Yellow ochre is little subject to adulteration, for it is too cheap a pigment to make it worth while to substitute other substances for it. But sometimes the golden and richer coloured varieties have been found to have had their colour enhanced by the addition of certain fugitive or semi-permanent yellows of artificial or organic origin. The majority of such additions may be detected by pouring a little liquor ammoniŠ mixed with spirits of wine upon some of the ochre placed on a filter-paper in a funnel: the liquid passing through will be colourless if the ochre be genuine. An ochre which when heated in a test-tube gives off, besides water, fumes which partially condense into a coloured or tarry matter on the glass, contains organic matter, naturally present or artificially added, and is generally of inferior permanence. Of late years a far more frequent adulteration of yellow ochre is the addition of chrome yellow - that is, lead chromate. This adulteration may be detected by boiling the suspected ochre with sodium carbonate solution, filtering, and adding to the nitrate enough acetic acid to neutralize it, and then a few drops of lead-acetate. A yellow precipitate indicates the presence of a chromate.
An artificial yellow ochre is made by acting upon solutions of iron salts with metallic zinc, and thoroughly washing the precipitate obtained.
Brown ochre is an approximately pure limonite: raw sienna is very nearly related to it (see farther on), but cologne earth, raw umber, Caledonian brown and vandyke brown are distinct substances. An artificial brown ochre is prepared by heating yellow ochre with 4 per cent. of common salt to a low red heat.
Under the name of cyprusite a peculiarly bright lemon-coloured earth has been imported from Cyprus as a pigment: it consists essentially of a hydrated ferric sulphate: it is not likely to prove a safe pigment for artistic use.
Cadmium Yellow: Orient Yellow - Aurora Yellow - Daffodil - Orange Cadmium - Sulphide of Cadmium - Jaime Brillant - Jaune de Cadmium - Kadmiumgelb.
The metal cadmium, which is nearly related to zinc both chemically and physically, was discovered by Stromeyer in the year 1817. To one compound only of cadmium, the sulphide, are due all the hues and tints from the palest lemon cadmium to the fiery orange-red. This compound is represented by the formula CdS, and contains 112 parts by weight of cadmium to 32 parts of sulphur. As commonly prepared, cadmium yellow is of an orange hue; when this compound separates slowly from a solution, or is made in any way to take a dense or aggregated form, it becomes of a decided reddish orange. The orange-yellow variety, when very finely ground, becomes less red and more inclined to yellow. Some of the palest cadmium yellows contain white pigments, or flour of sulphur, added to reduce their depth of colour: the presence of free sulphur is sufficient to make any pigment ineligible.
There are two well-known processes for making cadmium yellow. In one of these pure cadmium oxide is heated in a covered crucible with pure sulphur in excess. In the other process, which yields pigments of greater brilliancy and beauty, a soluble salt of cadmium, such as the chloride or sulphate, is precipitated in the presence of a little free acid, by means of a solution of sodium sulphide, or preferably, of a stream of sulphuretted hydrogen. The hue of the product inclines to red when the solution is strong, hot and faintly acid; to yellow when it is weak, cold, and neutral. It is necessary to state that all the materials used must be pure. Iron, lead, bismuth, and any metals giving a coloured sulphide, even in traces, are seriously detrimental to the beauty of the product. The precipitate of cadmium sulphide, after having been thoroughly washed with boiling distilled water until the wash-waters no longer redden blue litmus paper, is collected on filter-papers and dried in the water-oven. In order to remove any free sulphur that may be present, the dry cadmium yellow may now be digested in a suitable vessel with pure carbon disulphide. After this treatment the pigment is once more dried, and is then ready for grinding in oil or other vehicle.
Cadmium yellow, prepared by the process last described, presents a satisfactory degree of permanence, and has no action on white lead when both pigments are ground together in oil. But a curious change has been noticed when the orange-red variety of this pigment, ground in oil, was kept some time in the ordinary metallic collapsible tubes, which formerly contained some lead, although of late years they have been made of nearly pure tin. The interior surface of the tube became darkened, sometimes almost black, from the formation of lead sulphide. It is certainly strange that a similar action does not occur between white lead and these deep cadmiums. For I found that the same sample of cadmium-red in oil which had blackened the metallic tube, when some of it was laid upon flake-white in oil, and kept for years, had not darkened the lead compound anywhere, even at the surface of contact. Moreover, cadmium yellows mixed with flake-white prevent, as do many other substances, such as baryta-white, lead sulphate, etc., the ready darkening of this lead paint by sulphuretted hydrogen. On the other hand, the cadmium yellows act with great energy upon some of the pigments containing heavy metals.
Emerald green, for example, is rapidly ruined by cadmium sulphide, both in water and in oil; cadmium yellow and emerald green (Schweinfurt green) are absolutely incompatible. Chrome yellow and true Naples yellow are also darkened by admixture with cadmium yellow, at least after a time. With oil colours, a sample of yellow ochre, which was afterwards found to have been adulterated with chrome yellow to the extent of 8 per cent., became yellowish-grey after admixture with some cadmium yellow.
While the stability of what may be called the normal cadmium yellow or orange is pretty well assured, both as an oil and a water colour, a very different verdict must be pronounced upon pale and lemon cadmium when used in water-colour painting. When thus used these pigments do not merely fade, but acquire a somewhat greyish hue. The following observations throw some light upon these changes. During the year 1876 I prepared a number of samples of cadmium yellow and orange. All were obtained by the action of sulphuretted hydrogen upon solutions of cadmium chloride. The products ranged in hue from a lemon colour to a deep orange, according to the strength of the solution, the presence or absence of free acid, and the temperature at which the precipitation of the pigment took place. After due washing and drying the various samples were put into bottles and preserved in my laboratory. They were never exposed to direct sunshine. On examining them from time to time it was noticed that the specimens of medium depth, having a yellowish orange hue, kept their hue perfectly, while two or three of the orange-red varieties exhibited a curious phenomenon of alteration.