From Medieval Scriptoria to Today’s Studio: Friendly Tips on Gilding and Painting Media. Understanding Sizes and Binders in Illuminated Manuscript Art.
Illuminated manuscripts rely on two families of adhesive that are easy to confuse but serve distinct purposes.
A size (or ground) is the adhesive layer that attaches gold or silver leaf, or metal powder, to the page.
A binder (or medium) is the substance mixed directly into powdered pigment to make paint, holding pigment particles together and fixing them to the parchment, vellum, or paper.
Medieval scribes and illuminators relied on a short list of animal, plant, and mineral substances prepared by hand; many of these same materials, alongside new synthetic alternatives, remain in active use by calligraphers, conservators, and manuscript artists today.
Medieval Materials: Handy sizes for gilding
Gesso (Raised Gilding Ground)
What it is: A chalky, slightly elastic paste built from slaked plaster (gypsum) or “gesso sottile,” bulked and plasticized with white lead, and bound with a glue. Cennino Cennini’s fifteenth-century treatise lists the core recipe as gesso sottile, white lead, sugar candy, and clear water, sometimes with a fish-glue or parchment-glue binder added by later practitioners.
Function and use: Gesso is painted or piped onto the page with a pen or fine brush wherever a raised, mirror-bright gilded area is wanted, such as haloes, initials, or heraldic devices. Once fully dry it is scraped level with a knife, lightly dampened by breath, and gold leaf is laid over it and burnished with an agate or dog’s-tooth burnisher to a brilliant polish; the slight give of the dried gesso is what allows the leaf to take so high a shine. White lead in the mix fills small voids left by the plaster and historically discouraged mould growth, while sugar or honey keeps the dried gesso slightly hygroscopic so that a moistening breath can reliably reactivate its tack.
Preparation: Plaster of Paris is “slaked” (soaked and repeatedly washed in water to remove impurities and slow its set), then ground wet on a stone slab with white lead, sugar, and a little clear water or egg glair until perfectly smooth and free of air bubbles; Armenian bole (a greasy red clay) is sometimes ground in purely for colour, so that any bare patch shows red rather than white. The paste is dried into small “buttons” or cakes for storage and reconstituted as needed with a few drops of water or glair, ground patiently until it reaches a thin, cream-like consistency.
Gum Ammoniac (Flat Gilding Size)
What it is: A resin-gum exuded from the stem of the perennial herb Dorema ammoniacum, used as a thin adhesive for flat (non-raised) gilding; its preparation is described as early as the fourteenth-century treatise De Arte Illuminandi.
Function and use: Brushed or pen-drawn onto the area to be gilded and left to dry completely (a process that can take many hours), the dried film is reactivated immediately before gilding by breathing warm, moist
air onto it so that it becomes tacky again; gold leaf is then pressed on and burnished lightly. Because the dried size stays slightly flexible, flat gilding over gum ammoniac rarely reaches the hard mirror polish of raised gesso work, but multiple thin coats can build a gently raised texture.
Preparation: Raw ammoniac nodules (chosen over the cleaner gum balls for their stronger adhesion) are crushed, soaked in water, and left to dissolve; the liquid is strained repeatedly through cloth to remove grit and plant debris, then a little sugar, a little gum arabic solution, and a pinch of Armenian bole are added for working properties and colour. A few drops of egg glair are often added just before use to improve tack, and the strained solution is kept refrigerated and re-strained periodically to keep it clear.
Glair (Egg-White Mordant and Size)
What it is: The clarified, watery liquid produced by whipping egg white to stiff peaks and letting it collapse overnight; this breaks down the albumen’s long protein chains into a thin fluid. Beyond its major role as a paint binder (Section 4), glair itself is one of the simplest gilding sizes.
Function and use: Glair is brushed onto the design as a mordant, allowed to dry (roughly twenty minutes), then breathed upon with a tube or quill to raise a film of moisture; gold leaf is laid on while that moisture is visible and pressed down with cotton wool, then burnished. Because glair becomes water-resistant within about a day, gilding must follow its application promptly.
Preparation: Fresh egg whites are separated, beaten to a stiff froth with a whisk or fork, and left to stand (traditionally overnight) in a cool place; the clear liquid that weeps out at the bottom of the bowl is skimmed off and strained, discarding the foam. A drop of vinegar or a clove of garlic is sometimes beaten in to prevent putrefaction and to improve flow. Aged, strained glair was kept cool and restrained monthly to stay free of sediment.
Parchment Size, Hide Glue, and Minor Mordants
Dilute animal-skin glue (parchment clippings or rabbit-skin shavings simmered in water) served both as a plain gilding adhesive and as an ingredient mixed into gesso and gum-ammoniac recipes for extra strength. Garlic juice, pressed from a crushed clove, was occasionally used on its own as a simple, quick-drying flat-gilding mordant. Shell gold, meanwhile, is not a size at all but a paint: gold leaf is ground by hand with a little honey or gum arabic until it breaks into a fine powder, the honey is washed out with water, and the remaining gold powder is bound with gum into a paintable gold “ink” kept, historically, in a mussel or scallop shell, hence the name. It was brushed on like watercolour for fine lettering, hairline details, and small accents where applying leaf was impractical, but it never burnishes to the brilliance of true leaf.
