Stromeyer discovered metallic cadmium in 1817 but production of the cadmium pigments was delayed until about 1840 because of the scarcity of the metal. A natural mineral, green ochite, is known in nature but was not used for pigments. Cadmium sulfide was prepared with an acid solution of cadmium salt (either chloride or sulfate) which was heated with hydrogen sulfide gas until a powder was formed. Hues ranging from a lemon yellow to a deep orange were made in this way. The deeper varieties of cadmium yellow and orange were the most permanent. The paler varieties were known to fade on exposure to sunlight. All of the cadmiums were brilliant and the deeper shades had the greatest tinting strength. Field claimed that the best cadmiums were those produced without an excess of sulfur and that the permanence of a carefully made cadmium was improved when mixed with lead white using only an ivory knife. They were used in both oil and watercolor but could not be mixed with copper-based pigment.
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