little
below boiling point for a time, varying from a few hours in the "
softer " (or more absorbent) varieties such as the Brazilian, to a week
or more in the case of the "harder" kinds. After being washed, the
Agate is next placed in sulphuric acid, which dehydrates the sugar;
this results in a fine black deposit being left in the pores of the
absorbent bands. Staining is said to proceed more rapidly in radial
directions. Much of the material so treated is not true Onyx in the
mineralogical sense, but Agate, having alternate bands of Cachalong and
Chalcedony ; but where the bands are uniform, and capable of taking a
good stain, it is known under the trade name of Onyx.
Another
well-known method of staining is by treating the material in
hydrochloric acid at a moderate temperature for about two weeks, when
many of the bands assume a rich lemon-yellow colour.
Agate
may also be made to resemble a Carnelian Agate by allowing a solution
of ferrous sulphate to soak into the absorbing bands; this, on heating,
oxidises to ferric oxide, and gives the well-known carnelian-pink.
Green
is produced by treatment with chromic acid and subsequent heating, and
an apple-green tint may be induced by soaking in a solution of a green
nickel salt.
Blue
of various shades is imparted by treating first with a solution of
potassium ferro-cyanide, and afterwards by a warm solution of ferrous
sulphate. The tint varies from indigo to azure and ultramarine.
Many
of the methods, however, are kept as strict trade secrets, but other
methods, as the use of aniline dyes and the induction of coloured
chemical precipitates by double decomposition, will suggest themselves
to the reader.
