Gilding is the application of gold to some substrate be it metal, wood or, most often, a composite material such as gesso. The gold can be attached through electroplating, fire-gilding (mercury), or adhering gold leaf with an adhesive. Most wooden artifacts have gold leaf attached to a gesso ground using either an oil size or water size, each having a particular effect on the brilliance of the gold. Only water-gilding will allow the gold to be burnished to a high, brilliant polish. The technique involves applying a ground such as gesso on which a clay glue size (bole) is painted. The bole is mixture of clay and animal glue (rabbitskin glue or gelatin) that provides the adhesive for the gold and is capable of being smoothed so finely that it will produce that brilliant reflective finish we associate with polished metal. Usually gilt surfaces have a combination of matte and bright gold, sometimes employing both oil and water gilding for artistic effect.
Because both gesso and bole use a high-strength animal glue to bind the particles of whiting (gesso) or clay into a stable coating, water can easily damage the leaf and gesso. Care must always be taken to avoid getting water on the gilt surface—glass cleaner should never be sprayed onto the glass or mirror but rather onto the toweling used to remove dirt and smudges on the glass. And since gold leaf is gossamer thin, touching gilt surfaces should be kept to a minimum. Cleaning should be limited to dusting and performed infrequently using a soft brush with a vacuum held close by to remove the airborne dust. A piece of nylon curtain should be fastened over the nozzle to catch any dislodged gesso to be saved for reattachment.
John Coffey specializes in conservation gilding—preserving the original surface yet repairing losses in both the gesso and the gold leaf. Even surfaces that have been painted over with “gold” paint can often be restored with minimal losses to the original gilt surface, as seen below in a photo of wall mirror.