much. In the manufacture of high-grade and art pottery, however, it 

 is often worth while to go to any length in the effort to <>l>tain an even 

 standard of excellence. By means of grinding, washing, jind-sifting tin- 

 e-lays are reduced to the even and uniform texture which is absolutely 

 essential to the production of high-class work. In addition to these 

 processes it has been found that by allowing the clay masses which 

 have been worked up with water to the consistency of a pasty dough to 

 remain quietly in this condition for long periods, months or more, the 

 binding power and plasticity increase. This is called tempering or 

 ripening the clay and has been ascribed by some observers to bacterial 

 action. It will readily be seen, how-ever, that according to the colloid 

 theory, as already outlined, the effect gained is caused by the gradual 

 softening of the active particles under the influence of water. 



A number of researches have been carried on during the last few 

 years in an attempt to find cheap and economical methods for increas- 

 ing the useful properties of clays. One experimenter, having occasion 

 in 1901 to seek a clay of the highest possible binding power, soon 

 learned that the material suited to his purpose could only be obtained 

 in Europe, and more particularly in Germany. This experimenter 

 immediately began to seek for a method of treatment for local clays 

 which would develop the useful qualities necessary for his purpose. 

 After some experimentation it was found that by a preliminary soaking 

 in a 2 per cent solution of tannic acid, or the ordinary catechu of com- 

 merce, the binding power and plasticity of most clays w r ere very much 

 increased while the air shrinkage was diminished. Even infusions of 

 dried leaves and straw were found to produce the effect to some extent. 

 The process was called the " Egyptianizing of clay" on the somewhat 

 fanciful theory that the ancient Egyptians used straw in brickmaking 

 more on account of the strength given by the infusion than by the fiber. 



This subject was investigated in a general way in the Division of 

 Tests, and it was found that many substances have the effect of pro- 

 ducing increased binding pow r er and apparently increased plasticity in 

 some clays. This is shown by the tensile strength tests given in the 

 following table, made on a buff-baking brick clay. Each result is the 

 average of 30 to 40 briquettes, from 10 to 12 having been made by each 

 of three operators. 



Average tensile strengths of treated clays. 



T'aiinds per 

 square inch. 



Water 65 



Two per cent solutions of 



Tannic acid 153 



Dilute ammonia a 147 



Alum solution 98 



Iron alum solution 118 



Dilute hydrochloric acid 63 



Dilute caustic alkali (KOH)_.. 80 



