success has attended experiments with sodium sulphide, the colour often brightening noticeably.

It is curious that chemically pure sodium carbonate, or such as is made by the ammonia-soda process, is not well fitted for the manufacture of ultramarine; Leblanc soda, containing a little caustic, is distinctly preferable. Sprinkling the soda with a strong solution of sodium sulphide before use is a good plan, and one easy to adopt. The more silicious the mixture the more difficulties are encountered, but the product is a deeper, richer colour, and withstands the action of alum and weak acids better. Excess of oxygen must be guarded against; many a manufacturer has had a batch turn out a hard cold blue, instead of a soft rich colour, solely on account of a too-excellent draught, an accident especially liable to happen in winter time. So much dreaded is this catastrophe that some makers habitually limit the air supply—smothering the neighbourhood with smoke, and wasting coal. The need for exact control here indicated points to a probable advantage from the use of gaseous fuel. Considerable economy has resulted from the use of the waste gases from one furnace serving for the preliminary heating of another; a better plan would probably be the introduction of regenerative heating.

The crude ultramarine as it comes from the furnace contains a large proportion of soluble salts, notably 20 to 24 per cent. of sodium sulphate, which have to be removed before it is merchantable. Usually, after grinding, it is simply stirred up repeatedly with hot water and the aqueous extract is siphoned off. That such a crude method should be in vogue at the present time is very significant of the ample margin of profit that must exist. By systematic extraction and filtration under pressure the washing may be effected with so little water that the solution is sufficiently concentrated to pay for evaporation by the heat of waste furnace gases, the recovered sodium sulphate serving to replace part of the raw material.

The quality of ultramarine largely depending upon its fineness, it is graded by levigation, the coarser portions being filter pressed, and the finest “floating” quality, which remains in suspension for an inconveniently long time, precipitated by the addition of a trace of an ammonium salt, gypsum, or even hard water, and filtered by the aid of a suction tube on the principle of an ordinary Bunsen pump.

The first successful attempt to produce ultramarine violet was made by Professor Leykauf in 1859. By heating ordinary ultramarine with calcium chloride in the presence of air and moisture, he obtained a violet-toned pigment, but it was not a full colour. The active substance in this change was probably hydrochloric acid, produced by the decomposition of the calcium chloride. Later experiments with other reagents, such as chlorine and gaseous hydrochloric acid, led to the following methods being devised. In the first, ultramarine blue is spread out on stoneware shelves in iron chambers and treated with a mixture of chlorine and steam at a temperature of 300° F. to 480° F. for about three hours. In the second, the plant is very similar, but at the bottom of the chambers are stoneware dishes, into which hydrochloric acid is poured from time to time. As the temperature is raised, copious vapours arise from these, evaporation being aided by a strong draught, and the ultramarine blue, after being kept at 428°-446° F. for some seven hours, becomes converted into a dull violet, which brightens on continuing the process with a temperature gradually falling to 320° F. The ultramarine violet produced by either of the above methods resists the action of lime, and is of general applicability.

The pigment produced by a third and simpler process, consisting merely in heating ultramarine blue mixed with salammoniac and a little sodium nitrate, is unfortunately not so stable. Another shade of considerable interest is a pure bright light blue, formed by heating the violet variety in hydrogen to about 536°-554° F. It has not yet been prepared on a commercial scale, but certainly merits the attention of manufacturers. An ultramarine red has been made by acting on the violet produced by either of the first two methods with the vapour of either nitric or hydrochloric acid at 275°-293° F., the sole essential determining condition being the temperature. Iron vessels could be used in the case of nitric acid at this temperature, but if hydrochloric acid were employed stoneware would have to be substituted. In the manufacture of the violet the temperature is above the limit at which hydrochloric acid acts on iron.

It is now only necessary for some successful experimenter to put on the market yellow and orange shades of ultramarine for almost the whole of the spectrum to be represented. The problem of the cause of the colour of ultramarine, attempts to solve which have been repeatedly made, seems increasingly difficult when its protean character is considered; but this from the industrial point of view is of secondary importance, provided all required shades can be produced with ease and economy. Nevertheless, it is certain that here, as in other cases, substantial technical progress would follow from adequate scientific investigation.—(Industries.)

Ultramarine was also made the subject of a very interesting paper, by Herbert J. L. Rawlins, read before the Society of Chemical Industry, in December 1887.

