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

When the furnace is filled and the doors are bricked up, the heat is slowly raised to redness, at which point it is kept for six or eight hours. The combustible gases are evolved and consumed in the furnace as the bones begin to decompose, and by this means so much heat is produced that only a small quantity of fuel is needed to maintain the required temperature. When the carbonisation is complete, the doors are taken down and the crucibles are removed to cool, their places being immediately filled with fresh ones. The heat must be kept as uniform as possible throughout the process: if it be not sufficiently high, the bone-black will contain a portion of undecomposed organic matter, which renders it quite unfit for use; if, on the other hand, the temperature be raised too high, the bone-black will become dense and compact, whereby its efficacy as a decoloriser is much reduced. When the charcoal in the crucible has become perfectly cool, it is removed and crushed. When required for decolorising or deodorising purposes, it is only roughly broken up into small lumps, in which form it is most readily applicable. The crushing is effected by means of two grooved cylinders, consisting of toothed discs, alternately 10 and 12 in. in diameter. These are so placed that the 10-in. discs of one cylinder are opposite the 12-in. discs of the other, and thus, in revolving, the carbonised bones are crushed to fragments between them, but are not reduced to powder. They are passed successively through six of these mills, the cylinders of each couple being nearer to each other than the last. Finally the crushed bones are carefully sieved; the powder is placed apart from the lumps, again passed through finer sieves, and sorted out into different sizes.

A furnace such as that described above will carbonise four charges of bones in one day, each charge being more than half a ton in weight. With careful work, the bones will yield 60 per cent. of bone-black, or more than one ton daily.

If it be required to condense the volatile gaseous products of the carbonisation, this process is conducted in retorts similar to those used in the manufacture of acetic acid from wood: these are so arranged that the whole of the gaseous products are condensed and collected. The aqueous portion of the distillate is usually evaporated down to obtain salts of ammonia; the uncondensable gases may be employed for illuminating purposes. The manufacture of bone-black is usually carried on in the neighbourhood of large towns, where a good supply of bones may be readily obtained.

Ordinary bone-black has about the following composition: Phosphate and carbonate of lime, and sulphide or oxide of iron, 88 parts; charcoal, containing a small quantity of nitrogenous matter, 10 parts; silicated carbide of iron, 2 parts. The decolorising properties of bone-black are due solely to the presence of the charcoal.

When intended for use as a deodoriser or decoloriser, bone-black should be kept carefully excluded from the air, for by exposure it loses this power to a great extent, and becomes almost inert. That which has been freshly burned is therefore best for these purposes.

The cost of production of bone-black may be calculated as follows:—

Bone-black never has the depth or brilliancy of lamp-black, but it mixes well with either water or oil, and though a slow drier as an oil paint, is permanent and not high priced.

Frankfort-black or Drop-black.—This is a black powder obtained from dried vine-twigs carbonised to a full black and then ground very fine. On a large scale it is prepared from a mixture of vine-twigs, wine-lees, peach-stones, bone-shavings, and ivory refuse. It varies in shade according as the animal or vegetable charcoal is in excess; when the latter predominates, the powder is of a bluish colour; but when there is an excess of animal charcoal, it has a brownish tinge. It is customary to wash the powder well when first made, in order to remove any soluble inorganic impurities. The finest Frankfort-black is probably the soot obtained from the combustion of the materials mentioned above. It makes an excellent pigment, and is extensively used by copperplate engravers in the preparation of their ink. Drop-black is simply Frankfort-black ground exceedingly fine, mixed with a little glue water, and dried in pear-shaped drops for sale.

Ivory-black.—Ivory-black is a beautiful black pigment prepared by carbonising waste fragments and turnings of ivory. These are exposed to a red heat for some hours in crucibles, great care being taken to avoid overheating or burning. When quite cold, the crucibles are opened, and the contents are pulverised, the richest coloured fragments being kept apart for the best quality. The powder is then levigated on a porphyry slab, washed well with hot water on a filter, and dried in an oven at a temperature not exceeding 212° F. The product is of a very beautiful velvety black colour, superior even to that obtained from peach-kernels, and quite free from the reddish tinge which so often characterises bone-black. Ivory-black, like Frankfort-black, is employed by copperplate printers in the preparation of their ink. Mixed with white lead, it affords a rich pearl-grey pigment.

