Alberta Slip cone 6 lithium brown (GA6-G1) on a red burning clay (left Plainsman M390) and buff burning (right M340). Obviously this looks better on the former where iron from the underlying body variegates the entire surface and bleeds through on contours where the glaze is thinner, creating a breaking effect.
GA6-G1 - Alberta Slip Lithium Brown Cone 6 Low Expansion
Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com.
A decorative effect in ceramic glazes that exploits changes in color and character in certain glazes when their thickness varies.