I live in a condo that was built in 1980. I've replaced a few switches and outlets around the unit. Every single electrical box is filled with chips of this material, and the wires are coated (either a little or completely) with what I assume is sprayed-on primer that leaked into the boxes.
The material was already at the bottom of every box when I removed the cover plate.
I have limited experience with building materials, but it looks like plaster - it crumbles like chalk under pressure, snaps if broken and dissolves in water.
Is it likely to be plaster, or some kind of fire retardant or insulation that was once in fashion?
Should I clean out the existing electrical boxes? Most haven't been opened since the unit was built.
How did they get there? Is it just laziness on the part of the original electricians?
I know this is a bit vague, let me know what other detail I can add.