ORES METALS AND ALLOYS - Archaeological Chemistry - page 178

Chemical gilding methods used in antiquity included such processes as
fire gilding , depletion gilding , and dipping . Other chemical gilding methods, as
for example, electroplating, based on the use of electric currents, and vapor
deposition, using evaporated metals, were developed only as recently as
during the nineteenth and twentieth centuries (Budden 1991; Oddy et al.
1979). Fire gilding , also known as mercury gilding or amalgam gilding , was
widely practiced in antiquity to gild metals; it is based on the property of
mercury to evaporate from amalgams when heated, leaving behind the gold
(see text below). In practice, the object to be gilded, generally made of copper,
bronze, or brass, is first coated, using a brush or a swab, for example, with
a thin layer of freshly prepared gold amalgam; heating the amalgam-coated
object expels the mercury and leaves behind a well-adhered, thin layer of
gold on the surface of the object. Fire gilding was the major technique for
the gilding metals since 300 B.C.E. in China and 200 C.E. in Europe until the
invention of electroplating in the nineteenth century (Anheuser 2000, 1999;
Oddy 1991).
Depletion gilding , or depletion plating , as the process is also known, is a
process that was widely used in antiquity for gilding sheets of alloys of gold
and copper with a thin layer of gold. In contrast to the other gilding tech-
niques, which require the use of gold from an external source, in depletion
gilding the gold is initially an integral constituent of the original sheet of
alloy (La Niece 1995). To gild a sheet of a gold-copper alloy by the depletion
gilding technique, an object made of the alloy is subjected to a repeated
sequence of hammering , annealing , and pickling operations, so as to reduce its
thickness. Hammering reduces the thickness and shapes the sheet, but it also
hardens the alloy and, in order to soften and make it malleable again, the
sheet is annealed (see Textbox 29). Heating during annealing causes oxida-
tion of the copper in the surface of the alloy and its conversion to copper
oxide. Subsequent pickling of the sheet by immersion in an appropriate acid
(vinegar or fruitjuice in ancient times) dissolves the copper oxide, leaving
the surface slightly depleted of copper and enriched in gold. Repeating the
sequence of hammering, annealing, and pickling many times until the sheet
is modeled into a required shape removes enough copper and leaves on
the surface a sufficiently thick and continuous gilding layer. The process
was apparently used in places as far distant from one another as
Mesopotamia, Greece, and Peru. The Greeks seem to have used depletion
gilding to make Corinthian bronze, the most prized of all the copper alloys
of classical times (Jacobson and Weitzman 1992; Lechtman 1984).
Dipping , a process also known as flush gilding or wash gilding , was used
to coat the surface of objects made of base metals with a thin layer of molten
gold. Copper and its alloys were gilded by dipping in pre-Columbian South