As the vaporised zinc is emitted at the mouths of the retorts a in a partially ascending current, it immediately encounters a plentiful supply of air, and thereupon takes fire (undergoes combustion or oxidation). In this condition it enters the lower and funnel-shaped end of the sheet-iron flue e, by which it is conveyed into the series of settling compartments f.

While the bulk of the zinc oxide thus formed passes into the settling chambers, a portion of it is too heavy to do so; its specific gravity is such that the force of the draught is not sufficient to carry it up. This portion falls at once into a receptacle placed beneath the mouth of the flue e.

In order to obtain the necessary draught, the conduits h are open to the outside atmosphere, and introduce a supply of air just below the mouths of the retorts a, so that it impinges against the current of escaping zinc vapour. After passing through the settling chambers f, the superfluous air finds an outlet at i into a sufficiently capacious flue k, which communicates with the chimney stack d. Thus the draught created by the fuel consumed in the reverberatory furnaces is made to assist the current through the settling chambers.

These settling chambers f, are constructed of wood, and are usually about three in number, intercommunicating of course. The zinc oxide enters the first compartment through an aperture in the top of the side to which the discharge end of the iron flue e is attached. After traversing the first chamber, the stream of air and such oxide as has not yet settled passes into the second chamber through the orifice l, near the bottom of the partition dividing the two chambers. To reach the next compartment, the stream has to ascend again, the aperture being at the top of the partition, and this alternation is carried on to the end of the series, thus checking the through draught and facilitating the settlement of the zinc oxide. The floors of all the chambers are made funnel-shaped, with a door at the lowest point, so that the discharge of their contents may be as automatic as possible. The flue k contains screens hung at intervals for the purpose of hindering, as far as possible, the escape of minute particles of zinc oxide into the chimney, and thence into the outer air, whereby they would be lost.

A description of the process as conducted in Belgium, says that ingot zinc is placed in a series of retorts within one furnace, and the oxide is formed in an exhaust chimney, and then passes through a long series of passages and condensing chambers, in which are ranged tanks of sheet iron or cloth to collect deposits. At a certain hour of the day it is collected into casks, and after being tested as to quality it is ready for delivery. According to the purity of the metal various qualities are produced. The best is called “blanc de neige,” or snow white, and is of a very superior quality; No. 1 white is the most used, the ores for this quality being selected and purified by remelting; No. 2 white is the common variety. In the process of manufacture there is more or less waste material, imperfectly oxidised, deposited in the retorts and passages. This residuum is carefully ground, washed and dried, and is employed in painting in the place of lead.

In the American method of making zinc white they use the ore direct. This is cheaper than the Liège method, but its product is of inferior quality to that produced by sublimation. There are but two works in Belgium for making zinc white, the Vieille Montagne Company (at the Valentine Cocq works), which produce yearly 3000 tons by sublimation; the other is at Ougree, near Liège, where the American method is employed, but at present it is idle.

There are many other modifications in detail in different works. One may be noticed here as it is claimed for it that the pigment is gifted with greater covering power or body, the limited degree of which is the only drawback to zinc oxide whites. The plan consists in this, that the oxide is allowed to collect in the condensing chambers till it is of such a depth that a man entering stands waist deep in the pigment. The latter is gathered in pieces of sacking, which are drawn together and squeezed up tightly, so that the oxide, when newly prepared, is pressed into hard dense masses.

(2) Sulphide.—Prof. Phipson, in a paper read before the International Health Congress, at Paris, remarked that for several years efforts had been made to discover some white substance to replace white lead for painting buildings, ships, &c. He himself had devoted several months to this important subject, but without success. There has been found, it is true, in oxide of zinc a substance less poisonous than lead, and serving very well as a white pigment in oil painting; but its production is very expensive, and its mechanical properties as a colour in oil are not pronounced enough to allow it to compete in commerce with white lead. Such is not the case, however, with an invention of Mr. Thomas Griffiths, of Liverpool, who has succeeded in obtaining a very interesting product. This new preparation, which is being manufactured at the present time on a pretty extensive scale, has for its base sulphide of zinc (or an oxysulphide of that metal), the properties of which as an oil colour are of the most remarkable character. It is prepared by precipitating one of the salts of zinc by a soluble sulphide, and washing and drying the precipitate. The latter is then calcined at a red heat, with some precautionary measures, then taken from the furnace, and, while still warm, thrown into cold water. It is afterwards levigated and dried. The result is a white pigment, very fine, and of great beauty. Regarded from a hygienic point of view, Griffiths’ new white is infinitely superior to white lead, as it also is in its practical bearing; it possesses no injurious qualities; its manufacture and use do not affect the health of workmen; its durability in climates of the most diverse kinds is, so to speak, illimitable; it is altered neither by gaseous emanations nor by dampness; and its price is comparatively low. The most remarkable thing about this new white is that it covers as well as white lead, while it withstands the effects of all kinds of weather, so that its use is not only deprived of all danger to health, but it is much more economical than white lead. Prof. Phipson stated to the Congress that he regarded this new chemical preparation as being among the most ingenious and useful products that have been discovered in our time.

A later method, introduced by Griffiths and Cawley, consists in making an artificial sulphide of zinc by bringing the vapours of zinc and sulphur into intimate contact.

In carrying out this process, sulphur is melted in a jacket pan heated preferably by high pressure steam. The melting vessel is connected with a cast-iron still by means of a jacketed pipe, and the connection is regulated by means of a valve in the bottom of the melting pan. The latter should be at such a height above the still that the pressure due to the column of sulphur in the conduit pipe may be greater than the tension of the sulphur vapour in the still, so that when the valve is opened, the sulphur in the melting pans may descend into the still. The still is kept during the process at a temperature of incipient redness, so that when the sulphur reaches it, the sulphur is immediately vaporised and the resulting current of vapour passes to the chamber described below.

Metallic zinc is melted in a retort or crucible, heated preferably by means of a furnace on the Siemens or a similar principle, and raised to such a temperature that it begins to volatilise freely. When this takes place, the resulting zinc vapour is met by a current of sulphur vapour obtained as above described, and in excess of that required to form with the zinc sulphide of zinc. The reaction takes place according to the chemical equation Zn + S = ZnS.

The sulphide of zinc is in the form of white extremely light powder, which is carried along by the current of sulphur into the collecting chamber, such as is used for the manufacture of oxide of zinc. This allows of the separation of the different constituents of the products into different qualities; those parts that are carried the farthest are the whitest and best generally. Those portions nearest to the part of the apparatus where combination takes place may sometimes contain metallic zinc, if the sulphur supply has not been carefully attended to, but this may be separated from the sulphide by levigation.

The collecting apparatus should be kept at a temperature slightly superior to that of the boiling point of sulphur, in order that sulphur, which is necessarily in excess, may not be deposited with the sulphide of zinc, but may pass on in the vaporous form to a suitable condenser, where, after condensation, it may be collected and used again.

Before the sulphur vapour reaches the condenser, the last traces of sulphide carried along with it are collected by the interposition of metallic screens or sieves, placed between the sulphur condenser and the apparatus.

In carrying out the process, care must be taken to keep the collecting apparatus as cool as possible consistently with the fulfilment of the conditions above mentioned, viz. that no sulphur be condensed therein. In practice, this object can be effected with little difficulty. Impurities in the zinc and sulphur are of little consequence provided they are not volatile, and not of such a nature that they would detract from the whiteness of the sulphide of zinc formed.

The accompanying diagram, Fig. 29, is given as an example of a plant that may be employed with good results.

a is the sulphur melting pan with its steam jacket b, and steam pipe c; d is a cast-iron still, arranged within a gas furnace e; f is a crucible for melting and volatilising zinc, the said crucible being contained in a gas furnace g; h is an automatic apparatus for freeing the mouth of the zinc vessel from deposited sulphide of zinc; i is the collecting apparatus; and k, the condensing apparatus.

(3) Mixtures.—There are a number of compound pigments commonly known as zinc whites, which only deserve the name in so far as they contain a proportion, greater or smaller, of some zinc salt. At the same time it must be admitted that some of these combinations possess very good qualities, and that the foreign ingredients largely correct the weak points of the zinc compounds.

Freeman’s.—This pigment, when ground with oil in the customary way, forms a paint equal in body and covering power to the best white lead, while it is superior in colour, permanence, and density, and is free from odour and noxious qualities.

It is produced by grinding together “zinc white” (either oxide or sulphide), lead sulphate and barium sulphate, in certain proportions, in the dry state, in an edge-runner mill. By thus grinding the several pigments together, their particles become intimately incorporated and undergo changes in character. The barium sulphate not only cheapens the product by reason of its low cost, but also imparts a distinct feature in rendering the paint more free working. The proportions generally adopted, calculated by weight, are 5 parts lead sulphate, 2 of zinc white, and 1 of barium sulphate. The duration of the grinding will necessarily vary in accordance with various governing conditions, but it should be continued until the mixture has a density of about 200 lb. per cubic foot.

Orr’s.—The pigment known as Orr’s enamel or Charlton white, is a compound of oxide and sulphide of zinc and sulphate of strontia, or of baryta. It is prepared in two ways.

(a) Barytes is calcined for some hours at white heat with charcoal, and the calcined mass is lixiviated with water to wash out the barium sulphide; to one-half of this solution is added zinc chloride, which produces a precipitate of zinc sulphide, leaving barium chloride in solution. To this mixture of zinc sulphide and barium chloride is added the remaining half of the barium sulphide and some zinc sulphate, the result of which is a double precipitate of zinc sulphide and barium sulphate. This is water-washed, filter-pressed, dried, calcined at red heat, thrown immediately into cold water, ground very fine, and finally dried.

(b) The second process closely resembles the first, but celestine takes the place of the barytes.

Either form of Orr’s enamel is a good useful white pigment, very permanent, mixing well, of excellent covering power, and pure in colour.

Characters.—Zinc oxide, being an expensive pigment, is liable to adulteration; fortunately, all such adulterations are easily detected, and their nature ascertained by a few simple tests. Zinc oxide, if pure, should dissolve entirely without effervescence in nitric acid; any residue would indicate adulteration with barytes or china clay; the former may be distinguished by its weight and the yellowish green colour it imparts to the Bunsen flame, the latter is lighter and gives no colour to the Bunsen flame. Boiled with strong sulphuric acid, barytes is not acted on, while china clay is. If, after cooling, the mass be diluted with water, and ammonia be added to the liquor, if barytes is present no precipitate will be obtained, while if china clay is present a white precipitate is produced.

If the zinc oxide dissolves with effervescence, white lead or whiting may be present; the solution should give no precipitate of black sulphide of lead on passing sulphuretted hydrogen through it. On neutralising the solution in nitric acid with sufficient ammonia, and adding ammonia sulphide to precipitate all the zinc (the precipitate should be white, any other colour would show some impurity), filtering off and adding a little oxalate of ammonia, no white precipitate of calcium oxalate should be obtained; such a precipitate would show presence of whiting or gypsum.

The white pigments having as a base the sulphide of zinc, also contain barytes, oxide of zinc, sulphate of strontium, &c. They can be distinguished by evolving sulphuretted hydrogen gas, recognisable by its odour, on treatment with an acid. They are not entirely soluble in acids, the residue being mostly barytes, but may also be sulphate of strontium; it is immaterial whether the two be distinguished or not.

CHAPTER VIII.

YELLOWS.

Arsenic Yellow.—Another name for Orpiment (see p. 280).

