A PRELIMINARY REPORT ON CLAYS OE ELORIDA 
95 
bons, etc.), derived from the original minerals should have been con¬ 
verted into stable forms, . . . and nothing should be left except 
what may enter into a silicate solution. The normal beginning of the 
reaction in the vitrification process is from little spots or foci scattered 
throughout the body, . . . each focus being represented by some 
easily fusible mineral grain, or the juxtaposition of two or more min¬ 
eral grains which combine to form a, eutectic or the most fusible ratio in 
which these minerals can combine. . . . The spread of the glassy 
cement from focus to focus in a clay of good vitrifying character is slow 
and steady, and the proportion of grains which will not readily dissolve 
is such that they readily form a sort of skeleton or frame work, holding 
the mass in its shape, while the glassy cement slowly decomposes them 
and fills up the voids, causing the well-known phenomenon called 
shrinkage. . . . Practically all silicates when passing from the solid 
state to a state of complete fusion, give off some gaseous matter. It 
may be the gas which they have held in solution and which is then 
occluded, or it may be from remnants of volatile matter not hitherto ex¬ 
pelled, or it may be due to the swelling of gases caught in the intersti¬ 
tial voids of the mass during the shrinkage and unable to escape. Prob¬ 
ably all three causes are responsible in most cases. . . This swelling 
agency is at work as soon as the formation of glassy cement be¬ 
gins. ... If the process of fusion be carried along steadily until 
a fluid bath is obtained, the liquid will pass into a frothy stage in which 
the gas bubbles work their way to the top and escape, . . . but with 
continued heat and liquidity the bubbles finally cease to form. . . . 
It can thus be seen that the clay product, in reaching its point of greatest 
density, does not reach the point where the gases are fully expelled but 
only the highest point attainable without causing their evolvement to 
seriously begin. This maximum density is found at a point where 
the reduction in volume due to shrinkage is equalized by the ex¬ 
pansion due to gases evolved. One force balances the other and for a 
time the volume of the clay remains constant. This time may be long or 
short. In some clays of most excellent vitrifying habit, a heat treatment 
represented by five or six cones may occur with scarcely any change in 
size. In others the volume diminishes rapidly and at the minimum point 
begins at once to swell again, with no appreciable interval. Such clays 
cannot be burned profitably into hard products. There is no margin in 
which the burner can regulate his kiln, and a part of every kiln would 
