We recently decided to strip the wallpaper in the small office room of our pre-1900 era house. Upon starting to strip the wallpaper we realized that there was something odd under two of the walls in the room. Two of the walls had a large square (about window size) of some type of adhesive looking material that was on the plaster under the wallpaper (see pictures). One of these squares had a layer of an aluminum foil type material between the wall and the wallpaper. Upon removal of all of the wallpaper we noticed that both spots looked somewhat different but were similar shape and both ran brown streaks down the wall when wet (in this case with the DIF we used for the wallpaper removal). I cleaned both with a bleach solution and the general color of both squares did not change. The differences between the squares are that they are different shades of brown and one has sporadic spots of dark brown to black putty material throughout it. Can anyone tell what these are and/or how to possibly remove it? The color has us perplexed and I wanted to paint the walls and that clearly can't be done until this problem is solved.