How can you harden an oil based modeling clay?

Useful for many moldmaking and sculpting undertakings. Unlike water-based clay, oil-based clay will not harden and may be used repeatedly. It is ideal for temporary molds and models. Good for making dams when molding and casting. Soft enough to make impressions of leaves or other habitat materials. Several leading bird taxidermists, most notably Ed Thompson of Lilburn, Georgia, like to use oil-based modeling clay to set the eyes in their bird mounts. This gives them the freedom to slightly modify the eye angle or depth after the bird has begun to dry. Roma Plastilina Modeling Clay is the number one choice of discriminating sculptors everywhere. Grayish-green color, medium firm.

* http://www.taxidermy.com/cat/16/oilclay.html