Aureolin Yellow.—This colour is prepared by precipitating carbonate of cobalt from a solution of a cobalt salt with carbonate of potash; this precipitate is dissolved in acetic acid, and to it is added nitrite of sodium; nitrite of potassium and cobalt are thrown down as a yellow powder. Experiments made on the freshly-precipitated colour prepared in this way, have proved it to be invariably destroyed by caustic potash; but aureolin yellow prepared in another way, put into a flask with some caustic soda, has remained unchanged for several days, and proved to be perfectly stable under this treatment. Samples of the same yellow, exposed to the action of ammonia, soda, and potash for a considerable length of time, have not changed. This is really a most important fact, because aureolin yellow is so beautiful a pigment that one wishes it would stand for ever. It is, in fresco and silicious painting, a great desideratum to have colours that will stand the action of lime and caustic alkalies. Our colours for fresco painting are very limited, and if aureolin will stand the action of caustic alkalies, it may be safely used both for fresco and silicious painting.

Cadmium Yellow.—The pigment known as cadmium yellow is important on account of its permanence and brilliance. It is prepared by passing a stream of sulphuretted hydrogen gas through a slightly acid solution of a salt of cobalt, usually the nitrate or sulphate, whereby the sulphide of cadmium is precipitated as an impalpable powder. It is filtered off, well washed with water, and finally dried at a low temperature.

By this method there is considerable difficulty in producing any precise, shade of yellow, which is dependent on the proportion of precipitant used, and can only be controlled after long practical experience. Therefore some modifications have been introduced with the object of securing definite tints. The ordinary colour is a pure chrome yellow. For a lemon-yellow shade, a solution of yellow sulphide of ammonium is added to one of sulphate of cadmium. For an orange tint, the cadmium solution (chloride or sulphate) is made very acid by addition of hydrochloric acid before passing the sulphuretted hydrogen; or the cadmium solution, as for lemon-yellow, is boiled, and receives the ammonium sulphide solution while still boiling. But none of these modifications results in a really durable pigment, on account of the presence of traces of free sulphur or acid.

The best brands of this pigment being somewhat expensive, there is great inducement for adulteration, which usually takes the form of orpiment or of chrome. The former can be detected by the ordinary tests for arsenic, and the latter by the process described on p. 266.

Chrome Yellows.—A very important family of yellow pigments are the “chromes,” consisting mainly of chromic acid in combination with lead, iron, or zinc.

Chromates of lead, says Prof. Barff, are produced by precipitating a lead salt with a salt of chromic acid, and the difference in tint is owing to the different quantity of the chromic acid which is present in the salt. The orange chrome is a basic chromate of lead, and basic chromate of lead contains more of the chromic acid than is present in the lemon chrome. The lightest chrome contains some sulphate of lead precipitated with the chromate. All these colours contain lead, and are therefore liable to the influence of sulphuretted hydrogen. Now, if a chromate of lead is bought hap-hazard anywhere, it may or may not be pure, but generally speaking, unless the chromates are obtained from makers who are careful in the preparation of their colours, they contain many other substances besides chromate of lead. For instance, they contain a quantity of the solution, in a dry state, from which they have been precipitated, and are by no means pure. Obviously, also, there is an inducement to men who sell cheap colours to adulterate them, to bring them down with whiting, and so forth. In that case the chrome loses its body; if it is brought down with lead sulphate it has more body; but when chrome yellows are prepared and mixed with good oils, and put on carefully, and those oils have time allowed them to become oxidised, perfectly dry in fact—not dry in the sense in which an artist considers a painting dry, but perfectly hard—then the action of sulphuretted hydrogen in the atmosphere will not be of such great moment as if the colours are impure, or if they are submitted to the influence of that deleterious gas before they have become perfectly hard.

Another objection to the chromes, on the authority of Prof. Barff, is that they are soluble in alkali, and so are many other colours. If a painting is painted with chromes, and if that painting be washed with an alkaline soap, it is quite certain that some of the chromates will be dissolved. Consequently the minute and delicate touches, upon which the artist depended for some of the best effects of his picture, are removed with soap and water. No painting should ever be washed with soap and water at all; but there are certain colours which will withstand the action of soap, even if it is intensely alkaline, while others will not. Taking a precipitate of lemon chrome, if we act upon this with potash or soda, it dissolves up the yellow precipitate, and destroys it altogether.

According to Weber, the preparation of chrome yellow presents difficulties in practice, because products differing in shade and structure, although of uniform chemical composition, are obtained according to modifications of the method of manufacture or nature of materials employed. A special difficulty is the turning of colour, whereby a “turned” yellow has a dirty orange-yellow colour, which when mixed with barytes gives a yellowish-brown leather-coloured shade and not a light pure yellow. Other derived colours are similarly affected.

Lead Chromates.—The acetate and nitrate of lead are the soluble lead salts generally used. Basic carbonate of lead (white lead) is also much used, the yellows prepared from this material being cheaper, having a large covering capacity, and being particularly adapted for ordinary greens; but these have not the smoothness and lightness so much prized in some of the yellows obtained from soluble lead salts. The oxide, sulphate, and chloride of lead have been proposed for the manufacture of chrome yellow, but their treatment would be very tedious and the products obtained only of medium quality. In the manufacture of chrome yellow from acetate of lead and bichromate of potash, different proportions of these materials are given by different authorities, but some of these are obviously wrong where an excess of bichromate is to be used, for every light chrome yellow is liable to turn by the action of chromic acid or a chromate and thereby become of little value. Proportions should be used, whereby the acetate of lead remains in excess, and the reaction should take place in a solution as cold and dilute as possible. In this way a brilliant yellow tint is obtained.

