You might try throwing it into the freezer for a bit: usually plasticines which can be melted (kind of a weird thing to say, they all can be melted) will also harden if you chill them. You can control the hardness of the result by how long you chill it and how cold the environment is in which you place it (eg. refrigerator vs freezer). Hope this helps.
Thank you for your advice. Yes, it's good way to temporary measure but I'd like to make Claytoon hard permanently like Oven clay (Baked clay?).
If I can do it, I think it's very good for replacement heads or mouths......
Yesterday we released a book through Animateclay's store that covers all the techniques for hardening clay. It is available here: