﻿428 cox. 



The highest constant temperature obtainable in this country is at Zobel's 

 pottery kiln in San Pedro Macati. In it a temperature of over 1,300° 

 is attained, which is about the same as the highest temperature attainable 

 in the muffle furnace of this Bureau. The kiln was placed at my disposal 

 for experimental purposes and the temperatures of fusibility recorded 

 in Table VI were determined there by comparison with standard cones 

 and the temperature confirmed thermoelectrically. The fusing points 

 marked b either lay above the temperature obtained with the furnace 

 used, or a sufficient quantity of the sample was not at hand and therefore 

 they were estimated from their analyses given in Table I, by comparison 

 with a table of Seger cones and the fusion points actually determined, 

 and by our general knowledge of fluxing materials. 



77/ e color of a raw clay is usually due to the presence of organic matter 

 or of iron. In general, the former imparts a gray or black and the latter 

 a buff or red color. 



The color of a burned clay to a large extent depends on the proportion 

 of iron and the state of its oxidization. Its presence makes the result- 

 ing product buff-brown or red. The more iron the ckvy contains, the 

 deeper the red color. A quantity of iron as small as 1 per cent may 

 impart a slight, yellowish tint. Clay which bears practically no iron 

 burns white. In many cases the oxides of other metals are present and 

 these often modulate the effect of the iron. The intensity of the color 

 depends also on the temperature and the supply of air entering the kiln ; 

 in some cases it is possible to vary it from pink to reddish violet by chang- 

 ing these conditions and influencing the degree of oxidization. Sometimes 

 these means are used to produce a standard or varied shade in wares. 

 In terra cotta, especially, a certain percentage of iron is sought to give 

 the finished product a definite and desired color. A certain manufac- 

 ture is often known by the particular shade of the finished product. 

 Titanium 20 renders cla} r yellow on burning. This is visible only when 

 the material has a very low percentage of iron; when more iron is 

 present the yellow is entirely obscured. 



Below are given some physical properties of Philippine clays, includ- 

 ing the plasticity, breaking or tensile strength, the shrinkage from the 

 plastic condition in the air and with burning, and color. 



20 Attention is directed to the fact of the usual occurrence of titanic oxide in 

 Philippine clay (see Table I). Its determination has not been carried out in 

 every case, but not a single analysis has been made where it was not present and 

 it usually occurs to the extent of from 0.4 to 1.3 per cent. 



I have recently been interested in the titaniferous sands of these Islands and 

 find them of almost universal occurrence. The commonest titaniferous minerals 

 in these are ilmenite and rutile. These minerals occur only in small quantities in 

 the rocks from which the sands are derived, for the sands represent large con- 

 centration; however, their universal distribution clearly indicates that they are 

 the probable source of the titanium in clay. 



