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Oil Paint dries by oxidation and polymerisation. The process is catalysed by metal ions and light, and is temperature-dependent. A painting in a warm, bright room will dry faster than one in a cold dark room - the rate approx doubles for each 10 degrees C rise in temp (but I wouldn't heat the painting above 40 degrees C - excessive heat will cause yellowing and cracking). Paints vary in their drying rates, some are fast, such as umbers and earth colours; others are slow, such as many of the organics (phthalos and quinacridones), cadmiums, and titanium white. Using earth colours in underpaintings can help speed the drying of upper coats. Very small additions of driers (cobalt, manganese, zirconium) ("L&B courtrai") can help, as can using an alkyd medium such as W&N Liquin, or substituting slower-drying colours such as cadmiums or titanium white with their alkyd equivalent (W&N Griffin).