For the lighter chrome yellows, lead sulphate is precipitated simultaneously with the chromate by adding sulphuric acid or a soluble sulphate to the solution of the bichromate; these yellows have less tendency to change colour than the pure chromate of lead, if the above precautions are observed. Chrome yellow, precipitated from an excess of lead acetate solution, by means of potassium bichromate, corresponds to the formula PbCrO₄. When dried it forms light pieces, which show a conchoidal fracture. A still more voluminous product corresponds to the formula PbCrO₄, PbSO₄. The yellow having the composition PbCrO₄, 2PbSO₄, is very heavy and shows a smooth fracture. Lightness is often imparted to chrome yellow by the addition of magnesium carbonate.

Nitrate of lead offers no advantages over the acetate, and is generally more expensive to use. Free nitric acid is more objectionable than free acetic acid, because it may act as a solvent on chrome yellow and liberate free chromic acid, which is liable to “turn” the yellow. When using lead nitrate it is preferable to neutralise potassium bichromate with soda, to avoid the presence of free nitric acid. The yellows mostly in demand are the inferior qualities, prepared by mixing pure chrome yellows with white mineral matters, generally barytes, gypsum, and kaolin, usually stirred in with the bichromate solution before adding the lead salt. Barytes injures the colour least, but kaolin has the advantage that it does not increase the weight of the colour so much. Gypsum occupies an intermediate position; it is much more voluminous than barytes, and does not injure the colour so much as kaolin, but it is generally used together with barytes. It is not advisable to use this combination, for the reason that the colour on drying forms very hard pieces, which offer difficulties in grinding. Gypsum, too, from being more easily acted on by reagents than barytes or kaolin, tends to take part in the reaction by decomposing the potassium bichromate before the addition of the lead salt, and this is objectionable. (Weber.)

A writer in the Chemical Trade Journal gives the following formula for the production of various yellows:—

(1) For soluble lead salts:—

This mixture yields a yellow of the formula PbCrO₄, PbSO₄. To obtain good shades, the amount of water employed should not be less than 1000 litres, or double as much with the nitrate. In the latter case, it is better to neutralise the lead nitrate and to replace the acid by the sulphate of an alkali or of magnesium, or preferably of aluminium; for the neutralisation of the bichromate, the best substance is magnesite. The formula thus becomes—Lead acetate and bichromate as before; magnesite, 6 kilos.; aluminium sulphate, 27 kilos.

(2) The Basic Acetate Method.—Litharge, 76 kilos.; acetic acid (30 per cent.), 42; bichromate, 21·5; sulphuric acid, 21·5; water, 2000 to 3000 litres. To obtain a denser chrome, 10 kilos. of soda should be added to the acetate, and 5 of the sulphuric acid replaced by 10 of aluminium sulphate. For the production of an orange chrome, the following formula is given: Litharge, 76 kilos.; acetic acid (30 per cent.), 42; bichromate, 24; Solvay’s soda, 15; and caustic soda (100 per cent.), 5. Care must be taken that the temperature does not rise sufficiently high to spoil the shade.

(3) The White Lead Method.—In this case the white lead must be in the finest state of subdivision possible, and suspended in the water: White lead, 100 kilos.; nitric acid (36° B.), 12; bichromate, 13; and aluminium sulphate, 10: or nitric acid (40° B.), 44; bichromate, 24; sulphate, 20, the latter giving the more fiery shade. For the production of an orange: white lead, 100 kilos.; nitric acid (36° B.) 18; bichromate, 28; and caustic soda, 8, the latter being best added to the bichromate before precipitation, and the temperature kept between 150° and 165° F.

(4) The Basic Chloride Method.—The same proportions and temperature are suitable here as in the case of the white lead.

(5) The Sulphate Method.—Lead sulphate, 100 kilos.; bichromate, 24 to 25; Solvay’s soda, 8·75 to 16; ammonia (24 per cent.), 1 to 2; and acetic acid (30 per cent.), 5 to 10. The sulphate, in the form of a cream, is gradually added to the other ingredients after solution.

A new and improved process for manufacturing from galena chemically pure chrome yellow having great colouring power, according to the Paper Trade Journal, consists in first dissolving pulverised galena with nitric acid to produce liquid nitrate of lead, and then precipitating the chromate of lead by subjecting the nitrate of lead to the action of bichromate of potash, neutral chromate of potash or chromate of potash-soda.

The galena (sulphide of lead) is first pulverised by suitable means, and in case it contains foreign minerals or other impurities, it is washed or otherwise treated in a suitable manner to remove these substances. The pulverised galena is then placed in acid-proof vessels and is dissolved by adding nitric acid diluted with water, the entire mass being well stirred. A slow dissolving takes place at the ordinary temperature; but when the mass is heated artificially, either by heating the vessel, or by using hot water added to the nitric acid, or by the use of steam, more rapid solution is effected. The product obtained is nitrate of lead in a liquid state.

The quantity of nitric acid necessary for dissolving a certain quantity of galena depends on the percentage of lead contained in the ore, and to a certain extent on the amount and nature of impurities present in it, and also on the length of time in which the dissolving takes place. In treating 100 lb. of galena having 80 per cent. of metallic lead, about 90 to 100 lb. of nitric acid of 36° to 38° B. are used, and the nitric acid is diluted with 100 to 200 lb. of water. This mixture is left for about 24 to 36 hours, and is stirred up occasionally, as above stated.

After the galena is dissolved by the nitric acid, and the sulphide of lead is changed into liquid plumbic nitrate, then the sulphur which floats occasionally on the surface of the solution is removed, and the substance which remains undissolved is washed out and is also removed. The liquid nitrate is then passed through filters of felt, linen, hemp, flannel, &c., or is left standing for about 12 to 18 hours for settling and clearing.

