226 INDUSTRIAL MINERALS DIAGRAM 19 



are made of the paste formed by tlie mixture of clay and water . 

 Porcelain is simply made of a white kind of clay called kaolin. 

 All bricks and earthenware must be baked before they are suffi- 

 ciently solid to be fit for use. 



Fuller's earth is a greenish clay, greasy to the touch, which 

 dissolves in water, and makes it soapy. It is used to remove 

 the oil from cloth, for it is necessary to apply oil to wool before 

 it can be spun and woven. 



Gypsum, or plaster of Paris. Gypsum is a stone which has 

 much external resemblance to limestone, but it does not effer- 

 vesce with acids, and does not yield quicklime when burnt, but 

 plaster. Sometimes gypsum is found in the form of large crys- 

 tals shaped like spear-heads, which also yield plaster when 

 heated. This does not require so intense a heat as the manu- 

 facture of lime. When gypsum is taken from the kiln, it is easily 

 reduced to powder which again becomes solid when mixed with 

 water. If plaster has been exposed to the air before using it, it 

 is no longer serviceable, because it has absorbed moisture from 

 the atmosphere. Plaster is used either to join brickwork or 

 masonry, or to make ornaments and stucco on walls. It is also 

 run into moulds, and is taken out with the desired form. This 

 process is called moulding, and is used to reproduce statues and 

 busts, 



Rock salt. A large part of the salt which we use is collected 

 by evaporating sea water in shallow pools. This is called sea- 

 salt, or lay-salt. But salt is also found in the earth in layers. 

 It is sometimes deposited in thick beds like rock, from which it 

 is hewn like stone. It is then very white and transparent, and 

 called rode salt. At other times the salt is mixed with clay or 

 sand in the earth, and in this case water is allowed to flow into 

 the mines, which dissolves the salt, and afterwards deposits it 

 on evaporation. 



Salt is not only very useful for food, for salting provisions, 

 making bread, &c., but is also used to make soda, a substance 

 very useful in industry. We have mentioned that it can be ex- 

 tracted from sea-weeds, but the greater part is manufactured 

 directly from salt. 



