Scriptorium
Fire-Gilding (Mercury Gilding): How Gold Was Bonded to Silver and Copper
What fire-gilding actually is — painting a gold–mercury amalgam onto silver or copper and heating the object until the mercury burns off — how it differs from modern gold plating, why it was so dangerous, and its place behind the gilded surface of pre-modern metalwork.
Fire-gilding — also called mercury gilding — bonds a thin film of gold to silver or copper by spreading on a paste of gold dissolved in mercury and then heating the object until the mercury evaporates, leaving the gold fused to the surface. It was the dominant way of gilding metal from antiquity until the middle of the nineteenth century, when electroplating replaced it — and it is the technique behind the warm gold surface of most pre-modern silver-gilt and gilt-bronze objects.
The name is exact: the mercury is the part that is lost. Gold dissolves in mercury to form a soft paste — an amalgam — that can be painted onto a base metal; gentle heat then drives the mercury off as vapour and leaves the gold behind, bonded to the surface. What follows sets out what fire-gilding is, how the work is actually done, why the mercury made it one of the most hazardous of all crafts, how it differs from the modern gold plating that replaced it, and where it sits behind the gilded metalwork of the pre-modern world.1“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.” — W. A. Oddy, “Gilding: an outline of the technological history.”
What is fire-gilding?
Fire-gilding is a way of covering the surface of one metal with a durable film of gold using mercury as the carrier. Gold is first dissolved in mercury to make an amalgam — a soft, silvery paste. That paste is spread over the base metal, and the object is then heated gently: the mercury evaporates away, and the gold it held is left behind, fused to the surface as a well-bonded gold skin. It works, in the conservator W. A. Oddy's account, “well on either silver or copper.” Because mercury is the medium that is driven off, the method is also called mercury gilding or amalgam gilding.2“Fire gilding… uses gold amalgam… for gilding by spreading it over the base metal and heating gently to evaporate the mercury. This works well on either silver or copper.” — W. A. Oddy, “Gilding: an outline of the technological history,” AIC Objects Specialty Group Postprints, vol. 3 (1995).
The result is the deep, even gold surface seen on a vast range of historical metalwork: gilt-bronze mounts and clocks — known as ormolu when the technique is applied to bronze — gilded silver vessels and church plate, and the gilt fittings of arms and armour. For most of history, when an object was described as gilded, this was very often how the gold got there.
How is the work actually done?
There are two ways to lay the amalgam on. In the first, the gold–mercury paste is painted directly over the base metal. In the second, the worker rubs plain mercury onto the surface and then presses on gold leaf, which dissolves at once into the mercury to form the amalgam in place; sheets of leaf can be added until no more gold will dissolve, which makes it easy to build a thick layer. Either way, the object is then heated to drive off the mercury — and, Oddy cautions, the final heating must be done slowly, or blisters form in the gilding.3Fire-gilding “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… The final heating stage must be carried out rather slowly, otherwise blisters may form in the gilding.” — W. A. Oddy (1995).
A single application leaves only a very thin layer of gold, which on its own would resist little wear — but the process can be repeated any number of times to build the film up. One durable mark it always leaves is chemical: the gold retains a trace of mercury, and the way conservators confirm that an object was fire-gilt, rather than gilded by some other method, is precisely by detecting that mercury in the gold.4“Fire gilding normally gives a very thin layer of gold… but this gilding process can be repeated any number of times in order to increase the thickness… The key to identification of fire gilding [is] the detection of mercury in the gold by chemical analysis.” — W. A. Oddy (1995).
Why was it so dangerous?
The step that makes fire-gilding work is also what made it lethal. Heating the amalgam releases mercury as a vapour, and inhaled mercury vapour is severely toxic — it is described as causing serious health problems including neurological and endocrine damage, and inhalation is an especially efficient route into the body. Gilders working over the fumes, often for years, suffered accordingly; the danger was one of the reasons the craft was eventually abandoned in favour of safer methods.5Mercury vapour causes “serious health problems, such as neurological damage and endocrine disorders,” and “inhalation is a very efficient route” of exposure — the hazard that made fire-gilding notoriously dangerous. — Wikipedia, “Gilding” (drawing on the 1911 Encyclopædia Britannica). Single-source here; the toxicity of mercury vapour is independently well established.
