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Green Pigments
Brunswick Green.- This popular and widely used pigment is made from Prussian blue and chrome yellow, with barytes, the proportion of which varies according to the quality of the green.It is made in three tints-pale, middle, and deep-ranging from a light, rather yellowish green to a deep blue-green.
Brunswick greens, if of good quality, have excellent staining power and cover well.
They are, however, only moderately permanent on exposure to weather and tend to turn blue or yellow in time, depending on whether the yellow chrome or Prussian blue content fades the first.
Since both chrome and Prussian blue are readily affected by alkali, Brunswick green is unsuitable for distemper colours, nor should it be used in oil paints on surfaces containing alkali liable to be activated-as, for instance, new Portland cement.
Chromium Oxide.--This pigment is an oxide of the metal chromium and is prepared in various ways.
It has a slightly yellowish tinge but is remarkably permanent and unaffected by acids, alkalis, or heat, and has great opacity.
It mixes well with white and all other colours, and is used both for oil paints and for good-grade washable distempers.
Viridian.-This pigment, also known as Guignet'8_Green, is also an oxide of chromium, differing only slightly in composition and properties from chromium oxide.It is equally stable and permanent, but more brilliant in colour and rather more transparent in oil.
Permanent Green.-A name given to a mixture of viridian, zinc chrome, and barytes.
As the name implies, the product is fast to light and does not discolour on exposure.
Terre Verte, or Terra Verde.-This is a natural silicate of iron and magnesia, first discovered in Verona, but also obtained from Cyprus, the Mendip Hills, etc. It varies considerably in strength and tone, but always gives a good green with a slight bluish-grey tinge.
The natural mineral only requires to be ground and can then be mixed in water or oil and with most other pigments.
It is permanent to light, and, though containing iron, is not darkened by sulphur or sulphurous fumes. It is not brilliant and has but a poor body.
Terre Verte is also known as Veronese Green and Ochre Green.
Copper greens are sometimes substituted for the natural earth, but whereas the latter are not darkened by the action of sulphuretted hydrogen or dissolved by dilute acids, the copper greens are blackened by sulphuretted hydrogen and dissolved by acids.
Emerald Green.- This colour pigment is one of the most brilliant greens which can be obtained, but it possesses the disadvantage of being very poisonous, consisting, as it does, of a very large proportion of arsenious copper.
The colour is almost identical with the green of the solar spectrum.
The average composition may be taken at about 33 per cent. of copper oxide, and 58 per cent. of arsenious oxide, the balance being made up of various products.
The pigment must not be mixed with any other which contains sulphur, such as, for instance, ultramarine, cadmium yellow, etc.
Its actual use is largely restricted to that of an insecticide, known as " Paris green. "
Malachite, or Mountain Green, is a pigment prepared from carbonate of copper found in malachite stone.
It is ground in oil and water, is permanent and does not influence other colours when mixed with them.
It is, however, expensive, and only available for the finest work.
Cobalt Green, also known as Rinman's Green and Zinc Green, is a mixture of oxides of cobalt and zinc.
It has a bright tint.
The brightness is increased by the addition of a little phosphoric acid.
Cobalt green is permanent and can be safely mixed with other colours and can be used on lime.