92 
FLORIDA GEOLOGICAL SURVEY—15TH ANNUAL REPORT 
added it causes an increase in volume or swelling of the clay. This addi¬ 
tional 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 100° 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 com¬ 
bined 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 1 , in experi¬ 
menting with New Jersey clays, found: “That most of the volatile sub¬ 
stances, 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 600° C. Between 600° C. and 1100° C. there was a small but 
steady loss. Although the loss in weight between 500° C. and 900° C. is 
considerable, there is little or no shrinkage, so that after the volatile 
1 Ries, H., The Clays and Clay Industry of New Jersey, New Jersey Geological 
Survey Report, Vol. VI, p. 94, 1904. 
