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Dedicated to the memory of
Wi ll iam S. Rapson
From ancient times, gold has been
used mostly in decorative items, andthe colour of gold plays an important
role in this application field. D ifferentmethods have been developed to
objectively characterize the colour.T his paper describes the C I ELA B
system, which has gained acceptanceas an effective way to assess colour.
T he different coloured gold alloysknown today are also described, with
emphasis on the relationshipbetween their metallurgy and their
colour.T he history of gold goes hand in
hand with the evolution of humancivilization, influencing its extension
and development ever since the firstgold i tem was created. T he rich
yellow colour of gold has alwaysbeen attractive to mankind, and gold
has been used in all parts of theworld, since as far back as 3000 BC(1, 2).
G old and copper are the only twometals that exhibit colour. O wing to
this property, gold alloys can bemade to assume a range of colours by
varying the alloying additions. In 862BC the Lydians were using coins
made of a green gold-silver alloy
Coloured Gold AlloysCRISTIAN CRETU AND ELMA VAN DER LINGEN
Mi ntek, Pr iva te Bag X3015, Randbu r g 2125, Sou th Afr ica
known as electrum and composedof 73wt% A u and 27wt% A g. A red
gold alloy, the combination of70wt% Cu wi th 30wt% Au, was used
during the time of the Chimu Empire(ca 1300 AD ) by goldsmiths in what is
today northern Peru ( 3). T oday,various coloured gold alloys are
available, giving the opportunity ofcreating unique jewellery and
decorative items (Figure 1) .I t is the intention of this paper to
briefly review the formation of, andassessment of, colour i n metals, as
well as the different coloured goldalloys and their properties.
Formation of colour in MetalsWe perceive colour when thedi fferent wavelengths composing
white light selectively interfere withmatter by absorption, diffraction,
reflection, refraction or scattering, orwhen a non-white distribution of light
has been emitted by a system (4). T hewavelength of visible light is of theorder of 380-780 nm. Figure 2 shows
the energy of the visible spectrum.The formation of colour in metallic
elements and thei r alloys can be
explained by means of the bandtheory. When metal and light
interact, electrons from the metalsurface situated either below or on
the Fermi level absorb photons andenter an excited state on or above the
Fermi surface respectively (5) . T heefficiency of the absorption and re-
emission of light depends on theatomic orbitals from which the
energy band originated ( 4). A whitereflected light will result if the
different colours in white light arereflected equally well. In the case of
gold and copper, the efficiencydecreases wi th i ncreasing energy,
resulting in yellow and reddishcolours due to the reduction in
reflectivity at the blue end of thespectrum.
M easurements performed on goldalloys with various amounts of silver
( Figure 3) show a high reflectivi tyexhibited by fine gold for the low
energy part (below 2.3 eV ) of thevisible spectrum (6) ( Figure 2) . T hismeans that the yellow colour of gold
is formed by a selective reflection ofthe red-yellow wavelengths. As the
si lver content increases, a hi gherreflectivity is displayed for the high
energy part of the visible spectrum.Silver has a very similar electronic
structure to that of gold, but thetransition of electrons above the
Fermi level requires energy in excessof that of the violet end of the visible
spectrum, and thus all the visiblespectrum is reflected, resulting in the
characteristic white metallic colour(6).
Figure 1 Jewellery pieces producedby using coloured gold alloys.
By courtesy of Jenna C lifford
(Electron Volts)
Energy Colour Wavelength Frequency
(Nanometers) x 1014(Hertz)
10.000
2000
1000
700
600
500
400
300
250
00
2
4
6
8
10
12
1
2
3
4
5
IncreasingEnergy
Figure 2 Wavelengths in the visiblespectrum.
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Colour Measuring SystemIn some appli cations of gold, mostly
in jewellery and dentistry, the colourplays an important role. For a very
long time, the colour was estimatedvisually by the producer, but this is
no longer appropriate, as the humaneye i s subjective. T here are cases
when the colour is the sine qua noncondition, as when selecting a solder
for repairing a jewellery item. Suchrepai rs and todays mass produced
jewellery both require an objectiveway of measuring colour. Even small
variations in the colour of similari tems are a sign of non-
professionalism and poor quality,and can be very costly for jewellerymanufacturers.
