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The Surface Enrichment of Carat Gold
Alloys  Depletion Gilding*
MARK GRIMWADE
Consultant, UK
Introduction because the treatments described recipes for refining, testing, alloying
.
The techniques of surface enrichment have a relevance to some of the more and soldering of gold, for gilding
of jewellery and other artefacts made modern processes such as wet and silver and other metals, and for the
from alloys containing gold have dry colouring. preparation of base metal alloys. An
been practised for well over 2000 The fact that base metals and silver English translation of the text was
years. The techniques are known as can be removed by chemical means published in 1926 (4) and discussed
Depletion Gilding or Mise en Couleur from gold alloys is well known. For more recently by Hunt (5). Of
and, in essence, the process involves example, when gold is assayed by particular interest in the context of
the removal of base metals and silver cupellation (fire assay),  parting  with depletion gilding is Recipe No. 25:
from the surface layers to leave them nitric acid is used to remove silver  For treating gold or for thoroughly
considerably enriched in gold. This is leaving behind a residue of fine gold. purifying it and rendering it
obviously radically different from Similarly, refining by  inquartation is brilliant: misy 4 parts; alum 4 parts;
other gilding processes such as done by alloying gold scrap with salt 4 parts. Pulverize with water
electroplating, electroless plating and copper such that the gold content is (to make a paste) and having
the older techniques of water gilding not greater than about 25%. It can then coated the gold with it place in an
and fire gilding where gold is applied be treated with nitric acid to remove earthenware vessel put in a
from an external source onto a silver and the base metals to leave furnace and luted with clay (i.e.
substrate, usually of base metal or silver. gold. Many early cultures in different sealed with clay) and heat until
Similar changes in surface parts of the world used chemical above named substances have
composition to enhance the colour and means for the surface enrichment of become molten; then withdraw it
finish of carat gold jewellery are gold-containing alloys and even for (the gold) and scour carefully.
achieved nowadays using processes gold refining before nitric acid was According to Pliny, misy is either
such as Wet and Dry Colouring, bright available. In Europe, for example, iron or copper pyrites or their
acid dipping and pickling and chemical nitric acid has only been in use since oxidation products, i.e. the sulphates.
 bombing with cyanide/peroxide the twelfth century
mixtures although these do not Pre-Columbian Goldsmiths
necessarily all involve surface Mediterranean Cultures The pre-Columbian cultures of Central
enrichment of gold. The last named Analysis of fifth and fourth century BC and South America were highly skilled
process is particularly hazardous Greek  white gold and electrum coins at metal working, and archaeologists
because of dangers of spillage and has shown the existence of a gold- have drawn attention to the methods
toxicity and there are other enriched surface layer with that were used to produce gold-
disadvantages as will be seen later. concentrations of over 70% gold being enriched surfaces on items made from
There appears to be very little found in places, as compared to about alloys containing a low proportion of
recently published or readily 35% for the bulk of the metal (2,3). A gold. Gonzalo Fernandez de Oviedo
available practical information for cementation process was described by (1535-1548) wrote (6):
jewellery manufacturers on these Pliny in the first century AD for  ... they know very well how to
processes although Rapson has refining gold. In this process, impure gild the objects and items they
reviewed some of the scientific gold is placed in a crucible in intimate make from copper and low grade
reports on selective dissolution and contact with a mixture of salt and brick gold. And they have such ability
corrosion of gold alloys in acid media dust and then heated. Base metals and and excellence in this, and give
(1). This paper is an attempt to silver are gradually removed from the such a high lustre to what they
disseminate some of the information gold and absorbed into the brick dust gild, that it looks like good gold of
so that manufacturers are encouraged leaving behind a residue of relatively 23 carats or more... They do this
to investigate the use of alternative pure gold which can be separated with a certain herb, and it is such a
processes to chemical bombing. from the brick dust by washing. secret that any goldsmith in
Another source of information on Europe or in any other part of
Historical background ancient techniques is the Leyden
*This is based on a presentation given at the 13th
It is pertinent to briefly review the Papyrus X, a metallurgical text
Santa Fe Symposium on jewelry manufacturing
historical and archaeological written in the fourth century AD by a
technology, held in Albuquerque, N.M., USA in May
evidence for depletion gilding goldsmith, and which contains 1999 (see report, this issue).
