'Tis right Oz, toasting the sculpey wasn't a good idea. However, you could use plaster of paris for a break-away type mould, or cuttlefish shell (available at pet stores for birds) two of these shells and a bit of strong wire makes an excellent "clamshell" type mould as it can be carved with a good sharp knife (sprues), or "profiled" by clamping the original article between the shells and imprinting the design that way. (just make sure that your original is strong enough to withstand the process)
Hope this helped
-AzureLupine