194 CHEMISTKY. 



gles with the ashes. So, also, if there be much lime in 

 the clay that is used for making bricks, they will be apt 

 to be spoiled in burning, from the too great fusibility of 

 the silicate that is thus formed. 



266. Soluble Glass. Glass, as commonly made, is wholly 

 insoluble; but soluble glass can be produced by using a 

 very large proportion of alkali ; and a solution of it was 

 known a long time ago as the liquor of flints. Such a solu- 

 tion is sometimes employed as a fire-proof varnish for wood, 

 canvas, etc. 



267. Earthenware. All earthenware is made of clay, 

 which has as its essential ingredient silicate of aluminium. 

 There are mingled with this in different clays silicates of po- 

 tassium, sodium, calcium, etc. The coarsest clay employed 

 is used in making bricks and common flower-pots, and the 

 finest in making porcelain. The plastic nature of clay, and 

 its hardening by heat, are the causes of its peculiar adapta- 

 tion to the manufacture of earthenware. The moistened 

 clay, after being well kneaded, is shaped, either by pressure 

 in moulds, as in brick-making, or by the hand of the potter 

 as he makes it revolve with his lathe, thus pressing into 

 his service centrifugal force, as indicated in Part I., 213. 

 The articles are first dried in the sun, and then are baked 

 in furnaces, both of which processes cause considerable 

 shrinking, especially the baking. The reddish-brown color 

 of bricks and flower-pots is owing to the presence of ferric 

 oxide. The bricks of the Egyptians, in the making of which 

 straw was used as one of the constituents, were merely sun- 

 dried. The bits of straw mingled with the clay were of 

 the same use as hair is in mortar which is used in plaster- 

 ing. 



268. Glazing. Although earthenware by baking becomes 

 hard and firm, it is quite porous, so that water can exude 

 through it. This is not objectionable in the case of flower- 



