I feel so old ....
I stopped at two high end paint stores today, looking for rottenstone. It used to be a common product. At the first store, the young clerk had never heard of rottenstone. He was probably knowledgeable about latex paint, though. Or not. The second store had a clerk my age, maybe, but he had only started working in the paint store about ten years ago, ... so ... not a clue.
Rottenstone is one of the subtleties of fine finishes. I routinely mix a little dry pigment of various colors into some rottenstone to alter the overall color and effect of a finish. Once a tone has been applied, ( a colored wash ), and is dry, rottenstone may be dusted on, then off. I use one of the old style aluminum salt shakers to shake onto the surface, then brush it overall, and either vacuum, or blow off with compressed air.