Now in order to produce the chrome yellow from this nitrate of lead, bichromate of potash is dissolved in water, and a sufficient quantity of this solution is poured into the plumbic nitrate solution until all the plumbic nitrate is changed into chromate of lead, called “chrome yellow.” Instead of the bichromate of potash, neutral chromate, or chromate of soda may be used, and for the purpose of obtaining lighter tints they may be tempered with sulphuric acid or any other compound of sulphur. The liquid nitrate of lead is placed, preferably, in large open receptacles of wood, clay, earthenware, or other suitable material, and the chromate of potash solution is put into similar vessels, and then placed above the receptacles containing the plumbic nitrate. The chromate of potash can then easily be run into the lower receptacles containing the liquid nitrate of lead, and this mixture is constantly agitated by similar means until all the plumbic nitrate is changed into chromate of lead, which is precipitated on the bottom of the larger receptacles.

The chemical action which takes place by this changing of nitrate of lead into chromate of lead is that the chromic acid of the potash assumes the place of the nitric acid, which parts from the lead and combines with the potassium, so that the lead as chromate of lead is precipitated on the bottom of the receptacle, while the nitric acid of the plumbic nitrate remains with the potassium, which latter has parted with its chromic acid, and a quantity of water as solution above the chromate of lead.

To change the nitrate of lead recovered out of the 100 lb. of galena above mentioned into chromate of lead, about 56 lb. of bichromate of potash are used. This change usually takes place in from about 10 to 30 minutes, after which the chrome yellow (chromate of lead) is left for a few hours to settle, and then the solution standing on top of the chrome yellow is drawn off by suitable means, or run out of the vessel by opening a cock placed above the level of the chromate of lead.

The latter is then washed by adding pure water, which is poured upon the chrome yellow, and the mixture is stirred up, so that all the remaining liquid nitrate of potash is removed. After this is accomplished, the mass is left to settle, and the water is again drawn off from the precipitate, which then settles on the bottom of the receptacle. This washing is repeated as often as is deemed necessary.

The chrome yellow is next placed in suitable receptacles, and dried in the open air or in specially constructed drying rooms, after which it is packed in boxes, kegs, &c., and is then ready for use. The liquid nitrate of potassium, or saltpetre lye, removed from the receptacles in which the chrome yellow is precipitated, and the first water used for washing the chrome yellow, as above described, are placed in large open flat receptacles or excavations, so as to be exposed to the action of the air and sun; or the liquids may be operated on by a small graduation work, so that a great portion of water evaporates. The residue is then heated in suitable vessels or troughs by a slow heat until a salt crust is formed, which, when cooled off and left to dry, is nitrate of potassium or saltpetre in a pure state.

From 100 lb. of galena having 80 per cent. metallic lead, some 28 to 30 lb. of pure and dry saltpetre are produced by the above described process. The sulphur produced by the dissolving of the galena by nitric acid is melted in a small stove or furnace in the usual manner, and then refined, so as to produce bars of sulphur called “brimstone.” About 10 lb. of such sulphur are produced from 100 lb. of such galena treated in the manner described.

The chrome yellow thus produced is said to be chemically pure, and of great covering power, equal to the best chrome yellow in the market.

The process is very simple, and the crude lead ore is transformed into chrome yellow in from three to four days.

Characters.—Pure chromate of lead has an orange-yellow colour, and in whatever manner it be made it always has this tint. Commercially, chromes are made of a great variety of tints, from a pale-lemon chrome to a deep scarlet, through all the intermediate shades of yellows and oranges; in fact, most colour makers produce not less than eight, and some more shades of chromes, whence it is obvious that they cannot be chemically pure, but must be mixed with some other bodies.

In commerce, chromes are distinguished as “pure” and “common”: the distinction between them is that the “pure” chromes are made from a lead base, and consist of chromate of lead mixed to a larger or smaller amount with sulphate of lead, the paler shades containing most of the latter; while the “common” chromes are mixed with china clay, barytes, gypsum, or similar bodies.

To distinguish the two kinds of chromes, treat with boiling hydrochloric acid. If pure, the chrome will completely dissolve, the solution usually having a green colour. On cooling, crystals of lead chloride separate out. The liquor will give a white precipitate with barium chloride.

By taking a weighed quantity of chrome, dissolving in hydrochloric acid, adding excess of barium chloride, filtering, thoroughly washing the precipitate with boiling water, drying and weighing it, and making the necessary calculations, every one part of the precipitate being equal to 1·3 parts of sulphate of lead, the quantity of the latter in the chrome is obtained.

The “common” chromes, which mostly contain barytes, are not completely dissolved on boiling in hydrochloric acid, the barytes they hold being left as an insoluble residue. By taking a weighed quantity of the chrome, and filtering off, drying and weighing the residue, the amount of barytes present can be ascertained; the solution can be tested for sulphate of lead, which they sometimes contain, as described above.

Chrome yellows and oranges should be assayed for colour and tint, care being taken to compare them with a thoroughly reliable standard sample. Their colouring power and body should also be assayed. Another property which should be tested is the colour that they yield on mixing with Prussian blue. A great deal of chrome is used for making greens by mixing with Prussian blue, and as there is a very considerable difference between them in the shade of green they give, it is rather important to test this property, which can readily be done by mixing 100 grains of the chrome with 10 grains of Prussian blue, grinding them together in a mortar, and observing the shade of the green which is produced thereby; if the green is not bright and pure, the chrome is not fit to be made into greens, and should be rejected for that purpose.

Iron Chromate.—If a solution of chloride of iron acidulated with hydrochloric acid be added to neutral chromate of lead, a light orange powder is precipitated, which is chromate of iron. Dried at 104° F., it is found to consist of 65 to 65·11 chromic acid, and 34·58 to 34·78 oxide of iron. Chromate of iron is insoluble in water, dissolves easily in hydrochloric, nitric, and sulphuric acids, and decomposes when mixed with soda lye. Under strong heat, it melts into a brownish mass. It may be used in painting in oils, as a substitute for chromate of lead. Although inferior to the latter in brilliancy, it has certain advantages over it—it does not blacken with exposure to sulphuretted hydrogen, is not injurious to the user, and withal is cheaper.

