COLOURING AND BUKNING 87 



The tool used in cutting or engraving is a very small iron wheel fixed at the end of a 

 rotating axis in a lathe. The small wheel, which may be conical, hemispherical, or disc- 

 shaped, and often not more than a twelfth of an inch in diameter, is charged with moistened 

 diamond-powder. The stone to be engraved, after being cut to the required form and 

 polished, is placed in a holder and its surface pressed against the rotating wheel. The out- 

 line of the device to be engraved is obtained by moving the stone about while the cutting is 

 taking place, more or less prolonged working gives a deeper or shallower engraving. The 

 final polish must, as a rule, be given after the cutting process is completed ; this and the 

 last touches to the engraving are given by hand with a graving tool furnished with a 

 diamond point. 



Etching. — An alternative to the difficult, lengthy, and costly process of engraving is 

 furnished by the simpler, quicker, and cheaper process of etching. The process is, however, 

 only applicable to certain stones, and the figures produced are less clear and beautiful than 

 those created by the engraver. The method requires that the stone used shall be susceptible 

 to the action of some acid ; hence none of the more valuable precious stones are available as 

 material, since they are unacted upon by acids. Those stones, however, which consist wholly 

 of silica, namely rock-crystal, chalcedony, agate, &c., are acted upon by hydrofluoric acid as 

 easily as is glass, and by its means devices may be etched upon them. The polished 

 surface of the stone to be etched is covered with a thin coating of wax, upon which the 

 outline of the device is drawn with some sharp instrument ; the surface of the stone along 

 this outline is therefore laid bare while other parts are protected by the wax. The stone is 

 then placed in liquid or gaseous hydrofluoric acid, which eats away the surface of the stone 

 where it is not protected by a layer of wax. There soon appears a hollowing out of the 

 material along the outlines of the device, and this will be deeper the longer the acid is 

 allowed to act. After removing the wax and cleaning the stone the device (monogram, 

 crest, &c.) sketched upon the wax will appear as if cut in the stone. 



r. COLOURING AND BURNING. 



The methods adopted for altering or improving the natural colours of stones may con- 

 veniently be considered here. Such a change of colour, which has already been stated to be 

 possible under the general discussion of colour, is effected in various ways. We shall not 

 now discuss the change of colour produced in a stone already set and mounted by a surface 

 coating of colouring-matter, but only those methods by which a change in the colour of the 

 whole mass of the stone is effected. 



Those precious stones which have a porous structure can be artificially coloured through 

 their whole mass with great ease. The stone to be coloured is allowed to lie in the liquid in 

 which the colouring-matter is dissolved ; its porous structure causes it to become after a 

 time saturated with the liquid, which penetrates to the innermost parts of its mass. The 

 stone is then taken out and allowed to dry, during which process the solvent evaporates 

 and the colouring-matter in solution is left behind, lodged in the interstices of the stone, and 

 imparts its colour to the stone. This method is not infi-equently practised, and many agates 

 are remarkable for the ease with which they can be coloured by its use. In other cases the 

 use of two liquids is necessary to produce an artificial colour, the rationale of the method 

 being that by the interaction of the two liquids a chemical precipitate is formed, which is 

 deposited in the pores of the stone, and imparts to it the desired colour. In this method 

 the stone is first placed in one liquid, in which it is allowed to remain until completely 

 saturated : it is then taken out and dried and placed in the second liquid, when precipitation 

 takes place. The coloured precipitate so produced is regularly distributed in the interior of 



