GLASS IMITATIONS 99 



The amount of lead especially varies very considerably, and is sometimes entirely 

 absent, though a glass free from lead cannot be correctly termed a strass. Many recipes are 

 given for the preparation of glasses suitable for the imitation of precious stones. A few of 

 the best containing varying amounts of lead are given below : 



3 parts of fine quartz-sand, 2 of saltpetre, 1 of borax, ^ of white-arsenic. 

 9 parts of quai'tz, 3 of potassium carbonate, 3 of fused borax, 2 of red- lead, ^ of white- 

 arsenic. 



8 parts of white glass free from lead, 3 of rock-crystal, 3 of red-lead, 3 of fused borax, 

 f of saltpetre, ^ of white-arsenic. 



7^ parts of quartz, 10 of red-lead, I J of saltpetre. 



A mixture which is frequently used is 32 per cent, of rock-crystal, 50 of red-lead, 17 of 

 potassium carbonate, 1 of borax, and J per cent, of white-arsenic. 



The greater or less the amount of red-lead in the mixture, the more or less rich in lead 

 will the resulting glass be, and the other constituents will vary accordingly. The amount 

 of silica in lead-glasses varies between 38 and 59 per cent., potash between 8 and 14< per cent., 

 and lead oxide between 28 and 53 per cent. As an example of the chemical composition 

 of a lead-glass (strass) used to make an imitation of diamond, the following analysis may 

 be given: silica (SiOj) 41-2, potash (Kfi) 8-4, lead oxide (PbO) 50-4! per cent. 



The physical characters of these glasses vary very considerably with the chemical 

 composition, the amount of lead present having a specially marked influence in this direction. 

 When this element is present in smaller amount the hardness of the glass is rather greater, 

 but the specific gravity, as well as the index of refraction and the dispersive power, are 

 lower. These latter properties are increased with an increase in the amount of lead present ; 

 a glass very rich in lead, such as the one of which the percentage chemical composition is 

 given above, has an index of refraction and a dispersive power comparable with those of 

 diamond, and will therefore have the brilliancy and play of prismatic colours characteristic 

 of this stone. This, indeed, is the object of the addition of lead to glasses which are to be 

 used as imitations of precious stones ; the increase in the amount of lead also raises the 

 specific gravity of the resulting glass, this being sometimes as high as 3"6 or 3'8, higher, that 

 is to say, than that of diamond. 



The play of prismatic colours is even finer in glasses in which thallium replaces 

 potassium than in those in which lead is the only heavy metal present. The presence of 

 this heavy metal as a constituent of the glass very considerably increases its dispersion and 

 index of refraction ; such thallium-lead-glasses are, as regards their optical characters, much 

 superior to the ordinary strass of the composition mentioned above. The specific gravity is 

 also higher and reaches 4'18 to 5'6, increasing with an increase in the amount of thallium. 

 A glass containing a moderate proportion of thallium, and with a specific gravity of 4"18, 

 has a dispersion of 0'04!9 ; that of ordinary lead-glass (flint-glass of Fraunhofer) is only 

 0'037, the dispersion of diamond being 0'057. 



Different glasses, varying in their physical characters according to their chemical 

 composition, may therefore be employed for different purposes. A stone which is to imitate 

 the diamond must have a high index of refraction and a high dispersion, and for this 

 purpose a glass rich in lead, or, better still, a lead-glass containing thallium, will be used. A 

 precious stone possessing only a low index of refraction may, on the other hand, be imitated 

 by a strass containing but little lead, or even by one from which lead is altogether absent. 



The recipes given above should produce, when the materials are quite pure, a perfectly 

 colourless glass. A coloured glass is obtained by the addition to the strass of a colouring 

 substance. The substances usually employed for this purpose are metallic oxides, which 



