162 PRACTICAL LESSONS IN SCIENCE. 



quality from the heavy compact vitrified brick that will absorb 

 not more than two or three per cent, of water to the light porous, 

 fragile brick that will absorb half of its weight of water. Bricks 

 are made by hand as they were in Egypt four thousand years 

 ago, and they are made by different kinds of machinery. 



Pottery ware that is designed to hold water must be glazed, 

 which is done by dipping the ware in a mixture of fine sand and 

 water and heating it intensely, in a kiln into which a quantity of 

 wet salt has been thrown. The water, sand and sodium of the 

 salt from a sodium silicate glass, which fuses on the surface of 

 the ware, making it impervious to water. 



In the manufacture of porcelain great care is used in the 

 selection and preparation of the material, definite proportions 

 are used and all the manipulations approach mathematical 

 exactness. One very fine ware is made as follows : The purest 

 materials are selected in the following proportions: Kaolin, 

 Al 4 (Si0 4 ) 3 +4H 2 0, 62 parts, chalk, CaC0 3 , 4 parts, sand Si0 2 , 17 

 parts and feldspar AlKSi 3 8 , 17 parts. These materials are 

 ground up with water and allowed to settle. The water is 

 drained away and the mass well kneaded, then stored away in 

 a damp place for some months, during which time any organic 

 matter is oxidized and the texture of the mass seems to be im- 

 proved. It is then moulded, dried in the air, and strongly heated 

 in a kiln with a wood fire. After burning, the ware must be 

 glazed, care being used to get a glaze that will expand and con- 

 tract with the ware so that it will not crack or craze. The glaze 

 employed at Sevres is a mixture of finely ground feldspar and 

 quartz. When the ware is dipped in this mixture some of it is 

 absorbed, a thin coating remaining on the surface. It is now 

 baked the second time when the glaze fuses, forming aglass var- 

 nish, which covers the surface penetrating the ware to someextent. 

 If the ware is to have a uniform color, some mineral pigment is 

 mixed with the glaze. If designs are to be painted, glasses 

 colored with metallic oxides ground up with the oil of turpentine 

 are used. After painting, the ware is again burned so that the 

 paint is fused upon the glaze. 



