96 FLORIDA GEOLOGICAL SURVEY— 15 TH 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 pyrom¬ 
eter, or an optical pyrometer. 
The Seger cones consist of small slender pyramids of known com¬ 
position (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 tempera¬ 
ture 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 burn¬ 
ing 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 prod¬ 
ucts. They are used in several of the brick and pottery plants in Florida. 
Throughout the United States the following cone numbers are com¬ 
monly used for the classes of ware indicated i 1 
Common brick . 012-01 
Hard burned, common brick. 1-2 
Buff front brick . 5-9 
Hollow block tile and fireproofing . 03-1 
Terra-Cotta. 02-7 
iRies, H., Clays, Their Occurrence, Properties and Uses, p. 187, 1908. 
