The Colour Characteristics of Gold Alloys Errol F. I. Roberts and Keith M. Clarke Department of Metallurgy and Materials, City of London Polytechnic, London The characterisation by physical methods of the colour of gold alloys is of particular significance to industries which use theet on account of their aesthetic qualities. In this article, the authors describe the systematic determination of the colour of alloys in the gold-silver-copper system. It embodies unpublished data which should be useful for future znvestzgatzons. As a result of subjective assessment, different col- our descriptions may be applied to gold alloys of the same or very similar compositions (1). There thus appears to be a definite need to place the colour characterisation of such alloys — which are often used for aesthetic and decorative purposes — on a sound basis. Over recent years, colour standards have been developed that have relied upon a straightforward sample comparison technique. The most notable of these is the German DIN 8238 which is an extension of an earlier Swiss watch industry standard. This is based upon a series of 18 and 14 carat gold alloys ranging from red to white through green and yellow (Table I) and assessment is performed by visual com- parison with test pieces. It is simple and convenient but is qualitative, imprecise, highly subjective and does not provide a basis for judging colours outside the arbitrary standard specimens. More recently, this limited number of specimens has been examined quantitatively in a manner not dissimilar to that reported in this article. Few fundamental colorimetric studies of gold alloys are reported in the literature, possibly as a result of inadequate instrumentation being available in the past. In 1949, Josef Colour coding Leuser of Degussa in Pforzheim, Germany, published (2) a diagram of the ternary gold- silver-copper system in which 1N14* the alloy field was divided into 2N-18* areas with named colours. This report was based on a very exten- sive visual examination of manufactured alloys and Leuser mentions in this connection that some years earlier, 1089 dif- ferent alloys covering the full range of compositions of the ter- nary system had been prepared in the Degussa laboratories in Pforzheim. From each of these 1089 alloys a piece of foil, in the shape of an equilateral triangle, was fabricated and the pieces were assembled in a large triangle, 1 m in side-length. This historie colour chart of the gold-silver-copper system was unfortunately destroyed in World War II. Due credit should also be given to Tammann (3, 4) who made similar but much more limited observa- tions from 1917 to 1919. Subsequently, E. M. Wise (5) reproduced the Leuser diagram and, in transla- tion, a few modifications were made to the colour descriptions. This is not surprising as such descrip- tions are notoriously difficult. Since colorimetry (the science of colour measure- ment) is fast becoming a well-established and scien- tifically sound field, it seemed reasonable to try and apply conventional colorimetric procedures to the gold-silver-copper system. Indeed, Gardam (6, 7) did carry out some preliminary studies of a range of metals using a small EEL tristimulus photometer. He demonstrated the important facts that metals are Table 1 Standard Colours Adopted for Watchcases Colour Composition of the corresponding description Carat reference alloy, parts per thousand Au Ag Cu Ni Zn Pale yellow 14 585 265 150 — — Pale yellow 18 750 160 90 — — 3N* Yellow 18 750 125 125 — — 4N* Rose (pink) 18 750 90 160 — — 5N* Red 18 750 45 205 — — ON** Yellow-green 14 585 340 75 — — 8N** White 14 590 — 220 120 70 * Standards common to Germany, France and Switzerland **Standards common to Germany and Franse 9
Transcript
The Colour Characteristics of Gold AlloysErrol F. I. Roberts and Keith M. ClarkeDepartment of Metallurgy and Materials, City of London Polytechnic, London
The characterisation by physical methods of the colour of gold alloys is
of particular significance to industries which use theet on account of
their aesthetic qualities. In this article, the authors describe the
systematic determination of the colour of alloys in the gold-silver-copper
system. It embodies unpublished data which should be useful for future
znvestzgatzons.
As a result of subjective assessment, different col-our descriptions may be applied to gold alloys of thesame or very similar compositions (1). There thusappears to be a definite need to place the colourcharacterisation of such alloys — which are often usedfor aesthetic and decorative purposes — on a soundbasis. Over recent years, colour standards have beendeveloped that have relied upon a straightforwardsample comparison technique. The most notable ofthese is the German DIN 8238 which is an extensionof an earlier Swiss watch industry standard. This isbased upon a series of 18 and 14 carat gold alloysranging from red to white through green and yellow(Table I) and assessment is performed by visual com-parison with test pieces. It is simple and convenientbut is qualitative, imprecise, highly subjective anddoes not provide a basis for judging colours outsidethe arbitrary standard specimens. More recently, thislimited number of specimens has been examinedquantitatively in a manner not dissimilar to thatreported in this article.