After referring to the native form, lapis lazuli, Rawlins goes on to observe that “analysis could give no clue as to the cause of the blue colour. To prepare it artificially became a great object, and the efforts in this direction were stimulated by the offer of prizes, amongst which was one of 6000 francs, offered by the ‘Société d’Encouragement’ of France, to be awarded to the discoverer of a method of making ultramarine, provided it did not cost more than 90s. per lb. How strange it seems to think of this in these days when the value has fallen to less than half that price per cwt.!

“As early as 1814, two German chemists, Tessärt and Kuhlmann, had observed the formation of a blue product in soda kilns and calcination kilns, but Guimet, in 1828, first discovered how it was produced, and gained the 6000 francs prize. He did not, however, publish his method, and grew immensely rich, although the price sank to about 16s. per lb. In 1828 he was producing at the rate of 120,000 lb. annually.

“About the same time, or, as is positively asserted by some, even prior to Guimet, Gmelin made the same discovery and published his researches in full, thus perhaps laying the foundation stone of the present supremacy of Germany in this manufacture.

“In spite of the valuable discoveries of Hoffmann, Unger, and others, our knowledge of the chemical constitution of ultramarine is very limited and uncertain, many different theories having been advanced regarding the cause of the blue colour.

“According to Wilkins, ultramarine is composed of two portions, one of which consists of two silicates of alumina with sulphite and sulphide of sodium, and is constant in its composition; the other being a mixture of variable quantities of sand, clay and oxide of iron, with sulphuric acid. The blue colouring principle he considers to be a compound of sodium sulphite and sulphide. Another ingenious theorist, Stein, in two papers published in the Jahresberichte in 1871 and 1872, concludes that blue ultramarine contains sulphurous, and not thiosulphuric acid, that neither sulphites nor thiosulphates are necessary to its composition, and that it owes its colour to the presence of black sulphide of sodium, which is formed at high temperatures by the action of sulphide of sodium on alumina—admitting, therefore, that it is not a chemical compound, but merely a mechanical mixture, the blue colour of which is due to the bodies composing it.

“Brunner considers ultramarine to be a compound of aluminium silicate, with sodium sulphate and sulphide; while Brünlin regards it as a double silicate of aluminium and sodium, in combination with pentasulphide of sodium. Green ultramarine he considers to be the same double silicate in combination with bisulphide of sodium.

“Again, according to Ritter, ultramarine contains a double silicate, not only associated with polysulphide, but also with thiosulphate of soda; and Schülzenberger, on the other hand, considers that it is a mixture of a double silicate with sulphite and monosulphide of sodium.

“Endemann considers that the blue colour is due to a ‘colour nucleus,’ consisting of unchanging proportions of aluminium, sodium, oxygen and sulphur, in each variety of ultramarine the proportion being different, while the rest of the sodium and aluminium and the whole of the silica merely act as a vehicle necessary to the preparation and existence of the colour. He considers that this ‘colour nucleus,’ in the case of white ultramarine, which he calls the ‘mother-substance in the manufacture of blue ultramarine,’ has the formula AlNa₄O₂S₂. By the action on two molecules of this of sulphurous acid gas, Na₂O is removed, and green ultramarine Al₂Na₆O₃S₄ is formed, which then, by the action of oxygen, which forms sodium sulphate, passes into the pure green compound, having the formula Al₂Na₄O₃S₃. In the ‘indirect process’ of manufacture, green ultramarine is converted into blue by being burned with sulphur. By this means Endemann considers that more sodium and sulphur are removed, and blue ultramarine Al₂Na₂O₃S₃ is formed. He considers that the other portion, not included in the ‘colour nucleus,’ differs in different samples. In one which he mentions it has about the composition 3Al₂O₃.5Na₂O.16SiO₂.

