96 VLORIDA GEOLOGICAL SURVEY-15TH ANNUAL REPORT

surely be overfired and bloated, part would be at its best density, and part would be underfired and not up to its best density. Such clays are said to have a short firing range."
 The fusibility of clays may be measured in several ways. This is most frequently done by use of Seger cones, the thermoelectric pyrometer, or an optical pyrometer.
 The Seger cones consist of small slender pyramids of known composition (clay- and fluxes) which fuse at definite temperatures. They are arranged to represent a series of fusion points, each cone melting at a few degrees higher than the next one below it'in the series. In actual practice these cones are placed in a protected place in the kiln where they will not be in the direct flame and yet will receive the average heat from the fuel. They are placed in such a position that they can be watched through a peep-hole. If it is desired to reach a certain temperature in a kiln, a cone representing the temperature and two or three lower cones are placed in the kiln in such a way that they may all be observed. The lower cones melt at lower temperatures and therefore indicate that'the desired temperature is being approached. For example, cones 1, 3, and 5 may be used. When Nos. 1 and 3 are bent over in burning and No. 5 is still standing erect the temperature is between cones
 3 and 5.
 The cone cannot be regarded as an exact measure of the degree of temperature. It in reality measures pyrochemical effects instead and represents the conditions of time and heat which will. accomplish certain results in fusion and vitrification. The cones usually melt quite close .to their theoretic fusion point, if heated slowly.
 These cones have had a wide application in the ceramic industries and are almost invariably used by the manufacturers.of high-grade products. They are used in several of the brick and pottery plants in Florida.
 Throughout the United States the following cone numbers are commonly used for the classes of ware indicated:1

Common brick ....................................................... 012-01
Hard burned, common brick.......................................... 1-2
Buff front brick ...................................................... 5-9
Hollow block tile and fireproofing .................................... 03-1
T erra-Cotta .......................................................... 02-7
 'Ries, H., Clays, Their Occurrence, Properties and Uses, p. 187, 1908.