Lamp-black.—Lamp-black is an exceedingly light, dull-black powder, formed by the imperfect combustion of oils, fats, resins, &c. It may be prepared on a small scale by suspending a small tin-plate funnel over the flame of a lamp fed with oil, tallow, or crude naphtha, the wick being so arranged that it shall burn with a large and smoky flame. Dense masses of this light carbonaceous matter gradually collect in the funnel, and may be removed from time to time. The funnel should be furnished with a metal tube to convey the gases away from the room, but no solder must be used in making the connections.

An especially fine quality of lamp-black is obtained from bone-oil, deprived of the ammonia with which it is always contaminated. It is manufactured on a commercial scale by means of the apparatus shown in Figs. 3 and 4. The oil is contained in the lamp A and kept at a constant level by means of the globular vessel B, which is also filled with oil and inverted over A. The oil flows from the lamp into the tube C, which is bent upwards at the farther extremity on a level with the oil in the lamp. A cotton wick is supplied to the bent end of the tube, as well as a little spout D, for conducting away any oil that may overflow into the receptacle E placed beneath. A conical hood a surrounds the flame of the lamp and terminates in a tube b, through which are conveyed the sooty products of the combustion of the oil into the wide lateral tube c, arranged to accommodate the smoke from about a dozen such lamps placed at intervals of about 6 feet, as indicated in the figures. The effect of this wide tube c is not only to cool the smoke, but also to collect the water and other liquids condensed. The smoke and vapours pass hence into d, the first of a series of sacks made of closely woven linen, about 10 or 12 feet long and 3 feet in diameter, closed at the bottom with a trap or slide e, and formed at the upper and lower ends of sheet-copper tubing made funnel-shaped. The upper one of these is prolonged into an additional pipe f, by means of which the smoke arrives at the second sack g in the series, thence finding its way to the third, and so on till the last sack of the row is reached. In connection with the last sack of each row is placed a horizontal flue F, in which are arranged frames covered with wire gauze and mounted on hinges. Their purpose is to retain the small remaining portions of lamp-black passing out with the smoke from the sacks. The meshes of the gauze are constantly getting filled up with soot, which necessitates a periodical checking of the draught for its removal. This is done by means of the rod G, which, when raised and allowed to fall suddenly, jerks the accumulated mass off the gauze. The current of air passing through the entire apparatus can be regulated by a damper placed at the entrance to the chimney in which the flue F embouches. At regular intervals, the mouthpieces in the lower ends of the sacks are removed, and their contents are shaken out separately and collected according to their various qualities. That gathered from the first sack in each row should always be kept apart from the remainder, as it is much contaminated by the presence of resinous and tarry matters.

The old-fashioned method of preparing lamp-black from the incomplete combustion of gas tar is conducted in an apparatus resembling that shown in Fig. 5. The furnace a, lined with fire-brick, contains a kettle b, and is surmounted by a large thick cast-iron hood c, communicating with a stone or brick condensing chamber, divided by means of perforated partition walls into three unequal sized compartments d, e, f, wherein the black is deposited. A chimney g delivers uncondensed vapours into the atmosphere. In working, the furnace is first brought to a red heat, then the kettle b, charged with tar, is introduced. As a charge is finished, more tar is added, with occasional stirring, till the kettle becomes inconveniently full of residue, when it is withdrawn and a fresh one replaces it. The residue is chipped out and used as fuel. The black is removed weekly through the door h. It is of good quality and colour so long as the combustion is conducted with a minimum of air, admission of which is controlled at the furnace. The yield is about 25 per cent. of the weight of the tar; and one furnace should treat a ton of tar in a week. One workman can manage several furnaces.

An improved process has been introduced by Martin and Grafton for the preparation of lamp-black from coal-tar, which affords a very good product. The coal-tar is first stirred up energetically with lime-water in any convenient vessel, after which the mixture is allowed to stand until the coal-tar has subsided to the bottom, when the lime-water is drawn off. The tar is then well washed by decantation with hot water, and rectified in the ordinary naphtha still. Afterwards it is run into a long iron cylinder, which is placed over a furnace, and supplied with numerous large burners. Each burner has a metal funnel placed immediately above it, connected with a cast-iron pipe, into which all the fumes from each burner are conducted. The naphtha in the cylinder is heated almost to the boiling point by the furnace beneath. A series of smaller pipes lead away the fumes from the main pipe into a row of chambers, and thence into a series of large canvas bags, placed side by side, and connected alternately at top and bottom. The bags vary in number from fifty to eighty, the last one being left open to allow the smoke to escape, after traversing some 400 yards since leaving the burners. The best quality of lamp-black is found in the last bags, that near the furnace being much coarser and less pure. The bags are emptied whenever they contain a sufficient quantity.

