HOLMES] TEMPERING MATERIALS 47 



once started, coutinue througli the wall of the vessel in a direct line 

 without interference. In the tempered paste it would, in avoiding the 

 solid particles, or through interference of cavities take a siiuious 

 course or be led off in diverging- directions. 



Again, anj- condition or ingredient that reduces the amount of con- 

 traction resulting from drying out during the baking process must be 

 advantageous. It may be possible for a body of clay to contract so 

 evenly as to suffer no injury, yet. as a rule, there must be considerable 

 unevenness of contraction, with consequent danger, and it would seem 

 that the greater the contraction the greater the danger of disaster. 

 Clay contracts through the evaporation of water held between the mi nute 

 particles. The coarse particles of tempering may contain water, ))ut, 

 being rigid, they do not contract on drying out. The amount of con- 

 traction would thus be reduced in direct ratio with the increase of tem- 

 pering material, and this would seem a most important consideration to 

 the potter. 



It may be further surmised that the presence of foreign particles in 

 the clay may serve some purpose in connection with the distribution of 

 the heat in firing or in subsequent use over fire. The points reached 

 by a gi\'en degree of heat in pure clay may be on or close to a particular 

 line or plane and may thus give rise to distinctly localized strain, 

 whereas the foreign particles may tend to conduct the heat iinevenly 

 and distribute the strain. 



In reference to the function of the tempering material during the 

 subsecjuent use of the vessel, it might seem that the presence of large 

 fragments of hard substances would weaken the wall of the vessel so 

 that when in use it would readily be fractured by a strain or blow; 

 })ut the particles arrange themselves so that strong points alternate 

 with the weak ones in such a way as to increase strength rather than 

 to reduce it. It appears further that the particles of tempering, espe- 

 cially if coarse, must add greatly to the toughness of the paste during 

 the use of the vessel, much as they do during the drying-out process, 

 and it is not impossible for a flaw to extend entirely through and across 

 a vessel, and still not seriously impair its strength, as the particles of 

 tempering are so interlocked or dovetailed that separation can not 

 readily take place. It woidd appear, therefore, that the offices of the 

 tempering ingredient are almost purely physical, and not chemical. 

 In America the heat employed in tiring eartlienware was not sufficient 

 to serioush' alter any of the mineral constituents. It rarely happened 

 that the heat was sufficient to calcine the shell material with which the 

 clay in many sections was filled. 



The favorite tempering matei'ials were powdered shell and pulverized 

 crystalline rock. Sand, the grains of which were rounded, and various 

 other materials, so finely powdered as to be almost impalpable, were 

 often employed. In the piedmont i-egions of North Carolina and 



