AGATES AND AGATE-WORKING. 
35 
common property. The art of colouring, so as to produce good 
onyx from comparatively worthless stone, gave great impetus to 
the manufacture. 
As at present practised, the stones, having been well washed, 
are usually placed in a syrup of honey and water, or in some 
cases, in olive-oil. They are then exposed for some time — at 
least three days, and often longer — to a moderate heat, in a 
vessel standing in hot ashes or on a German stove, care being 
taken that the liquid does not boil. When removed they are 
well washed and placed in sufficient commercial oil of vitriol to 
cover them, and again exposed to gentle heat. After they have 
taken colour, they are removed and well washed ; and it is often 
the practice to finally lay them in oil, which improves the 
lustre. If too strongly stained, the colour may be “ drawn ” by 
the action of nitric acid.* The rationale of the process of 
colouring is extremely simple. Certain layers of an agate are 
found to be porous, whilst others are well-nigh impervious. 
When, therefore, such an agate is steeped in syrup or in oil, the 
liquid is absorbed by the porous layers only, and the subsequent 
treatment with sulphuric acid carbonizes the saccharine or 
oleaginous matter, and thus produces a deep brown or blackish 
colour in certain strata, by impregnation with carbon. Some 
agates never lend themselves to this treatment, and altogether 
refuse to take colour, whilst others colour in a few hours : the 
South American stones usually take the colour readily, and 
lienee their great value to the cameo-worker. 
It is clear that the essence of this process lies in the differ- 
ences of texture displayed by the various strata in an agate. 
Such differences are strikingly seen when a polished section of 
an agate is exposed to the action of hydrofluoric acid ; the dif- 
ferent layers are then corroded in different degrees, and a rough 
surface is obtained, from which Dr. Leydolt has been enabled to 
print perfect impressions, showing with fidelity every line in the 
structure of the stone.f 
At the same time that the porous layers of an agate are 
deepened in colour by the process of staining, the intervening 
non-porous strata appear to be brought out of purer white 
colour than before. This is probably in many cases the effect 
of contrast only ; but it is known that chalcedony of bluish tint 
may by the action of heat be converted into a pure white stone. 
When an agate has been properly stained it usually exhibits 
alternate bands of strongly-contrasted black and white chalce- 
dony, thus becoming a true onyx — a stone greatly prized by the 
* 11 The Science of Gems.” By Archibald Billing, M.D., &c. 1867, p. 62. 
This interesting work contains a view of Oberstein. 
f u Denkschriften d. k. Ak. d. Wissenschaften.” Vienna, vol. v. p. 107. 
d 2 
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