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.