In both methods the metal had to be carefully cleaned free of all grease and scratch-brushed to key the surface; then mercury was applied. In the more remote past metallic mercury was vigorously worked into the surface with a short, stiff brush until the surface was uniformly covered. From the 18th century, the usual practice was to dip the cleaned metal into a solution of mercuric nitrate, which precipitated mercury on to the surface in a thin, continuous layer. This process was known as quicking.
In the first method of fire gilding, thin sheets of gold were laid on to the prepared surface, vigorously burnished and then heated to drive off the mercury. This seems to be the process described by Pliny in his Natural History (xxxiii.32; Rackham, 1968, p. 77). In the second process an amalgam of mercury and gold was first prepared by heating mercury to about the temperature of boiling water and then adding about half the weight of pure gold filings, stirring with an iron rod until the amalgam had the consistency of butter (hence the term ‘butter of gold’). The health hazards of handling mercury were recognized even in medieval times, and the prepared amalgam was stored under water. The amalgam was then applied to the freshly quicked surface by brushing. When the piece was covered, it was gently warmed to evaporate the mercury, while the gilding continued to be worked with a stiff brush to ensure even coverage. Alternatively, the amalgam could be carefully applied and brushed in with a wire brush that had itself been quicked to promote even coverage, then heated in a closed furnace. In the latter process, however, the piece had to be kept under constant observation, periodically moved and the amalgam layer worked if necessary to keep the gilding even as it formed. The freshly gilded surface had a rather matt, granular appearance, which could then be further treated by scratch brushing, burnishing or by the use of chemicals to produce the desired color and texture.
The technique of fire gilding has been identified on metalwork from China in Han burials of the 3rd century AD (Lins and Oddy, 1975), and on some Hellenistic metalwork (Craddock, 1977), including items (Thessaloniki, Archaeol. Mus.) from the tomb of Philip II, King of Macedonia (reigned 359–336 BC), at Vergina (Assimenos, 1983). The use of mercury gilding at an early date in China is to be expected, given the importance attached to mercury and mercury-based gold elixirs (potable gold) by the Daoist philosophers and alchemists (Needham, 1980). Oddy (1982) has doubted the existence of mercury gilding in the West before the Romans, and certainly leaf gilding was prevalent until well into the Imperial period, when gold plating of bronze statuary and of architectural fittings became extremely popular. Fire gilding provided a much more durable bond than could be achieved with leaf gilding, and, although it could not be used on ordinary bronze, the greater resistance to corrosion and reduced maintenance costs on exterior metalwork in particular made fire gilding prevalent, although it never completely superseded the other methods.
Even in the early medieval period, when supplies of mercury must often have been difficult to obtain in Europe, fire gilding was the favored method for gilding metalwork and remained so until the mid-19th century, when, in common with most other traditional plating techniques, it was challenged and rapidly replaced by electro gilding. Although it continued for some time to be favored for applications where either its durability or the special surface effects were required on work of the highest quality, the process has become virtually extinct in Europe and North America because of the extra cost and stringent health regulations. Fire gilding is still used in Tibet, however, although in other countries where traditional metalworking methods still thrive, for example Nepal, and where amalgam gilding was practiced until the late 20th century (Oddy, 1981), it has now been replaced by electro gilding.
Theophilus: De diversis artibus (MS.; 12th century); ed. and trans. J. G. Hawthorne and C. S. Smith (Chicago, 1963/R 1979)
E. Spon: Workshop Receipts (London, 1882/R 1932)
E. Thorpe: Dictionary of Applied Chemistry (London, 1890–93)
H. Rackham: Pliny: The Natural History (London, 1968)
P. Lins and W. A. Oddy: ‘The Origins of Mercury Gilding’, J. Archaeol. Sci., ii (1975), pp. 365–73
P. T. Craddock: ‘Copper Alloys Used by the Greeks II’, J. Archaeol. Sci., iv (1977), pp. 103–23
J. Needham: Science and Civilization in China, v/4 (Cambridge, 1980)
W. A. Oddy, M. Bimsom and S. La Niece: ‘Gilding Himalayan Images’, Aspects of Tibetan Metallurgy, ed. W. A. Oddy and W. Zwalf, Brit Mus. Occas. Pap., 15 (London, 1981), pp. 87–103
W. A. Oddy: ‘Gold in Antiquity’, J. Royal Soc. A., cxxx (1982), pp. 1–14
K. Assimenos: ‘Technological and Analytical Research on Precious Metals from the Chamber Tomb of Phillip II’, Preprints of the Second International Symposium: Historische Technologie der Edelmetalle: Meersburg, 1983
K. Anheuser: ‘The Practice and Characterization of Historic Fire Gilding Techniques’, JOM, xlix/11 (1997), pp. 58–62
K. Anheuser: Im Feuer vergoldet: Geschichte und Technik der Feuervergoldung und der Amalgamversilberung (Stuttgart, 1999)