<P> Traditionally, paint was transferred to the painting surface using paintbrushes, but there are other methods, including using palette knives and rags . Oil paint remains wet longer than many other types of artists' materials, enabling the artist to change the color, texture or form of the figure . At times, the painter might even remove an entire layer of paint and begin anew . This can be done with a rag and some turpentine for a time while the paint is wet, but after a while the hardened layer must be scraped . Oil paint dries by oxidation, not evaporation, and is usually dry to the touch within a span of two weeks (some colors dry within days). It is generally dry enough to be varnished in six months to a year . </P> <P> Although the history of tempera (pigment mixed with either egg whites or egg yolks, then painted on a plastered section) and related media in Europe indicates that oil painting was discovered there independently, there is evidence that oil painting was used earlier in Afghanistan . Outdoor surfaces and surfaces like shields--both those used in tournaments and those hung as decorations--were more durable when painted in oil - based media than when painted in the traditional tempera paints . </P> <P> Most Renaissance sources, in particular Vasari, credited northern European painters of the 15th century, and Jan van Eyck in particular, with the "invention" of painting with oil media on wood panel supports ("support" is the technical term for the underlying backing of a painting). However, Theophilus (Roger of Helmarshausen?) clearly gives instructions for oil - based painting in his treatise, On Various Arts, written in 1125 . At this period, it was probably used for painting sculptures, carvings and wood fittings, perhaps especially for outdoor use . However, early Netherlandish painting in the 15th century was the first to make oil the usual painting medium, and explore the use of layers and glazes, followed by the rest of Northern Europe, and only then Italy . </P> <P> Early works were still panel paintings on wood, but around the end of the 15th century canvas became more popular as the support, as it was cheaper, easier to transport, allowed larger works, and did not require complicated preliminary layers of gesso (a fine type of plaster). Venice, where sail - canvas was easily available, was a leader in the move to canvas . Small cabinet paintings were also made on metal, especially copper plates . These supports were more expensive but very firm, allowing intricately fine detail . Often printing plates from printmaking were reused for this purpose . The popularity of oil spread through Italy from the North, starting in Venice in the late 15th century . By 1540, the previous method for painting on panel (tempera) had become all but extinct, although Italians continued to use chalk - based fresco for wall paintings, which was less successful and durable in damper northern climates . </P>

Where did painters perfect the technique of painting with an oil medium