Kodak Professional Fixer is a general-purpose hardening fixer for films and papers. This comes as a powder, and mixes to working strength.
The Brownie Camera Guy says:
When I use a hardening fixer, Kodak Professional Fixer is one of three that I use, Ilford and Tetenal liquids are the other two. Because it’s a powder, the shelf life is virtually endless making it great for emergencies. I split 3.8l packages, by weight, into either 1.9l or 0.95l zip-lock baggies so that I mix only what I need.