“But of all chemists who have worked on this subject, none has done more to increase our knowledge of ‘the blue marvel of inorganic chemistry,’ as he himself has called it, than Reinhold Hoffmann. His position of manager of the Marienberg Ultramarine Works, near Benscheim, in the Grand Duchy of Hesse, renders his acquaintance with the manufacture perfect, and his untiring researches on the subject have been well rewarded by results both interesting and valuable. He considers ultramarine to be a double silicate of sodium and aluminium, together with bisulphide of sodium, the variety poor in silica, characterised by its paleness and purity of tint, and easy decomposition by acids, having the formula 4(Al₂Na₂Si₂O₈) + Na₂S₄; while that rich in silica, characterised by its dark and somewhat reddish tint, and more difficult decomposition by acids, has the formula 2(Al₂Na₂Si₃O₁₀) + Na₂S₄. He also considers it very doubtful whether green ultramarine is really a chemical compound, and indeed it is now generally considered that the colour is only due to small traces of sodium salts in very intimate mechanical mixture with the blue variety, for by heating the green body for some time at 160° with water in closed tubes, it is converted into the blue product, and small traces of sodium compounds are found in solution in the water; and further, on heating blue ultramarine strongly with sodium sulphate and charcoal—that is, acting upon it with sodium sulphide—the green variety is formed.

“In a paper by Knapp, an abstract of which appeared in the Journal of the Chemical Society for March 1880, there are some curious facts recorded with regard to the colouring agent. It was noticed that when silicic acid was replaced by boracic acid, a blue, nearly as stable in its properties as that of ordinary ultramarine, was produced. It was found that a blue could be obtained without alumina being introduced. Hence silica without alumina, and alumina without silica, can be employed with a certain amount of success. The blue, however, formed without silica, is not so strong or stable as that formed with it.

“One very curious property which ultramarine possesses is its power of giving up its sodium in exchange for other metals. Thus, by heating blue ultramarine with a concentrated solution of silver nitrate in sealed tubes to 120° for fifteen hours, a dark yellow silver ultramarine is produced, containing about 46·5 per cent. of silver. This corresponds to about 15·5 per cent. of sodium, which is just about the amount that the original body contained.

“When this body is heated with an aqueous solution of sodium chloride to 120° in sealed tubes, about three-quarters of the silver is replaced by sodium, but the other quarter cannot be so replaced; in fact, blue ultramarine, when heated with silver chloride, takes up silver, and becomes green. But by heating silver ultramarine with sodium chloride in the dry way, at rather a higher temperature, the whole of the silver is replaced by sodium, but the ultramarine thus regenerated does not equal the original body in colour. The change is probably due to the loss of sulphur in the formation of the silver ultramarine.

“If in the above experiment potassium chloride be substituted for the sodium salt, and the temperature not allowed to exceed 400°, a bluish-green potassium ultramarine is formed. Barium ultramarine is a yellowish-brown product, zinc ultramarine is violet, and magnesium ultramarine is grey. These may all be obtained by acting on the yellow silver ultramarine with the corresponding metallic chloride.

“From the experiments of Dollfus and Goppelsröder some very striking differences have been brought to light between the three types of colour which they examined—namely, the blue, green and violet—in their behaviour with various reagents. Thus, an aqueous solution of caustic soda or potash does not act on the blue or green, but turns the violet to blue, and when heated with carbonic oxide the same result ensues. Many other reagents have the same effect on the violet variety, but when acted upon with sodium sulphide, the green turns grey, and when heated with potassium chlorate becomes darker and loses its brightness of colour. Dollfus and Goppelsröder attempt no explanation of these facts, but simply state them as results of their observations, and profess their inability to give any chemical formulæ for the three ultramarines, though they consider that there is sufficient proof that each has its distinct constitution. They give as their opinion, however, that they are double silicates of aluminium and sodium, in which a part of the oxygen is replaced by sulphur.

“Violet and red ultramarines are more bodies of scientific interest than of any practical use, as their colouring power is not sufficiently great. The violet variety may be prepared by exposing the underground blue product to chlorine gas under a high temperature, while the red may be obtained from the violet by acting on it, under a low temperature, by dilute nitric acid fumes.

“The first artificial method of producing ultramarine was that known as the ‘indirect process’—that is, first the manufacture of green ultramarine; and secondly, its conversion into blue. It was carried out as follows:—

“An intimate mixture of Glauber’s salts, china clay, and coal or rosin, finely ground together, was placed in crucibles and baked or burned in an oven for about six hours. It was then transferred to iron trays, and heated with flowers of sulphur to the point where the sulphur took fire, when it was allowed to burn itself out. By this second process the green was converted into blue. It was then washed, ground with water, and settled out, the first deposit being of a darker shade than the second, and the colour becoming lighter as the powder settled was finer in grind. This is essentially the method employed now at many German works—those at Marienberg, for instance—and produces what is known as “sulphate ultramarine,” distinguished by its pale shade and almost greenish blue tint.

