Hello My name is Ivor I hope that I can help you today.
In 1900 Victorian properties, walls were mainly constructed with lathe and plaster, also then covered with Lime plaster with horsehair mixed in it. Over time this plaster degenerates and can cause bubbling and a crystal substance coming through. The only thing to try is to cut back the bad places and replace with plasterboard over the wooden lathes and skim with a plaster coat. and reseal with a sealer as PVA.
I hope this helps you to solve your problem.
I believe the interior walls are brick.
I am sorry for the late reply, as I have been at work this evening.
If the walls are brick or stone, then maybe the old plaster as i described ealier is deteriorating and causing the problem, As it is upstairs I would not have thought any dampness is the problem unless there is little heating or air circulating. I think I would have to see it to give you any more ideas.
I know it will cause a little work but if you take off one of the bad pieces it may give you a better idea what lies underneath.
I would be interested in what you find.
I have loosened the burst plaster & revealed black ' old fashioned coke sustance' (not coal but coke). I suppose you could describe it as crystalised. Please advise.
Hello again Maureen.
Thanks for coming back to me.
My suggestion is this for further work Im afraid. Make sure you remove all the plaster from the area leaving a border where the 'coke' is sound. I would buy from a builders merchant a sealant that neutralises alkaline materials,( Carbon is an alkali.) Give two or three coats to the problem area. the area can then be replastered and painted.
I hope this helps you now solve your problem.
Please come back to let me know how you get on.