674 GLASS, COLOURED 



of imparting varieties of red, was found by varying degrees of heating at a high 

 temperature, and re-casting several times, to give a groat many tints, varying from 

 blue to pink, red, opaque yellow, and green. Charcoal in excess in a mixture of 

 silica-alkaline glass gives a yellow colour, which is not so bright as the yellow 

 from silver: and this yellow colour maybe turned to a dark rod by a second fire. 

 Tho author is disposed to refer these chromatic changes to some modifications of 

 the composing particles rather than to any chemical changes in the materials em- 

 ployed. 



It is not possible in the present essay to enter into the minute details of this 

 beautiful branch of glass-manufacture. In the following statement the materials 

 ordinarily employed to colour glass alone are named. 



YELLOW. Charcoal or soot is used for producing the commoner varieties of yellow 

 glass. 



The glass of antimony, which is obtained by roasting sulphide of antimony until 

 antimonious acid is formed, and melting it with about 5 per cent, of undecomposed 

 sulphide of the same metal. 



The antimonate of potash, a preparation similar to James's powder, is stated to 

 answer the same purpose. Bohemian glass is coloured yellow with glass of antimony, 

 minium, and oxide of iron. 



Silver imparts a very beautiful yellow colour to glass ; but it requires some caution 

 in its mode of application. It is believed that the presence of alumina is necessary to 

 the production of colour, since a fine yellow cannot bo produced unless alumina be 

 present. A mixture of powdered clay and chloride of silver is prepared, and spread 

 upon the surface of the glass ; the glass is then re-heated, and the silver penetrates to 

 a certain depth into the glass, before the latter softens. The coating is then scraped 

 off, and the fine yellow colour appears. If the silver yellow glass is held over the 

 flame of burning wood, a peculiar opalescence is produced upon the surface, probably 

 by the oxidation of the silver. 



Uranium produces the beautiful canary-yellow, which is found in many articles of 

 an ornamental kind. This glass possesses the very peculiar property of giving a 

 green colour when it is looked at, although perfectly and purely yellow when looked 

 through. This has been attributed to the presence of iron in the commercial oxide of 

 uranium employed ; but the purer the uranium is, the more beautifully will this 

 phenomenon be brought out. It depends upon a very remarkable physical peculiarity 

 belonging to uranium and some other bodies. See FLUORESCENCE. 



EED. A common brownish-red colour is produced in glass by oxide of iron, 

 added as ochre, or in the state of pure peroxide. Muller found ancient glass to 

 contain silicic acid, alkalis, lime, magnesia, alumina, protoxide of iron, and suboxido 

 of copper. 



Copper is more generally employed in colouring glass red. The use of this metal 

 for such a purpose dates from very high antiquity, and all through the middle ages it 

 was employed to produce the reds which we see in the fine old windows left by our 

 ancestors for our admiration. The ancient Hamatinone was a copper-red glass. Sub- 

 oxide of copper is used, either in the state of commercial copper-scale, or it is prepared 

 by heating copper-turnings to redness. If, during the fusion of the glass in the pot, 

 the suboxide unites with an additional quantity of oxygen, green and not red is the 

 result. This is avoided by combining some reducing agent with the molted substance. 

 Glass thus coloured does not exhibit its red colour on leaving the crucible ; it is 

 nearly colourless, or with a tinge of green even when cold ; "but if it is then heated a 

 second time it assumes the red colour.' H. Eose supposes that a colourless neutral or 

 acid silicate of the suboxide of copper is formed at a high temperature, and that the 

 subsequent softening of the glass at a lower temperature causes the decomposition of 

 this compound and a separation of a portion of the suboxide. We believe that no 

 such chemical change takes place, and that the alteration is duo merely to a change 

 in the molecular arrangement of the particles. The suboxido of copper possesses an 

 intense colouring power, so groat indeed that glass coloured with even a very small 

 quantity is almost impermeable to light ; hence it is usual merely to flash colourless 

 glass with this coloured glass, that is, to spread a very thin film of it over the colour- 

 less surface. A process for colouring glass red, after its manufacture, with sulphide of 

 copper has been introduced by Bedford. 



Gold can, according to circumstances, bo made to impart a ruby, carmine, or pink 

 tint to glass. Tho purple of ('assius was employed ; but Dr. Fuss first showed that 

 a mero solution of gold without the presence of tin, as in the salt named, is capable 

 of producing rose and carmine coloured gla.^. 



Similar changes to those already described with copper occur with tho salts of 

 gold. Perhaps the glass is colourless in tho pot, and it then remains colourless when 

 cold ; but when re-heated, the glass quickly assumes a light red colour, which rapidly 



