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A poultice, also called cataplasm, is a soft moist mass, often heated and medicated, that is spread on cloth over the skin to treat an aching, inflamed, or painful part of the body. It can be used on wounds such as cuts. It can also be a porous solid filled with solvent used to remove stains from porous stone such as marble or granite.
The word "poultice" comes from the Latin puls, pultes, meaning "porridge".
A poultice is a common treatment used on horses to relieve inflammation. It is usually used on the lower legs, under a stable bandage, to focus treatment on the easily injured tendons in the area. Poultices are sometimes applied as a precautionary measure after the horse has worked hard, such as after a cross-country run, to prevent heat and filling. They are also used to treat abscess wounds, where a build-up of pus needs to be drawn out.
Poultices may also be heated and placed on an area where extra circulation is desired.
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".
Poultices for removing stains are made from a malleable mass of a porous material (paper, whiting, diatomaceous earth, flour, limestone) filled with a solvent which can be applied to the surface of the stone. The solvent used (ammonia, acetone, alcohol, peroxide, etc.) depends on what substance caused 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 is kept moist and 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.