There is a historical building near where I live where the walls are covered in pencil writing. The notes were taken over centuries by multiple generations of the family that lived there. There are all kinds of things-- names, recipes, poems, sketches. (Some are quite historically unique-- for example, a Confederate child's drawing of a fanged, monstrous Abraham Lincoln.)
Several rooms in the house, however, have patches where the wall has been repaired, seemingly just by adding more plaster on top of the previous layer. Words disappear under the patches, leaving sentences unfinished. Clearly, there is more writing underneath. This is mysterious. I want to know what's under there.
Beings that graphite is a material with physical properties varying significantly from plaster, I wonder if there might be some way to image this writing. Graphite is a conductor, it's diamagnetic, acoustically and thermally anisotropic.
Unfortunately, my physics in this area is limited, so I'm not sure where this gets me. Would it show up on an ultrasound? Maybe a thermal camera? Can anyone think of a different scheme?