Gilding refers to the application of a thin layer of metal to a substrate. Gilding is commonly applied to illuminated manuscripts and book binding, china and useful objects such as clocks and chalices, accents to paintings and sculptures, and in architecture to provide glittering accents on both interior and exterior surfaces. Gilding can use gold, silver, copper, platinum, and a variety of “composite” materials made to either look like a pure metal, or to provide a distinct appearance. Gold is preferred for architectural purposes because it does not tarnish.
Gold as an element has been with us since the beginning of time. Its use in sculpture and architectural decoration is widespread, dating back to 3000 BC in Syria where silver nails were wrapped in gold foil. Tomb paintings in Egypt at that time depict men beating gold into thin sheets of foil. Early gold foil gilding was attached to the substrate with overlapping sheets and burnished until it attached to the metal substrate. This was obviously very laborious, and not very durable. A refinement was to carve grooves into the substrate, press the edges of the foil into the grooves, and beat the seams shut. While this method was applicable to wood and metal objects, it was not as effective for brittle stone. Eventually animal hide glue was used, but this deteriorated over time, leaving only the gold left in seams and crevices.
Eventually fire gilding was developed in 300 BC wherein gold was melted down and added to hot mercury and stirred with an iron stick until the gold dissolved. This liquid was further treated to form an amalgam. Once the amalgam was applied to the object, it was heated until bonded. This was the standard method of gilding until the Renaissance. In the mid-nineteenth century the method was abandoned due to the concerns with volatilized mercury affecting both the workers and the inhabitants surrounding the gold works.
Once gold refining methods were developed in Mesopotamia about 2000 BC, gold foil could be pounded down into thinner sheets. The purer gold allowed the “gold beaters” to pound the gold into sheets so light that they move under the breath. Gold leaf production has been the same since the 14th century. Imitation leaf was developed in the 19th century to address the rising middle class during the Golden Age, who wanted to show their wealth at a fraction of the cost.
Gold leaf is typically produced in small sheets, which are sold either loose or on transfer papers. Preparation products to install the leaf depend on the substrate, but the steps are similar. Sand the substrate smooth and remove all dust. Apply a suitable primer to the substrate and let it dry. Apply the adhesive (or “size”), letting it dry until just tacky for maximum adhesion. The loose gold leaf is transferred one at a time by a gilder’s knife. Transfer leaf is applied by placing the leaf with the transfer paper onto the surface, then peeling back the paper.
The type of ‘size’ used correlates to the substrate conditions. For surfaces with medium porosity, such as wood, plaster, stucco and paper, water-based size is used. For smooth and compact surfaces such as metal, plastic and glass, alcohol-based size is used. Water-based and alcohol-based are suitable for interior gilding. For exterior applications on either porous or nonporous surfaces, oil-based size is used.
To coat or not to coat?
With pure gold leaf, coating is optional since it doesn’t tarnish over time. If using gold with a high silver content, or silver, aluminum or composite leaf, a coating is recommended to prevent damage, especially in areas that can be touched, or are exterior, as they will all tarnish over time. De-waxed shellac natural varnish in several coats is traditional, using a colorless extra-clear type for silver or aluminum leaf to keep its original color.
Strengths and Weaknesses
Two actions damage gold leaf. Interior applications are typically marred by touch—hand soiling, impact damage, and exposure to moisture (roof leaks) and dirt accumulations. Exterior applications are more prone to weathering: wind driven particles, bird soiling, freeze thaw, and UV deterioration of the adhesive, all lead to loss of the surface. These actions take place over decades.
Annual observation can identify when such repairs as touching up scuffs or coating are needed. All locations of gold leaf should be identified by location. The observed conditions should be photographed and recorded by date and kept in a permanent record. This will track when the deterioration is accelerating, and a decision to trigger maintenance can be made at that time.
Routine maintenance can slow the surface loss. By maintaining an overall smooth condition of interior leafed surfaces, vandals won’t be attracted. By maintaining coatings, the exposure of the substrate will be limited. Gilding which is mostly intact can be repaired by infill gilding smaller areas over time. The process follows a similar method to new gilding and does not require the removal of the original gilding extant. Left without intervention, gold leaf can weather away entirely, with little remaining to tell of its form grandeur.
Gilding is a job best left to professionals, both in its original installation, and in the execution of repairs. There is a great deal of experience required in selecting the materials which work with the substrate and each other, and craftsmanship only comes from a person with a great deal of practice and experience. Specifications should require gilders to have at least 5 projects of similar size and complexity, with similar materials and substrates.
It is important to consider that, beyond the fact that pure gold doesn’t tarnish, it doesn’t require a coating which requires maintenance. Gilding has a higher proportion of costly labor to the material cost, despite the rising price of gold. When specifying repairs or new work, ensure that you place a reasonable time frame for the validity of a bid, due to the gold price fluctuation. Perhaps bidding with a stated per-ounce price for the gold will permit more consistent bidding, and with more fairness to the bidders. Who knows, this approach could be to your advantage should the price drop.
A human hair is 70 microns. Gold leaf starts from 0.15 micron thick. How does it get that thin?
Pure gold, refined from the mines, is melted down at temperatures over 1800° F with small amounts of other elements (copper, silver) for strength. The amount of other elements added determines its color, working properties, and its karat number. Pure gold (24K) is not strong enough for any functional purpose.
The liquid gold is poured into molds to make ingots and cooled. The ingots are then flattened by a series of steel rollers to a thickness of about 300 microns. The flattened gold is then tempered with heating and cooling to remove the tension in the metal, stretching it into ribbons about 25 microns thick. These ribbons are cut into a four-inch squares. The four-inch square, in turn, is divided into four equal pieces, and placed between parchment sheets. Mechanical hammers can pound these postage-stamp-sized down to three or four microns. The rest is manual labor.
Goldbeating is an ancient craft, frequently done by generations of one family. The flattened gold between the sheet of parchment are stacked into a block and delivered to the craftsman. The stacks are and beaten by hand. Goldbeaters are extremely skilled at using many different hammers of different weights. The strikes of the hammer fall in time to their breath, rendering the gold into smooth silky sheets of gold a micron thick that can be blown off the table with a breath.
Each golden film is then cut into four-inch squares that are transferred between the 25 sheets of books that are sold to the gilders. The offcuts? Carefully collected and put back into the melting pot.
www.manetti.com/en/ An informative website for an Italian company of 15 generations over 400 years that is a major manufacturer of gold leaf.
Becker, Ellen Gold Leaf Application and Antique Restoration © 1998 Schiffer Pub. Limited
Oddy, Andrew “Gilding Through the Ages—An Outline History Of The Process In The Old World” Research Laboratory, The British Museum, London, U.K. published in Gold Bull Magazine, 1981, pg 14
F.W. Rauskolb, History Of Gold Leaf And Its Uses, © 1915 (Company published book)