Also called a binding agent; a material that mixes with the pigment and causes it to adhere to the support An ingredient in paint or ink that binds the pigment and makes it adhere to the surface to be embellished. Clarified egg white (glair, clarea) was the principal binding medium used in manuscript illumination. Gum (such as gum arabic from the acacia), glue (such as ichthyocollon, a fish glue, and casein, a dairy-product glue) or other forms of size (parchment size or gelatine) were also used for this purpose as well as for gilding.
Michelle Brown , mzh
Michelle Brown. Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).