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Christendom, would soon become chlorides were then absorbed by the
a rich man from this manner of brick dust. Additional ingredients
gilding. may have been ammonium chloride,
potassium nitrate, copper and iron
The depletion gilding techniques sulphates, vinegar and urine. After
that were most likely to have been cooling, and subsequent washing to
used by the pre-Columbian Indians remove residues, the brilliance of the
have been investigated by Lechtman gold-enriched surface was improved
(7,8). The alloys used were generally by burnishing.
of two types. One type was known as An alternative method was to
tumbaga, reddish, bronze-coloured immerse an object to be gilded in an Figure 2 - Cross-section of Vicus disc
showing depletion gilded layer. x200
copper-gold alloys produced with aqueous paste or solution of alum,
differing gold contents, and probably iron sulphate and salt at room
containing silver as an impurity. The temperature. After about ten days, it
other type were pale greenish-white was taken out and washed in a strong
ternary silver-gold-copper alloys salt solution to remove surface scale.
containing a high proportion of The object was then heated to
silver. These were not dissimilar to convert the spongy, gold-enriched
the electrum of the Mediterranean surface to a smooth compact richly
world and appear to have been coloured surface. Both the
widely used in Peru. cementation and the aqueous paste
The methods used by the pre- methods work equally well on alloys
Columbian goldsmiths also fall into with a high silver content and with
two groups. The first is Mise en tumbaga.
Figure 3 - Vicus disc showing gold flecks
Couleur, a name given in Figure 1 shows discs which were
in corrosion products. x200
archaeological circles to processes made in Peru probably during the
for depletion gilding objects made early Vicus period. The classical Vicus
from tumbaga, in which the alloy was period was between 400 BC and 100 The Vicus culture was followed by
rubbed with the juice of a plant and AD, although there was a later period the Mochicas who excelled in a
then heated so that it assumed a gold which extended to about 700 AD. It is variety of metalworking techniques,
coloration due to the removal of thought that these particular discs are such as raising, forging, chasing and
copper from the surface layer. This from the earlier period because of the lost wax casting using mainly the
process could be repeated many lack of surface decoration. The discs high silver-containing alloys, similar
times to increase the thickness of the had lain buried for many centuries to electrum. They in turn were
gold-enriched layer and improve its and traces of black corrosion followed by the Chimu (1000-1470
colour. It is believed that the plant products may be seen in the AD) who were highly skilled in
was a species of oxalis and that the micrographs of cross sections of the depletion gilding objects made from
juice contained oxalic acid. It is discs, Figure 2. A thin, gold-rich layer tumbaga and electrum type sheet.
possible that ammonium carbonate is clearly visible, which is surrounded Jacobson and McKenzie (9) and
soaked in urine was also used. by corrosion products in places. Tiny Scott (10) put forward the argument
Lechtman has pointed out that flecks of gold can be seen within that low gold alloys containing
although depletion gilding of corrosion products, Figure 3. These copper were first heated in air to
tumbaga works well with these particular discs were made from produce a black copper oxide scale
processes, they do not remove any tumbaga having high copper and low and then treated to selectively
silver that might be present in the gold contents. remove the copper and silver.
alloy.
Gold alloys of the electrum type
containing high silver were probably
gilded using a cementation process
or by using special aqueous pastes.
In the cementation process, the
object to be gilded was placed in a
crucible and surrounded with a
powdered mixture which invariably
contained alum (potassium
aluminium sulphate), common salt
(sodium chloride) and brick dust.
The crucible and its contents were
heated and the mixture reacted with
the surface of the alloy to form
chlorides of silver, copper and other
Figure 1 - Depletion gilded tumbaga
impurity metals. The molten
discs from the Vicus Period, Peru
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Depletion Gilding Experiments experiments. Using electrically
and Workshop Trials heated ovens or muffle furnaces,
In 1981, the author of this paper, temperatures in the range 100 to
together with Teresa del Solar, an 600°C and treatment times ranging
archaeology graduate from Peru who from 15 minutes to several hours
was studying silversmithing and were investigated. The following
jewellery at the City of London trends were observed.
