I found a small white patch in a plaster wall. All of the other holes in the wall had been patched with joint compound. This one was patched with some other material that did not crumble into powder, but came out in chunks that looked a bit like styrofoam and were almost as lightweight.
Inside were electrical wires crossing over a two-by-four. The plaster and lath had been removed to make space for the wires between the two-by-four and wall. The space was too shallow to allow a drywall patch so it had been filled in with the foamy stuff.
I don't know what the foam filler was and why they didn't use joint compound. Is it because it goes on wet (although the wires are insulated)? Because it gets so hard it would be difficult to dig the wires out in the future?
How should I close the hole--should I chisel a groove into the two-by-four so there will be enough space to use a drywall patch? If the foam is the best solution, please tell me what the stuff is called.