I found an ineresting way to detail a mirror is to "float" it leaving a gap betweeen the mirror and the frame. The backer sheet is plywood , painted black. Mirror mastic is applied and the mirror is allowed to set with weight on till dry (2 days) The 1/4" plate mirror usually comes with a tiny bevel which is required since it is not covered with a frame. below are 2 examples. on heavier frames the plywood can set in a rabbit.