Hardness of the compacted sand mold.
A spring-loaded ball is pressed into the compacted sand mold and the hardness of the mold surface (compaction state) is derived from the penetration depth based on the change in spring length.
However, this system is imprecise as there are no springs with linear characteristics and the upper measured values are therefore distorted. In addition, the measurement may also be falsified by the elastic recovery of the sand in the highly-compacted molds typical today (see Pulsed compaction). It has become common practice, to preferably determine the mold strength (see Mold strength tester) instead of the mold hardness.
Mold sand testing
Mold material compaction