Paint is made by mixing a pigment, which is the source of colour, with a medium, which binds the particles of pigment together.
Among the different types of medium are egg tempera, watercolour, oil and glue size. Works in the National Gallery are mostly painted in oil but many are of egg tempera. The type of medium, the ratio of medium to pigment, and the optical properties of both pigment and medium influence the ways in which paint can be applied and the effects which can be achieved with it.