This section is from the book "Chromatography; Or, A Treatise On Colours And Pigments, And Of Their Powers In Painting", by George Field. Also available from Amazon: Chromatography, or A Treatise on Colours and Pigments, and of Their Powers in Painting.
Iodine Scarlet is a new pigment of a most vivid and beautiful scarlet colour, exceeding the brilliancy of vermilion. It has received several false appellations, but is truly an Iodide or Bi-iodide of mercury, varying in degrees of intense redness. It has the body and opacity of vermilion, but should be used with an ivory palette-knife, as iron and most metals change it to colours varying from yellow to black. Strong light rather deepens and cools it, and impure air soon utterly destroys its scarlet colour, and even metallizes it in substance. The charms of beauty and novelty have recommended it, particularly to amateurs; and its dazzling brilliancy might render it valuable for high and fiery effects of colour, if any mode of securing it from change should be devised; but it is out of the general scale of nature, - at any rate it should be used pure or alone, but in the recency of experience it ought not to be trusted by the artist. A similar pigment of a more crimson hue has lately appeared, having all the imperfections of the above, but in an inferior degree, and have both been improperly called vermilion, being among the most changeable of chemical substances. By time alone these colours vanish in a thin wash or glaze without apparent cause, and they attack almost every metallic substance, and some of them even in a dry state. When used in water, gum ammoniac appears to secure it from change; and it has been observed that, when gamboge is glazed over it, it preserves its hue with constancy.