Polytechnic, did some experiments " At temperatures in the range 100-
and workshop trials on depletion 150°C, enriched layers of up to 7 µm
gilding using cementation and in thickness were obtained but these
aqueous paste treatments similar to had a patchy grey surface
Figure 4 - 9ct gold-silver alloy treated 7
those described earlier. The aim of discolouration and grey inclusions h at 165°C in alum paste. x250
this short research programme was to were found within the layer. It is
.
see whether the techniques could be thought that these were due to the
successfully applied to 9 and 14 carat formation of silver chloride which
gold-silver alloys (11). could not be removed and absorbed
The alloys were prepared using either by the porous crucible or by
fine gold and silver grain by melting, brick dust at these relatively low
casting, forging and rolling to 1mm temperatures.
thick strip with intermediate and final " Better results were obtained in
anneals. The hardnesses of the the range 150-200°C with only a few
annealed strips were HV40.5 and 42.5 greyish patches being visible on the
for the 9 and 14 carat alloys, surface. Figure 4 shows a
respectively. The 9 carat alloy was microsection of a 9 carat sample after
Figure 5 - 9ct gold-silver alloy treated 15
white, whereas the 14 carat alloy had 7 hours at 165°C in the alum paste
minutes at 600°C in alum paste. x250
a distinct pale greenish-yellow tinge. without brick dust but in an
Two different pastes were earthenware pot. The layer thickness
prepared by mixing the powdered is 10-12 µm.
chemical compounds together with " At temperatures in the range 400-
sufficient distilled water to form a 600°C, there was depletion of silver
thick paste. These were: in the grain boundaries adjacent to
the surface layer, Figure 5.
Alum paste " Although both the alum and iron
Potassium Nitrate 10 g salt pastes yielded depletion gilded
Sodium Chloride 9 g layers, the latter tended to give
Potassium Aluminium Sulphate blackened surfaces as well as grey
(Alum) 5g patches and inclusions and superior
Iron salt paste results were obtained with the alum
Ferrous Sulphate 10 g paste.
Sodium Chloride 5 g Another variation in technique
Potassium Nitrate 5 g proved equally successful. In this,
samples were placed in the alum
Figure 6 - EPMA analysis showing gold
In some experiments, the pastes were paste with brick dust in an
distribution inwards from the surface of
used alone with samples cut from the earthenware pot and gently heated
9ct gold-silver alloy given 9 cycles of torch
alloy strips whereas, in the with a torch for 3 minutes such that
heating in alum paste and brick dust.
cementation trials, the pastes were the paste melted but did not dry out.
mixed in equal parts with brick dust. Although temperatures were not
Three types of crucible were used, measured, the paste had an
namely, glass, porcelain, and porous appearance similar to that when
earthenware pots used for assaying. furnace heating to 150-170°C. After
The glass crucibles were unsuitable washing, the treatment could be
as only thin depletion gilded layers of repeated, with intermediate washing,
poor quality were obtained. as many times as necessary to build
Porcelain crucibles were satisfactory up the thickness and obtain the
only when brick dust was present. desired surface.
The porous earthenware pots were Some of the sections were
necessary to absorb the molten subjected to electron probe
reaction products containing silver microanalysis (EPMA) to analyse the
from the surface layers in the absence variation of gold and silver contents
of the absorbent brick dust. in from the surface. Figures 6 and 7
It is only necessary to summarise show the gold and silver distributions
the results from the many inwards from the surface and Figures
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8 and 9 show the corresponding
changes in concentration. The higher
gold content at the surface can be
clearly seen and was calculated to be
69% for this particular 9 carat gold-
silver alloy sample, which had been
given nine cycles of torch heating for
3 minutes followed by washing.
Because the gold-silver binary alloys
were considered to be too soft for
general jewellery applications,
experiments were done also on a
ternary 9 carat gold alloy containing
57.5% silver and 5% copper prepared as
before. The annealed strip, which was
Figure 7 - As Figure 6 showing the silver
again whitish in colour, had a hardness
distribution.
of HV 70. The results with regard to the
quality and thickness of the depletion
gilded layers were similar to those
achieved with the binary alloys.
Additional treatments, not tried with
the binary alloys, gave interesting
results. A sample of the ternary alloy
was heated for 14 hours at 450°C in a
mixture of alum paste and brick dust
in an earthenware pot. The surface
layer produced was found to have an
intriguing structure, Figure 10. The
depleted layer was about 75 µm thick
but contained islands of relatively
undepleted material which EPMA
indicated contained more copper than
Figure 10 - 9ct gold-silver-copper alloy
did the bulk of the sample. This might
heated 14 h at 450°C in alum paste and
suggest that the silver is removed at a brick dust. x200
Figure 8 - EPMA analysis showing the x-
faster rate than the copper. The
ray intensity distribution of gold. Gold
average gold content for this depleted
peak calculated to be 69%.
layer was 51%. Similar structures have
been observed in studies on the
corrosion behaviour of gold-silver
alloys in nitric acid, as will be
discussed later.
