Is this plumber's putty or water putty? Plumber's putty doesn't
ever harden completely, but water putty, such as Durham's Rock
Hard, does harden. It it also used by plumbers and such, but for
different jobs. It's available at hardware stores, easy to mix and
easy to use. Kinda heavy, though.
I have used Man-in-a-Can also known as Rock Hard Water Putty by
Durham for forming and making hollow ware moulds, bead cores, etc.
The trick is to lighten it with the addition of gypsum available at a
painters supply. It is a sparkely looking flake that is added to
plaster to lighten it, and works similarly with Rock Hard putty. The
putty sets quite fast though and use only the minimum necessary in a
batch. You may add a bit of vinegar to it to slow down the hardening
time but too much and it can leave air pockets if poured. Buy the
finest grit gypsum flake you can find if you want to go that route.