BAND. 360 



partly in the state of a hydrous silicate like Raolin or Hal- 

 loysite. It owes its plasticity to the alumina, and ceases to 

 be called clay when the proportion of silica is too great for 

 plasticity. It is afforded by the decomposition of feldspar 

 and all argillaceous rocks. Oxyd of iron, carbonate of lime, 

 and magnesia, are often present in clays. 



Sand for glass manufacture should be pure silica, free 

 from a taint of iron. This purity is apparent in the clear- 

 ness of the grains, under a lens, or their white color. The 

 sand of Cheshire and Lanesboro', in Massachusetts, is a 

 beautiful material. 



In the manufacture of glass, the object is to form a trans- 

 parent fusible compound, and not an opaque infusible one as 

 in pottery. This result is secured by heating together to 

 fusion, silica (quartz sand or flint powder) and the alkali pot- 

 ash or soda. The ingredients combine and produce a sili- 

 cate of potash or soda — in other words, glass. 



Besides these ingredients, lime or oxyd of lead are added 

 for glass of different kinds. A small proportion of lime in- 

 creases the density, hardness, and luster of glass, producing 

 a specific gravity between 2*5 and 2*6 ; while with lead a 

 still denser material is formed — called crystal or flint glass — 

 whose specific gravity is from 3 to 3*6. 



From 7 to 20 parts of lime are added for 100 of silica, and 

 25 to 50 of calcined sulphate or carbonate of soda ; common 

 salt (chlorid of sodium) may also be employed. A good 

 colorless glass has been found by analysis to consist of silica 

 76*0, potash 13*6, and lime 10*4 parts, in a hundred. For 

 coarse bottle-glass, wood-ashes and coarse sea-weed soda, 

 called help, or else pearlashes, are used along with siliceous 

 sand and broken glass. For a hard glass, the proportion of 

 alkali is small. 



The best English crystal glass analyzed by Berthier, af- 

 forded 59 parts of silica, 9 of potash, 28 of oxyd of lead, and 

 1 *4 of oxyd of manganese . Crown glass contains, in general, 

 less alkali than crystal glass, and is superior in hardness. 

 The alkali, moreover, in England, is soda instead of potash. 

 Plate glnss also contains soda, and this soda (the carbonate) 

 is prepared with great care. The proportions are 7 parts of 

 sand, 1 of quickiime, 2£ of dry carbonate of soda, besides 

 cullet or broken plate. 



The materials are first well pounded and sifted, and mixed 

 into a fine paste ; they are then heated together in pots made 

 80* 



