QUARTZ (AGATE): ARTIFICIAL COLOURING 523 



in an aqueous solution of honey or sugar in a perfectly clean and new pot. The whole is 

 kept at a temperature below boiling-point for two or three weeks, during which time the 

 loss of water by evaporation must be made good in order to keep the stones covered with 

 liquid. The stones are then taken out, washed, and placed in another pot with commercial 

 sulphuric acid (oil of vitriol) and again warmed. The honey or sugar absorbed by the 

 stone dui-ing its immersion in the first liquid is decomposed by the sulphuric acid with 



Fig. 92. Antique intaglio. 



Fig. 94. Antique cameo. 



{See page 521..> 



Fig. 93. Antique cameo. 



separation of carbon, which imparts its black colour to the stone. The " softer " stones,, 

 even after a few hours immersion in the acid, acquire a deep black colour, others require an 

 immersion lasting for a day or several days, and some pieces only assume the black colour 

 after being immersed for a considerable time. When the stone is as black as it is possible 

 for it to be, it is taken out of the liquid and quickly dried in an oven. It is then cut and 

 polished, and finally rubbed with oil or allowed to soak in this liquid for a day, the oil not 

 absorbed being wiped off with bran. This final treatment hides small cracks and improves 

 the lustre of the stone. 



The fine black agates now bought and sold in the markets have been artificially 

 coloured in this way, as have also the onyxes (Plate XX., Figs. 5a, h), in which the black 

 layers, now alternating with the white, were originally greyish or bluish in colour. Not 

 only do highly porous layers become a deep velvety black, but less porous bands acquire a 

 more or less dark shade of brown. The shade of colour impai'ted artificially to a stone 

 depends, to some extent, on its original colour ; a red layer, for example, after being 

 artificially coloured will have a reddish tinge, and so on. It is a significant fact that, 

 according to the way in which the stone will acquire the black colour, a hundredweight of 

 agate may be worth £5 or ^250. When offering such stones for sale, it is therefore 

 customary to submit small fragments as samples for the purpose of testing whether they will 

 acquire a good colour. 



The tinting of agates with colours other than black is comparatively unimportant, but 

 a few details of the methods adopted are given below. 



A fine lemon-yellow, such as agate in its natural state never possesses, may be imparted 

 by placing well dried agates in a pot of hydrochloric acid and gently warming in an oven. 

 The colouring will be completed in a fortnight. 



