This beautiful blue color is made by mixing solutions of sulphate
of iron (green vitriol) and prussiate of potash together, when the
Prussian blue falls in the form of a precipitate, or moist powder,
of a pale greenish color, but which, by exposure to the air and
the addition of a little nitric acid, becomes intensely blue; but
the Prussian blue of commerce is never pure, for the solution of
prussiate of potash is used as it comes from the factory, before
being crystallized, and contains carbonate of potash, to get rid
of which, alum is added, and the result is, the formation of a
considerable amount of alumina, which is a white earth produced from
the alum, and which falls down in the precipitate mixed with the
Prussian blue, which is thereby increased in bulk and but little
injured in color; when a larger quantity of this alumina is purposely
produced, the result is a blue of a rather less intense color,
called “Antwerp blue.” The chemical name for pure Prussian blue is
percyanide of iron. Chinese blue is this substance in its purest
state.