|
In the twentieth century there can be little doubt that electroplating
is the best way to develop a gold coating on a metal object. It provides
successful results in many colors and can be adapted to one-of-a-kind
or mass production. Knowing this, it might seem peculiar to devote the
following section to the ancient art of fire gilding. It is more time
consuming than modern methods, it requires more material, there is a greater
risk of failure, and it is a health risk unless performed under ventilation.
Why then should it be included?
For one thing, it is vital that the field of goldsmithing remember its
origins in order to keep the sense of history alive. For another, when
antique work is to be repaired or an exact replica made, modern techniques
simply will not do. There is a clearly visible difference between the
intense saturated layer of gold created in this process and the mechanically
different layer that results from electroplating.
SAFETY NOTE: Mercury evolves poisonous,
invisible vapors even at room temperature. As mercury is heated the vapors
are even more aggressively released. These vapors are absorbed through
the lungs and stored in the body; they cannot be cast off and therefore
accumulate. This means that even minor exposure will result in a health
risk eventually. The first signs of mercury poisoning are gum and throat
inflammations, nervous disturbances, irritability, general aches and pains,
muscle tremors, and fainting.
Mercury should be stored in sealed containers under water. Because mercury
can be absorbed through the skin, direct contact should be avoided; always
were thick rubber gloves and tight protective clothing. Because the vapors
are so dangerous only ventilation systems with the highest rating will
allow safe use of mercury. If any symptoms appear, stop work and see a
doctor immediately.
Making the amalgam The first step in the process is to make a paste or amalgam of gold in
a vehicle that will later be driven off. The amalgam will be made by joining
fine gold and mercury. It is widely known that the phrase "mad as
a hatter" comes from the brain damage that was common among tradesmen
who made hats because of the use of mercury in that process. Though goldsmiths
seem to have avoided the bad press, they are just as susceptible to the
neurological damage of mercury. Industrial strength ventilation is mandatory
for this process.
In order to provide the maximum amount of surface area, a sheet of fine
gold is rolled as thin as possible and then cut into tiny pieces. The
process resembles the cutting of solder chips but the pieces should be
much smaller. If the pieces curl this has the advantage of preventing
them from lying flat on the floor of the crucible.
It is impossible to mix mercury and gold by melting them together because
the mercury vaporizes at a relatively low temperature. Instead the two
metals are put into a mortar and pestle and ground together. The process
will be assisted a little if the gold and mercury are first put together
into a closed container and allowed to diffuse into each other. In order
to reduce the danger of exposure to toxic mercury fumes the grinding is
done underwater.
The process is among those that was well documented by Theophilus, Pliny
and other early writers about metalworking. The proportions given in those
ancient sources, still relevant today, are 1 part gold to 8 parts mercury,
or 11.1% Au/88.9% Hg.
A phase diagram of the Au-Hg system shows that the amalgam of this composition
has a liquidus temperature of about 290C (550F). As the amalgam cools
to temperatures approaching 124C, mixed crystals of the concentration
of about 22% Au, 78% Hg precipitate out of the melt. At 124C these mixed
crystals have a proportion of the entire mass of only about 17%, which
is to say that despite these crystals, the greatest part of the amalgam
is still fluid.
Upon further cooling a structural change occurs: the b-mixed crystals
transform themselves and there develop crystals of the intermetallic phase
Au2Hg3 (a concentration of about 39% Au; 61% Hg), which swim in the mercur-rich
remnant.
At room temperature the mixture is about 37% solid Au2Hg3 and 63% fluid
mercury. The gold may be described as "floating" in the mercury
in the same way that crushed ice floats in water. This yields a spreadable,
pasty gilding material.
Even if the proportions are changed somewhat because of the burning off
of mercury, the mix will still work. If the proportion of gold were to
drop below 39% the mixture would start to harden, but this is very unlikely
as long as you start with initial mixture of 1 part gold; 8 parts mercury.
Working under a fume hood, the mercury and gold is heated to the boiling
point of the mercury (357C; 675F). More gold can be introduced into the
crucible at this point and the whole mass is stirred with an iron or titanium
wire until solid pieces of gold can no longer be felt.
The mix is poured into a basin of clear cold water at this point to force
a crystallization of the amalgam. Because of the rapid cooling the transformation
of the b-mixed crystals into the intermetallic phase Au2Hg3 is repressed
and the condition of the temperature zone between 290-1240 C is "frozen
in." It is this process that leaves the b-mixed crystals floating
in fluid mercury.
