For black & white films and prints, Sprint Record Alum Hardening Solution converts Record Speed Fixer into a hardening fixer, or Block Stop Bath into a hardening stop bath.
Unhardened emulsions are easily scratched when wet, and absorb moisture when dry. Alum hardening in the fixer is one method of toughening emulsions to be more resistant to abrasion and water absorption.
If used correctly, it will increase archival permanence by protecting emulsions. All B&W films require alum hardening in the fixer.
Resin-coated Prints require alum hardening to prevent scratching, and for a hard gloss on glossy finish papers.
Fiber-based Prints should only be hardened under certain circumstances: if prints are to be dried on hot drums, or if emulsion is very soft.
Fiber prints are hardened in manufacture, and by heat, light, and air, so additional hardening may be harmful.
Dilutions: Add 15 ml per liter of 1:9 fixer add 30 ml per liter of 2:8 fixer.