Watercolour produces an imprecise painting with soft and delicate colours. Its base is a series of pigments bound with gum-arabic or similar substances, rather than with oils as in the case of oil paints. This agglutinate means the paint is water-soluble. The paintbrush used to apply the colours must have an absorbent nucleus to soak up the coloured water. Watercolour paper has a special thickness and is highly absorbent to prevent it from curling up. It is usually white, although it can also be tinted in neutral colours. The extended colour is transparent and allows the texture of the paper underneath to be appreciated, as well as other layers of colour. Watercolour was discovered in 2nd century AD Egypt but it was a minor technique up until the 16th century when it was rediscovered Dürer and Van Dyck, who used it to finish parts and details of large works. The most important watercolour school was in 19th century England, when it became inextricably linked to the romantic landscape painters.