172 On the Porosity of Agates, Calcedonies, Sc. 
and then place them beside each other in a vessel, which is 
then covered and placed in a hot fire until they are red hot. 
The fire is slowly extinguished, and they are taken out when 
cool; by this roasting the oxyhydrate of iron* which the 
stones contained is freed from its moisture, and the colour of 
the oxide assumes a more lively hue, and is seen in the 
translucent mass in the proper colour of the carnelian. The 
smaller stones are burned before being polished; the larger 
ones are first cut into various shapes, e.g., dessert plates, 
bowls, vases, &c. Small pieces do not easily fly to pieces 
in the roasting, but large ones very easily ; so it is necessary 
to make them as thin as possible by grinding. 
There are many other dexterous manipulations necessary, 
which are known only to the polishers themselves, but I have 
collected the above processes from many sources; and my 
esteemed friend, Herr Tischbein of Herstein, in the Pala- 
tinate of Birkenfeld, has given me many particulars which 
assisted my studies on the agate very much. (I acknowledge 
them here with much thankfulness.) 
When once, however, the properties which these minerals 
(to which I have given the collective name of agate) possess 
of being quite penetrable to colouring fluids, in consequence 
of their porosity, are better known, it is probable that other 
colours may be given them; and, also, that many antique 
stones, presenting unusual colours, may have been coloured 
so artificially. This seems to me very likely, as many of the 
antique cameos and intaglios which I have seen in collec- 
tions seem to be so. 
Why should we not find the ancient coloured stones so 
good as we know the ancient pastes were ? 
* That iron is the colouring principle of carnelian cannot be doubted after 
the experiments of M. Heintz (Poggendorff’s Annalen. Band 60, s. 579). 
Gaultier de Claubry (in Poggendorff’s Annalen, Band 26,s. 562) has attempted 
to shew that the colouring principle is in the nature of the carnelian itself; 
but the critique of his experiments, and Heintz’ opposite conclusions, have 
shown the untenableness of his views. 
