Cassel Yellow, also known as mineral or Veronese yellow, has now a very restricted use; it has been replaced by the deeper and cheaper chrome yellow. Much of the Cassel yellow of commerce is nothing but chrome yellow shaded with barytes. As regards chemical composition, Cassel yellow has the following formula: PbCl₂.7PbO. It is obtained by heating litharge, red lead or white lead with ammonium chloride. To 10 parts of the lead compound 1 part of ammonium chloride is used; on melting, ammonia is set free, by which part of the lead oxide is decomposed, metallic lead separating. The melted mass is poured off from the lead into iron moulds, in which it solidifies to a very crystalline substance of a fine yellow colour. By grinding and levigating, the Cassel yellow is prepared for use. Pale yellow shades, obtained by admixtures of barytes, are occasionally encountered.
Montpellier Yellow consists, like the preceding pigment, of basic lead chloride. It is obtained by gradually mixing 400 parts of powdered litharge with a solution of 100 parts of common salt in 400 parts of water. After each addition of salt solution the pasty mass must be thoroughly stirred, or it will harden. When all the salt solution has been mixed with the litharge to a homogeneous white mass, the latter is treated with water to remove excess of salt, and the washed material dried and melted in earthenware crucibles. The melt, which has a bright yellow colour, is ground and levigated, when it forms a handsome pigment.
There are several other yellow pigments of similar composition, of which one only need be mentioned, obtained by treating a solution of zinc chloride with lead hydroxide.
Turner’s Yellow or English Yellow is prepared by two methods: either by melting 7 parts of finely ground litharge with 1 part of common salt; or by treating litharge with a solution of common salt and converting the white oxychloride into a yellow pigment by melting.
Naples Yellow.—This handsome pigment, which is, unfortunately, susceptible to the action of sulphuretted hydrogen, is known commercially under different names. Naples yellow takes its names from the fact that it was formerly exclusively made in Italy, where the method was kept secret, a secret which disappeared with the advance of analytical chemistry. Naples yellow is now known to be lead antimoniate.
Naples yellow is a handsome pigment. Its preparation is more tedious than that of chrome yellow, hence it is now rarely employed. The author has had practical experience that much of the so-called Naples yellow of commerce is nothing but a suitably shaded chrome yellow.
Naples yellow can be prepared by different methods. According to the oldest, given by Brunner, 1 part of pure tartar emetic is carefully and thoroughly ground with 2 parts of lead nitrate and 4 parts of common salt. The mixture is melted at a low heat in a Hessian crucible, and the fluid mass poured on a cold iron plate. After cooling, it is boiled out with water, when lead antimoniate remains as a powder of a more or less deep yellow colour. It is not easy to obtain this favourable result with certainty in every case. If a certain temperature is exceeded only by a little, a hard mass results, which by long boiling does not become a fine powder, but a sandy substance of little brilliance. Even when the operation succeeds, the product often varies considerably in shade, sometimes a sulphur yellow, at other times an orange pigment being formed. As a rule, the paler product is obtained at a lower temperature; by stronger heating, darker products of a red shade are obtained.
According to another recipe, 2 parts of tartar emetic are melted with 4 parts of lead nitrate and 8 parts of common salt. The mass is treated with very dilute hydrochloric acid for a long time, which extracts some quantity of lead oxide, a deeper product being thus obtained. Care is, however, necessary in this treatment; acid of too great strength would spoil the whole product.
The Paris method for Naples yellow is as follows: metallic antimony is oxidised by melting in air; to 12 parts of antimony, 8 parts of red lead and 4 parts of zinc oxide are used, and the mixture is melted at a low red heat.
A cheap, but not particularly bright product, can be obtained from old printer’s type. The metal, which is an alloy of antimony and lead, is powdered, mixed with 3 parts of saltpetre and 4 parts of common salt, melted, and the mass washed out with water.
Other formulæ which are said to yield a good result are as follows: 12 parts of white lead, 3 parts of antimony oxide, 1 part of ammonium chloride, 1 part of alum. Or: 16 parts of stibnite, 24 parts of lead, 1 part of common salt and 1 part of ammonium chloride. The intimate mixture of these materials is first gently heated with access of air, then more strongly, and the mass extracted with water. There are many other recipes for the preparation of Naples yellow, the majority of which are distinguished by an apparently arbitrary arrangement of the materials; for there is no scientific reason. If it were possible to accurately obtain any desired high temperature in a furnace, the manufacture of Naples yellow would no longer be a matter of skill, but the same product could be obtained at every attempt. Since this is not yet the case, the exact procedure for the preparation of this colour can only be found by careful experiments.