A merican Society for T esting and
M aterials standard on colour andappearance. CI ELA B is an
internationally recognized colourmeasurement system that was
developed by CIE ( the InternationalCommi ssion on I llumination) and
adopted in 1976.The CI ELA B method expresses the
colour as three-di mensional co-ordinates: L*, a*, and b*, where L* is
the luminance (brightness) ( 10) . A nL* value of 0 means that no light is
reflected by the sample, and an L*value of 100 means that all incident
light is reflected. The a* co-ordinatemeasures the intensity of the green
( negati ve) or red ( posi tive)component of the spectrum, while
the b* co-ordi nate measures theintensi ty of the blue ( negative) or
yellow ( posi tive) component. T hecolour of a sample can be defined byplotting these co-ordinates as a point
in the three-dimensional spacedepicted in Figure 4.
T he values of L*, a* and b* of a
T he need for accurate colour
measurement led to the creation ofdi fferent systems designed to assess
the colour. T he M unsell systemdescribes the colour by using three
co-ordinates: hue, chroma andvalue ( 7) . T he hue i s the colour
name described in words ( red, green,blue, etc.) , chroma denotes the
intensity of the colour as theseparation from a whi te-grey-blackaxis, and the value describes the
positi on on the whi te-grey-blackscale. T he M unsell system still relies
on the human eye, and colours aredescribed by visually comparing
them with standard files and findingthe closest match.
H owever, the jewellery industryrequi res a system w hereby a
reference point for colour can beestablished in order to have
uniformity between the variousmanufacturers. The D IN system used
as the European reference for goldcolour is based on physical colour
comparison with a standard goldpanel. D rawbacks of this system are
that colour identification stilldepends on the eye, and lower
caratage gold panels are subject todiscolouration due to tarnishing with
time.T he M anufacturing Jewellers &
Silversmi ths of A mericas Commi tteefor Color References has brought out
a G old Color R eference K i t that isbased on the CIELA B system (8, 9) .T his system offers the advantage of
describing colour mathematicallywithout the intervention of the
human eye, and forms part of the
100
80
60
ReflectivityPercent
Energy of incident light (eV)1
Visiblespectrum
2 3 4 5
40
20
5
43
2
1
Figure 3 R eflectivity as a function ofenergy of the incident light. 1 - fine gold,
2 - Ag50at% Au, 3 - Ag10at% Au,
4 - Ag5at% Au, 5 - fine silver (6).
+a-a
-b
+b
LWHITE
BLACK
Figure 4 The L*, a*, b* three-dimensional coordinate system.
Table 1. Compositions of Coloured Gold Alloys (wt%)
Alloy Au Ag Cu Al Fe In Co
1N-14* (9) 58.5 26.5 15.0 - - - -
2N-18* (9) 75.0 16.0 9.0 - - - -
3N* (9) 75.0 12.5 12.5 - - - -
4N* (9) 75.0 9.0 16.0 - - - -
0N**(9) 58.5 34.0 7.5 - - - -
R ed gold 50.0 - 50.0 - - - -
Blue gold 75.0 - - - 25.0 - -
Blue gold 46.0 - - - - 54.0 -
Purple gold 80.0 0 - 20.0 - - -
Black gold 75.0 - - - - - 25.0
* Standards common to Germany, France and Switzerland
** Standards common to Germany and France
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sample are obtai ned as di rect
readings from a spectrophotometerwhich is connected to a computer.
T he spectrophotometer has aresolving power between five and
ten times greater than the human eye( 7, 10) . A s the colour of a sample
depends on the illuminant, thesample itself and the observer, the
spectrophotometer uses an arti ficialli ght source that simulates naturallight, an array of photodiodes, and
the computer as the observer.D i fferent problems related to
measuring colour such as objectivity,accuracy and reproducibility are
resolved by thi s system.
