CLAY MANUFACTURES. 699 
kiln, and precaution must be taken to keep them from sticking together. 
They are excellent for making flues, pavements, boiler settings, chim- 
neys, etc., and as they can be well made by a machine they ought to be 
furnished at low rates. The burning of these various grades of brick 
demands considerable technical skill. The products exhibit many 
phenomena which are very interesting; iron, in particular, is noticed in 
the black blotches which its union with silica has caused. | 
Often a nail, bolt or some stray piece of iron gets into a kiln of 
brick. Its effects can be seen in a large, conical hole in the brick lined 
with the black cinder of iron and extending downward as far as the 
iron lasted. The blackening of the faces of the arch brick and those 
most exposed to the direct heat of the fires has often been mentioned 
by brick men as being the result of impure fuel and sulphur in the 
coal. This explanation is incorrect; sulphur, 1. e., sulphide of iron, 
when burnt in a grate would decompose to SO, or sulphurous anhydride, 
and in that state would pass into the kiln. The only effect the gas 
could possibly have on hot silicate of alumnia or any body likely to be 
present in clay would be to recombine in the same state from which it 
has just been expelled by a less heat than is met in the inside of the 
kiln. The true explanation is this: the heat on the brick that are 
blackened is more intense than on any other part of the kiln, and they 
are rendered softer and nearer to fusion; while in this pliable state, the 
draft from the fires just outside carries in a very appreciable amount of 
dust and ashes which lodge on these portions and flux outside to the 
black crust seen. The burning of terra cotta in the Ohio Valley gives 
some interesting points in proof of this view. 
Among other orades of brick made are several patent mixtures and 
various secret compositions called “silica” bricks, etc. Noticeable 
amongst them is the patent brick, made by Mr. J. Park Alexander, of 
Akron, which is used in the roofs of reverberatory furnaces and in steel 
works. The brick is made of a peculiar form of silica cemented by 
caustic lime. The bricks are very delicate, and are easily crushed by 
the hand, but once in place they are considered a very superior article. 
The great preponderance of silica in their composition, makes them 
expand under heat instead of contracting, as a pure clay does, so that 
when built in an arch so flat that there is trouble in keeping it up, the 
heat of burning causes the arch to bow upward from the expansion 
instead of sinking in as it would if the brick contracted. 
