CLAY WORKING INDUSTRIES. 83 



removed. If the cla} r is not washed but merely tempered with water, 

 this water becomes a solution of sulphate of lime and when the clay is 

 dried before and during the burning process, the sulphate is carried to 

 the surface of the ware in small quantities and deposited there as a white 

 incrustation. If the clay contains a small quantity only of the sulphate, 

 it is probable that the action of the water will prevent any of the bloating 

 effect being accomplished. 



There may be other agencies besides those which have been indi- 

 cated which are the cause of this second bloating of clays. The question 

 has not been, by any means, probed to the bottom as yet, but it seems 

 tolerably certain that in the cases mentioned the action is substantial^ as 

 has been described. 



The phenomena which clays show when undergoing the changes due 

 to excess of heat are very various. Some clays simply get soft and pasty 

 and become more and more fluid as the heat is raised. Such clays gen- 

 erally contain a good deal of the alkalies and alkaline earths. Many good 

 fire-clays, when their refractory qualities have been over-taxed show their 

 failure in this way. 



Other clays do not melt at any heat but seem to undergo a complete 

 disunion of their former chemical compounds and become perfectly 

 worthless. Clays in which iron is the chief impurity and in which it is 

 believed to be in chemical combination with the clay base, generally 

 behave in this manner. Some few clays cannot be made to undergo 

 chemical dissolution or even fusion under any heat which metallurgical 

 practice brings to bear on them. These clays fail by the fluxing and 

 scorifying action of the slags to which they are exposed. They are simply 

 dissolved away. 



In addition to the facts which are presented to us in the application 

 of heat to clay, there are other phenomena which are due to the chemical 

 character of the flames which give off this heat. If the clay is burnt in 

 an atmosphere which is always free from unconsumed carbon, the colors 

 it develops will be clear and bright. If the fire places are badly con- 

 structed and the fires badly managed, clouds of smoke will be produced, 

 which darken the color of the wares. Sometimes the coloring is done 

 intentionally, by what is termed "smudging" the kiln, or making a very 

 dense smoke for a time just before and during the glazing of the ware. 



Sometimes the idea is advanced that this darkening color is due to 

 the absorption of carbon from the smoky flames while the surface of the 

 ware is sticky with the glaze. This is not so. The variation in color 

 produced in clay wares by changes in the quality of fire employed is due 

 to the oxydizing or reducing action of the gases on the oxides of iron 

 present on the clay. 



It is not probable that the quality of the flame affects anything else 

 except other metallic oxides, artificially employed in the clay for fluxing 

 or glazing effects. 