Colour Measur ements
Figure 5 shows the a* and b* colour
co-ordinates of various coloured goldalloys. T heir compositions are listed
in Table 1.The colour di fference can also be a
measure of the tarnishing of an alloy.By calculating the distance between
two points in the CI ELAB system, thedifference in colour can be
numerically described. The D E* valuerepresents thi s distance and can be
calculated ( 11) with the formula:
D E* = [( L*2 - L*1)2 + ( a*2 - a*1)2 +
( b*2 - b*1) 2]1/2 ( 1)
The subscripts 1 and 2 refer to the
co-ordinates of the initial and thetarnished sample respectively; L*, a*and b* were defined above as being
the CI ELA B colour co-ordinates ofthe respective alloys/samples - see
text near Figure 4. T he DE* describedthe distance between two points
representing the colour of thesamples in the tridi mensional CIELAB
system. B y plotting the D E valueversus the heat treatment time, the
colour change can be visualized in agraph such as the one in Figure 6. In
this case, the tarnish behaviours oftwo gold alloys are compared with a
standard one.
Coloured Gold AlloysAlloying additions to gold and
copper can create vari ous colours,resulting in diversity in jewellery
applications. I t is less well k nown,however, that gold alloys can have
colours which are surprisinglydifferent to the conventional alloys,
as in the case of blue, black or purple
gold alloys ( Figure 7) . I n this paper
coloured gold alloys are classified inthe following three main
metallurgical categories:
Au-Ag-Cu SystemIntermetallic compounds
Surface oxide layers.
Au -Ag-Cu Syst em
T he Au-Ag-Cu system is the basis ofthe most common gold jewellery and
dental alloys used today, and datesback several mi llennia. Colour
variations of yellow, red and greencan be obtained by different ratios of
Au:A g:Cu. Figure 8 illustrates the rich,yellow colour of high carat gold.
White gold is also based on the Au-Ag-Cu system, and the whi te colour
results from alloying with elementsk nown for thei r bleaching
characteristics, such as nickel ( N i ) ,palladium ( Pd) , and manganese
( M n) . A lloying additi ons to theternary system are also used to
improve properties li k e castibi li tyand hardness.
Black goldBluegold
(In)Purplegold
Bluegold (Fe)
Red gold
PureGold
3N
1N-140N
Silver
Copper
-10
-5
0
5
10
15
20
25
30
35
40
-5 0 5 10 15 20
a* co-ordinate
b*co-ordinate
2N-18
4N
yellow
blue
green red
Figure 5 C IELAB co-ordinates ofvarious coloured gold alloys.
0
10
20
30
40
50
60
70
0 50 100 150
Heat Treatment Duration (min)
DEValues
Standard alloy
Alloy 1
Alloy 2
Figure 6 C olour differences of threealloys after the same heat treatment in
air. The closer the curves are to the
standard, the smaller the colour
difference.
Figure 7 Pendant produced with 18 ctyellow, white and purple gold,
Sandawana emeralds and diamonds.
C ourtesy of Hans M eevis, Serol-Wana
Studio, Botswana.
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Yel l ow , Gr een and Red Go ld
Alloys
A multitude of hues and colours can
be obtained in the Au-Ag-Cu systemalone by variation of composition as
shown in the ternary phase diagramin Figure 9. A dditions of copper give
a reddish tint to the alloy, andadditi ons of si lver mak e the alloy
greenish. I n accordance with bandtheory, the addition of silver to theAu-Cu alloy causes a widening of the
energy gap that the electrons have toovercome to reach an energy state
above the Fermi level. T he wider thegap, the higher the energy absorbed
from the incident light, and thereforethe reflectivity increases not only for
the red and yellow regions of thespectrum, but also for the green (6).
