When the enamel becomes separated from the metal, or when the iron bends away from the enamelled side, i.e., when the contraction of the enamel on cooling, is less than that of the iron, one of the following alterations must be made in the composition of the enamel: -

(a) Increase the amount of silica.

(b) Replace part of the boric acid by silica.

(c) If lead is present, replace part of the lead oxide by alkalies or alkaline earths.

(d) Replace part of the alkaline earths by alkalies.

(e) Increase the alkalies and diminish the boric acid.

(f) Instead of tin oxide, use bone ash.

The Relation Between Tin Oxide And Bone Ash Is Of Great Importance

(Monit. de la Ceramique et de la Verrerie.)