THE CLAYS. 651 
shape at the temperature at which they are used, must also hold a heavy 
charge of fluid glass as well, and the temperature with some variations 
of intensity must be maintained for weeks at a time. Such pots are 
made in England of the best Stourbridge clay, which has a specific 
gravity of 2.45 to 2.55. In our own country, we use a mixture of various 
native clays, with the famous Gros Almerode clay from Germany, which 
has a specific gravity of 2.2. The Missouri clays, much used for this 
line of work, are also very heavy. None of these clays owe their weight 
to their composition, but entirely to structure. From these points it 
would seem that fineness of division, entailing as it does superior density, 
is a benefit to a clay. 
Another property of a physical nature is plasticity. With the 
accompanying property of permanently hardening under heat, it is the 
quality which gives clay its widest use. Various explanations have been 
offered, but none are yet advanced which make clear all points. It has 
been ascribed to the impurities, to the alumina, to the combined water, 
and to other causes, against each of which, examples can be cited that 
seem to set it aside as inadequate. The impurities do not appear to 
cause the plasticity, for the sand acts unfavorably to it. The alumina 
is not responsible, or kaolins would be the most plastic of all, while the 
flint clays of Ohio are many of them approximately pure kaolins, and at 
the same time eminently non-plastic. The combined water exerts some 
influence it is evident, as its expulsion entails permanent loss of plas- 
ticity, but it cannot be the sole cause of plasticity, as clays equally 
hydrated are just as liable to differ in this respectastoagree. No theory 
is so well received at present as that advanced by Cook. He shows 
that the microscope reveals a crystalline structure which the eye does 
not detect, and that this structure varies greatly in degree of perfection 
in different samples. Some are composed of masses of hexagonal plates 
or scales piled up in long bundles or faces and masses of unattached 
scales nearly perfect. Such clays are always but little plastic, but may 
become so on mechanical treatment such as grinding and kneading; 
on re-examination the clay then shows the same elements of structure, 
but broken and confused, no bundles left intact, scales broken and 
a homogenous matrix of the crushed material derived from the still 
erystalline part. Clays are found in all states of this breaking up, from 
the highly crystalline mass to the homogenous matrix showing no plates 
at all; and on the degree in which the crystalline structure is retained, 
