The oxidation bars (lower) are typical of most ball clays. They burn off-white and darken slightly in color as they approach maturity (above cone 10). Ball clays are fairly refractory, often as much more more than common fireclays. Ball clays almost always have some soluble salts and these reveal themselves in reduction firing (top bar).
Gleason Ball Clay
In ceramics, this term is most often used to refer to kilns firing with an atmosphere having available oxygen to react with glaze and body surfaces during firing