Rotten stone, or rottenstone, is a form of powdered stone which is used to polish wood and some soft metals; you may also hear it referred to as tripoli. It is typically available from stores which cater to woodworkers, and it can also be ordered through specialty suppliers. This product can be useful to have around the home, as it can be utilized to buff out stains in addition to being used to finish wood.
Classically, rotten stone is made from soft, powdered limestone mixed with silica. The result is a soft abrasive powder which can be used towards the end of the finishing process to create a smooth, clean finish. Typically, this stone is applied after varnishing and a rough polishing with pumice powder, which buffs out the big uneven spots, making it easier to use the powder. After polishing, the wood should have a bright, glossy finish.
Rotten stone comes in powder form, meaning that people must mix it into a paste before using it. Many woodworkers use oil to make their rotten stone paste, although it is also possible to use water. The paste is smeared onto a cloth to make it easier to apply. As is the case with other abrasives, it is a good idea to be mindful about which direction the cloth is moved in, to ensure that the wood and finish do not develop a strange appearance or texture.
Rotten stone can also be used on some metals, like brass, in addition to wood. If you aren't sure whether or not the powder is safe to use with a metal, find a patch which is not readily visible or exposed, and apply a small amount of paste to it with a soft cloth. If the metal buffs into a smooth, glossy finish, you can go ahead and polish the rest of the object; if the rotten stone creates gouges or dulls the finish, you will need to find another metal polishing product.
When you use this substance to buff out stains, be aware that it can react poorly with some wood finishes if it is applied too dry or too roughly. Increasing the amount of lubricating oil or water used in the paste is a good way to reduce the risk of creating a gouge or messing up the finish. As is the case with metal, if you aren't sure whether or not you can use rotten stone on a stain, find a hidden patch on the object and do a spot test.