Restoring an old fireplace mantel can require a lot of messy work and if you are missing any parts you will have to either search for original parts or have them reproduced.  This mantel and a duplicate mantel on the parlor floor of the same building required 4 days of cleaning, scraping and a light sanding to remove all the yellow/black soot and dirt.  Tile Lab Heavy Duty Cleaner and Stripper was applied and then Soft Scrub w/Bleach was subsequently applied with water and a very fine sandpaper.  The sand paper did remove the polished finish but it was the only way to get the marble this white.  One mantel is missing the marble keystone and top.