The small scale free swelling index test is used to evaluate to what extent a coal will swell during coking, thereby helping to assess whether the coal is suitable for the production of coke and the amount necessary to fill the production plant. This involves heating a small sample of coal in a covered crucible to 800°C. The shape of the coke button obtained is classified by comparison with the outlines of a set of standard profiles. The results may be used as an indication of the caking characteristic of the coal when burned as a fuel.

SNF is designed to comply with BS ISO 501:2012 (Hard Coal – Determination of the crucible swelling number) or ASTM D720-15 (Standard test method for Free-Swelling Index of Coal)