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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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38

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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