544 NEW YORK STATE MUSEUM 



position but different plasticity; viz, waslied Zettlitz kaolin and 

 plastic refractory clay froni Miihllieini, near Coblenz. Botli have 

 only a small admixture of quartz sand, viz, about 1^/^, the balance 

 being nearly pure clay substance, while in the percentage of ferric 

 oxid they differ by only 1^. 



Briquets (air-dried) of the Zettlitz kaolin were loose, and rubbed 

 easily, while their porosity was 42^. Those of the Miilheim 

 clay were hard, and showed only 28^ porosity. If both are heated, 

 the latter gets thoroughly dense at 1100° — ■ 1150° 0., while the 

 kaolin retains its porosity up tO' a high temperature. The exhibi- 

 tion of density by kaolin is not tO' be regarded as even the be- 

 ginning of fusion, for the clay, after assuming it, retains it unaltered 

 up to a high temperature. Many plastic and hard drying clays act 

 in this respect like the Miihlheim material; they sinter however at 

 a much lower temperature. 



For the manufacture of glass pots, this is of high importance; 

 for it is not the most refractory clays that are the^ best, but those 

 which burn dense at a low temperature, and are consequently less 

 attacked by the molten glass. 



The same is true of brick used in coke ovens holding coal with 

 soluble salts. 



Plasticity, whatever its cause, is an important property from a 

 commercial standpoint, and interesting from a scientific one. 



The amount of water required to develop the maximum plasticity 

 varies. If toO' little is added, the clay cracks in molding and is 

 stiff and hard to work. If too much water is used, the paste be- 

 comes soft and retains its shape with difficulty. Lean clays usually 

 require less water to produce a workable mass than fat ones. 



Tensile strength 



The tensile strength or binding power of a clay often stands in 

 relation to its plasticity, but not always. It is, however, an im- 

 portant property, and exerts an important effect in connection with 

 the cracking in drying. One way of testing the tensile stiengtK ■ 

 is the briquet method mentioned under " Plasticity," p. 539. 



