THE PHYSICAL PROPERTIES OF CLAY. 97 



facilitate the escape of the moisture in drying and in the early 

 stages of burning, as well as enable the product to withstand 

 sudden changes of temperature. If sand is added for this pur- 

 pose, it may act as a flux at high temperatures, and this action 

 will be the more intense the finer its grain. 1 



Large particles of grog are undesirable, especially if they are 

 angular in form, because in burning the clay shrinks around 

 them, and the sharp edges, serving as a wedge, open cracks in the 

 clay, which may expand to an injurious degree. Large pebbles 

 will do the same, and at many of the common brickyards in 

 the State, the writer has seen numbers of bricks split open during 

 the burning because of some large quartz pebble left in the clay, 

 as the result of improper screening of the tempering sand. For 

 common brick, the type of sand used does not make much dif- 

 feren, as long as it is clean, but if sand is to be added to fire 

 brick mixtures, it should be coarse, clean quartz sand. Burned 

 clay grog is more desirable than sand for high-grade wares, since 

 it does not affect the fusibility of the clay, or swell with an in- 

 crease of temperature as sand does, but precaution should be 

 taken to burn the clay to its limit of shrinkage before using it. 



FUSIBIUTY. 



The changes occurring in the early stages of burning have 

 already been referred to on pp. 93-96, and in the table of tests 

 there given it was seen that the clay had become steel-hard. 

 The temperature at which this occurs varies with the character 

 of the material, impure, easily fusible clays becoming so at a 

 low temperature, such as cone 05, while others, such as kaolins, 

 will not become steel-hard before cone 5 or possibly 8. 2 



The attainment of a steel-hard condition represents the begin- 

 ning of fusion, not of the whole mass, but of some of the more 

 fusible elements in the clay, the result of this preliminary soften- 



1 See Chapter on Fire Clays and Fire-Brick Industry. 



2 The cones referred to are small pyramids of definite chemical composition- 

 and a theoretic fixed fusion point. Their exact nature and method of use are 

 explained on p. 101. 



7 CI, G 



