GLASS 649 



the proportion of its fluxes, and the more exact the equivalent ratios of its constituents. 

 When glass contains too much alkali, it is partially soluble in water. Most crystal- 

 glass is affected by having water boiled in it for a considerable time ; but crown-glass 

 being poorer in alkali, and containing no lead, resists that action much longer, and is 

 therefore better adapted to chemical operations. In general, also, potash-glass is more 

 apt to become damp than soda-glass, agreeably to the respective hygrometric properties 

 ot these two alkalis, and also to the smaller proportion of soda than of potash requi- 

 site to form glass. 



Air and light operate upon glass probably by their oxidising property. Bluish or 

 greenish coloured glasses become by exposure colourless, in consequence undoubtedly 

 of the peroxidation of the iron, to whose protoxide they owe their tint; other glasses 

 become purple-red from the peroxidation of the manganese. The glasses which 

 contain lead suffer another kind of change in the air, if sulphuretted hydrogen be 

 present ; the oxide of lead is converted into a sulphuret, with the effect of rendering 

 the surface of the glass opaque and iridescent. The more lead is in the glass, the 

 quicker does this iridescence supervene. By boiling concentrated sulphuric acid in a 

 glass vessel, or upon glass, we can ascertain its power of resisting ordinary menstrua. 

 Good glass will remain smooth and transparent ; bad glass will become rough and 

 dim. The conditions of decomposition, as it occurs in glass of great age, have not 

 been satisfactorily explained ; the glass of the Koman tombs decomposes from the 

 surface, exfoliating in a remarkable manner, film after film, of a pearly and beauti- 

 fully iridescent character, falling off one after the other. The same kind of change 

 is seen on the windows of our ancient churches. 



The brittleness of unannealed glass by change of temperature is sometimes very 

 great. This defect may be corrected by slowly heating the vessel in salt-water or oil 

 to the highest pitch consistent with the nature of these liquids, and letting it cool very 

 slowly. Within the limits of that range of heat it will, in consequence of this treat- 

 ment, bear alternations of temperature without cracking. 



It has been said that glass made from silica and alkalis alone will not resist the 

 action of water, but that the addition of a little lime is necessary for this effect. In 

 general 100 parts of quarteose sand require 33 parts of dry carbonate of soda for their 

 vitrification, and 45 parts of dry carbonate of potash. But to make unchangeable 

 alkaline glass, especially with potash, a smaller quantity of this than the above should 

 be used with a very violent heat. A small proportion of lime increases the density, 

 hardness, and lustre of glass ; and it aids in decomposing the alkaline sulphates and 

 muriates always present in the pearlash of commerce. From 7 to 20 parts of dry- 

 slaked lime have been added for 100 of silica, with advantage, it is said, in some 

 German glass-manufactories, where the alkaline matter is soda ; for potash does not 

 assimilate well with the calcareous earth. 



In many glass works on the Continent, sulphate of soda is the form under which 

 alkaline matter is introduced into glass. This salt requires the addition of 8 per cent, 

 of charcoal to decompose and dissipate its acid : a result which takes place at a high 

 heat, without the addition of any lime. 88 Ibs. of quartz-sand, 44 Ibs. of dry Glauber 

 salt, and 3 Ibs. of charcoal, properly mixed : and fused, afford a limpid, fluid, and 

 workable glass ; with the addition of 17 Ibs. of lime, these materials fuse more readily 

 into a plastic mass. If less carbon be added, the fusion becomes more tedious. 



By a proper addition of galena (the native sulphide of lead) to Glauber salt and 

 quartz-sand, without charcoal, it is said a tolerably good crystal-glass may be formed. 

 The sulphuric acid of the salt is probably converted by the reaction of the sulphuret 

 of lead into sulphurous-acid gas, which is disengaged. 



One atom of sulphide of lead = 1495-67, is requisite to decompose 3 atoms of 

 sulphate of soda = 2676. It is stated, on good authority, that a good colourless 

 glass may be obtained by using Glauber salt without charcoal, as by the following 

 formula : 



% 



Quartz-sand . . .100 Ibs. I Lime . . . .20 Ibs. 

 Calcined Glauber salt. . 20 [ Gullet of soda-glass . 12 



The melting-heat must be continued for 26 hours. A small quantity of the sand 

 is reserved to be thrown in towards the conclusion of the process, in order to facilitate 

 the expulsion of air-bubbles. The above mixture will bear to be blanched by the 

 addition of manganese and arsenic. The decomposition of the salt is in this case 

 effected by the lime, with which the sulphuric acid first combines, which is then con- 

 verted into sulphurous acid, and dissipated. Glass made in this way was found by 

 analysis to consist of 79 parts of silica, 12 lime, and 9'6 soda, without any trace of 

 gypsum or sulphuric acid. 



Glauber salt is partially volatib'sed by the heat of the furnace, and acts upon the 