The process employed in Germany for the manufacture of lamp-black is to conduct the products of the combustion of any resinous matter in a furnace into a long flue, at the end of which is placed a loose hood, made of some woollen material, and suspended by a rope and pulley. The lamp-black collects in this hood, and, when a sufficient quantity has accumulated, is shaken down and removed. In this manner about 6 cwt. of lamp-black may be collected in twenty-four hours.

One form of the apparatus is shown in Fig. 6. The circular structure a is lined inside with hanging cloths upon which the black can condense, and is covered with a conical roof from which depends a movable sheet-iron cone b, perforated at its apex to give egress to a current of air. This cone b is supported by a rope g passing over a pulley c and accessible from the outside. A fireplace d, containing a small iron dish e for holding the resin, is built against one of the side walls of the structure in such a manner that it can be fired externally. The rate of combustion is regulated by a small sliding damper on the door of the fireplace. When the black has accumulated in the chamber a to such an extent that operations must be suspended, the fire is let out, and the chamber is left to cool entirely, so that the black may not ignite on contact with the air. The cone b is then lowered, and in its descent scrapes the walls of the chamber a and causes the black to collect on the floor, whence it is removed through an iron door f which at other times is kept tightly luted.

In England, an inferior variety is sometimes obtained from the flues of coke-ovens. That known as Russian lamp-black is made by burning chips of resinous deal or pine wood, and collecting the soot formed; but it is objectionable, owing to its liability to take fire spontaneously when left for a long time moistened with oil.

A modified form of apparatus has been introduced by Thalwitzer, a German manufacturer, and is shown in Figs. 7 and 8. A vertical tube is provided at its upper end with a funnel, into which cooling water is poured and flows out through openings in the tube immediately above a circular plate of thin cast or wrought iron arranged horizontally and

secured at its centre to the tube. Round the periphery of this plate is a vertical rim of tin plate, at the top of which is a pipe through which the cooling water runs into a gutter round the top of the cylindrical casing, the water being carried off from this gutter by a pipe. The vertical tube is carried near its upper end in a bearing, and at that part is attached a worm-wheel geared into it by a worm driven by any suitable power. At the underside of the circular plate is fixed a scraper, the edge of which is formed with a strip of leather in contact with the lower surface of the plate. Opposite the scraper, at the bottom of the casing, is a burning lamp, which sucks up the oil for its consumption by a flat wide wick.

The operation is as follows:—The vertical tube is caused to revolve by the action of the worm-wheel, the circular plate thereby receiving a slow rotary movement; and a small stream of water being poured into the funnel at the top of the tube, this water passes down the latter and through the openings on to the circular plate, which is thus kept cool. The burning lamp filled with paraffin or other oil is brought as near to the circular plate as is necessary for the cooling of the flame and the most perfect extraction of the carbon, which, in the form of soot, attaches itself readily to the plate, owing to its coldness and to the condensation of the steam produced. The revolving plate presents continually to the flames a new and clean surface, in consequence of the lamp-black being scraped away by the scraper as soon as deposited, and brought away through a pipe or shoot into a collecting barrel.

The apparatus as shown in Figs. 7 and 8 consists of a round metal plate A, provided with a flange a, and fixed on a vertical shaft b supported by the bearing B, and carrying at its upper end a worm wheel d set in motion by a worm. The plate A is cooled by water admitted through a pipe g, and the flange a is provided with a discharge pipe h, through which the cooling water runs into the groove D, surrounding the whole apparatus. Underneath the plate A a number of lamps J are applied, which are fed with oil by a common pipe l. H is an oblique scratcher or blade, the working edge of which is formed by a strip of leather, and touches the lower surface of the plate A.