One sample of the ternary alloy
was heated in a boiling solution of
alum (40g alum per 100 ml distilled
water) for two hours. The resulting
depletion gilded layer had a thickness
Figure 11 - 9ct gold-silver-copper alloy
of about 2 µm but was compact and heated 2h in a boiling solution of alum.
x400
had a good gold colour, Figure 11.
The torch heating and washing
technique was equally successful with
the ternary alloy. Figure 12 shows a
sample given nine cycles of 3 minutes
in the alum paste/brick dust mixture
Figure 9 - As Figure 8 showing the x-ray
with intermediate and final washings.
intensity distribution for silver.
The coating is 8-10 µm and of good
quality. The advantage of these two
techniques is that they are relatively
easy to use in a workshop situation
and the time to produce a layer of,
say, 10 µm thickness is much shorter
Figure 12 - 9ct gold-silver-copper alloy
than with a single treatment in a
given 9 cycles of torch heating in alum
furnace. paste and brick dust. x400
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After the completion of the Wet and Dry Colouring suggest that considerable
experimental work, Teresa del Solar There are old established chemical improvement in colour can be
demonstrated the possibilities for treatments, known as wet or dry achieved in lower carat alloys using
depletion gilding using the torch colouring, which were widely used in similar mixtures and it is possible that
heating method on a series of the jewellery trade, if not today, for the important factors of process time
plaques made from the ternary alloy imparting a rich gold colour to the and temperature, mixture
with various designs on their surface. These processes have been composition and caratage and
surfaces. Areas where depletion described in detail by Staton Abbey composition within caratage require
gilding was not desired were covered (12) and Ernest Smith (13). In a more thorough investigation.
with a resist and this was extended to essence, they are depletion gilding Recently, it has been reported that a
give variations in the colour of the processes using the same types of colouring solution containing a black
enriched surface by applying the treatments as described above. Two sludge or powder is being used to
resist at different stages of the heating recommended mixtures* for wet improve the appearance of gold
.
/washing cycles. The reflectivity of colouring (13) are:- jewellery after immersion. It is said to
the enriched layer was further give an  Italian Colour to the surface.
enhanced by burnishing and also by No. 1 No.2 It is known that the solution is based
finally heating in the presence of Potassium Nitrate 100 g 100 g on sulphuric acid. Analysis of the
ammonium sulphide or sulphur to Sodium chloride 50 g 50 g solution and the black sludge by
give what is often called an  oxidised Hydrochloric Acid 91.3 ml 91.3 ml Inductively Coupled Plasma (ICP)
silver finish to the undepleted Water 45.7 ml 53.3 ml Spectroscopy showed what chemical
regions. In fact, the characteristic * Converted to metric measures elements were present but not their
brown-black surface colouration is compound form. Aluminium was
due to the formation of sulphides. It will be noted that the only found in both the solution and the
Examples of the plaques are shown difference is that Solution No. 2 is sludge, where the level was
in Figure 13 (a & b). slightly more dilute. Nevertheless, particularly high. Although trace
Smith claims that this is better for 14  amounts were found of a number of
18 carat jewellery. He states also that elements, there were significant levels
alloys as low as 9 carat can be treated of lead, tin and cadmium. It is possible
when suitable compositions are that these were impurities rather than
selected, preferably those with a low deliberate additions. It is not possible,
silver content. Some mixtures contain on the basis of these analytical results,
alum, e.g. potassium nitrate (100g), to be sure what the colouring solution
sodium chloride (50g) and alum contains but there is a probability that
(50g). Typical recommended aluminium is present as a sulphate in
procedures (12) are to gently heat, which case the solution is not too
melt and boil the powdered dissimilar from the solution described
compounds together with 40 ml above for depletion gilding and wet
water. Add 7 ml hydrochloric acid, and dry colouring.
reboil and then immerse the work to A  Rinse powder containing a
(a)
be coloured for, say, 4 minutes, mixture of detergent and an
remove and rinse in clean boiling unknown active agent was also being
water. Then 20 ml water is added to used by the same manufacturer.
