A substance added to something to make it harder or more durable; a person or thing that hardens.
From 'harden' plus '-er' agent suffix, used both in chemistry and materials science, and figuratively for anything that makes something tougher.
In chemistry, hardeners unlock new possibilities—epoxy hardeners revolutionized manufacturing in the 20th century by letting workers create permanent bonds, but the same word describes what grief does to a person's emotional walls.
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