Describes a test based on the progressive stiffening of rubber with time, caused by crystallization. It is limited to materials having an initial hardness at test temperature of 10 to 85 IRHD. The method is applicable to raw, unvulcanized (compounded), and vulcanized rubber. It is mainly of interest for rubber with a marked crystallization tendency at temperatures experienced in cold climates, such as, for instance, chloroprene and natural rubber. The method is not applicable to fast-crystallizing materials.