T he metallurgy of the ternary Au-Ag-Cu system, including the order-
di sorder phenomena, has beenextensively studied, and reviewed in-
depth by Pri nce et al ( 13) , Rapson( 14, 15) , and Y asuda ( 16) . A n
example of a green gold item isshown in Figure 10.
Zi nc ( Zn) additions of up to 15wt%can be added to alloys of the Au-Ag-
Cu system to change the red colour ofcopper-rich alloys to reddish yellow
or dark yellow ( 17) . These alloys arecharacterized by good work abili ty.
Z inc additi ons affect both AuC uordering and the extent of the two
phase 1 + 2 region and, therefore,affect the age-hardeni ngcharacteristics of the alloys ( 17, 18) . A
relationship between thecomposition and colour was
developed for 14 carat alloyscontaining zinc ( 19) . A fter many
colour measurements, a colour indexwas designed, which can be
calculated from the weightpercentages of silver, copper and
zinc using the following formula:
CuColour index = ________ ( 2)
Ag + 2 Zn
Cadmium (Cd) additions of up to4wt% have also been used for the
production of 18 carat green alloys.75A u-23Cu-2Cd results i n a li ght
green alloy, and 75Au-15Ag-6Cu-4Cdis the composition of a dark green
alloy (20) . Cadmium is mostly knownas an alloying element used for
production of solders. H owever, CdO
is considered very toxic, being an
i rri tant to the respi ratory system,causing irreversible damage to the
k idney, and i s classi fi ed by theInternational A gency for Research in
Cancer as a carcinogen ( 21 - 23) .
Whi te Gold
White gold alloys were originally
developed as substitutes forplatinum, and are commonly used inthe jewellery industry for diamond
setting, combination whi te/yellowjewellery i tems, and clasps
( electroplated with yellow gold) forhigh-quality collars due to their high
strength ( Figure 11) . T hese alloyshave been used less than the
coloured gold alloys in the past, butfashion trends and the increase in the
gold pri ce in 1972-1973, broughtsilver jewellery and white gold into
prominence ( 24) . I n Europe, a newtrend towards more white gold
jewellery has recently becomeapparent.
T he white gold alloys used in thejewellery industry are based primarily
on Au-Cu-Ni -Zn, and Au-Pd-Agcombinations. T he bleaching effect of
nickel or palladium i s a result oflowering the reflectivity of the alloy
for the low energy part of the visiblespectrum. A bsorption processes
become possible at energiesconsiderably lower than for pure
gold, and the reflectivity is reduced inthe red and infrared regions of thespectrum ( 6) . Fi gure 12 shows the
reflectivi ty curves for di fferent nickeland palladium-containing alloys.
Figure 8 Items produced with hard 24carat gold revealing the rich, yellowcolour of the metal.
Au
10
20
30
40
50
60
70
80
9010
20
30
40
50
60
70
80
90
10 20 30 40 50 60 70 80 90 AgCu
Copper
Weight
%GoldW
eight%
RY-Red Yellow
GY-Green Yellow
Silver Weight %
RY
18 Ct.
14 Ct.
10 Ct.
Red
Reddish
Whitish
GY
Yellowish
Greenish
Yellowish
Yellow
Figure 9 Relationship between colourand composition in the ternary Au-Ag-Cu
system (12).
Figure 10 G reen gold elephantsdonated to T he Three Tenors by Sarah
Da Vanzo of C onsolidated Bullion Ltd,
South Africa.
Figure 11 White gold and combinationwhite/yellow gold jewellery designed by
Jenna C lifford, South Africa.
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35
100
80
60
ReflectivityP
ercent
Energy of incident light (eV)1
Visible spectrum
2 3 4 5
40
20
1
45 3
2
Figure 12 R eflectivity curves for Ni-(left) and Pd-(right) containing gold alloys (6).Left: 1 - fine Au, 2 - Au2at% N i, 3 - Au5at% Ni, 4 - Au10at% Ni and 5 - pure Ni.
