I wanted a unique way to hang various items like coats and bags so I found different old doorknobs that I liked the look of and five different escutcheon plates. At the hardware store I bought five 4-inch long screws, some heavy-duty drywall anchors, and some quick-set epoxy.
I removed the set-screws from the side of the doorknobs to remove the shaft. I stuffed cotton or paper into the large voids inside the doorknob so that there was room for the screw with some generous wiggle room. I put epoxy into the void of the doorknob then set the screw deep down into the doorknob as far as it would go. I had to carefully hold the screw straight until the glue cured.
I installed drywall anchors then screwed the doorknobs into the anchor with the escutcheon plate sandwiched between. I think using a chain to hang the mirror above the fireplace on a doorknob is an interesting detail.