For manufacturing lamp-black, a slow rotary motion is imparted to the apparatus by means of the worm and worm-wheel, and a slight current of water is directed upon the plate A through the pipe g. The lamps J, filled with paraffin oil derived from lignite, or with any other suitable oil, are ignited and approached to the plate A as far as is necessary for cooling the flame, so as to deposit the greatest possible quantity of black. The latter adheres to the cold surface of the plate, which is also kept damp by the aqueous vapour formed during the combustion. The revolution of the plate serves to bring the flame continually into contact with new and clean portions of the plate, the black being continually scraped off by the blade or scraper placed opposite the flames, and conducted through a channel into a collecting trough.

There is a risk of overburning, causing a grey tint and a hard and granular texture.

A variety of lamp-black known as “carbon black” or “gas black,” has of late years assumed an important position among black pigments. It is produced in considerable quantities in the United States by the combustion of the natural gas issuing from the earth in the mineral oil regions. The soot arising from the imperfect combustion of the gaseous hydrocarbon is made to deposit itself on cooled iron surfaces. These at first were made stationary, but now take the form of revolving discs or cylinders, which are automatically cleansed of the black as fast as it is deposited. This type of lamp-black is remarkably free from mineral impurities and unburned oil, and of a full colour.

An improved lamp-black kiln has been introduced in which the use of water is dispensed with. It is shown in Figs. 9 and 10. The furnace A, which is preferably built double, as shown, is constructed of brick lined with firebrick, with a rear wall a that divides the furnace room from the condensing room, side walls b, front c, and central dividing wall d, that divides the furnace into two long and narrow fire

spaces. The bottom of the fire spaces e is formed by a sheet iron plate f that is supported by the walls, and the space below plate f serves as an air space through which air circulates by openings g in the front and side walls, this circulation of air tending to keep the plate f cool. The rear of the fire space e extends upward and communicates by an opening h through the wall with the condensing room. In the front wall c is an opening to each fire space e and a door i to each opening. The oil or other liquid is supplied by pipes k that enter from the outside near the rear of the fire spaces. The outer end of each pipe k is fitted with a cup-shaped receptacle l, into which the oil will run from the vertical branch of the main supply pipe m, so that the amount of oil running into each pipe k may be observed, and regulated by a cock. The pipe m feeds the oil to one or more furnaces, the supply of material to each furnace being separately regulated. In the fire spaces beneath pipes k are placed shallow cast iron drip pans o to receive the oil, and the oil running in faster than it will burn will drop while on fire into the pans o, and be spattered into small particles. These pans are changed frequently, access to them being obtained by doors i. A slide p is provided in each door i to allow of ventilation when required. The slides and doors should close air-tight. By constructing the fire space e long and narrow, the plate f is more readily kept cool, and the space in front of the point of combustion renders the smoke less liable to escape by the doors. The products of combustion pass through the opening h to the condensing room, which is lathed and plastered, and if the room is sufficiently large a number of furnaces may be fitted to discharge into the same room. This furnace is especially adapted for burning dead oil; but by using burners of suitable construction other oils may be burned, and a superior quality of lamp-black made from mitigated spirits.

A very large proportion of the lamp-black now made is derived from the combustion of creosote or anthracene oils from coal tar, or of the residues of shale-oil distillation. The form of combustion chamber varies in different works, but is typified by the following rough sketch of that in use at the Stampshaw Chemical Works (Fig. 11).

At these works a horizontal brick flue a about 18 inches square and about 10 feet long is provided. At one end it enters the black-house b, and is here provided with a damper c to shut it off when not working. The other end opens to the air, and here is a sliding door d which, when shut down, leaves an opening round a small pipe e, which enters in this situation from a main pipe that conveys oil in a similar manner to four burners of this description placed side by side. At the bottom of the flue is an iron tray f to catch any liquid that falls from the tube, and in this tray the oil is burned. The burning of oil in one of these flues is not allowed to go on for more than three hours, and, when the combustion is over, the communication with the black-house is closed, the entrance door of the flue is opened, and the cover is taken off the chimney g so that the flue may become cooled, and another flue is taken into use.

The black-house is a brick chamber into which the smoke passes, and where it deposits its sooty particles. In some works there is only one undivided chamber; in other works there are more than one, and the chambers communicate by flues through which the smoke passes from one to another. At other works the chamber is divided by vertical partitions, springing alternately from the two ends, so as to constitute one high zigzag flue, along which the smoke must travel to its outlet from the black-house. This chamber must needs have an opening somewhere to the outer air. The opening is sometimes a small chimney in the roof, and sometimes a short louvre tower. This is necessary to produce a trifling draught, just enough to carry the smoke into the chamber and no more.