the colouring pot and then the Analysis by X-ray fluorescence
immersion and rinse process showed the presence of oxygen
repeated as necessary, with more (72%), sulphur (15%), aluminium
water being added between each (5.9%), potassium (2%) and sodium
immersion. (3%) together with silicon and
Dry colouring is similar except that phosphorus. This suggests
the acid and water are not added to aluminium sulphate or alum is the
the melt. Items are immersed directly major constituent with phosphate
in the melt when it boils up and starts and silicones typical of modern
(b)
to fume, held for a few seconds until detergents.
the melt sinks down, withdrawn and Warning: Caution! Fumes arising
pickled in hydrochloric acid pickle to from the boiling mixtures and
remove the adhering mixture before solutions described above are
repeating the process. Final injurious to health and adequate
burnishing further improves the rich ventilation and use of a fume
brilliant gold colour. Both Smith and cupboard are essential. Again, care
Staton Abbey state that dry colouring must be taken when adding water to
can only be used satisfactorily with these mixture but provided these
Figure 13a-b - Decorative plaques given
18 carat or higher quality golds. precautions are taken, the processes
varied cycles of torch heating in alum
paste and brick dust. However, our experience would are suitable for use in a workshop.
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Acid Pickling obtain the desired gold colour. It is
Acid pickling has long been also noted that if chlorides are
practised, primarily as a means for present in the solution, silver chloride
removing oxide films and flux may be formed which is re-
residues after heat treatment and precipitated back onto the surface,
soldering operations. causing discoloured patches.
A variety of pickling solutions, in It has been practice in many
terms of the acid strength, type of workshops (15) to gold  boil the
acid and bath temperature, have higher caratage gold wares in pure
been used depending on the caratage hydrochloric acid to obtain a deep
and composition within caratage of gold colour. Since the acid is non-
the gold jewellery being treated. oxidising, the silver or gold is not
Smith (13) give a good account of attacked and silver chloride is not re-
common pickles and should be deposited. However, there is a
referred to if more information is danger that with severely cold-
required. worked, unannealed parts, the risk of
Importantly, acid pickles can stress corrosion cracking may be
selectively dissolve out the base increased.
metals and silver from the surface It should be noted that parts
layers. Rapson and Groenewald (14) should not be held by clamps or
state that hydrochloric acid, sulphuric wires of non-precious metal when
acid and aqua regia acid mixture immersed in acid pickles since these
Figure 14 - Corrosion and diffusion re-
have been used for colouring but that will be attacked and go into solution,
ordering model by selective dissolution.
special procedures have to be causing re-deposition and unsightly After Forty et al (15,16)
followed if more than superficial coatings of the more noble metals.
surface enrichment is to be achieved. A number of workers have studied means was patented which has
They state also that some gold is selective dissolution from noble become widely known as chemical
dissolved in addition to the other metal alloys. Notably, Forty and  bombing (19). Normandeau
metals but is re-deposited onto the Durkin (16) and Forty and Rowlands described the process in some detail
surface by electrochemical (17) have studied the selective at the 1990 Santa Fe Symposium (20).
replacement methods. The technique dissolution in gold-silver and gold- Essentially, jewellery wares are
works best with copper-rich alloys of copper alloys in nitric acid and placed in a hot sodium or potassium
14 carat or lower. proposed a model by which the cyanide solution at 80-85°C in a
These procedures have been surface is enriched with gold. This suitable reaction vessel. A roughly
outlined in one of a series of articles work was reviewed by Forty (18) and equal volume of 30-50% hydrogen
appearing in a trade journal discussed by Rapson at the 1995 peroxide (H2O2) is added and almost
published in Germany probably Santa Fe Symposium (1). Briefly, the immediately there is a violent
during the 1960 s (15). Relatively mechanism proposed is corrosion foaming and explosive reaction
strong acid solutions have to be used, tunnelling, whereby the silver or liberating oxygen gas. The reaction
otherwise only the copper is copper atoms are removed, leaving should be quenched with cold water
removed, leaving discoloured gold- gold atoms in the surface which form just before it boils over. It can be
silver surfaces. A solution of 1:1 islands and grow by a reordering appreciated that this is a hazardous
sulphuric acid: water is process involving surface diffusion, operation and, at the very least, it
recommended at 80°C. This dissolves Figure 14. Alloys containing more should be performed in an efficient
some silver from the surface. than 50% gold rapidly form a fume cupboard. The full safety
Warning: Caution! Always add the continuous gold surface and precautions taken when using
acid to the water in a thin stream with passivate, i.e. the reaction is cyanides must be observed.
