Fishscale is delayed chipping of the fired & cooled enamel finish. During firing, hydrogen enters the steel in the atomic form. Hydrogen is introduced into the steel from various sources, such as in the form of moisture in the furnace atmosphere, improperly dried bisque, water in clays, residue left during metal preparation, & products of combustion. During firing, the moisture reacts with the steel. As the temperature increases, solubility of atomic hydrogen (H) in steel increases to as much as 1000 times room temperature. As the enamel cools, the solubility of hydrogen decreases. Hydrogen that dissolved at the higher temperature now begins to diffuse & form molecular hydrogen (H2) at the steel/enamel interface. If pressure of hydrogen gas builds up enough to overcome the enamel adherence/strength of the enamel itself, then fishscale will occur.
