We have 7-8 picture frames (different sizes) which we had hung in our previous apartment, and when we moved, they had left dark grey-black marks on the wall where the bottom of the frame touched the wall (all along the line of the frame). It is not seen when the frame is against the wall, but when you remove the frame off the wall, it is very visible.
I am not sure what they are from, because they also appear for frames that are not black in colour (we have a couple of gold coloured frames), so it can't be paint from the frame transferring onto the wall (also, the back of the frame is intact, without any damage or loss of paint/colour). We really don't know what the marks could be from. Dust? But it doesnt wash off, as one would imagine with dust.
They come with only long term (a few months) of hanging a painting in the same spot. We are afraid to hang any frames in our current apartment because we dont want to leave marks and then not be able to move frames around. I thought of lining the bottom of the frames with sponge-sheets or bubble wrap, but I am not sure if this will protect the walls from the marks.
Any suggestions will help!