How do you make placticine go hard?

To make plasticine harden, you can expose it to air for an extended period, as some types will dry out over time. Alternatively, you can use a solidifying agent like a polymer clay or bake it at a low temperature if it's a suitable type of modeling clay that allows for hardening. However, traditional plasticine is designed to remain soft and pliable, so it's important to check the specific product instructions for best results.