[Arran]

I agree with all, freeing up the innards of old filter caps and even worse, potted transformers can be a dangerous, frustrating, and absoluetely wretching experience. First of all, safety glasses. Please. Outside, yes, always. No flip flops and t shirts need apply. You can't always predict where they will burst, and it may not be where the "safety" vents are. And of course you will permanently pollute any cooking appliance like a toaster oven.

But it is fun.

You need to soften it enough to pull out the contents not liquify every last blob of wax and tar in the can in fact if it melts too much that makes it messier and harder to remove. But to avoid any issues use a second hand hot plate or toaster oven, although with a hot plate I don't really know how you would foul it anyhow since it's basically an open electric heating element like a stove would have.
Regards
Arran