Damp plaster on fireplace covered wall

[AnyaB]

Any advice on what to try next??
My property is a 1903 Victorian detached house. I have recently begun renovations in the study which has required plastering all the walls.
Wallpaper on one of the walls had a layer of what seemed like foil? Was a very annoying task to remove.
After plastering, this same wall has a damp patch that does not seem to be drying! There has been a fireplace that has been covered up on this wall and originally I suspected some problem with the chimney, however this has been checked all correct. Any ideas on where to turn to next??
I have been heating the room periodically over the last 2 months in aid to help which will improve the damp slightly but it returns within the next few hours afterwards!

There was also a strange painting on this wall that I have attached a picture of as well for interested - was certainly a shock when I started stripping the paper and found this!

Thanks

 

[malcolm]

It's difficult to tell from the photos. Does the damp bit correspond to anything that was underneath? A photo of the whole wall and what is on the other side would be useful. I imagine the wall was covered in waterproof stuff for many years and has become wet. Drying time is one month per inch when the wall becomes breathable and a bit slower if the wall is not breathable.

 

[aockenden]

Did you plaster in Gypsum? Why not a lime putty based plaster instead to help pull the damp out of the wall more quickly like a poultice and let it evaporate off?

 

[Lacan07]

It could be salts from the soot and lime mortar supping up the moisture in the air. Can you trace the flu in that area?

That looks like gypsum plaster to me, did the plasterer PVA the wall first? That should have sealed it.

 

[johnbanshee]

My sweep said to me this week (as I have a very wet capped chimney break) that the water can only come from in the room or down the chimney...and most likely down the chimney.
Easiest way to investigate would be to hack of plaster and see whats happening behind...? Should be able to to path it up your self or get a plastered for an hour or two.