100 CIIEMISTKY FOR AGRICULTURAL STUDENTS 



correspond to the acid HaSiOg (that is, SiOoJIgO). Thus a silicate of 

 calcium that occurs naturally is CaSiOg (or SiOa-CaO). China clay or 

 kaolin, a nearly pure silicate of aluminium, is 2AlHSi04.H20 (or 

 2Si02.H20.Al203; H2O). 



Dilute a little of a weak solution of silicate of sodium with 

 water, and add a few drops of solution of alum. Note that the 

 liquid is rendered turbid with a gelatinous precipitate of 

 hydrated silicate of aluminium (clay). Make a mixture of clay 

 and distilled water, and divide into two parts. To one add 

 distilled water and to the other lime water. Note that in the 

 latter the clay becomes granular and separates from the liquid ; 

 in the other the clay remains suspended. 



To a portion of the strong solution of silicate of sodium add 

 a saturated solution of alum. Note that a solid jelly of 

 hydrated silicate of aluminium is formed. Heat the jelly on a 

 dish on the water bath. Note that the anhydrous silicate of 

 aluminium produced is porous and no longer gelatinous. Wash 

 the mass well with water. Then heat a little on the loop of a 

 platinum wire in the Bunsen flame. Note that it is infusible. 



Owing to these properties clay is used as the principal ingredient in 

 pottery and porcelain. Vessels made of pure clay are, when baked, porous 

 and friable. The clay is therefore mixed with a little fusible silicate, for 

 instance, silicate of lime, which, in the baking, penetrates into the pores of 

 the clay. The glaze on the surface is also produced by covering with a 

 fusible silicate. Bricks are made from "brick earth," a mixture of clay and 

 sand in such proportions that the bricks bake to a solid mass without 

 cracking and warping. Tiles are made from a purer clay, as they need to 

 be less porous than bricks ; being thinner, they can be more easily baked. 



Mix a very little of the syrupy silicate of sodium with a trace 

 of lime and fuse by means of a blow-pipe on the loop of a 

 platinum wire. Note that a bead of glass is produced. Dip 

 such beads into solutions of salts of manganese, chromium, and 

 cobalt, and heat again. Note the different colours imparted 

 to the beads — amethyst, green, and blue. 



