224 GEOLOGY OF OHIO 



The charge used to produce a brick of good physical strength and 

 moderate fire properties is often about 50 per cent, plastic fire cla> and 

 50 per cent, of calcined and raw flint clay. 



Often mixtures, based on these same considerations and compounded 

 with reference to the kind of work they have to do, are also made; some, 

 between the two just quoted and some, composed of still larger propor- 

 tions of plastic clays. 



There is a class of brick called mill brick which are used almost 

 wholly in lining puddling furnaces and like structures and these brick have 

 to stand quite severe abrasion from the tools of the puddler and firemen 

 and the scorifying influence of the iron slags. These bricks are gener- 

 ally composed of all plastic fire clay which makes a brick of great tough- 

 ness though of rather low heat resisting power. In some few places, 

 paving brick and mill brick are made from the same clay in the same 

 factor)-. This is a mistake, for it is a certain fact that a clay which will 

 make good paving brick cannot make good fire brick and vice versa. 



The compounding of the charge of these various clays is accom- 

 plished in most cases by use of a shovel. The clay is loaded into wheel- 

 barrows, so many shovelsful of each kind. In only a very few cases is 

 the compounding done by weight. 



The preparation of the clays. The makers of firebrick usually lay 

 considerable stress on the weathering of the clays, some urging that a 

 great purification ensues from so doing. 



The experiments and experience of potters have shown pretty 

 clearly just what purification may be hoped for by the exposure of clays 

 to weather. 



Weather acts in two ways, one physical and one mechanical. Phy- 

 sically it breaks the hard clays up into fine grains which greatly reduces 

 the labor of grinding. The soft clays are rendered tough and plastic by 

 their exposure to weather. Chemically, the atmosphere oxydizes sul- 

 phide of iron to sulphate which the rain water dissolves and carries 

 away. In addition to sulphate of iron, sulphates of lime and magnesia, 

 existing as such or formed by metathesis from the sulphate of iron and 

 carbonates of lime and magnesia, are soluble in water and are removed 

 by rain. Any changes in the other iron constituents or the potash and 

 soda compounds must not be expected, as it has been shown that wash- 

 ings clays in the pottery processes does not diminish these constituents 

 by solution. 



These oxydizing and washing effects are very gradual indeed; the 

 conditions under which they proceed most rapidly are very seldom pre- 

 sent in the stock piles of clay companies and as a matter of fact, the 

 benefits of weathering are almost wholly confined to the improved 

 mechanical condition of the clay for the machinery. 



