No it is not built up food. Where did you get that from?

In dentistry, calculus or tartar is a form of hardened dental plaque. It is caused by precipitation of minerals from saliva and gingival crevicular fluid (GCF) in plaque on the teeth. This process of precipitation kills the bacterial cells within dental plaque, but the rough and hardened surface that is formed provides an ideal surface for further plaque formation. This leads to calculus buildup, which compromises the health of the gingiva(gums). Calculus can form both along the gumline, where it is referred to as supra- gingival ("above the gum"), and within the narrow space ( sulcus) that exists between the teeth and the gingiva, where it is referred to as subgingival ("below the gum").