“There are, however, some objections to the indirect process, and it was considered advisable to find a plan by which ultramarine could be made in bulk in a muffle furnace. The following is a method which is employed at the present time in some of the German works:—

“A mixture of china clay, carbonate of soda, sulphate of soda, sulphur, sand and charcoal or rosin, finely ground together, are placed upon the floor of a muffle furnace, being pressed down so as to present an even surface. The mixture is then entirely enclosed with fire-clay tiles, the spaces between which are filled in with thin mortar. When the oven is so charged, the front is built up, a small hole being left for watching the temperature of the flue between the tiles and the top of the furnace, and for drawing samples during the process, which is done through a corresponding hole in the front of the fire-clay tiles, temporarily closed with a fire-clay stopper. The oven is now heated, slowly at first, and afterwards more strongly, so that at the end of eight or nine hours it is at a dull red heat. It is kept at this temperature for about 24 hours, when the heat is raised so that a clear red glow is obtained, which is kept up to the end of the operation.

“For the purpose of taking a sample, an iron spoon borer is introduced through the hole left in the enclosing tiles, turned round, and pulled out. The contents are laid on a clean tile, and quickly covered with another tile, on which a second quantity is placed, and allowed to remain exposed to the air. If the oven has been sufficiently heated the covered sample should appear of a bluish green, and no longer brown or yellow, while the second sample should be rather bluer. If this be the case, the oven is heated slowly for another hour, and then all communication with the outer air is cut off. It is allowed to cool and then opened, when the contents should appear as a beautiful blue mass, the lower portion of which, however, is of a greenish tinge. Both parts are now treated alike, but worked up separately, the greenish-blue portion making an inferior article. The finishing process is as follows:—

“The raw ultramarine is ground in upright mills, and then repeatedly boiled for about ten or fifteen minutes at a time in cast-iron boilers, being all the time agitated by a mechanical stirring arrangement. It is then allowed to settle, and the water is drawn off with a siphon. As soon as the powder settles into a hard compact mass, it has been sufficiently washed, and it is then dug out. The part next to the bottom of the boiler is generally coarse and of poor quality. It is carefully separated from the upper portion, which is transferred to wet mills of the ordinary description, and there ground for six to twelve hours, during which time about 150 lb. can be treated in each mill. The ground colour from these mills is then collected in a large tub, and allowed to settle for four hours, during which time the coarsest particles fall to the bottom. The liquid is then passed through a series of tubs, in each of which it is allowed to stand for a period of time, lengthening as the quality settled out becomes finer, the last settling requiring about three weeks. The various qualities are then dried and sifted, when they are ready for the market.

“The blue produced by this operation is of a good quality, but there are some objections to the process, which have given rise to another, in which the ultramarine is produced direct in crucibles similar to those used in the indirect process.

“This is conducted as follows:—The mixture of raw materials consists of about 100 parts of china clay, 90 of carbonate of soda, 110 of sulphur, 20 of charcoal, and a quantity of infusorial earth, varying according as the ultramarine produced is desired to be rich or poor in silica. These are finely ground together, in which process great care must be observed, as much depends upon its being properly carried out. The mixture is then filled loosely into crucibles provided with flat circular lids, which are fixed on with mortar containing clay. This is allowed to dry, and the crucibles are then ready for firing, which process is conducted in ovens, generally constructed so as to contain several hundred crucibles, which are arranged in rows one above another.

“The mixture undergoes a very curious change of colour while in the ovens. When put in it is greyish white, and during the process of burning it becomes successively brown, green, blue, violet, red and white, in the order named. These changes are, according to Guimet, due to oxidation. The brown appears with the blue flames due to the combustion of the sulphur, the green just after the sulphur flames have ceased, and the blue is first formed at a temperature of about 700°—i. e. a bright red heat. If, after this, heat be still applied and air freely admitted, the mixture becomes first violet, then red or rose coloured, and finally white. When this white body is heated to redness with carbon or other reducing agents, the red, violet, blue, green and brown colours (according to the amount of reducing agent employed) may sometimes be reproduced, though the reaction is by no means a certain one.

