CHAPTER XIV.
OTHER YELLOW PIGMENTS.
Cassel Yellow, also known as mineral or Veronese yellow, has now a very restricted use; it has been replaced by the deeper and cheaper chrome yellow.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.