R ight: 1 - fine Au, 2 - Au5at% Pd, 3 - Au10at% Pd, 4 - Au20at% Pd, 5 - Au30at% Pd and curve 6 - pure Pd
100
80
60
ReflectivityP
ercent
Energy of incident light (eV)1
Visible spectrum
2 3 4 5
40
20
1
56
43
2
Element Effect Comments
Colour Workability
Ag M oderate bleach Excellent Sulfur tarnishing, bleaches only low caratage alloys
C u T oo red (more than 3% ) Improves work ability -
Co Very slight bleach (1.5% ) Excellent Increases liquidus, hardness, allergen
C r Excellent bleach (13% ) Impossible 2% C r has no colour influence; increases solidus
temperature above 1100CFe Nearly complete Poor to Poor corrosion resistance; increases solidus temperature
bleach (16% ) satisfactory in combination with 4-5wt% Pd, when more than 10wt%
causes excessive hardness and oxidation during lost-wax
casting
In M oderate bleach (5.5% ) Impossible 2% has no colour influence, lowers hardness
Sn M oderate bleach (5% ) Impossible Embrittlement
Zn M oderate bleach (6% ) Brittle (more than 6% ) Volatile, unsuitable for casting, increases tendency for fire-
-cracking, high proportions can cause sensitivity, less than
2wt% improves the lost-wax casting behaviour but still
without fully satisfactory results
Al Very slight bleach (1.5% ) P oor Strong oxide former
M n None (1.5% ) Poor Strong oxide former, high reactivity when more than 10wt% ,
appreciable whitening effect especially in combination with P d,
susceptibility to stress corrosion when used in important
quantities
Pt Bleaching effect - An expensive precious metal
similar to Pd
T i Just perceptible (1.3% ) P oor Strong oxide former
Nb - - Increases liquidus
Ta Just perceptible (7% ) P oor Increases liquidus, high reactivity
V Strong bleach (25% ) Impossible Increases liquidus, toxic, high reactivity
Table 2. Potential Alloying Elements for White Gold Alloys and Indications of their Bleaching Effects (27 -29, 32, 33).
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Nickel has traditi onally been used
for white gold alloys because of itslow cost and strong bleaching effect.
In the gold-copper-nickel-zinc alloys,nickel i s the primary bleaching agent,
and zinc is used as secondarybleacher to compensate for the
colouring effect of copper. N ick el-containing white gold alloys are
characterized by their hi gh as-casthardness, rapid work hardening andtendency to fi re-cracki ng. T he
commercial alloys undergoseparation into gold-rich ( Au-Cu) and
nick el-rich ( N i -Cu) phases, whichproduces considerable hardness (25) .
As these alloys have poor formabili ty,frequent intermediate annealing
treatments are required (26) . Copperis added to improve plasticity and
work abili ty. T he use of nickel-containing gold alloys in the
jewellery industry has become lesspopular, due to the allergic reaction
caused by nickel when in contactwith the sk in ( 27) .
Comparison between the
properti es, advantages anddi sadvantages of ni ckel- and
palladium-containing white goldalloys has received considerable
attention i n the li terature ( 28 - 31) .The palladium-containing white gold
alloys are characterized by low as-cast hardness, excellent ductil ity and
cold work ing properties, good colourmatch with sufficient palladiumadditions, and no tendency to fire-
crack ing. T he colour of the A u-Pdalloys ranges from yellow gold to
white after the palladium contentreaches 15wt% . H owever, palladium-
containing gold alloys areunattractive due to their high liquidus
temperature and high density, and inparticular because of the increasing
cost of palladium.Primary requirements desirable in
white gold alloys include (32) :Suitable white colour and high
reflectivityA reasonable hardness in the
as-cast and annealed states(arbitrarily 25%
elongation)A relatively low liquidus
temperature (arbitrarily
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T he Au-Al phase diagram (Figure
14) contains a number ofintermetallic compounds formed at
compositions situated close to thegold-rich end of the diagram. A uAl2
has a gold content close to that of an18 carat alloy ( 75 per cent gold) ,
which makes it possible to produce ahallmarkable 18 carat purple alloy.