“If brown ultramarine be removed from the oven, and allowed to remain exposed to the air, it immediately takes fire and burns to an inferior blue colour. The same thing occurs with the green body. Even if the brown product be completely cooled before being exposed to the air, it will, as soon as the air is allowed to reach it, get hotter and hotter, until it is glowing, when it will burst into flame and become blue. Attempts have been made to preserve the brown colour, which is of a beautiful chocolate tint, but have always failed. In one instance, when this was tried, the colour was put immediately into water, and treated like the ordinary blue variety, and as long as it was kept moist no change was apparent. After being washed and wet ground the moist powder was put into a cask, where for some time it was allowed to remain undisturbed. At the end of about three weeks it was noticed that the mass was hot, and on being turned out of the cask and broken up it was found to be at a glowing heat in the interior.

“After the oven has been fired for several hours, it is carefully closed at every point where air might enter, and allowed to cool for four or five days. The exact length of time during which the ovens are fired, and the amount of air admitted, depend upon various circumstances, one important one being the state of the weather. Thus, on a dull, foggy day, when the draught in the chimney is not good, a longer time is required. Of course, no rule can be given for this, and it is the experience required in the management of the oven that makes the manufacture so difficult to carry out successfully, the early efforts of a manufacturer not unfrequently resulting in the loss of a whole ovenful of raw material. As soon as the oven has cooled, the crucibles are taken out, and the contents of each are turned out in a solid mass, which must be carefully cleaned with a knife of any badly burned portions, and afterwards broken up and thrown into a cask along with the contents of other crucibles.

“This forms what is known as crude raw ultramarine. It contains about 15 per cent. of sulphate of soda, which must be removed before the colour is fit for sale.

“For this purpose it is washed with hot water in large tubs, after which it is ground in wet mills to an impalpable powder, and allowed to stand for about an hour in a large tub, in order to remove the coarsest particles and dirt which are sure to be present. It is then removed to another tub, where it settles for four or five hours, and from this it passes to others, where it stands for various lengths of time, increasing, of course, as the powder to be settled becomes finer, the last settling occupying three or four weeks, and producing the strongest quality that can be obtained—that is to say, it will bear mixing with more of a reducing medium, such as mineral white, than would a former settling for the mixture in each case to be of the same depth of colour.

“The water, after the final settling, still contains about 5 per cent. of ultramarine. This would take five or six months to settle, and as this time could not generally be given to it, it is precipitated with lime water, which has a sort of coagulating influence upon the particles, which can then be removed by filtration. It is a curious thing that this last quality is quite different from the one preceding it, being very inferior in both colour and strength.

“After settling, all the various qualities are dried in kilns, and sifted through fine brass wire sieves by means of a fan, which breaks up the lumps and forces the particles through the meshes of the sieve, which must be very close—about 100 to the inch—in order that the ultramarine may be perfectly smooth and free from lumps or grit of any sort. When finished, it should be in the form of an impalpable powder—the finer qualities so fine, indeed, as to feel almost buttery when rubbed between the fingers. After this process the different qualities and shades are mixed to certain standards, and are then ready for sale.

“The uses of ultramarine in the arts and manufactures are very numerous and important. The most important, from the point of view of quantity, is the manufacture of ‘square blue’ for washing purposes. In the preparation of this article the ultramarine is generally mixed with bicarbonate of soda and some glutinous material, to help it to retain its shape, and is then pressed into the well-known form of small square or oblong blocks.

“It is also used largely in the manufacture of blue paint and printing ink, and in the preparation of blue mottled soap. The way in which it is employed in the last-named manufacture is worthy of remark. It is added to the soap while it is in a molten state and just before it is allowed to cool, and thoroughly mixed with it, so that the whole mass is of a pale blue tint. If a small quantity of this be removed from the boiler and cooled quickly, it remains of a uniform tint, but in the case of the whole boilerful, where the cooling is very slow, the action is entirely different. Just at the point of cooling, when the soap is going to set hard, the ultramarine—to use a technical expression—“strikes,” and goes into the form which gives to blue mottled soap its well-known appearance.

“In the manufacture of paper, ultramarine also plays an important part. It is here used not only for producing blue shades, but also as a bleaching agent, to counteract the yellow when white paper is made.

“Another important use is in the calico manufacture, where it is used both in the printing of blue patterns and in the finishing of goods. In the case of calico printing, it is mixed with albumen and printed on to the calico, which is then subjected to the action of steam, the albumen being by this means coagulated and each grain of ultramarine surrounded by an insoluble envelope, so that it cannot be washed out of the calico.

