A PRELIMINARY REPORT ON CLAYS OF FLORIDA 93 
elements have been driven off, the clay must be very porous, and remains 
so until the fire-shrinkage begins again. In these tests, with one ex¬ 
ception, no shrinkage occurred between 600°C. and 900°C., but between 
900° C. and 1000° C. there was a decrease in size and a still greater re¬ 
duction between 1000° C. and 1100° C. It can be seen from this that up 
to 600° C. a clay should be heated slowly, but from that point up to 
1000° C. the temperature can be raised quite rapidly unless much carbon¬ 
aceous matter is present. Further heating should be done slowly as the 
shrinkage recommences at the last-mentioned temperature.” 
In some clays having an excessive fire-shrinkage and where severe 
losses occur from' warping and cracking a substance having no fire- 
shrinkage, a sand or grog (ground bricks, etc.), is often added. Sand, 
however, may act as a flux at high temperatures. 
FUSIBILITY 
All clays fuse or melt at some temperature and the temperature 
at which this action takes place is quite variable in different clays. In 
fact, clays are often classified on the basis of their refractoriness. Every 
mineral has a definite temperature at which it will fuse and this point is 
usually different for different minerals, but in the case of a mixture of 
minerals the point of fusion may be different from that of any mineral 
in the mixture. Thus the fusion point of clay, which is a mixture of 
several minerals, may fuse at a temperature different from the melting 
point of any of its mineral components. In such cases the minerals act 
as a flux on each other. Clays soften slowly due to different mineral 
grains entering into fusion at different temperatures. 
Ries 1 says that in the case of clays “the temperature of fusion de¬ 
pends on (1) the amount of fluxes; (2) the size of grain of the refrac¬ 
tory and non-refractory particles ; (3) the homogeneity of the mass ; (4) 
the condition of the fire, whether oxidizing or reducing; and. (5) the 
form of chemical combination of the elements contained in the clay.” 
Three stages are commonly recognized in the heating of a clay to 
its fusion point. The first is incipient vitrification, in which there has been 
sufficient softening to cause the grains to stick together. In this stage the 
individual grains can no longer be recognized. All the pore spaces, how¬ 
ever, have not closed. The second stage is known as complete vitrifiica- 
iRies, H., Clays, Their Occurrence, Properties and Uses, p. 166, 1908. 