T he microstructure of an 18 caratpurple gold alloy ( Au / 25wt% Al) is
shown in Figure 15, where the lightgrey phase represents the purple
colour ( 38) . A s the composi tiondeviates from AuAl2 towards the
aluminium-rich side of the phasediagram, the colour approaches that
of aluminium. T he volumepercentage of the purple intermetallic
phase decreases, wi th an increase inthe second-phase consisting of analuminium-rich solid solution. When
the composition deviates from theAuAl2 to the gold-rich side of the
phase diagram a new intermetalliccompound, AuAl, forms at 88wt% Au
content and replaces the AuA l2,altering the alloys colour. T he purple
colour is preserved until thealum ini um content decreases to
15wt% ( 40) .T he crystal structure of A uAl2 was
determined in 1934 to be the simplecubic CaF2 structure ( Figure 16) .
M ore recent research ( 41) revealedthat the aluminium sublattice is
incompletely occupied and that theactual composition of A uAl2 is closer
to Al11Au6.
Blue GoldT wo other intermetallic compounds
that are known to produce colours ingold alloys are A uI n2, which has a
clear blue colour, and AuG a2 (6 ) ,
which displays a slight bluish hue.
Figures 17 and 18 show the Au-In andA u-G a phase diagrams. T he gold-
indium intermetallic compoundAuIn2 forms at 46wt% Au, and AuG a2at 58.5wt% A u. B oth have a simi larcrystal structure to the purple gold
compound which is based on theCaF2 prototype. T he blue colour i s
diluted in a similar manner to that ofpurple gold.
T he intermetalli c compounds
behave in some ways li k e puremetals, whi ch makes i t possible to
calculate their band structures. T hereflectivity falls in the middle of the
vi sible spectrum and ri ses againtowards the violet end, giving
distinctive colours in each case (43) .Figure 19 shows the reflectivity as a
function of the energy of the incidentlight for AuAl2, AuIn2, and AuG a2.
Figure 16 C rystal structure of C aF2 -prototype for AuAl2, A uIn2 and AuGa2 (42).
1200
1000
800
600
400
200
00
0
10 20
2010 30
30 40 50
5040
Atomic Percent Indium
Weight Percent Indium
Temperature
C
InAu60
60
70
70
80
80
90
90
L
(Au)
(In)
1064.43C
649.25C
509.6C
224.3C
540.7C
495.4C454.3C
~156C 156. 634C
641C
487457.5 55.3482
35
39
337
275
374.6
364.5
AuIn8
AuIn
1
1
1
100
100
1
Figure 17 Au-In phase diagram (39).
1200
1000
800
600
400
200
-2000
0
10 20
2010 30
30 40 50
5040
Atomic Percent Gallium
Weight Percent Gallium
Temperature
C
GuAu60
60
70
70
80
80
90
90
L
(Au)
(Ga)
1064.43C
29.7741C
30C
491.3C
409.8C375
461.3C
339.4C286C282C
348C
415.4C448.6C
274
348.9
AuGa
AuGa2
1
1
1
100
100
0
Figure 18 Au-G a phase diagram (39).
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SpangoldCertain Au-Cu-Al alloys form aninteresting surface texture resulting in
a new fami ly of gold alloys known asSpangold ( 44, 45). T wo di fferent
colours are produced by theseintermetallic compounds: a yellow
76A u-19Cu-5Al alloy, and a pink76Au-18Cu-6Al alloy. T he decorative
spangli ng effect is the result of achange in the crystal structure whichcreates many small facets on the
previously poli shed surface ( Figure20) . D uring cooli ng from the high
temperature phase, the alloysundergo a quasi-martensi tic
transformation, which involvesordering of the atoms and shear. T he
observed behaviour can be explainedby assuming that the alloys exi st in
two structures: a hi gh-temperatureordered B2-type phase, which is
body-centred cubic, and an orderedphase stable at lower temperatures,
which has a body-centred tetragonalstructure (46) .
