Dr. Buck has collected thousands of paint samples from historic buildings and knew that traces of wallpaper can sometimes be found along the edges of windows, doors, and chimneypieces.  These traces are created when the wallpaper hanger slightly overlaps the wallpaper onto the sides of the architectural trim.  Later, when the paper is removed, these pieces of overlapping wallpaper sometimes remain attached to the architectural trim and so are entrapped under later coats of paint.  Because these layers of paint can often be dated after enough research, it is sometimes possible to date the wallpaper fragments found entrapped in the paint.  The hard part, however, is finding the paper fragments in the first place.