172 Frank Carney 



largely determining the change. The purpose of firing clay is to 

 make it impervious and thus prepare it to better resist weathering. 

 After the clay has been prepared, by grinding and mixing with 

 water, so as to admit of molding or pressing into the form desired, 

 this water is removed by slow drying. Next, burning removes 

 the water of hydration, and further burning increases the hardness 

 and density of the clay. But, to make the product enduring, 

 the manufacturer must know at what point to stop the firing; 

 if fired too long, the product crumbles easily. Mankind early 

 learned the art of clay working. Burned clay products form the 

 best evidence of prehistoric civilizations, and their success in thus 

 treating clays is seen in the freshness of pottery that doubtless has 

 been buried many thousand years. 



Kinds of clay in Ohio. Complexity of the clayey compounds 

 classified under the head of "clays," renders it impossible to, 

 draw fast lines between different types. In practice, a clay is 

 named in accordance with the use made of it. The same clay, 

 however, is sometimes used for different purposes ; hence clays are 

 not yet definitely classified on the basis of their constituents. 

 Indeed, two clays which are quite similar in the percentage of 

 different clay-making constituents may behave very much unlike 

 when burned; students cannot explain just why this is. Use is 

 the final criterion in handling a clay. 



The term ''fire clay" applies to those horizons that provide 

 a clay which does not melt under a very high temperature, as 

 perhaps 1600° C. This term is generally used for the shale horizons 

 that often underly coal beds. This form of cla}^ is used in making 

 fire brick, crucibles, furnace linings, and other refractory wares. 

 The "coal measures" supply the large percentage of our fire clays. 



Another form of this resource is called "pottery" clay because 

 it is used extensively by potteries. Pottery clays include a wide 

 range of materials, which are varied according to the product 

 desired. Summit Count}', perhaps, has the best reputation for its 

 potter's clay. 



Ohio produces large amounts of brick and tile. For this work, 

 a great variety of claj^s may be used; as a result, bricks vary 

 much in color and in other characteristics. Yevy many kinds of 

 clay are used in this state for making brick and tile. The compe- 

 tition does not admit of freight expenses in hauling the raw mate- 

 rial. Brick and tile plants use a clay found near the plant. 



