this
imitation is by no means a success. It is a paste formed of marble dust
and isinglass. The colour is given by a mixture of vermilion and minium
incorporated with the mass.
ARTIFICIAL COLOURING OF HARD STONES USED BY ENGRAVERS.
The
hard stones preferred by engravers are those which offer different
tints or strongly-contrasted colours. As these vari-coloured stones are
much more costly than stones of the same nature of a single colour,
chemistry has been applied to for an artificial colouring; and the
result is, that the greater part of hard stones engraved at the present
day are artificially coloured.
The
stone to be coloured is steeped in oil, and notwithstanding its
apparent impermeability, is easily penetrated by the liquid. After
being soaked a certain time, it is taken out, and, however perfectly
it may be dried, a certain quantity of the oil always remains in its
pores. It is then placed in a capsule, covered with sulphuric acid, and
heated to boiling point: this heat is maintained until the sulphuric
acid is evolved, when the stone is withdrawn and washed, and is found
to have become black.
If the stone is of a quite homogeneous texture,
