132 GEOLOGY OF OHIO. 



As a rule, the conditions which afford the easiest success and the 

 least trouble from this cause are when two or more clays can be economi- 

 cally mixed, one of which shall give to the mass a plasticity and obedience 

 to the influence of pressure which will allow it to be worked without 

 great expenditure of mechanical energy and with satisfactory smoothness 

 of surface and finish, while the other ingredient shall, by its rough, gritty 

 and granular nature, prevent the too free and rapid motion of the parti- 

 cles on each other, and the ensuing faults of structure. It is rarely that 

 a clay or shale is found which in itself unites these fire qualities and 

 plasticity, while the number which can be made satisfactory by judicious 

 mixing is very large. 



3d. In addition to these all-important qualities, the amount and 

 character of the iron is worthy of mention. On the state and quantity of 

 the iron depend two things (a) the color of the ware, (b) the smoothness 

 of the surface. 



If the iron be too low in quantity it is impossible to obtain a good 

 red brown or dark colored ware, and it is the general demand of the 

 makers and ihe buyers of vitrified goods that they shall be dark in color. 

 Many people will wrongly reject an article on the ground of its light ap- 

 pearance or light colored fracture, while at the same time it may be 

 vitrified past the point of its best qualities. 



Also, if the iron be present as grains of iron ore or sulphide, it not 

 only does no good to the general color of the ware but it does great harm 

 by the formation of blotches of silicate of iron which destroy the surface 

 smoothness and damage its color and appearance. This holds good more 

 of sewer pipe than it does of paving brick, because smoothness of surface 

 is a really valuable quality in securing the flow of thick and sluggish 

 fluids like sewage. 



These three qualifications are the important ones to consider in the 

 selection of a clay for use in making vitrified wares. The conclusions as 

 to these questions may be, perhaps, indicated by analysis, but must come 

 to actual trial to form a proper basis for confidence. 



The clays which are actually found in use in the state for these pur- 

 poses may be divided into three classes : 



1st. Shales, Carboniferous and others; 2d. Impure fire clays; 

 3d. River cla^-s, or sedimentary deposits of recent geological date along 

 the valleys of the large rivers. 



The drift clays, brought here by glacial agencies, do not figure as 

 available material, for it is only in small patches and isolated cases that 

 these cla3^s are used at all. They are, as a rule, too readily fusible, or 

 they fuse too quickby when they begin to fuse, and in addition they are so 

 contaminated with lime in small pellets that they are likely to ruin the 

 wares in which they are used. 



The following analyses have been made in the Laboratory of the 

 Surve3 7 , and collected from other sources, which illustrate the character 

 of shales and clays used for vitrified ware: 



