We are not accustomed in our world of art to sudden, breakthroughs so large that they instantly antiquate how we work and think as artists. Nothing as jolting as this has happened to our art since motion pictures took sole possession of the painter's storytelling role. And before that we must go back to the invention of photography itself to find such a telling breakthrough. Perhaps the greatest change ever in painting occurred about 1400 in the country known then as Flanders (part of Belgium). It was such an important invention (discovery?) that it was held for quite a number of years as something close to a state secret. History credits the van Eyck brothers, Huybrecht and Jan, as the inventors of oil painting.
To understand what a tremendous discovery pigmented linseed oil truly was its necessary to come to grips with the clumsy ways and means painters had to contend with previous to this breakthrough. There was fresco, of course, which was something of an improvement in picture-making over mosaic wall murals, from which it derived. Frescos were originally the poor-man's mosaic. However frescos were not portable. A painting to hang on the wall was first a flat piece of wood (no small task in this pre-plywood era). After the planks were painstakingly cut, planed, joined, and smoothed, they were covered with linen to which a mixture of plaster and rabbit skin glue was added (in several layers). The Italians called it gesso. This had to be sanded, smoothed, and polished (by apprentices) until it looked and felt like polished marble (and no doubt weighed almost as much). Over this was added an underpainting of green or brown pigment to which was then applied, layer after tedious layer of egg tempera, one of the most unforgiving painting mediums ever known to man. Of course I needn't recount canvas preparation or oil painting because we are all too familiar with that.
Unlike the painters of Italy who descended from mosaic artists, the Flemish painters came from a long line of manuscript illustrators, which gave them a whole different approach to painting. Use to working with coloured inks on paper, their painting style was tight, highly detailed, exquisitely jewel-like in overall effect. Their patented use of oils allowed them to explore photographic realism 400 years before photography was even invented. And they practically re-invented portrait painting. But the very portability that made oil painting so valuable also guaranteed that the news (indeed the paintings themselves) would quickly spread, first to Germany, then south to Italy where the Florentine artist Andrea del Castagno was accused of murdering his fellow artist, Domenico Veneziano (who had learned of this secret painting method) in order to keep it from becoming widespread in Florence and thus replacing his more traditional form of painting. (He was later found innocent).