Zinc Chromates.—The before-quoted writer in the Chemical Trade Journal, speaking of zinc chromates, says that although it is possible to prepare lead chromates having shades varying imperceptibly from the palest lemon to a deep granite red, in the case of the zinc compounds scarcely any variation from the normal is possible, this being, however, different from anything obtainable with lead. Zinc yellows fall considerably below the ordinary chromes in their colouring power, but they are faster in light and are less poisonous. More than 80 per cent. of the amount annually made in Germany is used for the production of zinc green by mixing with Prussian blues, of which substance Holland, Switzerland, and Hungary are the greatest consumers. The zinc yellows met with in commerce vary in their constitution considerably, being usually acid chromates of zinc and potassium, basic zinc chromates being rare. Ordinary salts of zinc invariably contain small amounts of iron, which must be removed before they are used in the manufacture of colours. The simplest method is to heat them with the quantity of permanganate theoretically necessary to convert all the iron present into the ferric state, adding zinc hydrate, which need not be free from iron; after thorough stirring, the whole is allowed to settle, and filtered, when the liquid will be found to contain not a trace of iron.

On the addition of chromate to such solutions, a yellow precipitate (ZnCrO₄) falls, but owing to its great solubility in the liquid this process is valueless. By using an excess of bichromate, the zinc chromate combines with some of the alkaline salt, forming the compound (ZnCrO₄)₃.K₂Cr₂O₇, which may be washed without loss; but on drying yields an extremely hard, sandy powder, possessing, in spite of its fine colour, no value as a pigment. By neutralising the two solutions before precipitating, a much higher yield of chromate is obtained, but still so much chromic acid is lost as to make the process too expensive to pay. Formerly an addition of calcium chloride was made to the neutral solutions, so as to precipitate as calcium chromate some of the acid which remained mixed with the zinc salt. The best results, both in regard to yield and colour, are obtained by adding to the zinc salt sufficient alkali to decompose one-quarter of it, so that the chrome may have the formula (ZnCrO₄)₃.ZnO. To the bichromate, enough alkali should also be added to convert it into the normal salt. It is to be remarked that the nature of the metal combined with the chromic acid has the greatest influence on the shade of the zinc yellow, so much so that in manufacturing “acid” zinc yellow, the use of sodium bichromate is inadmissible. A basic zinc yellow prepared from the sodium salt has a redder and more cloudy shade than one made from the potassium compound, but the difference is hardly noticeable when sodium-potassium chromate is employed.

Modern zinc yellows are invariably prepared from acid solutions, and consist of a double salt of zinc chromate and potassium bichromate, mixed with a varying amount of unchanged zinc oxide, which must not be regarded as an adulteration of the pigment, for its presence gives the substance “body.” As previously stated, sodium bichromate is inadmissible, as it does not form similar double salts.

The raw material is usually zinc oxide, which is met with in a state of great purity; by the addition of sulphuric acid, this is converted into basic zinc sulphate; potassium bichromate solution is added, and the whole is stirred vigorously for an hour. At the end of this time, the zinc chromate, which was previously in a state of partial solution, begins to separate out in the form of a brilliant yellow scum on the surface of the liquid, consisting of (ZnCrO₄)₃.K₂Cr₂O₇, while the solution rapidly becomes almost colourless. Suitable proportions are:—Zinc oxide, 100 parts; sulphuric acid (66° B.) 60; and potassium bichromate, 100. Although it is hardly possible that any hydration takes place, it is found advisable to soak the zinc oxide in water for 24 hours before the other reagents are added, the sulphuric acid after dilution being added gradually. Great care must be taken that the solutions are all cold, and the stirring is continuous, to avoid the pigment being deposited in a hard sandy form. Zinc yellows thus prepared are not liable to change during washing in a manner analogous to the lead compounds.

Zinc chrome is not much used, partly because it is expensive, partly because it cannot compete with the lead chromes in brilliance, depth of colour, and body. Still, owing to the fact that it can be mixed with sulphur pigments without change, it is often employed in the place of the lead chromes. Pure zinc chrome is completely soluble in sulphuric acid without any effervescence, but a slight effervescence may be disregarded. Any residue may be put down as adulteration, and its character can be ascertained by a few simple tests: it may be chrome yellow, barytes, &c.

Gamboge.—Gamboge is a product of several trees of Eastern Asia: viz. Garcinia Morella var. β. pedicellata [G. Hanburyi], a native of Cambodia, the province of Chantibun in Siam, the islands on the east coast of the Gulf of Siam, and the south parts of Cochin China; G. Morella, growing in the moist forests of Ceylon and Southern India; and G. pictoria, of Southern India, by some considered identical with G. Morella. G. travancorica, of the southern forests of Travancore and the Tinnevelly Ghâts, is capable of affording small supplies of the pigment for local use, but not for export.

When the rainy season has set in, parties of natives start in search of gamboge-trees, and select those which are sufficiently matured. A spiral incision is made in the bark on two sides of the tree, and joints of bamboo are placed at the base of the incision so as to catch the gum-resin as it exudes with extreme slowness during a period of several months. It issues as a yellowish fluid, but gradually assumes a viscous and finally a solid state in the bamboo receptacle. It is very commonly adulterated with rice-flour and the powdered bark of the tree, but the latter imparts a greenish tint. Sand is occasionally added. The product from a good tree may fill three bamboo joints, each 18 to 20 inches long and 1½ inches in diameter. The trees flourish on both high and low land. Annual tapping is said to shorten their lives, but if the gum-resin is only drawn in alternate years, the trees do not seem to suffer, and last for many years.

