Illumination is the art which decorates medieval documents, including initials, marginalia, illustrations, and cadels. Technically, illumination refers only to art which is ornamented with gold or silver leaf, but the term is usually used in a broader sense. There are many broad styles, and many approaches to creating illumination.
Pigment is mixed with binder to produce paint. Common binders of the era are egg-white glair and gum arabic, both with a touch of honey to improve the flexibility of the paint. The preferred paint of most modern scribes is gouache and, to a lesser extent, watercolor, both of which have gum arabic as the primary binder. The two paint types vary only by the addition of white pigment in gouache to aid in opacity. Gum arabic based paints have the advantage that they can be quickly reconstituted, and can be left on a palette for a long time in an immediately usable form. Glair paint, however, has short period of usefulness, quickly gumming up a paint brush, and must be mixed in very small batches for immediate use.