PROBLEM TO BE SOLVED: To provide a method for preparing an impression material for dentistry, a livestock nipple pack, and the like.SOLUTION: The method for preparing an alginate hardenability composition comprises forming a hardened substance by a gelling reaction under an aqueous medium using: calcium sulfate a hardening material paste consisting of polyglycerol fatty acid ester having an HLB of 2.0-6.0 and a poorly water-soluble dispersion medium and a substrate paste containing alginate and water, by mulling using a specific manual extrusion mulling machine 50.