632 



GEMS, ARTIFICIAL 



the greatest caro. It should be remembered that the most perfect mixture can only 

 be effected by numerous siftings, and that a separate sieve should bo used for each in- 

 gredient, and never be made to servo for different substances. When mixed, the 

 materials should be melted in a crucible, placed in the middle of a cylindrical furnace, 

 terminated in a dome, the height of which should bo 7 feet 6 inches, and its diameter 

 4 feet 3 inches. The fuel should consist as much as possible of thoroughly dry 

 wood, chopped very small. The melting should be effected by means of a heat 

 raised by degrees, and then steadily maintained, especially at the maximum tempera- 

 ture : then when once the melting has been thoroughly accomplished, which cannot 

 o in less than from twenty to thirty hours, the crucible must be allowed to cool very 

 ,owly. 



The art of imitating precious stones in paste has amazingly improved since the 

 time of Strass, as was shown by the results of the great Paris Exposition of 1850. 

 The imitations, especially as regards certain colours, leave little to be desired ; but 

 there is something still in that respect in which the imitation is far from being perfect. 



Now that it is proved that the alkalis and vitrifiablo earths are oxides of the metals, 

 all that has to be done to obtain the finest effects is to combine them skilfully with 

 other artificially-prepared metallic oxides, which have undergone the process of 

 vitrification. 



Experiments ought to be made with all oxidisable and verifiable substances, with 

 the different salts, fluates, phosphates, phosphoric acid, &c. 



The following are some of the mixtures generally known, but, it must bo observed 

 here that each artist has his own processes, ingredients, and proportions : 



Mixtures for Strass. 



Common Strass. 

 Litharge, 77*16; white sand, 5773; potash, 771. 



Strass of Douhaut-Wicland. 



Sifted rock crystal 

 Boracic acid . 

 Minium (purest) . 



Calcined flints 

 Pure potash 



2897'5 



181-18 

 4451-37 



Deutoxide of arsenic 

 Potash (purest) . 



English Straas, 



962-5 I Calcined borax 

 481-25 I Fine white lead 



Strass Bastenaire, 



4-92 

 1608-5.3 



. 301-9 

 . 120-89 



Topaz: No. 1. 

 Whitest strass, 842*079 ; glass of antimony, 36-421 ; purple of Cassjus, 0738. 