In some works, the black from the black-house is also calcined, the object of the “calcination” being to get rid of all greasiness, a point of great importance when the lamp-black is to be used for making fine pigment. This process is conducted in circular iron pans, usually about 2¼ feet high and 2¼ feet diameter, which are provided with removable iron covers. A pan of this size will hold about 2 lb. of lamp-black. A bowlful is first put in and lighted by a red-hot iron; more and more is added from time to time as the ignition proceeds. When the pan, being full, leaves off smoking, the calcination is known to be complete, and the pan is then covered and its contents are allowed to cool. The loss undergone in this process is about 25 per cent. The smoke which comes off is acrid and very irritating to the eyes, like that proceeding from boiling oil, and it is difficult for a person unaccustomed to it to remain many minutes in the chamber where calcining is going on. This process is sometimes conducted within a chamber, but frequently under a shed or even in a building freely open to the air.

There are three sources from which nuisance may arise in lamp-black making: 1. The smoke which issues from the chimney of the black-house, small as it sometimes is, often constitutes a nuisance to near neighbours; but the nuisance is not a very serious one, and it does not extend very far from the works, never to a greater distance than about 50 yards. The odour, even when but little smoke escapes, is oppressive and suffocating in character, and resembles that diffused in a room by a smoking table-lamp. It occasions headache, but is not otherwise injurious to health. 2. A similar nuisance of suffocating smoke sometimes proceeds from the burners, but this is when they are leaky or when there is a deficiency of draught through the black-house, or when the doors of the burning chambers do not shut closely, and when there is much wind blowing past them. This nuisance chiefly occurs when the burners are open to the air and merely protected by an open shed. 3. The escape of acrolein and other offensive vapours from the calcining house.

The best mode of preventing nuisance from the black-house is so to elongate the chamber as to give abundant opportunity for the soot to deposit in the course of the smoke along it to the outlet, and by taking means to consume by fire what little smoke escapes deposition. A most effectual arrangement for the accomplishment of these ends is to have a black-house 150 feet long, and so divided by partitions within as to cause the smoke to traverse a distance of altogether 500 feet before it finds an exit; the exit from the chamber communicating with a fire, in which the last of the smoke is consumed, and which serves to assist in regulating the draught through the chamber.

The regulation of the draught through the burner and black chamber is of importance in order to avoid the escape of smoke from the burners. If the draught be too great, too much black is lost from the chamber, but if, on the other hand, it be too little, the smoke instead of passing into the chamber will come out into the works and create a nuisance, especially where the burners are erected in the open air, under circumstances in which variation in the force of the wind cannot fail to interfere with due regulation of draught. This part of the manufacture should be conducted within a building of some sort.

The best mode of preventing nuisance from calcination is in operation at Shackell & Edwards’ works, in Hornsey Road, Islington. At these works the black is calcined in a chamber 20 feet square and 25 feet in greatest height, with a paved floor and arched roof. In the centre of the roof is the opening where a fire was formerly placed, but which is now closed by a sky-light, capable of being raised. The calcining pots are ranged round this chamber, and a fan, employed to draw off the vapours from the oil-boiling pans, is further utilised to draw off also, from the upper part of the calcining-house, the vapours arising from the calcination, and to drive them into the boiler fire, where they are consumed. Calcination should always be conducted in a closed building duly ventilated so as not to create nuisance.

The transport of lamp-black is effected in barrels or bags; when in the latter, these should be previously soaked in water containing some clay in suspension, which stops up the pores of the sacking, and thereby prevents loss.

The particular virtue of lamp-black as a pigment lies in its state of extremely fine division, which could not possibly be attained by artificial means; this quality renders it invaluable as the basis of black pigments, all of which contain it in a greater or less quantity. Indian ink and printers’ ink are also composed principally of this substance.

Unimportant Blacks.—In addition to the recognised blacks already noticed there are a number of other sources of black pigment which have been drawn upon to a limited extent, or have been suggested as substitutes for the standard articles. They only merit a short description.

Aniline black is prepared by adding an acidified (sulphuric) solution of bichromate of potash to an aqueous solution of hydrochlorate of aniline, and washing the precipitate. The cost is prohibitive.

Candle black is candle smoke condensed on a cold plate.

Charcoal black is finely-ground wood charcoal.

