This section is from the book "American Library Edition Of Workshop Receipts", by Ernest Spon. Also available from Amazon: American Library Edition Of Workshop Receipts.
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.
(Monit. de la Ceramique et de la Verrerie.)
 
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