Andrew Oddy
Abstract
For the purposes of this paper, gilding is defined as the overlaying of a metal surface with a layer of gold. Many different techniques have been used to effect a gilded surface, from the use of gold foil in the remote past, to the use of gold vapor in a vacuum in the late 20th century. This paper will deal with gilding up until the invention of electro-gilding in the 1840’s.
Gilding must be distinguished from inlaying, in which a recess is cut into the surface of the metal object and pieces of gold hammered into it. The following techniques of gilding will be discussed with examples.
Foil gilding
On most types of objects this involves pressing gold foil to the surface and holding it in place by riveting, or by bending the edges of the foil round the edges of the object, or by inserting the edges of the foil into grooves cut into the surface. This method of gilding can be traced back at least to the beginning of the third millennium BC. The gold would be burnished down and gently hammered into position, and a roughened surface on the base metal substrate would increase the adhesion of the foil.
Once the gold foil was in intimate contact with the substrate, particularly if that was made of silver, heating the blank to a temperature well below the melting point of silver would increase the adhesion by causing interdiffusion of the two metals to take place. It is, however, impossible to tell from a visual examination whether heating was used to increase adhesion; this point can only be ascertained by a destructive metallographic examination of the object.
Foil gilding is distinguished from leaf gilding by the thickness of the gold when it is visible round the edges of any losses. The definition of “foil” is a sheet of gold which is thick enough to support its own weight, and the minimum thickness for a particular alloy will depend on its composition.
Leaf gilding
Gold leaf is so thin that it cannot support its own weight. It has obvious economic advantages when used for gilding, but must be attached by a suitable adhesive to the underlying surface. Several naturally occurring organic adhesives are suitable for this purpose. This technique was widely used in antiquity for embellishing decorative metalwork, but the gilding is not very durable.
Diffusion bonding
Gold leaf can very easily be made to stick intimately to silver by rubbing it down with a suitable burnishing tool, often of agate or ivory, and then gently heating for a few seconds. This eauses interdiffusion of the gold and the silver to take place, thus creating a strong bond between the gold and the silver. It seems to be less easy to make a successful bond with copper.
Although the bonding to the silver is very robust, the thinness of the gilding means that it will not stand much wear and tear. Although diffusion bonding is postulated as a method of gilding silver in antiquity, it seems not to have been described in the surviving technical literature of antiquity and the early Middle Ages. There is, however a brief description of the technique in the 18th century by D. Coetlogan, and subsequently in other encyclopaedias of a similar type. Burnishing gold leaf on to lead and tin is, however, described in the early Medieval Compositiones Variae, although there is no mention of subsequent heating. In view of the low melting point of tin and lead, this is hardly surprising.
Fire gilding
This technique uses gold amalgam (ie, a ‘solution’ of gold dissolved in mercury) for gilding by spreading it over the base metal and heating gently to evaporate the mercury. This works well on either silver or copper and gives rise to a well bonded layer of gold on the surface. The process is sometimes known as wash gilding or water gilding, although the latter name is more correctly applied to the process of laying gold leaf on wood or stone with a water-based adhesive.
Fire gilding normally gives a very thin layer of gold, which would have very little resistance to wear, but this gilding process can be repeated any number of times in order to increase the thickness of the gold layer. The key to identification of fire gilding if the detection of mercury in the gold by chemical analysis.
Although fire gilding is usually carried out by applying a gold amalgam to the base metal surface, it can alternatively be done by rubbing mercury on to the base metal and then applying gold leaf. The gold leaf at once dissolves in the mercury layer to form a gold amalgam in situ, and heating then drives off the mercury. It is very easy to apply quite a thick layer of gold by this technique, as sheets of gold leaf can continue to be added until no more gold will dissolve. The final heating stage must be carried out rather slowly, otherwise blisters may form in the gilding.
Electro-chemical gilding
This is one of three methods of depositing gold from a solution. The other two are electroless gilding and the decomposition of organic gold compounds.
Electro-chemical gilding, sometimes also known as immersion or replacement gilding, involves immersing the copper or silver object into a solution containing a soluble salt of gold. An electrolytic reaction takes place at the surface of the base metal, which undergoes simultaneous anodic dissolution and deposition of gold. This technique will only deposit a very thin layer of gold because as soon as the base metal is completely covered the process stops.
The gold layer produced by this technique is not very durable and because of the difficulty of getting gold into solution it seems unlikely that it was used in antiquity, although it has been postulated, on the basis of the thinness of the gilding, as a technique which was used in preColumbian Peru. However, it is possible to achieve equally thin gilded layers by the technique of diffusion bonding.
Electrolytic gilding
This technique only became possible after the invention of an electrolytic cell capable of generating a reasonably constant current. The process was developed during the first half of the 19th century and became commercially viable in the 1840’s, and was already being described (as Voltaic Gilding) in Cooley’s Encyclopaedia of 1845. It is the most common technique used to gild other metals in the 20th century.
Gilding with gold powder
One of the last of the ‘medieval style’ of craftsman’s manuals for silversmiths and goldsmiths was published in 1903 and described only two methods of gilding: fire gilding and the use of gold powder. It is interesting that, even at this late date, the author did not describe electrolytic gilding!
The gold powder method involves soaking linen rags in a solution of gold chloride, allowing them to dry, and then burning them. The ashes, which contained finely divide gold, are then rubbed on to thoroughly clean silver with a piece of damp leather, when the gold gradually forms a layer on the surface. The ashes are rinsed away and the gilding is finally burnished.
Electroless gilding and the decomposition of organic gold compounds
These are relatively modem techniques which give rise to weakly bonded layer of gold. Nineteenth century (and earlier) encyclopaedias and books of receipts list a number of methods of gilding which might belong to one of these headings. Examples are Grecian gilding, Talbot’s process, and variations of a method for gilding iron and steel in which the gold is deposited from an organic solvent. However, it seems unlikely that any of these techniques would produce a very durable layer of gilding.
Depletion gilding
The technique of dissolving the alloying elements from the surface of a base gold object so that it appears to be made of good gold is very ancient, but by the early Middle Ages it had been largely superseded for the most part by actual gilding. With coinage, however, depletion gilding can be expected to continue in use, and in particular, it may well have been carried out on official coinages in times of inflation and by forgers who could make a base gold blank and enrich the surface by depletion gilding before striking the images with (false) dies.