u/alotofdrugsandbeans

Hello! Long time user, new alt account. We have a house in the Midwest built around 1915. There have been many questionable upgrades over the years and we are working on restoring some of its historical charm.

Today we were removing a cheap, builder's grade facade from our fireplace. The faux stone facade was on some cement board mounted into a frame over the fireplace. (Last photo is an in progress photo)

This is what the bricks look like under the facade. I have never seen anything like this and Google hasn't been helpful. What is going on with these bricks? Are they interior, fire-grade bricks? Can I sand them smoother?