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It was always hoped that a painting would last for a long time, so everything had to be very carefully prepared. A board had to be chosen that would not crack or warp (bend), and which did not have any knotty bits that might fall out. The board was sanded smooth and then coated with a gluey paint called "size" which would hold the coloured paint well, but stop the expensive colours from being soaked up by the wood. The artist would prepare the paints very carefully. Some of the colours were made from gound-up semi-precious stones. Others were made from chemicals.