Agree with some things Hope said. It's basically impossible to harden plasticine-type clay, although you could coat with polyurethane or another water-based clear finish (including water-based nail polishes) and then be careful never to move the clay since the dried "shell" of finish on the clay would crack, etc.
There is one guy somewhere who claims to have a way to harden plasticine-type clay but can't say how well his method works and in which situations, how much trouble it is, etc.
In the future, if you want a clay that can be hardened, use another oil-based clay like polymer clay or epoxy clay, or a water-based clay like any of the air-dry clays.
Polymer clay (brands like Fimo, Sculpey, Cernit, etc) has the advantage though that it won't cure/harden until you do it intentionally by exposing it to sufficient heat (e.g., in a home oven for a short time).
Air-dry clays will automatically dry with air-exposure, and epoxy clay will cure after its two parts have been mixed together.