318 CLAYS AND CLAY INDUSTRY. 



present alone, but are accompanied by silica, and the question 

 arises what possible effect this combination may have. 



In order to throw some light on this problem, the same series 

 of mixtures represented by the upper curve in Fig. 41 was made 

 up, but 5 per cent, of the silica was replaced in each case by 5 per 

 cent, of very finely ground titanium oxide as follows : 



Table showing composition of various kaolin- silica-titanium mixtures. 



Titanium 



Kaolin. Silica. oxide. 



VIII, 90% 5% 5% 



XI, 70 25 5 



XIII, 56.6 384 5 



XVI 30 65 5 



XVII, 10 85 5 



XIX, 7 88 - s 



The fusion point of these mixtures was then determined in the 

 Deville furnace and plotted as a dotted curve, Fig. 41. On com- 

 paring this curve with that of kaolin and silica alone it is evident 

 that the substitution of 5 per cent, titanium oxide in place of a 

 part of the silica rendered all the samples more fusible. For ex- 

 ample, No. IX (kaolin 90, silica 10) fused slightly below cone 

 34, whereas No. VIII (in which 5 per cent, titanium oxide had 

 been substituted for an equal amount of silica) fused 2^ cones 

 lower. So, too, No. XV (kaolin 30, silica 70) fused about cone 

 26, while No. XVI (kaolin 30, silica 65, titanium oxide 5) fused 

 between cones 24 and 25. The addition, therefore, of titanium 

 oxide to a mixture of kaolin and silica lowers the fusion point ma- 

 terially. The reason for the irregularity of the curve at No. XIII 

 is not quite apparent, for no doubt was felt regarding the deter- 

 mination of the fusion point. 



This curve can be combined with that of Fig. 25, since No. VI 

 (kaolin 95, titanium oxide 5) on the latter is the same as No. VI 

 on the former. If this is done we have a graphic illustration, first, 

 of the effect upon the fusion point of kaolin of the addition of 

 from 1 to 5 per cent, of titanium oxide, and second, the further 

 effect of adding silica to this mixture with a corresponding de- 

 crease in the amount of kaolin. For example, when no> silica was 