Naples yellow is, as has been said, a handsome colour, and offers a great resistance to varied reagents. It is only changed by one of them, sulphuretted hydrogen, by the prolonged action of which it is turned completely black.
Antimony Yellow is very similar in composition to Naples yellow. It consists of a mixture of lead antimoniate with the oxides of lead and bismuth. It is prepared by the process recommended by Meromé by intimately mixing 3 parts of finely powdered bismuth with 24 parts of powdered stibnite and 64 parts of saltpetre, melting the mixture and shaking it whilst molten into water. The brittle mass is finely powdered, washed and dried, then melted with 128 parts of litharge and 8 parts of sal ammoniac. The mass obtained has a fine pale yellow colour; when powdered it is antimony yellow. This pigment has almost fallen into disuse because of its instability and the high price of bismuth.
Calcium Chrome Yellow.—Calcium forms a yellow pigment with chromic acid, which, although far surpassed by the lead chromes in fineness of shade, has the advantage over them of greater stability and cheapness. For purposes for which cheap and at the same time permanent colours are required calcium chrome yellow can be recommended. It is most simply prepared from potassium chromate and calcium chloride, which, as a by-product of many chemical operations, is obtainable at very low prices. The deepest pigment is obtained when the precipitation is done with a boiling solution of the chromate. Calcium chromate, in addition to its use alone, may be employed instead of white pigments to produce pale shades from deep lead chromes. This addition should not be carried to a great extent or the chrome will be made too light, since calcium chromate has a much lower specific gravity than lead chromate.
Barium Yellow, Yellow Ultramarine or Permanent Yellow.—This pigment consists of barium chromate. The finest product is obtained when a solution of a barium salt, generally barium chloride, is precipitated boiling by a solution of potassium chromate. The very finely divided precipitate has a pale yellow colour very similar to that of pale lead chromes. This handsome pigment is distinguished by the valuable property of being practically unaltered by the atmosphere; it is only attacked by strong acids and alkalis. By long heating, the colour of this compound is gradually changed to a handsome green, which consists of a compound of barium and chromic oxides, and has occasional use as an artists’ colour. In order to obtain this pigment, the heating must be intense and long continued. According to the author’s experiments, it is not sufficient to heat for a short time to a very high temperature; in that way a mass is obtained of very unequal colour. The best result was obtained by spreading barium yellow in a thin, even layer in a flat porcelain dish and heating to whiteness for 10 hours.
Zinc Chrome Yellow.—Zinc chromate is inferior to lead chromate in beauty, but has the advantage of permanence. It does not blacken in an atmosphere of pure sulphuretted hydrogen, and resists very well the action of other agents. Zinc yellow may be prepared by the immediate precipitation of a solution of zinc sulphate by a solution of potassium chromate, both being boiling, but the very bright precipitate obtained in this way is not stable; on washing, it gives up chromic acid continually to the wash water, and only a pale yellow residue remains. A very fine colour is obtained in the following manner: zinc sulphate is dissolved in water and boiled for half an hour with 1 per cent. of white zinc whilst stirring. This operation effects the separation of iron oxide and the neutralisation of the free acid generally present in commercial zinc sulphate. When the solution has cleared by standing, it is precipitated by a solution of potassium chromate, the precipitate collected on a filter and allowed to drain completely; it is then washed with very small quantities of water and finally dried. A pure yellow precipitate is only obtained when all the iron oxide has been removed by boiling the zinc sulphate solution with white zinc; if the liquid contains only a very small quantity of iron, it has yet a very considerable influence on the colour, the yellow is not pure, but has a brownish tinge. Zinc yellow is used alone, and mixed with other pigments. Chrome yellows of all possible shades may be obtained in this way. Chrome yellows are often found in commerce which consist essentially of zinc chromate.
Cadmium Chrome Yellow.—When a solution of cadmium sulphate, or any other cadmium salt, is mixed with a solution of potassium chromate, a precipitate of cadmium chromate, CdCrO₄, is formed. This pigment has a beautiful, deep yellow colour, in no way inferior in shade to the finest lead chromes, and having the great advantage over the latter of being entirely unaltered by the atmosphere; it is thus to be highly recommended for artistic purposes. The high price at which it is sold prevents its general use, though now that cadmium compounds are to be obtained at so much lower prices than formerly, the price of cadmium chromate appears to be excessively high.
Cadmium Yellow is cadmium sulphide, CdS; in nature it occurs as the somewhat rare mineral greenockite. Cadmium yellow is obtained by dissolving metallic cadmium in sulphuric acid, and precipitating the solution with sulphuretted hydrogen. The solution of cadmium sulphate must be digested for some time with excess of cadmium, in order to separate the foreign metals present as impurities; the colour is not so fine when a quite pure cadmium solution is not used.
