Rotten stone

Rotten stone, sometimes spelled as rottenstone, also known as tripoli, is fine powdered porous rock used as a polishing abrasive for metal smithing, historically for the grinding of optical lenses and in woodworking.

It is usually mixed with oil, sometimes water, and rubbed on the surface of varnished or lacquered wood with a felt pad or cloth.

Plates used in daguerreotypes were polished using rotten stone, the finest abrasive available at the time.

[1] Rottenstone has been extensively worked in South Wales along the outcrop of the Carboniferous Limestone, particularly within the Brecon Beacons National Park.

Innumerable workings were initiated and later abandoned during the course of the nineteenth century, leaving a characteristic terrain of humps and hollows.