﻿GLAYS FROM LUZON. 427 



of the process, although they show normal shrinkage at other periods 

 during the burning. Many clays shrink to such an extent that they 

 warp, crack, or blister when burned. When the fire shrinkage is large, 

 other substances of small or no fire shrinkage must be added. It is 

 also necessary to know the air shrinkage in order to estimate the size 

 of a mold necessary to give a finished product of given dimensions. 



The shrinkages expressed in Table VI are given in per cent of the 

 length of the briquette when freshly molded. The clays were all burned 

 at a uniform temperature of about 1,100° C. (slightly above the melting 

 point of gold), which is well above that at which all water of constitution 

 passes off. 



Fusibility is either increased or decreased by each constituent of a 

 clay. Fire clay is practically pure aluminium silicate together with 

 silica, and it will bear intense heat without melting. The presence of 

 different quantities of iron alone varies the fusibility of the clay and there- 

 fore the use which can be made of it. The less the iron content the more 

 refractor}' the clay. Other common fluxing materials are magnesia, 

 lime, sodium and potassium compounds, and I have arranged them in 

 the order of their fluxing power. The presence of any of these with 

 the iron helps to form a flux and greatly to lower the fusion point. 

 For example, if more than 2 or 3 per cent of lime or potash is present 

 in a clay, good brick will not result as these substances cause the bricks 

 to run into glass while they are being burned. The fusion point of a 

 clay can best be determined by means of a pyrometer. It may also be 

 satisfactorily judged by comparison with test pieces of known composi- 

 tion (standard Seger cones), 18 etc. The elaborate work on the composi- 

 tion and fusing point of Seger cones will be of assistance in deciding 

 the fusibility of a clay from its analysis, as well as in actual comparative 

 tests made by placing cones and clay side by side in the furnace. Cone 

 number 1 melts at 1,150° and cone number 20 at 1,530°. The difference 

 between any two successive numbers is 20° and the highest number is 

 36 ; this cone is composed of absolutely pure kaolin. The temperatures 

 usually obtained in porcelain furnaces lie between the melting points of 

 cones numbered 1 and 20. 19 



18 Reis, H.: U: 8. G. 8., P. P. (1903), 11, 24; Bourry, E.: Loc. cit., 396. 

 19 The following data taken from Bourry's "Treatise on Ceramic Industries" 

 give an idea of the numbers of the Seger cones which are used in various potteries : 



Muffle firing for decoration 0.022 to 0.010 



Burning fusible bodies 015 to .01 



Burning slightly fusible bodies 1 to 10 



Stone ware burning 5 to 10 



Burning the body for fine faience 3 to 10 



Burning the glaze for fine faience 010 to .01 



Burning fire-clay ware and porcelain 10 to 20 



The tests from 20 to 3C are used only in experiments for fusibility. The work 

 of Simonis, Sprechsaal, 6, 7L Ghem. Abstracts (1907).. 1, 2166. shows that cones 

 20-25 melt within 1S° of each other and that their use seems unnecessary. 



