The one color the alchemists could not conjure up for painters was the one they labored the hardest to devise. Struck by slanting rays of the sun, gold set the medieval altarpiece ablaze with light. In Byzantine churches like the sixth-century San Vitale in Ravenna, golden mosaic tiles create a dome shimmering with holy radiance. Whatever the cost of ultramarine or vermilion, gold has ancient associations that make its value transcendental.
Gold is the substance of royalty, so what could be more pious than to offer it up to God in sacred art? And unlike silver and other metals, it was seemingly immune to the passing years- it did not tarnish or lose its splendor.
The use of gold in Medieval art shows us more clearly than anything else how the nature of materials took precedence over any concern for realism. Until at least the fourteenth century,holy figures on altar panels are framed not by nature’s skies or foliage, not by draperies or masonry, but by a golden field that permits neither depth nor shadow.
In later ages this metallic sheen was pushed back onto the gilded frame that held the canvas, but for the medieval artist gold was a color in its own right. It was applied to the gessoed panels in the form of thin sheets; gold leaf. There was no need to visiti the apothecary to procure this color, for it was to be found in the purse of every wealthy person. The craftsmen of the Middle Ages , unrestricted by laws protecting currency, made thier gold leaf by hammering and hammering at golden coins, transforming them to sheets so thin as to feel almost weightless.
This task was carried out by professional goldbeaters who, even into the twentieth century, measured the wieght of gold leaf by the ducat, the gold coinage of medieval Italy. The thickness of the foil was determined by the number of leaves (each about three and a half inches square) beaten from a single ducat.