HOLLOW WARE FOR STRUCTURAL WORK. 279 



material vary somewhat in their character, many of them are 

 dark, bluish-black clays, often strongly laminated and containing 

 much organic matter, mica, and scattered nodules of pyrite and 

 limonite. They are red-burning, usually of low refractoriness, 

 and their fusion point lies not uncommonly between cone 12 and 

 15, or even lower. To> these a certain proportion of a No. 2 fire 

 clay is sometimes added. Shale is used at one locality. The 

 mixtures used burn steel-hard at cone 01 to cone 1. The general 

 physical characters of the clays can best be judged from the tabu- 

 lated statement given below. 



