What material did flemish artists almost exclusively use in the 15th century?

The so-called Flemish Primitives were the first to popularize the use of oil paint. Their art has its origins in the miniature painting of the late Gothic period. Chief among them were Jan van Eyck, Hans Memling, Hugo van der Goes, Robert Campin and Rogier van der Weyden.

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artists need to create a realistic interpretation so that people in general can relate to the artwork. for a common people, an artist's impression gets increased only if he can create something which would look realistic. an abstract is difficult for a common people to understand whereas if a person can draw a girl with simple pencil sketch and if that looks like a real figure then the latter will be praised more than the former.