Manuscript illumination was expensive because of what it used: ground gold, beaten thinner than a hair and laid over a raised bed of gesso, with a burnisher to press it smooth. It does not fade, which is why a thousand-year-old initial can still be the brightest thing in the room.
Incipit liber — Here begins the book of the green field, in which are set down the hours, the sowing, the coming of the rain, and the names of those who asked after the oxen. Item, that on the ninth day of the month the mill did not turn, and the reason was not recorded, though the miller was questioned twice. Item, that the north wall is to be limewashed before the feast, and that the lime is to be brought by the younger brother and no one else.
Nota — The leaves of this book are vellum, and the writing upon them is that which the monastery buys by the dozen: a good brown ink, well tempered with gum, which does not crawl. The initial upon this leaf was gilded by the gold-beater, and it is to be burnished with the agate until it shines like a mirror, and no man is to touch it after that with anything but the cloth.
Gold leaf cannot be laid flat on a page and survive. It has to be bedded into a raised area of gesso — chalk and glue, built up in several layers and polished — and then burnished with an agate until the gold is continuous. Where the gesso dips, the leaf tears, and you get the fine cracks that run through old gilded initials like dried mud.
The leaf was beaten to about a tenth of a micron. It is so thin that the colour of the gesso beneath shows through wherever the burnishing has failed, and the play of light across a single initial changes completely as the reader moves.