Dr. Jamie, of Singapore, who has gamboge-trees growing on his estate, says that they flourish most luxuriantly in the dense jungles. He considers the best time for cutting to be February to April. The filled bamboos are rotated near a fire till the moisture in the gamboge has evaporated sufficiently to permit the bamboo to be stripped from the hardened gum-resin. The gamboge is secreted by the tree chiefly in numerous ducts in the middle layer of the bark, besides a little in the dotted vessels of the outermost layer of the wood, and in the pith. It arrives in commerce in the form of cylinders, 4 to 8 inches long and 1 to 2½ inches in diameter, often more or less rendered shapeless. When good, it is dense, homogeneous, brittle, showing conchoidal fracture, scarcely translucent, and of rich brownish-orange colour. Inferior qualities show rough, granular fracture, and brownish hue, and are sometimes still soft. The pigment consists of a mixture of 15 to 20 per cent. gum with 85 to 80 per cent. resin. Its chief uses are in water-colour painting, and in varnishes.

King’s Yellow.—A familiar name for the trisulphide of arsenic, also known as orpiment (see p. 280).

Naples Yellows.—This group of pigments embraces several combinations of the oxides of lead and antimony, derived from various sources, and prepared by sundry methods. Two of the most useful formulæ are as follows:—

(a) Mix 3 lb. powdered metallic antimony, 1 lb. oxide of zinc, and 2 lb. red-lead; calcine, grind fine, and fuse in a closed crucible; grind the fused mass to fine powder, and wash well.

(b) Grind 1 part washed antimony with 2 parts red-lead to a stiff paste with water, and expose to red heat for 4 or 5 hours.

There are a great many modifications both in the ingredients and the processes, and a great variety of shades in consequence.

Taken as a whole, the Naples yellows are unsatisfactory pigments, very prone to deteriorate in impure air, and necessitating great care in their preparation to avoid contact with iron, which turns them green. They cover well, are fairly brilliant, and mix readily with water or oil, but their application is declining rapidly.

Ochres.—The large class of mineral pigments known collectively as ochres or sienna earths possess considerable importance, notably on account of their remarkable durability and their reasonable price. They all consist essentially of an earthy base coloured by oxide of iron or of manganese, or of both. Some authorities differentiate between ochres and siennas, and ascribe the latter name only to those earths which contain manganese, but this seems to be an arbitrary proceeding, because the term sienna, or more properly Siena, is derived solely from the name of the Italian province in which these minerals are worked. They are of widespread occurrence, both geographically and geologically, and the methods of mining and preparing them are not subject to much variation.

Consul Colnaghi, in his report on the mineral products of the province of Siena, says that Siena earths, known also under the names of ochre, bole, umber, &c., are considered by some mineralogists to be ferruginous clays, by others, minerals of iron. They are chiefly found in large quantities in the communes of Castel del Piano and Arcidosso. The yellow earths and bole found on the western slopes of Monte Amiata are true lacustrine deposits found amid the trachytic rocks, of which it is principally composed. They lie under, and are entirely covered by, the vegetable soil. Varying in compactness and colour, they are termed yellow earths when of a clear ochreous tint, and terra bolare, or bole, when of a dark chestnut colour. Each deposit consists for the greater part of yellow earth, beneath which bole is found in strata or small veins. The mineral being very friable, its excavation is easy, and is generally conducted in open pits.

The different qualities are separated during the process, the bole, which has the higher commercial value, being the more carefully treated. After the first separation the bole is further classed into first, second, third, and intermediate qualities—boletta, fascia, cerchione, &c. Its most important characteristic is termed, in commercial language, punto di colore, or tint. The value of the bole rises as its tint deepens. Thus bole of the third quality is lighter than that of the second, and the second than that of the first. After the third quality comes the terra guilla. The yellow earths, after excavation, are exposed to the open air for about a year, by the pit side, without classification. The bole, on the contrary, is placed in well-ventilated storehouses to dry for about six months. This diversity of treatment is owing to the fact that exposure to the elements brightens the colour of the yellow earths, and raises their value, while it would damage the bole by turning its darker tint first into an orange yellow, and, if continued, into an ordinary yellow earth. It also loses in compactness and crumbles up under exposure.

In addition to the punto di colore, the size of the pieces influences the commercial value of the bole, which increases with their volume. Thus the classification is bolo pezzo, bolo grapolino, and bolo polvere. The yellow earths are classed as giallo in pezzo, giallo commune, and giallo impalpabile, the impalpable being worth more than the common yellow. The production of the Siena earths is estimated at about 600 tons per annum, of which amount about 50 tons are calcined, and the rest sold in the natural condition. The value of the trade is estimated at from £4000 to £6000.

The European trade in these earths is very large. Rouen exports some 5000 tons yearly, and Havre about 1500 tons.

Similar deposits occur in America, where they are known as “paint-beds,” and the earths are called “metallic paints.” A prominent example is the paint-bed at Lehigh Gap, Carbon County, Pennsylvania, which was originally opened as an ironstone mine. The mineral proved valueless metallurgically, but remarkably useful as a pigment, since it contains about 28 per cent. of hydraulic cement, which hastens the drying and causes the paint to set without any addition of artificial dryers, thereby making it eminently fitted for all outdoor application.

Along the outcrop of the paint, the beds are covered by a cap or overburden of clay, and by the decomposed lower portion of the Marcellus slate, which is 50 feet thick at the Rutherford shaft.

Beginning with the Marcellus slate, the measures occur in the following descending order:—

a. Hydraulic cement (probably Upper Helderberg), very hard and compact.

b. Blue clay, about 6 inches thick.

c. Paint-ore, varying from 6 inches to 6 feet in thickness.

d. Yellow clay, 6 feet thick.

e. Oriskany sandstone, forming the crest and southern side of the ridge.

