Lay the mirror on a flat surface protected by a drop cloth or newspaper with the reflective side down.
Scrape off the gray backing using a sharp razor blade or by rubbing it with muriatic acid.
Spray a commercial antiquing product on the silver backing to "distress" it or spritz bleach on the mirror. This creates a pattern of black dots on the surface of the mirror called "foxing."
Wipe the product using a soft rag across the back of the mirror to create a cloudy surface.
Drizzle and pour the solution on the back of the mirror to create a marbled look using a cup.
Seal the back of the mirror using paint or shellac after the effect has dried.