“The growth in the manufacture of ultramarine has been very remarkable, especially when it is considered how little the process is understood chemically, and what care and patience—to say nothing of the equally important item of capital—are required in the starting of a manufactory. Commencing less than 50 years ago in the works of Guimet, at Lyons, who produced 120,000 lb. annually, there are at the present day nearly 40 manufactories at work in various parts of the world—chiefly in Germany—producing about 20 million lb. per year. The following figures will give some idea of ten years’ growth of this industry—from 1862 to 1872:—

“From the above numbers it will be seen that in these ten years the manufacture more than doubled itself, the fact being due, however, not so much to the increase in the number of works, which was only one-third, as to the enlarged capabilities of those existing in 1862. Thus, in the works of Dr. Leverkus, near Cologne—the first works ever started in Germany—the number of men employed had, during these ten years, more than doubled, while the output had trebled; and in the case of the Marienberg Works the difference was even more striking, the number of hands employed and the quantity turned out per annum having nearly quadrupled.”

In reply to various questions which were asked in the discussion which ensued, Mr. Rawlins said that, with regard to the use of ammonia soda, it had frequently been used in the manufacture of ultramarine, and was constantly used he understood, but he himself had not much experience of it. As far as he could make out, it certainly produced ultramarine, but of a darker shade than that made with Leblanc soda. It could not be supposed, in works where the Leblanc soda was used, that ammonia soda could conveniently be substituted, for of course a works when established had to adhere to its known standards and shades, and it would not do to change the raw materials, though the ammonia soda produced a very good ultramarine. As regards the discovery of ultramarine, the first works started anywhere were Guimet’s. He had with him a little historical list containing the dates at which the various works established before 1866 or a little later had been started. It was drawn out by Hoffmann, who, as he stated before, was the manager of large ultramarine works, and he put down Guimet’s, which were started in 1829, first on the list. Dr. Leverkus started in 1834. He knew that the discovery of ultramarine had been attributed to different people. He had mentioned Guimet because it had generally been considered, as far as he had heard, that Guimet and Gmelin were the two who discovered it from a manufacturing point of view. He had heard of crystals of ultramarine, but had never seen any, and he knew they were very difficult to prepare, and very rare. He had mentioned that the grinding had to be done very thoroughly, because the better it was mixed and the finer it was ground, the better was the ultramarine produced. If it was badly mixed it was quite fatal to getting a good result. Grinding lightens the colour. Raw ultramarine must be ground before it was practicable to use it at all. For instance, a coarse ultramarine could not be used for printing calico. Therefore it was necessary to grind it both for the sake of the colour and for the sake of the way in which it was applied. It was increased in value by grinding because it made it stronger and finer. Before grinding it was of a dark colour, but after grinding it became lighter and brighter.

The materials employed in McIvor’s process for making ultramarine are kaolin or other suitable clay, a solution of sulphide of sodium, in which sulphur in the form of flowers of sulphur is dissolved to saturation, and caustic or carbonate of soda.

The preparation of the solution is effected by adding the sulphur to boiling sulphide of sodium liquor of maximum strength until it ceases to be taken up. The clay and soda are first roasted together at a red heat, so as to effect a partial double decomposition, and the product, after grinding, is made into a thick paste with “sulphur liquor,” i. e. the sulphide of sodium solution of sulphur. This latter operation may be carried out in an ordinary pug-mill. The paste so formed is dried in an oven or other convenient way, and the dried mass (being broken into small pieces) is roasted without access of air in a closed earthenware retort, first at about 480°-570° F. for an hour, then at a red heat for eight hours, and finally at a moderate heat just below dull redness, in presence of a slow current of air, which enters through a series of holes or small openings in the front of the retort, the current being regulated by means of a damper or an adjustable slide. The retort should be allowed to become quite cold before being opened, otherwise the tint of the product will be injured.

McIvor has found the following proportions of the raw materials used in the process to yield excellent results, viz:—

These quantities yield about 2 cwt. of ultramarine blue.

The following communication from the pen of J. B. Nejedly, of Vienna, appeared in the Chemiker Zeitung, during 1888:—

“Animated by various articles and notes in your journal under the heading of ‘The Present Position of the Manufacture of Ultramarine,’ I would like to draw out of obscurity a little work on this industry which contains much that is true, and furnishes at the same time many comparisons with regard to the present position of the ultramarine industry in Germany.

“The work above referred to was printed in the year 1840 and bears the title:—