92 FLORIDA GEOLOGICAL SURVY-I5TH ANNUAL REPORT

added it causes an increase in volume or swelling of the clay. This additional water absorbed by the clay is in the form of a film surrounding each particle of clay.
 After a clay is mixed with water and molded, its water begins to evaporate. As evaporation progresses the particles composing the clay come again in contact, resulting in a shrinkage of the mass. This will continue until all the water forming a film around the clay grains has escaped and the clay particles are in contact with each other. This is the point of maximum air-shrinkage if the water lost is the shrinkage water. The only moisture remaining in the clay is the pore water which can only be driven off by heating the ware to 1000 C. for a few hours.
 The air-shrinkage in clays ranges from less than one per cent to more than fifteen per cent. Six or seven per cent is about the average. Sand is often added to clays to reduce an excessive shrinkage.
 All clays shrink to some extent during certain stages of the burning process. The fire-shrinkage varies within wide limits in different clays and ranges from one or two per cent in some to more than forty per cent in others. At certain temperatures some clays may expand to some extent. Fire-shrinkage results from the driving off of any organic matter present, decomposition of some of the chemical compounds and the volatilization of certain substances as water in the hydrous minerals and carbon dioxide in the carbonate minerals present, etc.
 Fire-shrinkage probably begins at the point where chemically combined water begins to pass off and continues, but not uniformly, until the point of vitrification is reached, which is the point of maximum density.
 After the expulsion of the volatile elements the clay is left in a porous condition until the fire-shrinkage recommences. Ries', in experimenting with New Jersey clays, found: "That most of the volatile substances, such as chemically combined water contained in the hydrous aluminum silicates, mica, or limonite, and organic matter, pass off before 500' C. and that an additional appreciable amount is expelled between 500' C. and 6000 C. Between 6000 C. and 11000 C. there was a small but steady loss. Although the loss in weight between 5000 C. and 9000 C. is considerable, there is little or no shrinkage, so that after the volatile

 lRies, H., The Clays and Clay Industry of New Jersey, New Jersey Geological Survey Report, Vol. VI, p. 94, 1904.