﻿CLAYS FROM LUZON. 425 



tion of the final material would be next to impossible. Because of the 

 variation needed in making different wares it is seldom that a single 

 clay is used for the purpose of manufacture ; as a rule, several are mixed 

 together. One kind of clay by judicious admixture of other materials 

 is often adaptable to the manufacture of several varieties of wares. A 

 practical potter, if he has a complete knowledge of the chemical compo- 

 sition of a clay, is able to mix different varieties and materials to suit a 

 given purpose; that is, a chemical analysis shows him what ingredients 

 must be added or removed to give a desired result. In itself, a chemical 

 analysis does not show whether or not a clay is useful for a given purpose, 

 for of the impurities in a clay some are easily melted, others are infusible, 

 some are plastic and others are brittle, and slight differences in the 

 plasticity, tensile strength, shrinkage, fusibility, color, etc., which are 

 not controlled by the chemical composition, may very largely affect the 

 commercial value of a clay. 



Plasticity, that common property of all clay to form a moldable mass 

 when wet, varies with different samples, forming an entire series, from 

 those of low to the ones of very high plasticity. This property can not 

 be inferred from the chemical analysis. Up to a certain point there is 

 a coincidence of plasticity and bonding power and several of the other 

 physical properties stand in close relation to the former. Manufacturers, 

 as a consequence, carefully study the means of changing the degree. of 

 plasticity of clays ; those which are commonly known are : The reduction 

 by adding sand or less plastic clay, while materials of too low plasticity 

 may often have this property increased by washing and thereby removing 

 a part of the sand. Eohland 15 gives the following : 



A. The ways of increasing the plasticity of clays : First, by contact with spring 

 or river water, by means of which colloids are formed by hydrolysis. Thus 

 silicic acid, aluminium, and iron hydroxides are produced. The action is expe- 

 dited by fine grinding. Because the hydroxyl ions resulting from the hydrolysis 

 tend to decrease the plasticity, pure water is not suitable for the above purpose, 

 but owing to the acid fermentation of the organic constituents present, spring 

 or river water, assisted by the organic matter of the clay or substances added 

 intentionally, neiitralize the action of the hydroxyl ions. Second, by lowering 

 the temperature which has also to do with the colloids of clay. The effect is a 

 very slow one. Third, by the addition of colloids and organic substances like 

 dextrin, tannic acid, catechu, etc. 



B. The ways of decreasing the plasticity, as: First, by the addition of hydroxyl 

 ions, lime water being the cheapest reagent for this purpose. If the concen- 

 tration of the hydroxyl ions of the lime water is too low for some clays, it may 

 be increased by the addition of sodium hydroxide solution or any strong base 

 combined with a weak acid. The latter class is represented by phosphates, 

 silicates, etc. Neutral salts like sodium chloride, Glauber.'s salt, magnesium 

 chloride, etc., appear to be indifferent. Borax weakens the effect of the hydroxyl 

 ion and potassium carbonate strengthens the action. Second, by raising the 

 temperature. Just as gelatin, agar-agar, etc., liquefy on heating to a certain 



15 Sprechsaal (1906), 42, 1371; Chem. Abstracts (1907), 1, 90. 

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