To remove excess mercury the mass is collected and set onto a square
of chamois leather that is then pulled up into a pouch and twisted tight
at the neck. Small drops of mercury will be wrung from the mass and should
be scraped off the leather. This is a very dangerous process because mercury
can be taken in through the skin. Wear heavy rubber gloves and work in
a fume hood! The finished amalgam should be pale yellow and as spreadable
as butter.
Metals able to be gilded Mercury gilding works very well on silver/copper alloys like sterling
and it is no surprise that this was its most common use. These objects
- a layer of gold applied to sterling - are still called vermeil (ver-MAY), after the French term for fire gilding. It is also possible
to deposit a layer of gold onto copper, iron and bronze, but the surface
must be specially prepared first. In principle brass can also be gilded,
but the results are often spotty. To prepare iron and brass, a layer of
copper is deposited, either by electroplating or by immersing the work
in a saturated pickle. In the case of bronze, steel wire is wrapped around
the piece to hasten the plating reaction.
Preparing the article The article to be gilded must be perfectly clean, a state that is best
achieved with a bright dip of diluted nitric acid. The precise mix will
depend on the alloy being used but something in the range of a 50-50 mix
is typical.
Immersion of a few seconds is usually sufficient to remove all grease,
dirt and oxidation. Longer dips are not recommended because they eat away
from the surface and leave it matte. The microscopic craters that result
from this etching will require extra gilding to fill in and are therefore
something to be avoided.
Quicking the surface Amalgam will adhere well to alloys that contain over 3/4ths silver. For
metals of lower silver content, including copper and copper alloys like
brass, a preparatory step called quicking is required. The preparation
of this chemical is described in Chapter
11, Section 2. In this process a layer of mercury is laid onto the
article first to provide a better adhesion of the gold amalgam.
Applying the amalgam The amalgam paste is spread onto large objects with a brass brush. For
smaller objects it can be more convenient to use a spatula-shaped steel
rod. Quick the end of the rod and pick up a mass of amalgam then spread
it onto the piece as you might spread butter.
It is wise to work over a dish that will catch the excess as it falls
off, and of course it is worth repeating that this process can only take
place under ventilation. Remember that mercury is always releasing harmful
fumes!
Evaporating (literally, "smoking off") The traditional way to drive off the mercury is to warm the piece over
a charcoal fire. This will of course still work and has the advantage
of creating a gentle uniform heat. It would be possible, for instance,
to build a fire in an outdoor hearth, to set the work on a steel grill
and in this way remove the mercury. It would be unsafe to use this grill
for cooking food after this use.
In the case of ventilation systems, mercury causes a special problem
because it often precipitates onto the walls of the ventilation pipes
after it has been removed from the immediate vicinity. Even a properly
functioning ventilation system can be contaminated by mercury in this
way unless special filters are used. As mentioned at the outset, it is
no wonder that this process has been superseded by electroplating!
Over the fire the amalgam gives up the mercury, possibly turning into
a fluid coating if the heat is high enough. This is not required for the
process to work, nor does it cause a problem, but in large pieces like
vessels the work must be rotated to prevent the fluid from flowing down
the sides of the form.
When the mercury has been completely dissipated the surface will be left
with a pale yellow matte covering. If the coating does not appear complete,
additional amalgam can be added at this stage. The area is re-quicked,
fresh amalgam is applied, and the piece is heated. When the covering seems
adequate the piece is allowed to cool.
It is normal for 5-15% of the mercury to remain behind permanently bound
up with the gold, but if more than this remains the result is a pale color.
To avoid this the amalgam is heated further, but care must be taken not
to go so far that the gold fuses into the surface where it will make an
alloy with the silver of the article. Finishing treatment A soft golden color can be obtained through scratchbrushing, but it is
more common in fire gilding to seek a bright gold. This can be achieved
by depletion gilding; that is, by heating the article and immersing it
in pickle. A more uniform coating can be achieved by mixing the following:
- 40% potassium nitrate
- 35% table salt
- 25% alum
Mix the three ingredients together with enough water to make a paste,
heating the whole to a boil to complete the blending. Apply the paste
to the gilded surface and suspend the work over a fire (or set it on a
screen and heat it with a bushy flame) until the paste has solidified
into a salty crust. The piece is then quenched in water, which will cause
the crust to break away and dissolve. The piece is then cleaned in a dilute
nitric acid bright dip and rinsed.
|
