A PRELIMINARY REPORT ON CLAYS OE ELORIDA 
97 
Conduits . 7-8 
White earthenware .... 8-9 
Fire bricks .. 5-14 
Porcelain . 11-13 
Red earthenware .. 010-05 
Stoneware .... 5-8 
Electric porcelain . 10-12 
For the composition and fusion temperatures of the Seger cone 
series see Appendix B. A comparison of various temperatures used in 
the different kinds of products is shown in Fig. 1. 
The thermoelectric pyrometer is to a great extent replacing the use 
of the Seger cones in measuring temperatures. The greatest difficulty 
with its use is the cost of the apparatus. The type most widely used con¬ 
sists of two wires, one of platinum and the other of an alloy of platinum 
and rhodium or platinum and iridium, which are fused together at one 
end while the free ends are connected with a galvanometer. One of the 
wires is encased in a small quartz tube to insulate it from the other. 
The two wires are then placed in a larger quartz or porcelain tube which 
is inserted into the kiln. The heating of the thermopile in the furnace 
generates an electric current which is conducted to the galvanometer 
which measures the intensity of the current. The degrees of temperature 
are measured by the amount of deflection of the needle of the 
galvanometer. The galvanometer may be installed in the office some 
distance from the kiln and the temperature may thus be observed at will. 
Some types of this instrument have an automatic recording device. 
The optical pyrometer is used to some extent in measuring tempera¬ 
tures. There are several forms on the market. The principle of some of 
these is to compare the intensity of light in a furnace with that from 
some other source. In others the light emitted from the furnace is re¬ 
fracted by means of prisms and a comparison is then made with some 
standard. 
POROSITY 
The porosity of a clay is the volume of pore space existing between 
the clay particles and thus depends upon the size and shape of the 
particles making up the mass. Spherical grains would give a maximum 
porosity, but clays of this type are probably unknown. In clays the grains 
vary in size and are irregular in shape, which greatly reduces the poros¬ 
ity. It has been shown, however, that increasing fineness means increas¬ 
ing pore space. Porosity determines the amount of water a clay will 
