Noun
English Wikipedia has an article on:ball clayWikipedia
ball clay (usually uncountable, plural ball clays)
A type of clay that commonly consists of 20-80% kaolinite, 10-25% mica, and 6-65% quartz.
This clay has many of the characteristics between fire clay and ball clay, having finer grain, like ball clay but is more heat resistant like fire clays. Source: Internet