When the vapors cease to burn and no further blue smoke can be observed, readjust the burner and hold the heat as at the beginning so as to make the bottom and lower part of the sheet-iron crucible a cherry red, and maintain for exactly 7 min. The total period of heating shall be 30 +- 2 min,which constitutes an additional limitation on the tolerances for the pre-ignition and burning periods. There should be no difficulty
in carrying out the test exactly as directed with the gas burner of the type named,using city gas ( 20 to 40 MJ/m3 ),with the top of the burner about 50 mm below the bottom of the crucible. The time periods shall be observed with whatever burner and gas is used