GEOLOGY OF OHIO 229 



In theory, it is much better to use fire brick which have already 

 passed through severe heat in burning, for most purposes, because their 

 shrinkage is thus brought to a very small amount, and shrinkage of the 

 brick of large pieces of brickwork is a frequent cause of very expensive 

 repai s. 



The shrinkage to clay bricks of all kinds is the main reason for their 

 abandonment for use in steel furnace roofs which are now built almost 

 entirely of silica brick which expand when heated. 



The use of soft burned fire bricks is sometimes justified by the theory 

 that clay ware will stand just so much heat, that each time bricks are 

 brought to that heat their life is so much shortened, etc. 



Another reason why soft brick in a certain proportion are useful is 

 found in the case of cutting and trimming them to fiit in odd shaped work. 

 Fire brick laying has a great deal of fitting and adjusting to do and hard 

 bricks offer great trouble and delay to the operations of the bricklayers. 



The effect of high heat in burning fire brick ought never to produce 

 any signs of vitrification in its particles ; if it does, the clays are not of 

 high grade. Bricks like the Benizette, Woodland, Mount Savage, Solid 

 Crown, etc., cannot be made to show the effects of heat in any such tem- 

 perature as is developed in the burning kiln. 



The temperatures actually employed in burning fire brick are very 

 much the same that are used in burning other clay wares, but the range 

 of temperatures is greater for the obvious reason that the excess temper- 

 atures will do no harm other than the waste of a little coal. 



The heat which fire brick receives in burning, however, is sufficient 

 to effect some changes in the impurities. Iron, for instance, in any hard 

 burned fire brick is apt to indicate its presence by a black blotch of cinder 

 or silicate of iron. The ends and exposed portions of the bricks often 

 show a black or brown incrustation, commonly ascribed to the "sulphur"' 

 in the coal, which is in reality a fused layer of fine coal ashes drawn into 

 the kiln by the draft and attached to the surface of the brick while sticky 

 by heat. 



The regular manner of fire brick manufacture has been briefly set 

 forth. One important addition to this statement has now to be made. 



This is the application of the dry press machinery to fire brick mak- 

 ing. The origin of this departure from the old customs of the fire brick 

 business probably came about through the use of dry press machinery 

 by several firms who are engaged in making fine buff and other light 

 shades of building brick from fire clays. 



However, this may be, the process is in operation at the works of 

 the Dover Fire Brick Co. near Strasburg. The clays used are a very 

 hard flint clay from the lower Kittanning horizon and various plastic fire 

 clays found associated with the flint clay and in other horizons. 



The mixture is ground in a dry pan and screened to a rather fine 

 powder and is then run through a "Steamer" which is a tempering device 

 patented by Mr. C. Arnold, the manager of the works. 



