Scheme for the recognition of blue and green pigments

The scheme on p. 389 indicates reactions for the ready distinction of the principal blue and green pigments (when these are not complex mixtures). Below are described the methods of complete analysis of some of the more important of these. 

This is a complex compound of aluminium and sodium silicate and sodium sulphide. The typical and most common ultramarine is blue ultramarine, which forms a very fine powder (microscopically crystalline) of a pure blue, reddish blue or greenish blue colour according to the proportions of the components and the method of preparation ultramarines of other tints, especially green and violet ultramarines, are also made. Ultra-marine may be adulterated with gypsum, chalk, clay, heavy spar, zinc white and magnesium carbonate and may also be mixed with glycerine and glucose. 

It is subjected mainly to certain technical (see 1-5) and qualitative tests (see 6 and 7) quantitative analysis (see 8) is rarely required. 

Quality and Intensity of the Colour.—The colour is compared with that of a standard ultramarine by pressing a pinch of each on a piece of white paper with a spatula. 

To measure the intensity of the colour, a scale of comparison is prepared by making intimate mixtures of 1 gram of the standard ultramarine with different amounts of an indifferent white substance (kaolin), e.g.,with 10 grams for the normal intensity, and with 9, 8, 7, 6. .. or n, 12, 13, 14. . . grams for greater and less intensities. A mixture of 1 gram of the ultramarine to be examined with 10 grams of the same white substance is made and compared with the scale of mixtures. 

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Systematic Scheme for the Recognition of Blue and Green Pigments... 

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