652 GEOLOGY OF OHIO. 
its plasticity depends. This theory is certainly plausible, and is sup- 
ported by the fact that we always subject our clays to secure increased 
plasticity 10 mechanical disturbance which has the effect that the micro- 
scope reveals. This view harmonizes with more points than any other 
advanced as yet, and offers a fair solution of the different degrees of 
plasticity which plastic clays exhibit, but it does not explain, nor at- 
tempt to explain, the differences which exist between flint clays and 
plastic clays, as Professor Cook’s examinations were entirely confined 
to the latter. : 
There is no theory worth consideration at present concerning the 
difference between flint and plastic clays. Suffice it to say, itis no point 
in chemical composition, or water of hydration, and, as it seems, it must 
depend on the physical structure of the clay. Possibly it could be 
studied to the best effect through the microscope. 
It has been already mentioned in connection with plasticity, that 
this property is permanently lost by heating to redness, or by expelling 
the water of constitution. It is this point which brings in water as an 
essential to plasticity. The changes which accompany heating of a clay 
are as follows: First, the complete drying or loss of moisture, which is 
accomplished by maintaining the temperature at 212° F. for several 
hours; next, the elimination of the water of constitution, during which 
the clay becomes rigid and stone-like. When once expelled it can 
never be taken back. Rehydration is impossible. Though water may 
be absorbed by brick it can all be expelled at 212° F. After and dur- 
ing the expulsion of the combined water, the organic matter burns out. 
The matter is not ordinarily found agglomerated enough to cause po- 
rosity on expulsion, but is distributed in such a way that its absence 
merely shows the natural color of the clay. 
The expulsion of combined water is nearly always accompanied by 
a diminution in volume, which varies directly as the water, or the purity 
of the clay. Pure kaolin shrinks as much as one-fourth of its bulk, it 
is stated, sometimes even more. The sandy clays used in making sewer- 
pipe and stoneware shrink from the temrered state from one-ninth to 
one-sixteenth, usually about one-twelfth. The shrinkage of the raw clay 
would be very much less, probably not over 3 or 4 per cent. 
A clay, when all the water of crystallization is expelled, will not 
shrink any more at red heat, out with increased heat will shrink more 
and more up to the moment of fusion. A pure kaolin apparently shrinks 