The recipes give a sense of how much mercury was involved. By the figures reported from the older technical literature, the amalgam was mixed with roughly six to eight parts of mercury to one of gold, and even the finished, heated surface could still hold something on the order of thirteen to sixteen percent mercury. Those exact proportions come from a single technical source and are best read as typical period figures rather than a fixed formula — but they show why the fumes were so heavy.6“The proportion of mercury to gold is generally six or eight to one,” and the fixed film could remain an “amalgam containing 13 to 16% mercury.” — Wikipedia, “Gilding” (1911 Encyclopædia Britannica figures). Single-source; stated here as reported period figures, not a fixed recipe.
How is it different from modern gold plating?
Fire-gilding is not the same as the gold plating used today. Modern plating is electroplating: gold is deposited onto an object electrochemically, from a solution, by passing a current — no mercury, no fire. Electro-gilding was invented in the first half of the nineteenth century and became commercially viable in the 1840s, and it steadily supplanted fire-gilding, which was more dangerous and more wasteful of gold. So when a pre-modern object is gilded, the gold was almost certainly laid on with mercury and heat; when a modern one is, it was almost certainly plated.7Fire-gilding was in general use for gilding “up until the invention of electro-gilding in the 1840’s” (Oddy); it was “generally supplanted by the electroplating of gold” (Wikipedia, “Gilding”). Electro-gilding “became commercially viable in the 1840’s.” — W. A. Oddy (1995).
Fire-gilding and pre-modern silver-gilt work
For as long as objects were gilded — from antiquity to the nineteenth century — fire-gilding was the workhorse method behind the gold surface, on silver and on copper alloys alike. That places it behind the whole world of pre-modern silver-gilt metalwork: the gilded chalices, reliquaries, book covers, and crosses of Christian church treasuries, the Armenian liturgical silver among them, whose warm gilt surfaces belong to exactly this pre-modern gilding tradition.
One honest caution is worth keeping in view. When a museum labels an object “silver-gilt” or “gilt bronze,” that records the gold surface — not, on its own, the method that put it there. Confirming that a specific object was fire-gilt (rather than gilded by leaf, foil, or later electroplating) means finding the tell-tale trace of mercury in its gold, through a conservation analysis. So fire-gilding is best understood here as the dominant technique of the pre-modern gilded object in general, and the method very likely behind most early silver-gilt church metalwork — rather than a claim proven for any one named piece.
Frequently asked questions
- What is mercury gilding?
- Mercury gilding is another name for fire-gilding: gold is dissolved in mercury to make an amalgam, the amalgam is spread onto a silver or copper object, and the object is heated so the mercury evaporates, leaving a bonded film of gold. It was the dominant gilding method for most of history.
- Is fire-gilding the same as gold plating?
- No. Modern gold plating is electroplating — gold deposited electrochemically from a solution, using a current. Fire-gilding instead uses a gold–mercury amalgam and heat. Electroplating, commercially viable from the 1840s, largely replaced fire-gilding because it is safer and uses less gold.
- Why is fire-gilding no longer used?
- Chiefly because it is dangerous: heating the amalgam releases toxic mercury vapour, which caused severe harm to gilders. It was also wasteful of gold. When electroplating became commercially viable in the 1840s — safer and more economical — it steadily replaced fire-gilding.
- Were Armenian church objects fire-gilt?
- Almost certainly many were, since fire-gilding was the dominant way of gilding metal in the pre-modern period, and Armenian liturgical silver belongs to that silver-gilt tradition. But a museum label of “silver-gilt” records the gold surface, not the method — confirming that a specific object was fire-gilt requires a conservation analysis detecting mercury in its gold.