Hardener n. : One who, or that which, hardens; specif., one who tempers tools.

Example:
What does Hardener in fixer do to salt prints?

Taxonomy
hardener, Nail Hardener, gelatin, substance, varnish, paint, Dictionary, bath, potassium alum, fixing, swelling, adhesive, organic chemistry, Unanswered questions, Dental Dictionary, emulsions, Photography Encyclopedia, Dictionary of the English Language.

Defination from the web:
hardener n. One that hardens, especially a substance added to varnish or paint to give it a harder surface or ... Veterinary Dictionary: hardener. Top. Home > Library > Animal Life > Veterinary Dictionary. A substance used to harden the gelatin of the emulsion on an x-ray film. ...