A process used for stain removal generally from stone floors particularly granite and marble. From a chemical standpoint, a porous stone becomes stained when a solution containing a solute penetrates its surface and then evaporates leaving the solid solute behind within the stone. Alternatively, grease may penetrate the porous surface and remain within the stone without evaporating. In either case, the stone will become visibly ‘stained.’ A poultice is a malleable mass of a porous material (paper, whiting, diatomaceous earth) filled with a solvent which can be applied to the surface of the stone. The choice of solvent (ammonia, acetone, alcohol, etc.) should be something likely to dissolve the solute or grease causing the stain. As the solvent penetrates the surface of the porous stone containing the stain, it forms a single continuous solution between the stone and the poultice on the surface. The poultice needs to be kept moist and is usually covered to allow time for the solvent to sufficiently penetrate the stone and dissolve the staining material, be it grease or solute. The solute will then equilibrate by passive diffusion between the stone and the poultice. After an adequate time for this process to occur, the poultice is removed and with it the solution containing a portion of the dissolved solute or ‘stain’ Multiple repetitions of the process will eventually decrease the concentration of the solute or ‘stain’ within the stone until it is invisible or minimally visible. Reproduced from: http://en.wikipedia.org/wiki/Poultice