Before an artist can paint on a board or canvas, they must prepare it with a "ground" or "undercoat" of plain white paint. Then the artist can sketch a picture onto the surface using charcoal, or paint that is made thin and quick-drying with turpentine or mineral spirits. The artist often works in a brownish or bluish colour, to suggest where the "tone" (light and dark) will be in the finished painting. Then the colours and details are put on in layers.