214 BULLETIN 42, UNITED STATES NATIONAL MUSEUM. 



ThefolJowinf/ analtises taken from the report of the New Jersei/ GeuIogicaJ Survey iqiuu t<hi\i 

 deposits show the uide rariatiou which oceurtt in the chemical composition of different 

 charactei'istie clays. 



Physically clays vary from soft plastic or unctuoas irnid, as found iu 

 moist loiatious, to dry, hard, aud rock-like deposits as often found below 

 the Coal Measures. 



The only general property belonging to all clays is, that with water 

 they form an adhesive paste. Some clays show this in their natural 

 condition or on being moistened with water, while others have to be 

 ground in order to make a good paste. This property is called plas- 

 ticity, and varies very much in different clays. On being heated so as to 

 drive out the water of combination the plasticity is destroyed. Upon 

 these two facts depends the value of clay for most purposes. Its plas- 

 ticity allows the easy molding of the material iuto any desired shape, 

 while subsequent heating or baking permaneutly sets or fixes the form. 



According to their uses clay may be divided into pottery clay, re- 

 fractory or fire clay, and brick and tile clay. 



Under the head of ])otteiy clays are generally included earthen- 

 ware, including stoneware, ironstone, white, china, and porcelain clays. 

 Pure white clays, free from oxide of iron, constitute the ironstone, 

 china, white, and porcelain ware clays. In use other materials are 

 generally mixed with them to secure the proper composition. Less pure 

 clays, and those containing iron, constitute the earthenware claj's. 

 They may be of various colors, and when burned are some shade of red 

 or brown. When sufficient iron is present to partially fuse during burn- 

 ing it becomes a stoneware clay. 



Refractory or fire brick clays generally contain much quartz, and they 

 must be able to stand a very high temperature without fusing. They 



