Oil paint
Oil paint contains particles of pigment applied using a drying oil, such as linseed oil. The conventions and techniques of using oils demands a knowledge of drawing and painting, so it is often used in professional practice. It is necessary to size the print first to prevent absorption of the colours into the paper. In the past, photographic lantern slides were often coloured by the manufacturer, though sometimes by the user, with variable results. Usually, oil colours were used for such slides, though in the collodion era – from 1848 to the end of the 19th century – sometimes watercolours were used as well.