STATE GEOLOGIST. Silbli 
its pores. The third kind of clay, called flint clay, is a very hard 
infusible clay showing but slight plasticity, but great density. Its com- 
position is almost identical with that of the kaolin. The latter must be 
ruled out owing to his porosity and we are to deal therefore with the 
plastic bonding clays and the flint clays. The latter, in themselves dense, 
would appear as the ideal refractory clay material, but there is this prac- 
tical difficulty to be considered that, owing to their slight plasticity, they 
cannot be made into bricks by the ordinary processes. ‘They require a 
certain amount of plastic clay to allow the bricks to be molded. ‘The only 
method by which the amount of bond clay can be decreased is to press 
the bricks made from flint clay with a little plastic clay on a dry press. 
But this method, lacking the plastic bond developed by the use of water, 
results in a more or less porous brick. It might now be suggested that 
the flint clay be ground so coarse that its grains are difficultly attacked 
by the calcareous slag, but this again is not feasible, as then the bricks 
would become too friable and would not stand the mechanical abrasion 
they receive in the rotary kiln. 
We are reduced, hence, to the use of a mixture of flint clay with as 
much plastic clay as will produce a body which is dense, but not brittle, 
and it is essential that the bricks are burnt as hard as possible in order 
that all the shrinkage may be taken out and they will not shrink in the 
rotary kiln. The addition of burnt clay grog to the brick mixture is 
not necessary unless certain shrinkage flaws show in the bricks. The 
sizing of the clay grains is not given the attention by fire clay 
Ie IChEntiemSUDTeCtm@esehves une n DOdyamtst: not woes made 
up of all coarse grains nor of all fine, but of a combination of both with 
a preponderance of the coarse grains. By coarse are meant grains 
between 10 and 20 mesh sieve, by fine those passing the 40 mesh.. 
In regard to the chemical compositon it is important to note that 
Seger’s results require for an ideal refractory clay the composition (when 
burnt) : 
Alumina, 46.2% =ALO.aSIO. 
Silies, BA S% 
Any imcerease im silica above this amount will reduce the refractoriness, 
down to 17 equivalents of silica to one of alumina; at this point the re- 
fractoriness again begins to rise, finaly reaching the fusing point of silica. 
Seger also expresses the comparative refractoriness of a fire clay by the 
formula : 
a 
Refractory quotient=(a+-b)-— 
b 
where a= the molecular ratio of the alumina to the fluxes (RO), that is, 
equivalents of alumina divided by equivalents of fluxes (RO) and b=the 
molecular ratio of the silica to the fluxes. 
Seger, however, makes a special point of cautioning against the neglect 
of the importance of the physical condition and calls attention to the 
