I live in an old house with marble fireplace mantels. The opening of the fireplace is square and the original design was to have a cast iron arch surround the opening.
We had these fireplaces retrofitted with a Bellfire fireplace retrofit insert. The installer did a great job and told us that the cast iron arch was actually needed as it made the opening smaller and allowed for a better draw, less smoke etc.
He attached it with plaster of paris which actually held up surprisingly well, but after 3 years it started to crack and eventually the arch fell off. Since then, I have been reattaching with plaster of paris every year (I guess I'm not as good as he was!) and frankly I'm tired of it.
So, what else can I use? I was thinking about fire-rope and high temp silicon? furnace cement?