The best way to hang a heavy mirror on a plaster wall is through the use of anchors according to expert contractor Bob Villa. Screws can be used but only if the mirror will be located at a stud point in the wall. There are various types of wall anchors and screws that can support heavy pictures or mirrors on walls made of plaster.
Wall anchors are helpful if the location of the mirror will be in a space that does not have a stud behind it. The stud is a part of the wall's frame that runs vertically. To place the mirror at a stud point, use at least a three-inch screw. For open sections of plaster, choose an option like a winged anchor, molly bolts or self-drilling screw anchors for best results. An option that is rated for a heavy load should be able to support 25 to 50 pounds.