AbstractThe fiber method developed by Schwalbe, Badger, and Silverman for comparing the tensile strengths of glasses was used to compare the tensile strengths of sheet‐iron ground‐coat enamel glasses whose feldspar, borax, and quartz contents varied within workable limits. For purposes of comparison, this method uses that value of the unit stress at failure which exists in a fiber whose fractured surface shows a smooth portion constituting 1% of the total cross‐section. For the enamels tested, these values of tensile strength ranged from 30.0 to 32.7 kg. per sq. mm. (42,000 to 46,000 lb. per sq. in.). An increase in quartz or feldspar tended to raise the tensile strength slightly, while an increase in borax tended to lower it slightly, although the total variation was small. From these data, it was concluded that insofar as the enamel strength was concerned mechanical failures of the enamel, such as chipping, fishscaling, or crazing, could not be aided by variations of the feldspar, borax, and quartz contents of sheet‐iron ground‐coat enamel glasses within workab