Cadmium yellow is a very bright yellow. Several shades are obtained according as the solution of cadmium sulphate used in its precipitation is neutral or acid. The reason of this difference in shade lies apparently in the different size of the crystals of which the precipitate is composed. The deep, pure yellow colour becomes still deeper by fusion, which takes place at a white heat. Weak alkalis, acids and sulphuretted hydrogen do not alter cadmium yellow; it is thus to be regarded as a durable artists’ colour. It can be mixed with ultramarine without decomposition, when a fine green is formed; but mixed colours cannot be made from cadmium yellow and blue copper pigments, since these would blacken in the light.
Lead Iodide.—On precipitating a solution of lead nitrate with potassium iodide, lead iodide is formed. This is but slightly soluble in water, and, when dry, has a handsome, deep yellow colour. Unfortunately it is not permanent, but is decomposed on exposure to light. It can be used for bronzing, but other and cheaper pigments are available for this purpose.
On account of the great solubility of lead iodide in a solution of potassium iodide, it is prepared in another way, and accurately weighed quantities are used. Calcium iodide may be used instead of the potassium salt; 100 parts of iodine, 15 parts of fine iron filings and 25 parts of lime are mixed with sufficient water to form a thin paste, which is warmed until all the iodine is dissolved, when water is added, the liquid filtered, and the residue washed in order to extract all the calcium iodide. The solution and wash waters are united, then a solution of 152 parts of lead acetate is added, when all the iodine is precipitated as lead iodide.
A simpler method is to dissolve equal parts of lead nitrate and potassium iodide separately, each in 20 parts of hot water, to mix the solutions and cool quickly, when lead iodide separates in very small crystals. When pure lead iodide is melted in the absence of air, and the fused mass powdered, a product of yet finer colour is formed. It is necessary to completely imbed the crucible in which the fusion is performed in the fire. The action of air on the melted mass would produce a basic iodide. The fine golden yellow colour of lead iodide adapts it especially for the production of gold bronzes on wall papers and fabrics.
Mars Yellow, which is generally reckoned among the best artists’ colours, is usually a mixture of ferric oxide and calcium sulphate or alumina. The pigment is prepared by mixing a solution of ferrous sulphate with milk of lime, when ferrous oxide is precipitated, which becomes yellowish brown on exposure to air, in consequence of the oxidation of the ferrous oxide. By heating the precipitate, according to the temperature different shades are obtained, varying between yellow and red. In addition to Mars yellow, Mars orange and Mars red are found in commerce.
The manufacture of this pigment is very simple: 1 part of ferrous sulphate is dissolved in 10 parts of water, and the solution mixed with milk of lime made from 1 part of quicklime and 40 parts of water. If it is desired to produce a darker shade, and especially a product to be afterwards converted into Mars orange, the amount of ferrous sulphate is increased to 2 parts. When the mixture has been made, it must be stirred for a long time, in order that the reacting substances may come thoroughly into contact. The precipitate, which at first is greenish grey, soon acquires by the action of the air the colour of ferric hydroxide, which becomes deeper on drying.
When dried and finely ground Mars yellow is heated in thin layers, it changes to dark yellow, and finally to orange red, a similar alteration taking place to that occurring when ferric hydroxide itself is heated.
A Mars yellow of a deeper shade, consisting of a mixture of ferric hydroxide and alumina, is obtained by precipitating with caustic soda a solution of ferrous sulphate and alum. The sodium sulphate which is formed at the same time must be removed as completely as possible by washing with boiling water.
By calcining Mars yellow for a long time at a high temperature, Mars brown is produced, a fine brown pigment. The value of Mars yellow and the pigments obtained from it lies not only in their fine shade, but in their permanence, which distinguishes the majority of the iron colours.
Siderin Yellow.—This not very handsome yellow consists of ferric chromate; it is obtained by adding a neutral solution of ferric chloride to a strong boiling solution of potassium bichromate so long as a precipitate is formed. Siderin yellow is said to be used both in oil and water, and to be particularly adapted for use in sodium silicate paints, since in the course of time it forms a stony mass with that substance.
The low price of iron salts would make it desirable to employ chromates of iron, but it appears to be difficult to obtain a compound of constant composition. In experiments with this object the author did not succeed in obtaining products of the same shade. Others have probably been equally unsuccessful, for siderin yellow has never been used in quantity, as it would have been were there no difficulties in the way of its preparation.
