Plaque is a colorless or pale yellow sticky film of bacteria that continually forms on teeth surfaces. This biofilm consists of living microorganisms, their byproducts, and components from saliva. While plaque is generally translucent and difficult to see, it rarely turns black on its own. The dark discoloration people observe is caused by materials absorbed into the plaque structure or reacting with its mineralized form on the tooth surface.
Plaque Versus Calculus
The difference between plaque and calculus lies in their texture and removability. Plaque is soft and easily removed through daily brushing and flossing. If left undisturbed, it begins to harden through mineralization, absorbing minerals like calcium and phosphate from saliva.
This hardened deposit is known as calculus or tartar, and its formation can begin in as little as 48 hours. Calculus is rough, crusty, and adheres firmly to the tooth, making it impossible to remove with a toothbrush alone. Calculus typically presents as a darker yellow or brown stain. It can appear dark green or black if it forms below the gum line (subgingival calculus), often due to blood or other pigments infiltrating the porous structure.
True Causes of Blackening
Black discoloration stems from several distinct sources beyond the color of hardened calculus. Extrinsic staining is a common cause, where dark pigments from external sources adhere to the enamel or the plaque/calculus layer. Frequent consumption of beverages like coffee, tea, and red wine, or the use of tobacco products, introduces strong chromogens that bind to the tooth surface. Certain medications, such as liquid iron supplements or chlorhexidine mouthwashes, also cause dark, external staining.
A unique type of extrinsic discoloration is Black Line Stain, which appears as a dark, thin line following the contour of the gum margin. This stain is caused by the metabolic activity of chromogenic bacteria, primarily Actinomyces species. The black pigment is believed to be an insoluble ferric sulfide compound, formed by the reaction between bacterial byproducts and iron in the saliva or gingival fluid. Interestingly, individuals with this stain often have a lower prevalence of dental decay.
When black color appears as spots or deep shadows within the tooth structure, it typically indicates dental decay (caries). This intrinsic issue occurs when acid-producing bacteria break down the tooth’s enamel and dentin. The dark appearance results from the discoloration of the dentin layer beneath the enamel as the decay progresses deeper. A tooth may also turn black due to internal trauma or the death of the nerve, causing internal tissue breakdown and discoloration.
Removal and Prevention Strategies
Once plaque has mineralized into calculus or true dental decay is present, simple at-home brushing is insufficient to remove the dark material. Professional dental intervention is required for both conditions. A dental hygienist uses specialized instruments, such as ultrasonic devices and manual scalers, to physically scrape the hardened calculus from the tooth surface. If the black stain is decay, the dentist must remove the affected tooth structure and restore the area with a filling or other material.
Preventive measures focus on controlling initial plaque formation and limiting exposure to staining agents. Brushing thoroughly for two minutes twice a day and flossing daily helps remove plaque before it hardens into calculus. Reducing the intake of highly pigmented foods and beverages, or rinsing the mouth with water immediately after consuming them, limits extrinsic staining. Regular dental checkups, typically every six months, allow for the early removal of developing calculus and the monitoring of potential decay.