or gouging the metal surface should be avoided because if has been found that such surface indentations can cause spalling it is desirable to apply sufficient glass slip or slurry on the metal surface to achieve an enamel thickness of at least 10 mils with a practical upper limit of about 20 mils it has been found that spraying is the preferred method of application and that the enamel must be sprayed in a fine spray however the sprayed slip tends to run or slide on the metal surface when the wet thickness exceeds 4 or 5 mils by careful application techniques it is possible to apply 6 to 8 mils of enamel prior to each firing it has been found that multiple passes with the spray gun laying one coat over the other and careful attention to the moisture content of the slip will permit application of an optimum thickness of enamel for each firing cycle thus two coatings and firings will achieve the desired porcelain enamel thickness of at least 10 mils the enamel coating bisque on the metal surface should be permitted to dry before firing the firing parameters will vary depending on the shape and surface area being fired by way of example a 4 u00d7 4 u00d7 1 4 inch sample plate was satisfactorily fired at 1320 u00b0 f for 8 minutes also a 4 39 u00d7 6 34 u00d7 1 4 34 tubular member was satisfactorily fired at 1100 u00b0 f by alternately firing for 8 minutes with 2 minutes removed for.