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Webster 1913 Edition

Noun.

Making hard or harder.

That which hardens, as a material used for converting the surface of iron into steel.

ppr.

Making hard or more compact; making obdurate or unfeeling; confirming; becoming more hard.

Noun.

The giving a greater degree of hardness to bodies than they had before.

hardening

hardening ‎(plural hardenings)

The process by which something hardens.
That which hardens, such as a material used for converting the surface of iron into steel.

process

that which hardens