Removing a Hearth Bed

[schlafsack]

I'm currently in the middle of replacing some old, cupped floorboards with ply. I pulled up the old hardboard this afternoon and came across a hearth from a bricked up fireplace. There was a 2cm layer of cement on top that came up easily. However there is a slab of material I don't recognise underneath and I'm not sure of the best way to remove it. It stands about 1cm proud of the joists so I either need to take it up or thin it down somehow. The material is grey and powdery but appears quite well bonded together. It's flecked with what looks like charcoal and has a dull thud when tapped with a trowel. Is there any chance that it's furnace slag?

The flat is on the third floor of a 1890's conversion, and the ceiling beneath is newish plasterboard.

 

[SFK]

Dear schlafsack,

As a DIYer I have hit that stuff before. For me it appeared to be soil and ash mixed together (100 Year old house). I presumed that was used as it worked as a good thermal insulator to stop heat of the hearth damaging ceiling below.

As I wanted to ensure that if I ever took down the ceiling below that I would not have all of that stuff raining down on me, I used a a garden trowel to carefully break into it and scoop it into a bucket. I removed it all down to the lath and plaster and then used a dustpan-and-brush

As I found it made a very fine dust so I wore a mask.

Hope that helps, SFK