Watercolour is produced by mixing powdered pigments with a water-soluble gum, such as gum arabic, a natural secretion from acacia trees. The gum ensures that the pigment spreads evenly through the water, rather than settling or gathering in lumps. It also makes the pigment stick to the paper once the water has evaporated. Watercolour is transparent, so that the white of the paper remains visible through strokes and washes of colour, giving them a freshness highly valued by artists, especially for landscape painting. Like lithography, the technique was particularly associated with British artists.