The first, and perhaps most decisive, phase of the pictorial revolution in Flanders is represented by the Master of Flemalle.  His finest work is the Merode Altarpiece, which he must have done soon after 1425.  Here, for the first time, we have the sensation of actually looking through the surface of the panel into a spatial world that has all the essential qualities of everyday reality:  unlimited depth, stability, continuity, and completeness.  The Master of Flemalle has undertaken to tell the absolute truth.

    His objects, overly foreshortened, tend to jostle each other in space.  But  with an almost obsessive determination, he defines every last detail of every object to give it maximum concreteness:  its individual shape and size; its color, material, surface textures; its degree of rigidity; and its way of responding to illumination.  This is the earliest Annunciation in panel painting that occurs in a fully equipped domestic interior, as well as the first to honor Joseph, the humble carpenter, by showing him at work next door. 

     The Master of Flemalle was faced here with a challenge of transferring supernatural events from symbolic settings to an everyday environment, without making them look either trivial or incongruous.  He met this challenge by the method known as  "disguised symbolism", which means that almost any detail within the picture, however casual, may carry a symbolic message.  Thus the flowers are associated with the Virgin:  in the left wing, the roses denote her charity, and the violets, her humility; the lilies in the center panel, meanwhile, symbolize her chastity.  The shiny water basin and the towel on its rack are not ordinary household equipment either, but further tributes to Mary as the "vessel most clean" and the "well of living waters".  The candle, next to the vase of lilies, is perhaps the most intriguing symbol, as it was extinguished only moments before, which we can tell from the glowing wick and the curl of smoke.  It is supposed that the divine radiance of the Lord's presence has overcome material light, or that the flame of the candle itself represents divine light, now extinguished to show that God has become man, that in Christ, "the Word was made flesh".  The box-like object on Joseph's workbench (and similar one on the ledge outside the open window) has been identified as a mousetrap, intended to convey a specific theological message.  According to St. Augustine, God had to appear on earth in human form so as to fool Satan:  "The Cross of the Lord was the devil's mousetrap."

    In comparing the Merode Annunciation with earlier panel paintings, it is clear that the jewel-like brightness of the older works, their patterns of brilliant hues and lavish use of gold, have given way to a color scheme far less decorative, but much more flexible and differentiated.  The subdued tints of muted greens, bluish- or brownish-grays show a new subtlety, and the scale of intermediate shades is smoother and has a wider range.  All these effects are essential to the realistic style of the Master of Flemalle.  They were made possible by the use of oil, the medium he was among the first to exhibit.