CLAY WORKING INDUSTRIES. 141 



Shales, enormous in extent and area, difficultly plastic, but easily 

 vitrifiable. 



The fire clays, less in volume and accessibility, but still present in 

 large quantity; fairly plastic, but more difficultly vitrifiable. 



Sedimentary clays, limited in area but of enormous bulk in their 

 district, the cheapest of all to dig and prepare — too plastic and easily 

 vitrifiable. 



There are instances in each division of clays which fill the wants of 

 the manufacturers just as they are naturally found. It can hardly fail, 

 however, that as the industry gets older and the qualities of the output 

 become further tested by use, that the brick makers of the future will 

 avail themselves of the advantages of mixture. Each class of minerals 

 is deficient in some quality. Any two can be united to better effect than 

 either alone, and in large areas, the geological conditions admit of this 

 being easily and cheaply done. 



The prepa?'atio?i of clays — As in the selection of clays for the fabri- 

 cation of vitrified ware, we have seen that no line can be drawn between 

 the various industries represented, so also in the preparation of clays for 

 the actual manufacture of the different wares. The methods of prepa- 

 ration are common to all. 



The treatment varies more with the peculiarities of the clays them- 

 selves, than it does from the character of ware to be made. For instance, 

 it is true that more tempering and plasticity is required in making sewer 

 pipe than brick, yet the variations in tempering different clays for brick 

 are greater than the general differences between the tempering in sewer 

 pipe shops and paving brick factories. 



Both shales and fire clays are minerals in which the natural plasticity 

 has been to a large degree lost, and it can be brought back only by the 

 expenditure of power. 



The general treatment for both involves grinding the material in its 

 dry or natural condition to a powder, more or less fine, and tempering 

 the powder to the desired consistency with water in pug mills or wet 

 pans. 



Grinding — Dry grinding is universally accomplished in Ohio by use 

 of the machine called a Dry Pan. Preliminary to the grinding, the clays 

 are sometimes crushed to a uniform size, but this is unusual and in most 

 places unnecessary. 



The Dry Pan is a horizontal iron pan, revolving on a central vertical 

 shaft and driven by a heavy gear wheel above or below. In the pan are 

 placed two heavy iron mullers or wheels, with faces from six to fourteen 

 inches wide and weighing from 2,000 to 6,500 pounds each. These are 

 held in position by horizontal axles which are arranged to slide in 

 grooves up and down so that the wheels may run up on the top of any 

 thickness of clay that may be introduced into the pan. These wheels 

 revolve only by the tangential friction of the rotating pan floor, and 



