36 AGATE 



ground, the stones are polished with tripoli on a cylinder of hard wood, or on a plate 

 of tin or lead. 



An important branch of the agate trade is that of artificially colouring the stones 

 an art which has attained great perfection in recent times, but is far from being a 

 modern invention. Pliny states (bk. xxxvu. cap. 75) that in Arabia agates are 

 found which are purified and prepared for the cutter by being heated in honey for 

 seven days and seven nights. Evidently the mere absorption of this saccharine matter 

 into the pores of the stones would be insufficient to materially alter the appearance of 

 the agate ; but if the absorbed honey could have been carbonized by the action of 

 sulphuric acid as at present practised the dark colour of the brownish layers 

 would have been considerably heightened, and the stones improved for the cameo- 

 worker. Believing that this was really the process described by Pliny, though he 

 was ignorant of the entire secret, some authorities have argued in favour of the know- 

 ledge of oil of vitriol by the ancient Romans, while others have suggested that the 

 native sulphuric acid from volcanic emanations was probably employed ; others, again, 

 have maintained that the honey was merely charred by exposure to heat, as is said to 

 be still practised in the East. Be that as it may, it appears that the secret of arti- 

 ficially producing a black colour in agates was for ages handed down traditionally 

 by the Italian cameo-workers. From time to time, Italian travellers visited Idar, and 

 purchased stones, which were taken to Rome and there coloured. At length the secret 

 of this art oozed out under peculiar circumstances, and, being once known jn Idar, 

 rapidly spread, and has not only been the means of greatly developing the agate- 

 industry, but has, to a large extent, caused the removal of the seat of stone-engraving 

 from Italy to Germany. 



All artificial colouration of these hard stones depends for its success on variations 

 in texture and density presented by the different layers. Sections of agate under the 

 microscope often exhibit distinct pores, and the unequal texture of the several layers 

 is well seen by the action of hydrofluoric acid, which readily attracts certain strata 

 while it is resisted by others, thus producing an uneven surface, from which an im- 

 pression of the agate may be naturally printed a process which has been beautifully 

 carried out by Dr. Leydolt. Not only do the component layers of an agate absorb 

 liquids with different degrees of facility, but the stones as a whole exhibit like diffe- 

 rences ; some stones absorbing the colour rapidly, others requiring months to do so, 

 and others, again, entirely refusing to take colour. The South American agates, as a 

 rule, lend themselves with peculiar facility to this artificial colouration, and are much 

 more porous than the true German agates. 



Black or dark brown colours are those commonly developed in agates these dark 

 strata, when alternating with dense white layers, forming beautiful onyxes well fitted 

 for cameo-work. To produce the dark colour, the stones, having been well washed 

 and dried, are placed in honey, thinned with water, and are exposed in a warm place 

 for several days, in some cases as long as three weeks. The vessel containing them 

 is heated by being placed in hot ashes or on a stove, but the syrup is never allowed 

 to boil. After having lain in the warm honey for a sufficient time, depending on the 

 texture of the stone, they are removed, well washed, and placed in a vessel with 

 sufficient commercial oil of vitriol to cover them ; the vessel being covered with a slate, 

 and exposed to a moderate temperature. The sulphuric acid carbonises the saccharine 

 matter previously absorbed by the porous layers of the agate, and produces a black 

 or a deep brown colour according as the action is more or less intense. Olive oil is 

 used by the Italians instead of honey, but the chemical reactions are, of course, 

 essentially the same in the two cases. Some stones blacken in a few hours, others 

 require several days, while bad stones never take colour. When sufficiently tinted 

 the stones are removed from the acid, washed, dried, and polished ; they are then 

 generally laid in oil to improve the lustre, and are finally dried in bran. Whilst the 

 charring of the syrup or oil gives a dark colour to the porous layers, it is said that 

 the white colour of the denser strata is also heightened ; but this is probably merely 

 the effect of contrast. If the darker parts of the stone should be too deep, the colour 

 can be ' drawn,' or made lighter by the action of nitric acid. 



The art of agate-colouration has reached so advanced a state that stones can now be 

 tinted to almost any desired hue ; some of the processes are, however, still kept secret 

 by the polishers. In 1845 the method of colouring agate blue was first introduced. 

 This is now effected in various ways. One of the best modes is to submit the stone 

 successively to the action of solutions of yellow prussiate of potash (ferrocyanide of 

 potassium), and of a per-salt of iron, thus causing a precipitate of Prussian blue to be 

 thrown down in the pores of the stone. Or, red prussiate of potash and common 

 green vitriol may be used ; or an ammoniacal sulphate of copper may bo formed, by 

 placing the stone in a solution of blue vitriol and then in ammonia. A green colour was 

 formerly produced by nitrate of nickel, but it is now obtained by using chromic acid. 



