Plastilina clay, also known as plasticine, is an oil-based, non-hardening clay, which means it is designed to remain flexible and malleable. Unlike water-based clays or polymer clays like Sculpey, Plastilina clay does not harden when exposed to air or heat.

However, if you're looking to temporarily harden Plastilina clay for sculpting or to keep its shape, you can try the following:

1. Chill the Clay: Place the sculpture in the refrigerator or freezer for a few hours. Cold temperatures will make the clay firmer and more rigid, but it will become soft again when it warms up to room temperature.

2. Support the Structure: For more permanent pieces, some people build an internal armature (a wire or skeleton frame) to provide support. This won't harden the clay itself but will help it hold its shape better over time.

Unfortunately, Plastilina clay cannot be baked or chemically hardened, so it’s best suited for temporary or reusable projects where flexibility is needed. If you want a permanent sculpture, you may need to switch to an air-dry or polymer clay that hardens naturally or can be baked.

Plastilina is one brand of plasticine-type clay (sometimes called *true* modeling clay), but no plasticine-type clays can be hardened because although they are oil-based like some other kinds of clay that can be hardened, they also contain an added wax added which means they'll just melt if exposed to even low heat.

Does plasticine Harden? Plastilina clay is a strictly modeling material with oil and wax in its composition. The wax and oil make it permanently pliable – there's no way to permanently harden it. The best way to temporarily harden it is by placing it in the fridge for around half an hour.

If you're talking about water-based pottery-type clay, you can harden it by firing it in a kiln. There are bake-hardening modeling clays such as Sculpey and a few other brands, which harden by baking in an ordinary kitchen oven.

Plastalina and Plastilina are two brands of true modeling clay (plasticine-type clay) so they can never be hardened