Artificial Coloring of Hard Stones

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acid, and heated to boiling point: this heat is maintained until the sulphuric acid is evolved, when the stone is withdrawn and washed, and is found to have become black.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, the blackness will be uniform; but if, as often happens, its constitution is not very regular, the most porous parts absorb the greater quantity of oil, and varied effects of colouring are produced, which furnish to the artist the desirable opposition of tints.

This operation can be easily explained. The oil being composed of the three elements--carbon, hydrogen, and oxygen, it follows that if the hydrogen and oxygen are removed, carbon remains. It is precisely this removal that sulphuric acid effects. Penetrating in the track of the oil into the pores of the stone, it determines the union and elimination of the hydrogen and oxygen, and leaves carbon diffused through the stone in a state of excessive division. It is these minute particles of carbon that impart a definite colour to the stone.

So far as regards the colouring, this process furnishes excellent results; but when it is asked, Should the prepared stone be considered equal to those coloured by nature? we must answer in the negative, notwithstanding the opinion generally held.

No doubt, in ordinary conditions, the fixity of carbon is absolute, but in this case we must remember that it is in a state of the most extreme division; and seeing that the natural porosity of the stone is increased by the sulphuric acid, and that porosity is highly favourable to the combination of bodies, it seems to be not impossible that in course of time the carbon may undergo a slow combustion, and the colour of the stone be more or less destroyed.

It is impossible, too, that a substance so corrosive as sulphuric acid should not make some impression on the stone. Silica, it is true, is not attacked by this liquid, but it is altered in a remarkable manner; and then precious stones of the agate class are not formed exclusively of silica, but contain small quantities of different substances on which sulphuric acid has a very decided effect.