constant stirring. Never add water to effectively stopped whereas with Faccenda suggests that the process is
the acid as heat is rapidly evolved, lower caratage alloys, the enriched gradually being abandoned and that
leading to boiling and violent surface will remain spongy and relatively few companies are using it
ejection of acid, giving the risk of a porous and continue to increase in and then only with special  closed
serious accident to personnel. thickness. It will be recognised that circuit equipment (21) to ensure
For gold-silver-copper alloys, the this is the principle of inquartation safety.
following etchants are recommended referred to earlier, whereby excess
(15). Water and chloride-free nitric silver or copper has to be added to
acid mixed in the proportions 1:1 or higher caratage gold alloys to obtain
2:1. Both copper and silver dissolve fine gold when boiling in nitric acid
in nitric acid but Reference 15 says for assaying or refining.
that the copper dissolves earlier so
that, after a short etching time, the Chemical  Bombing
colour appears too light. It is In the early 1970 s, a procedure for
necessary to etch for a longer time to brightening gold alloys by chemical
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Various concentrations of cyanide solder surface was depleted of gold acid and 20 g water for thinner
in solution have been reported and that the anticipated surface articles and one of 100 g potassium
ranging from 13  110 g/l but since enrichment as a result of depletion of nitrate, 65 g sodium chloride and 80 g
cyanide is potentially very base metals was not found in the hydrochloric acid for general use.
dangerous, the use of the lower presence of the corrosion product. Filigree work will necessitate a
concentrations is recommended (21). This might indicate that surface further dilution, i.e. 50 g hydrochloric
Indeed, Schneller has emphasized enrichment during  bombing occurs acid and 50 g water.
that there is no advantage in using by re-deposition of gold onto the It may well be that some research
higher concentrations and that the surface from the solution. and development work is needed to
process can be more efficient at the Normandeau concludes that a simple establish the best additives and
lower concentrations (22). He has solution to the problems of corrosion concentrations for particular
stated that 13 g/l is not only quite and the dangers of spillage and toxicity caratages and alloy compositions.
adequate but is safer. The reaction, in would be to eliminate chemical The advances in analytical
.
terms of weight loss, reaches a  bombing as a finishing process. procedures will greatly assist in
maximum at about 25 g/l and understanding the mechanisms of
reduces at higher concentrations. Concluding Remarks depletion and surface enrichment.
According to Normandeau and The time has come, perhaps, to re- It is hoped that this paper will
Faccenda, the base metals are examine some of the colouring and stimulate an interest in re-discovering
dissolved in the cyanide leaving a surface enrichment processes in use techniques for surface enrichment
thin surface layer enriched in gold. It in workshops earlier this century. At and colouring, particularly if they can
may be necessary to repeat the the time they suffered from a replace the hazardous operation of
process a number of times to achieve disadvantage that the surfaces chemical  bombing .
the desired result, particularly if obtained were matt or porous and it
lower concentrations of cyanide are was necessary to burnish articles, References
used. Rapson and Groenewald say usually by scratch brushing with soap 1 W.S. Rapson,  Advances in
that the process can be used to lubricants. This may be acceptable Knowledge Relating to Gold
restore the colour of white golds if for a few items but it hardly lends Alloys and their Use in
they have developed a yellow tinge; itself to modern mass production Jewellery , Proc. Santa Fe
presumably nickel-white golds due processing and was, perhaps, one Symposium, p65, 1995. Ed. D
to phase separation. reason why chemical  bombing Schneller, Met-Chem Research
Surprisingly, little seems to have appeared to be an attractive Inc., Boulder, Colorado.
been published on the mechanisms alternative. However, considerable 2 J.F. Healy and A.S. Darling,
of dissolution in chemical bombing. advances have been made in recent Nature, 231, p443, 1971.
