A PRELIMINARY REPORT ON CLAYS OF rLORIDA 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 exception, no shrinkage occurred between 600'C. and 900*C., but between 9000 C. and 10000 C. there was a decrease in size and a still greater reduction between 1000* C. and 11000 C. It can be seen from this that up to 6000 C. a clay should be heated slowly, butt from that point up to 1000* C. the temperature can be raised quite rapidly unless much carbonaceous 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 fireshrinkage, 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.
 Ries1 says that in the case. of clays "the temperature of fusion depends on (1) the amount of fluxes; (2) the size of grain of the refractory 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 vitriication, 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, however, have not closed. The second stage'is known as complete Vitrihica1Ries, H., Clays, Their Occurrence, Properties and Uses, p. 166, 1908.