celain and other forms of white ware. Ball clay is the name given to 

 very fat plastic clays which can be formed and molded into shape and 

 which will dry out to a mass of high binding power without suffering 

 deformation. Used by themselves ball clays will rarely stand the fire 

 well, but for mixing with good burning clays of low binding power and 

 little or no plasticity they are very necessary. Florida supplies these 

 clays to a considerable extent, but for special reasons they are at pres- ' 

 ent very largely imported . from England and Germany. Kaolins are 

 made up of fine particles of the mineral kaolinite which results from 

 the decomposition of feldspar. Kaolin, which, chemically speaking, is 

 a silicate of aluminum, is supposed to be the essential ingredient in all 

 clays, and it is not uncommon to read in various publications that kaolin 

 furnishes the active clay base in all plastic clay. Recent researches 

 made in the Division of Tests of the Bureau of Chemistry, however, 

 show that such statements do not express the whole truth of the matter. 

 Fire clays, as their name signifies, are those which do not fuse or melt 

 except at very high temperatures, 3,500 F. and over ; although these 

 clays are generally used in the manufacture of fire bricks, muffles, and 

 linings for furnaces, they can be used for decorative purposes, as their 

 very stability in the fire makes them easy to burn. 



PHYSICAL PROPERTIES OF CLAY. 



We may now turn to the consideration of the physical properties of 

 clay, taking them in the following order: (1) Plasticity; (2) binding 

 power (tensile strength); (3) slaking; (4) air shrinkage ; (5)~ firing 

 qualities (a) fire shrinkage (distortion), (b) fusibility, (c) color ; 

 (6) absorptiveness. 



PLASTICITY. 



It is a matter of common observation that the particles of all wet 

 powders to a greater or a less degree cohere. If we take ordinary clean, 

 white, sea sand, grind it to an impalpable powder and mix it with a cer- 

 tain amount of water, we shall have a mass just sufficiently coherent to 

 ball together. If, however, we try to mold the wet mass into a form it 

 quickly crumbles and shows itself quite unworkable. From this 

 extreme up to the most plastic of the ball clays which will respond to 

 every touch of the artist's hand and preserve every line of the gravci ' s 

 tool, we find among the fine-grained materials on the earth's surface 

 every grade of variation. 



The cause of this wonderful property has been much discussed by 

 students of clay structure. Investigations carried on in the Division of 

 Tests show that the effect is due to the presence of a certain proportion 

 of particles which actually soften and become sticky under the action of 

 water. It has been noted that purely crystalline particles do not make 

 plastic masses, but there is another kind of matter than that which 