East of the Rutherford shaft the sandstone forms the top-rock of the bed. This is due to an overthrow occurring between the Rutherford tunnel and shaft.

The paint-bed is not continuous throughout its extent. It is faulted at several places; sometimes it is pinched out to a few inches and again increases in width to 6 feet. A short distance south of Bowman’s there is a fault striking; north-east in the Marcellus slate, which has produced a throw of about 200 feet. The measures dip from 10° to 90°. The dip at the Rutherford shaft is about 79° south, whereas at the tunnel it is 45° north. The ore is bluish-gray, resembling limestone, and is very hard and compact. The bed is of a lighter tint, however, in the upper than in the lower part, and this is probably due to its containing more hydraulic cement in the upper strata. The paint-ore contains partings of clay and slate at various places.

At the Rutherford shaft there are fine bands of ore, alternating with clay and slate, as follows—Sandstone (hanging-wall), clay, ore, slate, ore, clay, ore, clay, ore, slate, ore, cement, slate (foot-wall). These partings, however, are not continuous, but pinch out, leaving the ore without the admixture of clay and slate. Near the outcrop the bed becomes brown hematite, due to the leaching out of the lime and to complete oxidation. Occasionally, streaks of hematite are interleaved with the paint-ore. In driving up the breasts, towards the outcrop, the ore is found at the top in rounded, partially oxidised and weathered masses, called “bombshells,” covered with iron oxide and surrounded by a bluish clay. In large pieces the ore shows a decided cleavage.

The method used in mining is a variation of panel-work. Nearly the same system of working is employed by all of the companies who have developed their mines either by means of tunnels or shafts. Tunnels are preferred whenever equally convenient, because they involve no expenses for pumping and hoisting machinery, fuel, repairs to machinery, &c.

The following description of the operation of the Rutherford mines is typical of all the workings in the vicinity.

The Rutherford tunnel is 6 feet high and 600 feet long. The gangways are driven along the foot-wall of the cement side, 6 feet high, and are heavily timbered and lagged at the top and on the clay side. The sets of timbers are 3½ feet apart, and usually of 9-inch timber. The width at the top is 3½ feet, with a spread of 5 feet at the bottom, the extra width being cut from the clay. Where the cement-rock is firm, the collar is hitched 6 inches into it and supported by a leg on the clay side. The cost of the timber is 54 cents (2s. 3d.) per set, including the lagging. The monkey gangway, which carries the air along the top of the breast from the air-shaft, is 2½ feet high, 1½ feet wide at the top, with a spread of 2½ feet at the bottom. Wooden rails with a gauge of 18 inches are spiked to the cross-ties.

The gangway is not driven continuously, but after being driven about 55 feet on either side of the shaft, the breasts are started 25 feet from the shaft, a pillar being left to protect it. The breast is then opened up to the face of the gangway, and when one ore-breast is worked out, the gangway is driven ahead about 30 feet, and a new breast is opened and worked out before commencing a third. The air-hole is first driven to the surface, then the breast is opened to its full width of 6 feet. The thickness of the bed of ore here varies from 4 to 6 feet, depending upon the thickness of the partings of clay and slate. The clay and slate are left on the bottom, which is made sloping to allow the ore to roll down to the shute; this is 6 feet wide and 4 feet long and heavily timbered. Small props or sprags are hitched into the cement, and wedged with a lid on the clay side to prevent falls of rock.

The holes are drilled by hand in the clay-partings. They vary in depth from 1 to 4 feet, and the charge of dynamite is varied correspondingly, according to the amount of ore it is desired to throw down. The loose ore is wedged down with crowbars and picks, and is then freed from any adhering clay and thrown down the shute. It is there loaded into boxes holding about half a ton each, which are pushed to the shaft on a truck. The ore-boxes have four rings at the corners, to which are attached four chains, suspended from the wire hoisting-rope. At the top of the shaft the boxes are detached and placed on a truck, which is run to the dump. Thirty cars, averaging 15 tons, are extracted in a day of two shifts, the day-shift working nine hours and the night-shift eleven. The pay of the miners is 5s. per shift. The cost of mining the ore averages 7s. per ton.

The ore, as it comes from the mines, is free from refuse, great care having been taken to separate slate and clay from it in the working places. It is hauled in 2-ton wagons to kilns, which are situated on a hill-side for convenience in charging. The platform upon which the ore is dumped is built from the top of the kiln to the side of the hill. The ore is first spalled to fist-size and freed from slate, and is then carried in buggies to the charging-hole of the kiln.

The slate, when burned, has a light yellowish colour, which would change the colour of the product. Figs. 30 to 32 represent a front elevation of the kiln and two sections at right angles to each other. The kiln is 22 feet high and 16 feet square on the outside. The interior is cylindrical, 5 feet in diameter, with a fire-brick lining a of the best quality. The interior lining slopes from the fire-place b to the door c, by which the charges are withdrawn; this facilitates the removal of the calcined ore. The casing d is of sandstone, 5½ feet thick, and tied together with the best white-oak timber e. When charged, a kiln holds 16 tons of ore, and the kiln is kept constantly full. The heat passes from the fire-places b—of which there are two, placed diametrically opposite each other—through a checker-work f of brick into the centre of the charge. The charge enters at g and is withdrawn by a door c in the front wall, 2 feet long and 18 inches high. The ashpit is at i. The fire is kept at a cherry-red heat, and about one cord of wood is burned every twenty-four hours.

The kiln works continuously, calcined ore being withdrawn and fresh charges made without interruption. The ore is subjected for forty-eight hours to the heat, which expels the moisture, sulphur and carbon-dioxide. About 1½ tons of calcined ore are withdrawn every three hours during the day. The outside of the lumps of calcined ore has a light brown colour, while the interior shows upon fracture a darker brown. Great care is necessary to regulate the heat