BS 5930 (1999)+A2 (2010) classifies the consistency of a sample of clay as follows: very soft (exudes between fingers), soft (easily mouldable), firm (strong finger pressure required to mould), stiff (can be indented slightly by thumb; crumbles when rolled into a 3mm thick thread), very stiff (indented by a sharp object) and hard (difficult to indent but can be scratched by thumbnail).

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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