the Colouring of Glass hy Metallic Oxides. 44<1 



ture below its fusion : during this exposure the copper is re- 

 duced in the form of metallic crystals, and the glass being 

 coloured only by oxide of iron, takes a broivnish-i/ellow colour ; 

 and the greater the reduction of copper, the yellower is the 

 glass. 



Proceeding now to the usual circumstances of colouring 

 glass by oxide of iron, we find that at a temperature not very 

 high, for instance in covered pots for flint-glass, oxide of iron 

 gives a green colour approacliing nearer to yellow than to blue. 

 It is generally by mixing oxide of iron with oxide of copper 

 (giving blue) that all the tints of green are produced. The 

 greenish colour of bottle-glass must also be attributed to the 

 oxide of iron combined with the carbonaceous matters con- 

 tained in the mixture. But when we melt at a high tempe- 

 rature, for instance in the manufacture of window-glass, we 

 remark that the addition of a small proportion of oxide of 

 iron to the mixture produces a glass of a bluish colour. It is 

 known also by the manufacturers of bottle-glass, that when 

 the glass is cooled in the pot, it becomes opake blue before 

 being devitrified. 



We have shown by the preceding remarks that glass receives 

 all the colours of the spectrum from oxide of iron ; and at the 

 same time, it will be observed that these colours are produced 

 in their natural order in proportion as the temperature is in- 

 creased. 



2. Manganese. 



It is generally known that oxide of manganese gives to glass 

 a purple or pink colour, which property is used not only for 

 the production of purple glass, but especially as glass soap, for 

 neutralizing the light greenish colour produced by slight por- 

 tions of iron and carbonaceous matters existing in the materials 

 used in making white glass or flint-glass ; but it is very re- 

 markable, that the light pink colour given by oxide of man- 

 ganese is very apt to fade : if the glass remains too long in 

 the melting-furnace, and afterwards in the annealing kiln, 

 the purple tinge turns first to a light brownish-red, then to 

 yellow, and afterwards to green. 



I shall mention also a remarkable fact relative to the pre- 

 sence of manganese in the composition of glass. White glass, 

 in which a small proportion of manganese has been used, is 

 liable to become yellow by exposure to light. Having melted 

 for the celebrated Augustin Fresnel the glass for the first poly- 

 zonal lenses he made, and for which the whitest glass was 

 desirable, these prismatic pieces of glass became yellow after a 

 short time without losing their transparency and polish of 



