Dentures will become black for a number of reasons, including the following: 1, metal ions combined with the gums and make the soft tissue staining: porcelain crowns of the base crown for the metal crown, which contains metal ions are very active, can be combined with long-term contact with the gums, so that the soft tissue stained to form a black line, affecting the aesthetics. 2, adherence to exogenous pigment: movable dentures of artificial teeth for the resin teeth, the surface finish is poor, the structure of the laxity, easy to adhesive Smoke stains, tea stains, coffee stains and other exogenous pigments, presenting black, through the polishing can be removed after pigmentation, restore the original color.3, cavity retention of food: implant denture crowns are connected to the implant through the central screws, the crowns of the central usually left with a cavity, the use of composite resin or resin cementum, etc. closed, easy to retention of food debris, soft scales, so that crowns are blackened.