Coal black has been suggested by grinding coal, but lacks the requisite qualities of a pigment.

Cork black is a very fine pigment prepared by calcining cork refuse. Limited supply.

German black is Frankfort black.

Iron black is ground black sulphide of iron.

Lead black is prepared by boiling lead fume in sulphide of soda solution. It would probably be unstable on account of oxidation.

Manganese black is ground oxide of manganese. It is costly, and dries too quickly.

Prussian black is calcined Prussian blue. It is not of a good colour, nor economical.

Prussiate black is the carbonaceous residue from making yellow prussiate of potash. Used chiefly for decolorising syrups, &c.

Spanish black is cork black.

Tannin black is proposed to be made by exhausting the tannin from refuse leather and tanning agents, and adding alum and sulphate of iron. The colour is blue-black, weak, and unstable.

CHAPTER III.

BLUES.

There are several methods of preparing cœruleum:—

(1) A solution of stannate of potash is added to one of cobalt. A blue precipitate is thrown down, which, on washing, becomes first light-red and then brown. When calcined at a white heat it assumes a blue colour.

(2) A solution of stannate of soda is mixed with a solution of nitrate of cobalt, and the resulting precipitate calcined to bright redness forms a blue pigment.

(3) Solutions of cobalt and tin are mixed and precipitated by soda, the precipitate washed free from soda being calcined as in all the other cases. The silicate of soda is the most satisfactory sodium salt for a precipitant.

Cobalt Blue.—This rich pure blue pigment is not alone permanent, but actually develops its full intensity only after exposure to the air. With age, however, it acquires a greenish tendency, and in artificial light it inclines to a violet tint. It is proof against acids and alkalies, and mixes particularly well with water. In combination with other pigments it is unaltered and has no effect on them. Its tones are different from those of ultramarine. It slowly decomposes when heated in strong sulphuric acid, yielding a violet solution and a white precipitate, which latter dissolves and affords a blue liquid on dilution with water. It consists principally of about 80 per cent. alumina, and 15 per cent. oxide of cobalt.

There are several ways of preparing it:—

(1) By Thénard’s original process, roasted cobalt, from Tunaberg, Sweden, is dissolved under heat in an excess of nitric acid. The solution, evaporated nearly to dryness, is boiled in water, and the deposit of arseniate of iron is filtered out. Into the filtrate is poured a solution of basic phosphate of soda, which throws down a precipitate of basic phosphate of cobalt, varying in hue from violet to pink. This precipitate is washed on a filter, and, while still gelatinous, 1 lb. of it is intimately mixed with 8 lb. of hydrated alumina, recently precipitated by ammonia from a solution of potash-alum. The mass is first dried to brittleness and then calcined in a covered clay vessel for half an hour at a cherry-red heat. The resultant blue pigment is stored in glass receptacles.

The preparation of the gelatinous alumina is conducted as follows. The potash-alum is dissolved in at least three times as much water as is necessary, and is then precipitated by an abundant excess of ammonia, with frequent stirring. When settled, the supernatant liquor is siphoned off, and the precipitate is thoroughly washed several times on a filter.

Thénard blue will vary in tint according to the proportions of alumina used. A pure colour is obtained with 4-5 parts alumina to 1 of phosphate of cobalt; a greenish hue with equal parts of alumina and cobalt salt; and almost any intermediate tone by varying the proportions between these limits.

(2) To 16 parts of gelatinous alumina add 1 part of arseniate of cobalt, obtained by precipitating the solution of cobalt by arseniate of potash.

(3) A richer and more velvety blue is got by using oxide of cobalt and substituting phosphate of lime for the alumina (Boullai-Marillac).

(4) Binder’s process is as follows:—Dissolve by boiling 6 lb. alum, free from iron, in a leaden or earthenware vessel, and filter it into a vat 5½ ft. high and 3 ft. across, one-third full of clean water. Precipitate the alumina by solution of potash, fill up the vat with water, settle, decant the clear liquor, and wash repeatedly till barium chloride gives no precipitate. Dissolve ½ lb. sesquioxide of cobalt in 1½ lb. hydrochloric acid at 22° B., and evaporate to dryness. Dissolve the residue in 3 lb. hydrochloric acid, and pass a stream of sulphuretted hydrogen through it, to throw down any foreign metals. Filter clear, evaporate again to dryness, and dissolve the residue in enough water to produce 4½-5 lb. of solution. Next precipitate the cobalt solution (3 to 6 lb., according to depth of tint required) by ammonia, avoiding excess. Wash the precipitate, and add it to the water, holding the gelatinous alumina in suspension, stirring thoroughly for ½ an hour. A reddish tint in the supernatant liquor shows that some of the cobalt has been dissolved. Add a little ammonia, and allow the precipitate to settle. Decant and add new waters repeatedly. Finally collect on a fine filter cloth, drain, press, stove dry, and calcine for 2-2½ hours at red heat in clay crucibles: then cool, grind first in a mill and then on a slab, and sift.

