What is between brick and plaster?

[iamchris]

Hi all, bought my house a while back and when I moved in there was a damp patch around one of the windows and damaged plaster in the area. The source of the damp has been fixed and windows replaced. The damp has not come back since.

Anyway some of the plaster sounded hollow. I pulled it with my hand and a chunk came off all the way back to brick. The chunk feels like some sort of render with plaster on top. How do I repair this? I plan to pull all the rest of the hollow chunk off. But what do I put over the brick before plaster? Pictures attached so you can see what it is:

 

[Munroast]

it just looks like thick plaster (browning?), we had a property that was a mixture of this and even older horsehair stuff. I would probably just fill it back up with plaster

that green paint is likely to contain arsenic

 

[iamchris]

Thanks I'll just fill it in and see how it goes. Thanks for the tip with th green paint I'll be careful of that.