8. Procedure
8.1 Shake thoroughly the sample to be tested, first heating to 50° more or less 10°C for 0.5 h when necessary to reduce its viscosity. Immediately following the heating and shaking, filter test portion through a 100 mesh screen. Weigh to the nearest 5 mg a 10-g sample of the oil to be tested, free of moisture and other suspended matter, into a tared porcelain or silica crucible containing two glass beads about 2.5 mm in diameter. Place this crucible in the center of the Skidmore crucible. Level the sand in the large sheet-iron crucible and set the Skidmore crucible on it in the exact center of the iron crucible. Apply covers to both the Skidmore and the iron crucible, the one on the latter fitting loosely to allow free exit to the vapors as formed.
8.2 On a suitable stand or ring, place the bare Nichrome wire triangle and on it the insulator. Next center the sheet-iron crucible in the insulator with its bottom resting on top of the triangle, and cover the whole with the sheet-iron hood in order to distribute the heat uniformly during the process (see Fig. 1).
8.3 Apply heat with a high, strong flame from the Meker-type gas burner, so that the pre-ignition period will be 10 more or less 1.5 min (a shorter time can start the distillation so rapidly as to cause foaming or too high a flame). When smoke appears above the chimney, immediately move or tilt the burner so that the gas flame plays on the sides of the crucible for the purpose of igniting the vapors. Then remove the heat temporarily, and before replacing adjust by screwing down the pinch-cock on the gas tubing so that the ignited vapors burn uniformly with the flame above the chimney but not above the wire bridge. Heat can be increased, if necessary, when the flame does not show above the chimney. The period of burning the vapors shall be 13 more or less 1 min. If it is found impossible to meet the requirements for both flame and burning time, the requirement for burning time is the more important.
8.4 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 more or less 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.
8.5 Remove the burner and allow the apparatus to cool until no smoke appears, and then remove the cover of the Skidmore crucible (about 15 min). Remove the porcelain or silica crucible with heated tongs, place in the desiccator, cool, and weigh. Calculate the percentage of carbon residue on the original sample.
