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Blue Pigments
Ultramarine.-Natural ultramarine is prepared from the ground colouring matter of lapis lazuli.But this is very costly and is only used for artists' colors and in ceramics.
The characteristic colour is a deep, brilliant blue, often having a slightly greenish hue, and in some samples changing to violet.
By heating while exposed to air a red pigment is obtained.
Ultramarines have good body and are permanent.
The ultra marine of commerce, used by decorators, is an artificial preparation of china clay, soda-ash, sodium sulphide, sodium sulphate, and rosin.
The mixture is roasted, ground, and washed.
Ultramarine is fast to light and unaffected by alkalis, but not by acids.
It is widely employed both for oil paints and also in the preparation of
distempers and water paints.When used in oil paints, it tends to settle and cake in the container and is also liable, on occasion, to separate out from the medium after application, producing a somewhat streaky finish.
Having relatively little opacity, it is extensively used for glazes.
Another use to which it is frequently put is in small additions to white pigments to counteract any tendency to yellowness.
Prussian Blue.- This pigment, so called because it was discovered by a Berlin colour maker in 1704, is prepared by mixing a solution of ferrous salt with one of potassium ferrocyanide.
Its most notable characteristic is its great staining power.
It is fast to light and can be mixed with most pigments.
It withstands acids but is very susceptible to alkaline attack, 'which causes it to turn a brownish shade.
For this reason it is unsuitable for use in the preparation of distempers or for use in oil paints intended to be applied over a ground containing active alkali. It discolours, also, when exposed to heat.
It was also known in former times as Berlin Blue, but this name has now almost fallen out of use.
Chinese Blue.-A name given to the finest qualities of Prussian blue.
Chinese blue has a slight bronze sheen.
Brunswick Blue.-A mixture of Prussian blue and barytes. Pale tints of this mixture are sometimes sold under the name of Celestial Blue.
Antwerp Blue.-This blue is similar in general properties to Prussian blue, but it is paler in colour.
It has, to a large extent, been superseded by Brunswick blue.
Cobalt Blue.-This pigment is an oxide of the mineral cobalt and is notable for its brilliance and purity of tone.
It is extremely stable and is not affected by acids, alkalis, or heat, besides being fast to light. It can be mixed with other paints and ground in oil or water.
In oil, however, it is very transparent and is thus employed frequently for colour glazing.
It is, however, too costly as a rule for general work.
Cerulean Blue.-This differs slightly in composition and method of preparation from cobalt blue, which, however, it much resembles, though. it is paler than the latter and has a slight greenish cast.
Monastral Blue.-This is a relatively new pigment, introduced in 1935, since when it has been fairly extensively used in the preparation of high-grade finishes.
It is a beautiful colour, with a staining power greater even than that of Prussian blue.
It has the advantage of being fast to light, unaffected by acids or alkalis, and of being able to withstand high temperatures without changing colour.