To the yellow pigments belong also the Ochres, King's Yellow, Orpiment and Naples Yellow.
Red Oxides of Iron, by far the most impor tant series of this group, vary in colour from a deep bluish-red to a pale red-brown according to their purity and the percentage of ferric oxide,
which they contain. Ferric oxide occurs naturally in numerous varieties of the red iron ore, haematite, in red ochre, red bole or raddle, in crude red iron-stone and micaceous iron. The conversion of these ferruginous minerals into pigments is relatively simple and consists generally in grinding and levigating the material to remove impurities and to attain the required degree of fineness; in many cases the shade of the product needs to be modified by calcination whereby any ferric hydroxide is changed into the red ferric oxide with consequent reddening of tone (cf. OCHRES). Large quantities of iron compounds, by-prod ucts of industry, are now economically worked up for the produc tion of artificial iron oxides of great purity, their shade depending on their method of treatment and the temperature at which they are "burnt" or heated. Iron oxides are marketed under various designations according to colour and purity, such as English Red, Indian Red, Persian Red, Turkey Red, Caput Mortuum, Col cothar, Rouge, Venetian Red and many others. In certain cases, e.g., Madder Indian Reds and Tuscan Reds, the oxides are bright ened by the addition of a permanent red lake ; such toning is readily detected by chemical means. When of high ferric oxide content, they possess considerable covering power and opacity and are highly resistant to atmospheric influences ; they are used extensively as oil colours, as a pigment in rust preventive coat ings, in calico printing, for colouring porcelain and glass and in many other industries.
Red Lead is a lead tetroxide represented by the formula pb304, produced by heating litharge (PbO) in air at about 480° C. The commercial varieties usually contain from 6 to 12% of free litharge. It is a bright scarlet-red pigment possessing good cover ing power, opacity and permanence, and on account of its anti corrosive property is extensively employed as a priming coat for iron and steel structures. It exerts a strong drying action on linseed oil and its hard-setting properties render it valuable in this medium in making cements, lutes and packing for steam joints. It is used as an oil colour, as a base for organic lakes, for
manufacturing lead and optical glass, enamels, in other industries.
To the red pigments also belong Vermilion, Carmine, Cadmium Scarlet (cf. Cadmium Yellow), Antimony Vermilion, antimony trisulphide, Chrome Red and a large number of organic lake pigments.
Two main classes of green pigments are in use : the "elementary" and the "composite." (a) Elementary Greens.—Chrome Oxide Green, chromium sesquioxide,
is produced by the reduction of potassium dichromate at a high temperature, by calcination of ammonium dichromate or chromic chloride, and similar processes. It is one of the most permanent pigments known, and is unaffected by heat, light, acids, alkali, chlorine and other reagents. It is used as a water, lime and oil colour; in printing inks, in which it possesses non-actinic properties; as a mordant in calico printing and dyeing; as an abrasive for polishing steel and precious metals; and in many other industries. Guignet's Green (Viridian) is a hydrated chromium sesquioxide produced by heating potassium dichromate in the presence of boric acid or other flocculating agent. It is a pigment of great permanence, possessing similar properties to chrome oxide green, from which it may be distinguished by its solubility in boiling hydrochloric acid. Its covering power and strength are only moderately good ; it is used for chrome oxide green and also admixed with chrome yellow as a pale green for landscape painting. Cobalt Greens are not true chemical com pounds but solid solutions of zinc and cobaltous oxides obtained by calcination in a manner similar to the cobalt blues. They possess great permanence and are unaffected by heat, light, sul phuretted hydrogen, etc., are non-poisonous and are used as artists' oil and water colours. Green Earth is a natural alkali aluminium–magnesium–ferrous silicate of variable composition. It is an inert and permanent pigment deficient in colour and is used as an artists' oil and water colour, in fresco painting, and as a substratum for organic lakes, basic dyes, fixed by its silicic acid content, being in some cases rendered fast to light.