The porcelain glaze is a borosilicate glass coating, which is usually applied and then melted on metallic surfaces, i.e. cast iron, steel, aluminium blends. The complex system made by the metal and its porcelain glaze is generated during firing and it shows two materials joined together, but with different chemical and physical properties. Thissituation causes a state of tension because of diffent thermal behaviours of the two layers. Thiw issue was worled out by Timoshenko in 1925 who developped an equation to calculate bimetallic rods deformation at temperature variation. The different behaviour of these rods is due to their different coefficient of thermal expansion. However, if you study a more complex system like metal-glazed tile, you must take into account also the physical transformation of the glaze.