The higher the viscosity of a substance, the more difficult it is to separate out foreign matter from it. The viscosity makes it difficult both to see any foreign matter and to remove it easily.

In cosmetic and pharmaceutical products such as, for example, creams and gels, toothpaste and soaps, as well as foodstuffs such as milk products, spreads, and intermediate food products including dough, fats, and spice pastes, contamination from metallic foreign matter can occur in the production process.