Smalts.—This pigment has not maintained its position in competition with artificial ultramarine. Formerly it was very largely used to correct the yellow tone of cottons, papers, and pottery. It has a pale violet-blue tint, which, however, is not constant in artificial light. Being a silicate it is very permanent, and proof against the action of acids, alkalies, and sunlight, besides being inert when mixed with other pigments. It can be used with either water or oil as a medium, but is not a successful paint owing to its weak colouring power. It is virtually a double silicate of cobalt and potash, or a cobalt glass, containing a few impurities, of which the chief are aluminium, iron, and lead oxides. The colour varies somewhat according as these impurities fluctuate, and the finest ground sample is always the palest. It is hardly ever adulterated, and the chief point to secure is that it be ground to the finest possible degree.

Its manufacture is most extensively and successfully carried on in Saxony. The raw materials used are cobalt speiss (an arsenide of cobalt and iron), potash, and sand. The ore is broken up into convenient sized pieces and roasted at red heat in a reverberatory furnace provided with a tall shaft for discharging the sulphurous and arsenical fumes at a high altitude. When the evolution of these fumes has ceased and the mass begins to assume a pasty consistence, the roasted ore is removed from the furnace, cooled, reduced to a fine pulverulent condition (then known as “zaffre”) and passed through a silken sieve. Should it be necessary,

the cobalt ore is first spalled and hand-picked to remove the ores of foreign metals which are associated with it; and then reduced to a very fine state in an edge runner or mortar mill, and freed from earthy impurities by washing. The concentrated ore is then dried and dead-roasted in small charges at a time (about 4 cwt.) in a specially designed reverberatory furnace such as shown in Fig. 12, of which a is the hearth on which the ore is spread; b, the fireplace, the products of combustion from which pass over the ore on the hearth, and thence into the flues c, which repeatedly circle round the furnace so as to provide abundant opportunity for the arsenious oxide derived from the combustion (oxidation) of the arsenic in the ore to condense; this highly poisonous arsenious oxide is collected in a solid form from the flues at convenient intervals by means of the doors d. The ore is charged and discharged at the door e. The roasting should not be carried to such a point that the whole of the sulphur and arsenic are removed when making smalts, as by leaving a portion of these substances in the ore at this stage, the ultimate purification is better accomplished.

The next stage is to fuse the roasted ore with potash and silica so as to form a blue glass. The proportions in which the ingredients are mixed depend upon the depth of colour in the zaffre operated upon and the tint desired in the finished smalts; hence it is always determined by a preliminary experiment, and is then most carefully adhered to, each material being accurately weighed out. Only the best potash can be used, as it must be quite free from soda, and iron or other metal; the effect of soda is to render the blue greenish tinted. Quartz affords the requisite silica, and is hand-picked to ensure freedom from alumina, iron, and lime, which import dullness into the colour, and then ground to a fine powder in an edge runner mill. The duly weighed quantities of the several ingredients are intimately mixed in wooden or cement lined vessels, so as to preclude the possibility of any metallic iron finding its way in; and as a further protection against this risk a little white arsenic is often added so that the iron may be carried down in the regulus which is formed during the fusion in the crucible.

These crucibles are of refractory earthenware quite free from lime, and measure about 18 inches across at top, gradually diminishing to 14 inches at bottom, so that an ordinary charge is about ¾ cwt. They are placed in rows in a furnace which generally bears a close resemblance to a glass furnace, the operation being very similar. The form of furnace common in Saxony, where most of the smalts is made, is shown in Fig. 13. By means of a series of openings a in the walls of the furnace the pots b are introduced on to the hearth of the furnace, whereupon the openings a are bricked up again and remain closed during the operation. The ingredients are charged into the pots b