Plasticine

[Shipstabber]

Is there any way of making old, hard plasticine soft again?

[heathfield]

Not an easy task. What's probably happened is that the linseed oil used in the manufacture has dried out over time. You could try melting the Plasticine in a pot over a very low heat, (keeping it moving - don't let it burn!), and add a few drops of linseed oil, stirring it in well. As to how much...trial and error, I guess.
Others have had this sort of problem, and discuss it here