100 GENERAL CHARACTERS OF PRECIOUS STONES 



must be in as pure a state as the other constituents of the strass. In the manufacture of a 

 coloured glass, the colourless strass already prepared, and the requisite amount of metallic 

 oxide, are both reduced to a state of fine powder, and are then intimately mixed by being 

 passed through a sieve. The mixture is then fused at a moderate temperature, and allowed 

 to remain in this condition for about thirty hours, after which it is very slowly cooled. 

 Only a very small amount of the metallic oxide is necessary to produce any required colour, 

 the actual amount differing with different oxides. The depth of colour imparted by any 

 given oxide will, of course, depend on the amount of it used ; a light colour will be given 

 by a very small amount, while a larger amount may produce a colour so deep that thick 

 pieces of the glass will appear black and opaque. Between these two extremes every 

 gradation of colour is possible. As an example of the intense colouring power possessed by 

 some metals, it may be stated that one part of gold will impart a vivid ruby-red colour to 

 10,000 parts of strass, while this same amount will impart an unmistakable rose colour to 

 20,000 parts of strass. 



The substances employed for the production of differently coloured strass are many and 

 varied. Cobalt oxide or smalt produces a blue colour, to which a tinge of violet may be 

 given by the addition of a small quantity of manganese oxide. Yellow is produced by 

 silver oxide or chloride, and by antimony oxide or the so-called red-antimony, which is a 

 mixture of the oxide and sulphide. The addition of a small amount of coal also produces a 

 yellow colour, the intensity of which varies from a light honey-yellow to yellowish-brown 

 according to the amount used ; a beautiful golden-yellow is obtained by adding in addition 

 to the coal a little manganese oxide. The use of coal for the production of a yellow colour 

 is, however, only possible in glasses free from lead. Chromium oxide and copper oxide each 

 produce a green colour, to which a bluish tinge may be imparted by the addition of a little 

 cobalt oxide, while a yellowish-green is obtained by adding red-antimony. A mixture of 

 cobalt oxide and red-antimony produces a green colour, due to the combination of the blue 

 of the cobalt and the yellow of the antimony. Red may be obtained by the addition of 

 various substances, namely, cuprous oxide (CugO), gold oxide, gold chloride, or purple of 

 Cassius, the last named being used for the production of ruby-glass, so called from the 

 resemblance of its colour to the red of the ruby. A red colour inclining to violet is 

 obtained by the use of manganese oxide which should be as free as possible from iron, a 

 pure violet being obtained by the use of a little cobalt oxide in addition to this ; a larger 

 amount of cobalt oxide produces a reddish-brown colour. Black glass, which remains black 

 even in the thinnest layers, is produced among other methods by adding a large amount of 

 tin oxide and afterwards fusing again with a mixture of manganese oxide and hammer-slag 

 from iron-works. 



An opaque, white glass, that is an enamel, is obtained by the addition of a small 

 quantity of either tin oxide, calcium phosphate, or bone-ashes. This opaque glass is capable 

 of colouration by metallic oxides, and hence imitations of opaque stones such as turquoise 

 are possible, the blue colour of this stone being imitated by adding a little copper oxide 

 and cobalt oxide. The appearance of opal, chalcedony, and other translucent stones, and 

 even to a certain extent the colour bandings of agate, may be imitated in glass by methods 

 very similar to those above described. 



It is not by any means to be supposed that pastes can be produced at a very low cost. 

 The production of a strass suitable for making good imitations of precious stones is a 

 comparatively expensive operation, the price of materials of the necessary purity being high, 

 and the appliances and apparatus used in the manufacture necessitating the outlay of some 

 considerable capital. For this reason, only the more costly precious stones are imitated in 



