698 GEOLOGY OF OHIO. 
if a clay fills the other conditions required, it is liable to be impure like 
a stone-ware clay. 
There are three grades of fire-brick which can be recognized. The 
first or No. 1 brick is the best and most refractory, and is intended for 
the severest use, such as the hearth and boshes of the blast furnaces, 
the exposed parts of puddling furnaces and steel-mill plants. Its 
application enforces the use of a very large proportion of calcined and 
flint clays with the least possible plastic clay which will bind them to- 
gether. In several places the mixture is composed of about half and 
half of each of these with no plastic, and the mixture is ground very 
severely in a heavy wet mill for a long time. 
It is claimed by the makers of this class of brick that any flint 
clay, being reduced to a fine enough state of division, under violent 
friction, will become plastic, and it is certain that the clays which they 
use do so, but is still an open question if all flint clays will. The more 
usual charge for a No. 1 brick consists of about 45 per cent. of both 
flint and calcine and 10 per cent. plastic. 
The cohesion among the particles of such a mixture is very slight, 
and very light friction suffices to shell the brick up into its component 
parts; it is of course unfitted for use in any position when friction will 
be brought to bear, but at the intense heat at which they are used, the 
softening of the clay makes them as cohesive as need be, and in that 
state the friction of matter as highly heated as the brick has but little 
effect. 
The next well-marked grade of brick has neither name nor number 
among its makers. It is composed of about equal amounts of both flint 
and calcine, and about three times as much plastic as the No. 1 brick. 
Its proper uses are the same general kind as those for the No. 1 
brick, but the product is a little inferior to it. The next grade, or No. 
2 brick, is made of about 50 per cent. of plastic, 20 per cent. calcine 
and 30 per cent. flint clay; it has a homogeneous appearance on its 
fracture, is close-grained and emits a sharp ring when struck with 
another brick. Such a brick will sometimes stand a very fair heat with 
no symptons of fusing, but, as a rule, it is not fitted for any responsible 
position. It is excellent material from which to make kilns, etc., except 
the hottest parts. What might be denominated a No. 3 brick consists 
of a mixture of several plastic clays, or else a body made of one plastic 
grade. ‘They are generally vitrified slightly in the burning heat of a 
