THE PHYSICAL PROPERTIES OF CLAY. 101 



5. Nonrefractory clays, fusing below cone 20. 



Determination of fusibility. — The temperature at which a clay 

 fuses is determined either by means of test pieces of known 

 composition, or by some form of apparatus or mechanical pyro- 

 meter, the principle of which depends on the expansion of gases 

 or solids, thermoelectricity, spectrophotometry, etc. 



Seger cones. — These test pieces consist of a series of mixtures 

 of clay with fluxes, so graded that they represent a series of 

 fusion points, each being" but a few degrees higher than the 

 one next to it. They are so called because originally introduced 

 by H. Seg"er, a German ceramist. The materials which he used 

 in making them were such as would have a constant composi- 

 tion, and consisted of washed Zettlitz kaolin, Rorstrand feld- 

 spar. Norwegian quartz, Carrara marble, and pure ferric oxide. 

 Cone Xo. 1 melts at the same temperature as an alloy composed 

 of one part of platinum and nine parts of gold, or at 1100 C. 

 (2012 : F.). Cone Xo. 20 melts at the highest temperature 

 obtained in a porcelain furnace, or at 1530 C. (2786 F.). 

 The difference between any two successive numbers is 20 C. 

 (36° F.), and the upper member of the series is cone 36, which 

 is composed of a very refractory clay slate, while cone 35 is 

 composed of kaolin from Zettlitz, Bohemia. A lower series of 

 numbers was produced by Cramer, of Berlin, who mixed boracic 

 acid with the materials already mentioned. Hecht obtained still 

 more fusible mixtures by adding both boracic acid and lead in 

 proper proportions to the cones. The result is that there is 

 now a series of 58 numbers, the fusion point of the lowest being 

 590 ° C. ( 1094" F.), and that of the highest 1850 C. (3362 F.). 



As the temperature rises the cone begins to soften, and when 

 its fusion point is reached it begins to bend over until its tip 

 touches the base, Plate XIV. 



For practical purposes these cones are very successful, though 

 their use has been somewhat unreasonably discouraged by some. 

 They have been much used by foreign manufacturers of clay 

 products and their use in the United States is increasing. The 

 full series can be obtained from Messrs. Seger and Cramer, of 

 Berlin, for $0.01 each (or about two and one-half cents apiece, 

 including duty and expressage), or numbers .010 to 35 can be 



