The crystallisation of a glass-ceramic enamel system in which two modes of crystallisation are observed is described in some detail. During the coating fusion, uncontrolled, spontaneous devitrification of kalsilite occurs throughout the glass. In a post heat treatment process a controlled, intentionally nucleated crystallisation of the matrix occurs. The preferential diffusion of iron from the substrate, through the glassy phase of the biphasal system during fusion is observed. The implication of this behaviour on adherence is noted.