How should I support a stone mantel on a brick fireplace?
That is a pretty big piece of rock.

It would probably be better to distribute the load by using brackets.



In the photo above, the mantle is wood, but the principle for stone is the same.

What you want to do is build the brackets into the chimney as deeply as possible. So, you actually remove entire bricks and insert the brackets deep into the chimney superstructure. Then you just rest the mantle on the brackets. You can drill 1/2-inch holes in the bottom of the mantle and the top of the brackets and insert close-fitting steel pins to join the the mantle with the brackets, so it will not slide off.