cast iron fireplace - backfill question

[WireWolf86]

Hi everyone, We recently moved into a property which has a lovely cast iron fireplace. the cast iron firebrick (a piece of cast iron that slides into place at the back) seems to have warped and cracked. I have a replacement ordered offline to match our make/model but i noticed it mentioned that they really should never need replacing if the back filling has been done correctly. So i assumed this was previously done poorly.

Once i got the part, i removed the old broken part (slides in/out to be replaced easily) and noticed the entire backfill was dry loose vermiculite.

I was intending on removing all of this and replacing with some old bricks from a relative who recently did some work. I was just going to stack the bricks in the empty void leaving a 3" gap between the backplate and the bricks, fill the 3" gap with the loose vermiculite, then create a slope to cap it all off with a mix of 5:1 vermiculite and cement and some water.

Does this sound safe and correct?

 

[WireWolf86]

no one?

 

[Nige F]

Not sure but ...........vermiculite is an insulator so maybe that causes the iron to get too hot ? In the Olde days it would have been filled with old bricks ( like you have ) and a weak lime/sand mix with a little cement. Not so insulating, more heat absorbing. Just my theory ( and having taken out old fireplaces.