Colonies of bacteria grow in warm, moist environments that supply plentiful food. A host organism's mouth provides a home to multiple strains of bacteria, some of which form a pasty film that dentists call plaque. If plaque settles on teeth and remains undisturbed for long enough, it hardens via mineralization and becomes calculus or tartar. Calculus means "stone" and describes the rock-like substance's texture. Porous calculus provides more surface area for bacteria, leading to further dental problems.
Within minutes after brushing or cleaning teeth, a thin membrane or pellicle, forms on tooth surfaces. This pellicle arises not from the action of bacteria but from proteins within saliva that naturally deposits on teeth as a protective coating. Bacteria exploit this membrane, growing in it as readily as they grow on Petri dishes in a laboratory. As the pellicle gains a freight of bacterial colonies, it loses its solubility; the stabilized film allows more bacterial colonies to grow on teeth, particularly in fissures and crevices.
Bacteria produce their own biofilm (a slippery matrix in which the microbes grow undisturbed) that adheres to teeth. These biofilm layers are the earliest stages of plaque formation. Plaque -- a mixture of bacteria and bacterial waste products -- becomes visible within hours after tooth-brushing as a whitish paste on surfaces within the mouth. As these bacterial colonies grow, they produce acidic by-products that destroy tooth enamel, leading to tooth decay. Brushing and flossing dislodge immature plaque less than 24 hours old but as plaque matures it becomes more difficult to remove.
Within 24 hours, plaque begins to mineralize into calculus. Acids and bases interact to form salts; this process forms part of the composition of calculus as calcium and phosphates in alkaline saliva interact with acidic bacterial wastes. In its earliest phase of development, brittle calculus deposits fall away under a toothbrush or strand of dental floss. If left undisturbed, the mineralization process continues and builds around defunct bacteria, fossilizing them. Within approximately twelve days to two weeks, calculus matures.
Dentists can readily see mature calculus formations as whitish or pale yellow structures affixed to teeth. The largest calculus deposits typically occur on the lingual (nearest the tongue) surfaces of the lower front teeth but tartar can occur anywhere in the mouth. Sub-gingival calculus grows below the gums, while supra-gingival calculus grows on visible tooth surfaces above the gums. Although it feels solid, porous calculus plays host to multiple colonies of bacteria that thrive on the increased surface area that the mineralized plaque provides.