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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.
The loose friable lumps into which the powder had aggregated were distinctly paler on the outside than in the interior, while the parts of the contents of the bottles which had been most exposed to light were paler than those which had been comparatively shaded. But a still more marked change had taken place in the samples to which the term 'pale' cadmium might be applied. These had generally become still paler, almost straw-coloured, especially where most exposed to light; but in some of the specimens orange specks were observed, resembling in hue what is usually called 'middle' cadmium. From the above observations it would seem that there is a tendency in differently tinted 'wet process' cadmium yellows to return to what we may call the normal or medium hue, but that the palest varieties are most subject to change. This change seems to arise in part from oxidation and hydration, for the bleached specimens gave indications of containing some white cadmium hydrate, when heated giving off a little water, and becoming brownish from the formation of the brown oxide of cadmium.
Such a bleaching of pale cadmium, if my explanation be correct, is in a measure explicable if we recollect that this variety occurs in a very fine state of division, and on this account is more liable to chemical change. In water-colour painting, where there is no effective protection through the presence of a hydrofuge medium, this fading of 'wet process' pale cadmium is notorious. In oils this cadmium, like the others, is generally thought to be permanent. My faith in the inalterability of cadmium pigments, even in oil and allied media, has, however, been somewhat shaken during recent years. Cadmium orange has almost perished where used in Leighton's lunette 'Arts of Peace,' in the Victoria and Albert Museum, a work executed in spirit-fresco. I regard the passage of the pale and of the deep cadmium yellow when in powder into the normal or middle variety as dependent chiefly, if not entirely, upon molecular changes. Moreover, the pale cadmiums are rarely found free from admixture, and their alterability may be in part owing to the foreign ingredients they contain. More recent researches by G. Buchner and N. von Klobukoff confirm the conclusions drawn from my early experiments.
There can be no doubt that cadmium sulphide exists in two if not in three molecular states, differing not only in colour but in crystalline form and in specific gravity. Thus pale cadmium has the specific gravity 3.9 to 4.5, while the red modification is denser - 4.5 to 4.8. And when the pale variety, dry and in powder, is rubbed strongly with a piece of agate, its colour deepens and reddens in a very decisive manner. The same change occurs when a water-colour wash of cadmium yellow is exposed for a year or so to sunlight in a perfectly dry atmosphere. This phenomenon is clearly analogous with that shown when the yellow mercuric iodide is altered into the scarlet form by pressure. It is perhaps safer to employ an ivory palette knife rather than one of steel in manipulating the cadmium pigments.
Aurora yellow is a bright and beautiful pigment consisting essentially of cadmium sulphide. It has more opacity than most of the other varieties of cadmium and possesses a pure yellow hue. Its stability is greater than that of many other varieties of this pigment. Daffodil yellow is the name given to another variety of cadmium sulphide, prepared at a red heat and containing a small quantity of magnesia. Neutral orange is a mixture of cadmium yellow with Venetian red.
Cadmium yellows are sometimes adulterated with Indian yellow, baryta and strontia chromates, and chromates of lead. Indian yellow shows its presence by blackening and giving off tarry fumes when the pigment, in the state of dry powder, is strongly heated in a test-tube. The chromates may be detected by the green colour produced when the sample is warmed with alcohol and dilute sulphuric acid. The lead chromates or chrome yellows, and the orange and red basic chromates of the same metal will blacken when the substance in which they are present is moistened with weak ammonium sulphide. Free sulphur in pale cadmium yellows comes off as a vapour when the sample is heated, but it may be better detected by the solvent action upon it of carbon bisulphide. Baryta-white may be detected by its insolubility in hot strong hydrochloric acid, in which cadmium sulphide dissolves.
Cadmium red and cadmium orange are slightly translucent when compared with the paler and yellower varieties of this pigment, and possess very full and glowing hues. They work well as oil and water colours. Mixed with zinc-white or flake-white, deep and middle cadmiums yield several beautiful colours, some of which closely resemble the different varieties of true Naples yellow, and are now employed very largely in lieu of the latter pigment. Pure cadmium yellow, when heated moderately, becomes orange-red or red, but regains its pristine hue on cooling. If, however, the heat be considerably raised in the presence of air, some of the sulphur in the compound burns, and the residual mass presents a dull brown colour. 'Manganese oil' accelerates the drying of the cadmium colours, which is sometimes inconveniently slow.
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