130 GEOLOGY OF OHIO. 



the shape of foundation blocks, roofing tiles, and even bricks for ordinary 

 construction are either required to be vitrified or become specially valu- 

 able by possession of this quality. 



In making a scientific study of the chemical qualities of clays for 

 producing vitrified materials, it is found that there is nothing about the 

 character of the clays of any one branch like paving bricks or sewer 

 pipe to distinguish them from the clays used for vitrified wares in gen- 

 eral, and it is therefore necessary to consider under one general head all 

 the clays used in producing material of this kind. 



Before making use of the word "vitrified" any further, it is proper 

 to carefully define what is meant in this technical use of the term. 



In its general acceptation, vitreous means glassy; a vitreous fract- 

 ure means a glassy fracture and nothing else. 



But in discussing the burning of clay wares, the word vitrified has 

 come to mean rather that the material has entered into the preliminary 

 stages of fusion, and that its chemical ingredients have begun, if not 

 thoroughly completed, the process of chemical union by heat, rather than 

 that the fracture of the material under question is a glassy one. The 

 appearance of the ware cannot be made the test of this condition. A 

 glassy fracture is rather the exception than the rule in vitrified wares. 

 Many wares are vitrified in this special meaning of the term, which are 

 strong in fracture. 



The test of vitrification is the ability of the material to absorb 

 water. Glass, as the type of a vitrified body, will absorb none. Clay wares 

 absorb water freely even after the shrinkage which comes with the 

 expulsion of the combined water has taken place. But when the chem- 

 ical union of the clay ingredients begins, the appetite for water begins 

 to cease, and in many clays will absolutely cease as the vitrification 

 becomes perfect. In other wares, more sandy and coarse grained in 

 nature, it is impossible to bring about such chemical union by heat as to 

 destroy the power of the material to still absorb water. 



Therefore in the special sense in which this word is used, a piece of 

 clay ware which still absorbs water freely after exposure of a high heat, 

 is not vitrified. It is as much vitrified as it will ever become by the use 

 of heat, but it does not contain those elements which make a fusible com- 

 pound. 



It therefore becomes necessary to set a standard of absorption 

 which must not be exceeded if the ware is to be called vitrified. What 

 this amount must be will be carefully considered under the head of Tests 

 for Paving Material. 



The physical appearance which enables us to conclude in a general 

 way as to the right of a ware to the name vitrified is mainly its smooth 

 conchoidal fracture, and the absence to the eye of pores in the substance. 

 There may be cracks, and fissures, and holes, for which the machine or 

 process of manufacture is responsible, but the mass must not be porous. 

 As to the fracture, the breaks or surfaces exposed follow no regular 