One might expect that the base years in mass finishing equipment 3 J.H.F. Notton  Ancient Gold
metals and silver and gold will all be and processing. Refining , Gold Bull., 7(2), p50,
selectively dissolved but that the rates The use of pastes and absorbent 1974.
of solution may vary with cyanide materials such as brick dust are 4 E.R. Caley,  The Leyden
concentration. There is an extensive unlikely to be suitable for factory Papyrus X , J. Chem. Ed., 3,
review of the reactions with cyanide operations, although they may be for p1149, 1926.
in the book  The Metallurgy of Gold small workshops, but the use of 5 L.B. Hunt,  The Oldest
by Rose and there it is stated that the solutions for immersion followed by Metallurgical Handbook , Gold
rate of dissolution of gold goes rinsing and re-treatment, if necessary, Bull., 9(1), p24, 1976.
through a maximum with increasing should not present a problem. A final 6  The Gold of El Dorado ,
cyanide concentration and is reduced burnishing treatment using modern Catalogue of Exhibition held at
at high concentrations (23). machinery may produce the desired the Royal Academy, London,
Normandeau has discussed also a finish in terms of gold enrichment 1979.
further problem with chemical and colour. 7 H. Lechtman,  Ancient Methods
 bombing when using 10 carat There are a number of variations of Gilding Silver , Science and
cadmium-free solders which contain when considering solutions. Boiling Archaeology. p2, 1971. Ed. R.H.
indium (20). Indium cyanide may be in a solution of 40 g/100 ml of Brill, M.I.T. Press, Cambridge,
deposited as a discoloured corrosion potassium aluminium sulphate Mass, U.S.A.
product on the jewellery, requiring (alum) for two hours certainly 8 H. Lechtman,  The Gilding of
removal by additional finishing produced an acceptable result with Metals in Pre-Columbian Peru ,
operations such as abrasive the 9 carat gold-silver and gold-silver- Proc. Third Sem. (1970) on
polishing. This, in turn, may produce copper alloys and it is probable that Application of Science in the
a toxic dust and precautions would some of the other solutions given Examination of Works of Art,
be required against inhalation. The earlier in the paper will give equally Boston, Mass., 1973.
corrosion problem was not observed good results in perhaps a shorter 9 D.M. Jacobson, and J.S.
with 14 and 18 carat solders time. Reference 15 quotes an earlier McKenzie,  Transmutation of
containing indium. worker who recommends an etch of Base Metals into Gold ,
Energy dispersive X-ray analysis 100 g of potassium nitrate, 65 g Interdisciplinary Science Rev.,
showed that the chemically  bombed sodium chloride, 60 g hydrochloric 17(4), p326, 1992.
22
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" WOR/6615 Grimwade Tech web 18/8/99 3:57 pm Page 23
10 D.A. Scott,  Depletion Gilding 16 A.J. Forty and P. Durkin,
and Surface Treatment of Gold Phil.Mag., 42, p295-318, 1980
Alloys , J. Hist. Met. Soc., 17(2), 17 A.J. Forty and G. Rowlands,
p99, 1983. Phil. Mag., 1980, reviewed in
11 T. del Solar and M.F.Grimwade, (18) .
 The Art of Depletion Gilding , 18 A.J. Forty, Gold Bull., 14 (1),
Aurum, 12, p46, 1982. p25-35, 1981.
12 Staton Abbey,  The Goldsmiths 19 G. Mulnet, French Patent
and Silversmiths Handbook , 2,137,296, 1972.
2nd Ed. Publ. by The Technical 20 G. Normandeau,  Cadmium-
Press, London, 1968. Free Gold Solders: An Update.
13 E.A. Smith,  Working in Indium Toxicity and Potential
Precious Metals . Re-published Workplace Exposures , Proc.
by N.A.G. Press, Colchester, Santa Fe Symposium, p239,
Essex, England, 1978. 1990. Ed. D. Schneller, Met-
14 W.S. Rapson and T. Chem. Res. Inc., Boulder,
Groenewald,  Gold Usage . Colorado.
Publ. by Academic Press Inc., 21 V. Faccenda,  Handbook on
London, 1978. Finishing in Gold Jewellery
15 Anon.,  Kleine Manufacture , Published by
Edelmetallkunde: Kursus für World Gold Council, 1999.
Gold und Silberschmiede, 22 D. Schneller,  Safety
Section 2.1.4.1. (This course Overviews . Given at each
appears in a series of articles in annual Santa Fe Symposium.
Gold und Silber, Uhren und 23 T.K. Rose and W.A.C. Newman,
Schmuck extending over a  The Metallurgy of Gold ,
number of years.) Chap.XIII, 7th Ed. Re-published
by Met-Chem. Res. Inc.,
Boulder, Colorado, 1986.
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