Aureolin is a double nitrite of cobalt and potassium, Co(NO₂)₂.3 KNO₂. This pigment is prepared by adding excess of potassium nitrite to a solution of cobalt nitrate acidified with acetic acid. As the liquid cools, a deep lemon yellow crystalline powder separates, which, when dry, is known as Indian yellow or aureolin. It is distinguished from other yellow pigments by being unaffected by sulphuretted hydrogen.
The potassium nitrite required in the preparation of this pigment is most easily made by melting saltpetre in a thick iron vessel and stirring in fine iron filings in small quantities as soon as the saltpetre begins to decompose. The iron glows brightly and burns to oxide, the saltpetre changes to potassium nitrite. The mass is dissolved in a little hot water, the solution filtered and cooled, when most of the undecomposed saltpetre crystallises out, whilst the nitrite remains in solution. After further evaporation and separation of another crop of potassium nitrate crystals, the solution can be used to precipitate the aureolin.
It is advisable to use strong solutions in the precipitation of aureolin; the finest precipitate is obtained in this way. If dilute solutions are used, the precipitate forms gradually; it is then coarse and has little covering power.
According to the method of Hayes, aureolin is prepared by passing into a solution of cobalt nitrate the vapours produced by pouring nitric acid over copper and allowing air to enter. Caustic potash is added to the liquid from time to time. In this way all the cobalt can be obtained in the form of a yellow precipitate.
Tungsten Yellow.—Finely powdered tungsten is introduced in small quantities into fused potassium carbonate so long as effervescence occurs. After boiling with water and filtering, calcium tungstate is precipitated from the filtrate by means of calcium chloride. The moist precipitate is added to hot dilute nitric acid until the liquid is only slightly acid, when it is boiled for half an hour and allowed to cool. The precipitate, after washing with a little water and drying, is a deep lemon yellow powder.
Nickel Yellow consists of nickel phosphate. It is obtained by adding sodium phosphate to a solution of nickel sulphate or nitrate, and heating the pale green precipitate to redness. Nickel yellow has a pleasing shade, and is distinguished by great permanence. Up to the present it has found little use as an artists’ colour, but on account of its permanence, which does not distinguish many yellow pigments, its use is to be recommended.
Mercury Yellow or Turpeth Mineral is a basic mercuric sulphate of the formula Hg₃SO₆. It is obtained by heating 10 parts of mercury with 15 parts of sulphuric acid in a porcelain dish in a good draught, until a white crystalline mass of neutral mercury sulphate remains. This salt, HgSO₄, is decomposed in contact with water into free sulphuric acid and a basic salt of the above composition. The decomposition is effected by treating the finely powdered neutral sulphate with hot water so long as the washings are acid, when a handsome lemon yellow substance remains. The wash waters contain acid mercuric sulphate. They are allowed to stand with mercuric oxide so long as this is dissolved, and the solution then used to prepare new quantities of mercury yellow.
Turpeth mineral has a very bright shade and great covering power, but it has little permanence. Sunlight soon turns it grey, and air containing sulphuretted hydrogen in a short time turns it quite black, mercury sulphide being formed.
Yellow Arsenic Pigments.—The extremely poisonous character of arsenic pigments has practically banished these handsome and cheap colours from use. In many countries their use is justly illegal. The majority of arsenic pigments have, therefore, merely historic interest. The two yellow arsenic pigments are found in nature as realgar and orpiment; though these are not rare minerals, the artificial products were generally used as pigments, and when they were in common use they were generally made in metallurgical works, in which minerals containing arsenic were treated.
Realgar, As₂S₂, has an orange red colour, whilst orpiment, As₂S₃, is a pure yellow. When these substances were used as pigments they had the same drawbacks in regard to mixing with other pigments as other sulphur compounds. King’s yellow is finely powdered natural or artificial orpiment.
Lead Arsenite is a permanent deep yellow, but extremely poisonous. It can be made by fusing an intimate mixture of 100 parts of white arsenic with 75 parts of gold litharge, grinding and levigating the mass. Cadmium yellow, which has still more permanence and is less poisonous, replaces this pigment.
Thallium Pigments.—Thallium is a metal which exhibits certain similarities to lead. By precipitating a solution of a thallium salt with potassium chromate or bichromate, according to the proportion between the quantities of the two salts, precipitates are obtained of yellow, orange or deep red colour, or, after fusion, brown. By the addition of a mixture of potassium chromate and ferricyanide to a mixture of a thallium salt and ferrous sulphate an olive green pigment is obtained. On account of the rarity of thallium compounds, technical employment is out of the question, and the sensitiveness of thallium pigments towards sulphuretted hydrogen prevents their use for artistic purposes.