The sample is heated in a covered crucible under standard conditions to a final temperature of 820 +- 5 °C. The coke button obtained is classified by comparison with the outlines of a set of standard profiles. The number of the profile most closely corresponding to the coke button obtained is the crucible swelling number. Gas or electrical heating may be used. When using an electrically heated apparatus, the results obtained should be the same (within 1/2 unit) as those obtained by the gas heating method.