By cooling the material below20C, the high-temperature phase is
converted to the structure of theordered phase i rrespective of the
cooling rate. T he spangling heattreatment is normally conducted by
heating the item in a bath of hot oil atbetween 150 and 200C for 10
minutes and then cooling it to belowroom temperature.
Sur face Ox id e Layer s
T he coloured gold materials
described above have the commonfeature of preserving the same colour
in cross section. I n the quest fordifferent colours, another technique
is used, which involves the formationof a coloured oxide layer on the alloy
during heat treatment in air. A s golddoes not discolour in air, the metal is
alloyed wi th a base metalcharacterized by a low oxidation
resistance. T he oxide layer is only asurface film, and has limited
thickness, which could result i n thecoloured layer being damaged when
used in high wear applications.Surface oxidation is performed as the
final treatment after the article hasbeen buffed and polished.
Liver of sulfur (potassium sulfide)can be used to colour gold alloys
containing a significant quantity ofcopper (18ct and lower), and a range
of colours varying from brown to
black can be obtained ( 20) .
V A Blue G old SA of G enevalaunched, in 1988, an alloy that turns
to a rich sapphire blue with heattreatment at 1800C ( 47, 48) . T he
gold, with content between 20 and 23carat, is alloyed with ruthenium,
rhodium and three other alloyingelements. T he blue surface layer is
between 3 and 6 m in thickness. An18 carat gold alloy, which consists of24.4wt% Fe and a maxi mum of
0.6wt% N i ( 49) , forms a blue oxi delayer on heat treatment in the
atmosphere between 450 and 600C( Figure 21). T he oxide layer turns to a
blue-green colour with an increase ingold content to 85wt% .
B lack gold generally containscobalt, which forms a black cobalt
oxide layer on the surface with heattreatment between 700 and 950C
( 50, 51) ( Figure 22) . O ther alloyingelements also k nown to give a
black ish layer on ox idation arecopper, iron and titanium. Black gold
alloys also contain at least one of theplatinum-group metals, silver, or
nickel.Van Graan and Van der Lingen (52)
improved the wear resistance of 18carat cobalt-containing gold through
chromium additions. A n electrolytichardening cycle such as that used
after the colouri ng of stainless steelwas incorporated ( 53) . T he addition
of chromium results in a thinneroxide layer which consists mainly ofCr2O 3 and has an olive-green hue.
T he wear resistance of anA u15wt% Co10wt% Cr was
significantly better than that of abinary Au25wt% Co alloy, although
100
80
60
ReflectivityPercent
Energy of incident light (eV)1
Visiblespectrum
2 3 4 5
40
20
1
2
3
Figure 19 The reflectivity as a functionof the energy of the incident light for the
intermetallic compounds: 1 - AuAl2,
2 - AuIn2, 3 - AuG a2 (6).
Figure 20 An 18 ct dagger, withSpangold blade, finished with red ivory
wood, rubies, red tourmaline, and yellow
gold.
Figure 21 The H earts collection inblue gold by Ludwig M uller, S witzerland.
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the oxide layer was approximatelyfive times thinner ( Figure 23). T he
microstructure of the alloy in Figure23 is composed of a gold-rich phase
containing ca94wt% Au and a cobalt-rich phase containing ca90wt% Co.
Between the CoO layer and thematrix, is a cobalt-depleted zone
containing only ca 2.4wt% Co,resulting from cobalt segregation to
the surface during oxidation.
The Future of Coloured GoldAlloysG old jewellery has enchanted peoplefor thousands of years. H ues and
colours are only variations on thesame arti stic theme of beauty and
bril li ance. G old wi ll always bedistinctive due to its unique yellow
colour, but the need for diversity andoriginality will cause some of the
coloured gold alloys to remain infashion. N ew designs and ideas wi ll
certainly arise, and with todays
k nowledge and technologies, new
solutions wi ll most likely be found forthe challenges posed by some of the
coloured gold alloys.
AcknowledgementsT his paper is publi shed by
permission of M intek.
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