My house has several canister lights that aren't working. I'm pretty sure the problem is in the sockets. I can see that one light's socket is filled with some kind of white powder (plaster? deteriorated attic insulation?) that covers the hot contact so i can't even see it.

I need to replace the sockets, but I can't get them out. They're generic porcelain sockets that pass through a hole in each canister's back plate. It seems to me that when the screw that attaches each back plate's mounting bracket to the interior of the canister is removed, the back plate and socket should drop below the rim of the canister so I can get at the wiring and replace the socket, but it doesn't happen. Instead of the back plate and mounting bracket holding the socket in place inside the canister, the socket seems to be more or less holding the back plate in place.

I think I need to remove the porcelain ring that attaches each socket to the back plate so I can see what's holding the sockets in place and fix it. But I haven't been able to remove any of the rings with any amount of force that I can apply by hand.

Is there some kind of tool I can use to remove the porcelain rings without breaking them? Or is breaking them what I need to do? If it is, what kind of tool should I use to do that?