Gilding is a process in which a decorative thin layer of metal, often gold or silver, is applied to the whole or partial surface of an object.
Various types of media are gilded, including wood, metal, plaster, and glass. Common gilded objects include paintings, works on paper, sculptures, frames, and furniture. There are various types of gilding processes, which usually include priming the surface and adding an adhesive. Each process creates different types of gilded surfaces for care or repair.
A historical gilding process used to gild three-dimensional objects. Gold or the desired metal is dissolved in a solution of nitro-hydrochloric acid, which is then soaked into cloth. The cloth is burned, and the ashes, containing the metal particles, are deposited onto the object’s surface through rubbing. (Langeveld, 2012)
An ancient process used to guild sculptural objects with a thin coat of tumbaga, a gold-copper alloy. Theories include heating the object in a mineral-based or plant-based solution, dissolving the copper from the tumbaga surface and leaving a layer of gold. Scanning electron microscopy studies of artifacts gilded in this method show artifacts were immersed for long periods of time, or multiple times, resulting in thicker layers of gold. (Museum of Fine Arts, Boston, 2011)
When creating a gilded surface, often on frames, a coating of diluted animal glue is applied over the wood, and then many layers of gesso are applied until an even surface is achieved. The layer that is applied on top of the gesso is known as a bole- a mixture of clay and animal glue that is similar to gesso but harder. Gold leaf is then adhered by wetting the bole to momentarily reactivate the animal glue. (Peterson & Podmaniczky, 2009)
Contemporary water gilding processes may include the use of synthetic polymers. A recent study suggests the dramatic temperature and /or relative humidity fluctuations will cause stress and cracking in surfaces gilded using B-72 material, which becomes more brittle over time than either polyvinyl acetate resin or acrylic dispersion films. (Sawicki & Thomas, 2011)
Oil Gilding is similar to water gilding in that layers of gesso and bole are applied. Instead of re-activating the animal glue through moisture, a varnish is applied for adhesion. The process receives its name from the linseed oil component used in the varnish. (Peterson & Podmaniczky, 2009)
Langeveld, M. (2012). Historical Techniques: Cold Gilding. "EXARC Journal, vol 2012/3". Retrieved from http://exarc.net/issue-2012-3/ea/historical-techniques-cold-gilding
Museum of Fine Arts, Boston. (2011). "MFA Highlights Conservation and Care of Museum Collections". MFA Publications, Boston, MA. 2011
Peterson, M. & Podmaniczky, M. (2009). Gilded Frames. In Rollins, O. (Ed.) The Winterhur Guide to Caring For Your Collection. (pp. 135-139). Hanover, NH: University Press of New England
Sawicki, M. & Thomas, R. (2011). Tendency in aging behaviour of gilded surfaces formed with synthetic polymers. Canadian Conservation Institute. Retrieved from http://www.cci-icc.gc.ca/discovercci-decouvriricc/PDFs/Poster%20-%20Sawicki%20and%20Thomas%20-%20English.pdf