82 GEOLOGY OP OHIO. 



vitrify at so low a heat that the opportunity of expelling the combined 

 water before the outer layers of the clay become dense is much diminished. 



In pottery and pipe-making, less trouble is experienced on account of 

 the thinness of the section of the ware. This change of structure in a 

 clay by bloating may be very marked or very slight. If the action has 

 been very slight no change in structure can be detected by the naked 

 eye, but a change in color is apparent, perhaps in a narrow band or small 

 area furthest from the surface of the ware. As the action increases, the 

 deepness of the color increases, the size of the areas affected increases, 

 and soon the discolored areas are seen to be minutely porous or full of 

 holes and cells. Further increase of the action is marked by an increase 

 of all these symptoms and the object begins to swell out of its proper 

 shape. The last stages are so excessive in some cases that the clay has 

 become so porous and cellular that it will float on water. 



In this discussion, combined water has been mentioned as the cause 

 of these phenomena. It is the common cause, without doubt. Iron 

 pyrites or any form of sulphur which is decomposed by heat at the same 

 temperature is able to cause the same phenonena and is harder to control 

 than water alone. It is possible that organic matter may have the same 

 action in some cases. 



If the clay has been safely brought through to this stage of its burn- 

 ing with no changes of physical structure except the shrinkage and 

 closing up of its pores and general consolidation of its particles, there is 

 generally no further fear of trouble in burning it. The heat may be 

 raised as rapidly as is convenient and as high as the fusibility of the clay 

 will allow without danger to the structure. There are, however, excep- 

 tions to this general line of facts. If a clay contains sulphate of lime or 

 any other sulphates not decomposed at low red heat, they are likely to be 

 decomposed at a still higher heat, to give off sulphuric acid and cause a 

 set of phenomena very similar to those caused by the combined water. 

 There is this difference, however, that bloating from combined water or 

 sulphides nearly always proceeds from the center of the mass outwards, 

 while porosity produced by sulphates or sulphuric acid begins on the out- 

 side and proceeds inward as the temperature sufficient to cause it pro- 

 ceeds by absorption toward the center of the mass. Instances have been 

 seen of this kind of bloating where a still unaltered core was surrounded 

 by a porous envelope, when the burning process was stopped before com- 

 pletion of the bloating. This is due to the fact that the body of the clay 

 had become compact and solid before the heat necessary to decompose the 

 sulphate had been attained and gases generated near the surface of the 

 clay were unable to make their escape, which is never the case where 

 combined water is the cause of the bloating. 



The presence of sulphate of lime in small quantities is not likely to 

 do any harm, for it is comparatively soluable in water and if the clay is 

 washed, as in the pottery process, small quantities of this mineral will be 