Few fundamental colorimetricstudies of gold alloys arereported in the literature,possibly as a result of inadequateinstrumentation being availablein the past. In 1949, Josef
Colour codingLeuser of Degussa in Pforzheim,Germany, published (2) adiagram of the ternary gold-silver-copper system in which 1N14*
the alloy field was divided into 2N-18*areas with named colours. Thisreport was based on a very exten-sive visual examination ofmanufactured alloys and Leusermentions in this connection thatsome years earlier, 1089 dif-ferent alloys covering the fullrange of compositions of the ter-nary system had been prepared
in the Degussa laboratories in Pforzheim. From eachof these 1089 alloys a piece of foil, in the shape of anequilateral triangle, was fabricated and the pieceswere assembled in a large triangle, 1 m in side-length.This historie colour chart of the gold-silver-coppersystem was unfortunately destroyed in World War II.Due credit should also be given to Tammann (3, 4)who made similar but much more limited observa-tions from 1917 to 1919. Subsequently, E. M. Wise(5) reproduced the Leuser diagram and, in transla-tion, a few modifications were made to the colourdescriptions. This is not surprising as such descrip-tions are notoriously difficult.
Since colorimetry (the science of colour measure-ment) is fast becoming a well-established and scien-tifically sound field, it seemed reasonable to try andapply conventional colorimetric procedures to thegold-silver-copper system. Indeed, Gardam (6, 7) didcarry out some preliminary studies of a range ofmetals using a small EEL tristimulus photometer. Hedemonstrated the important facts that metals are
Table 1
Standard Colours Adopted for Watchcases
Colour Composition of the correspondingdescription Carat reference alloy, parts per thousand
Au Ag Cu Ni Zn
Pale yellow 14 585 265 150 — —
Pale yellow 18 750 160 90 — —
3N* Yellow 18 750 125 125 — —
4N* Rose (pink) 18 750 90 160 — —
5N* Red 18 750 45 205 ——
ON** Yellow-green 14 585 340 75 — —
8N** White 14 590 — 220 120 70
* Standards common to Germany, France and Switzerland**Standards common to Germany and Franse
9
(1) Highly desaturated(2) Of a limited range of dominant wavelengths(3) Of very high luminance.
Desaturated colours are pale in hue. Any given col-our can be reproduced by mixing `white' light with anappropriate proportion of spectrally pure light. Dif-ferent proportions give a range of saturation fromwhite, pastel shades to the spectrally pure shadewhich is the most intense colour of that particular hueavailable. The term saturation is used here in thesense of the proportion of spectrally pure colour inthe mixture. The dominant wavelength is thewavelength of the particular spectrally pure colourneeded to match the colour and hence describes thehue. Luminance is a technical term which will bemathematically defined below and which correlateswell with brightness. In black and whitephotography, objects having higher luminance willrecord as whiter than objects having lower luminance.Colour terminology is a confusing area and the in-terested reader would be well advised to consult astandard text such as (18).
Reflectance spectrophotometers are now availablethat can give both specular and diffuse reflectancemeasurements from metal surfaces and reflectivitydata have been reported on a number of binary goldalloys (8 to 1 l). In the present work, use was made ofthe spectro-ellipsometer to collect data upon specularreflections. A systematic series of suitable alloys wasprepared and the optical characteristics of standardsamples made from the alloys were measured. Thesystem chosen to be examined was the gold-silver-
Au
o ONE PHASE SAMPLE10 --- 90
TWO PHASE SAMPLE
20 80
Que 30 q( 70 ^-o
40 ^ \ 60 ^F
50 - 50\
60 -lr— 40
70 -7- - -r— --- ^t 30
80/ - - .. - ,-- - 20
10
Ag 10 20 30 40 50 60 70 80 90 Cu
COPPER WEIGHT PER CENT
Fig. 1 Relationship between the composition andthe number of phases present in binary and ternaryalloys of the gold-silver-topper system after waterquenching from 600°C. The dotted line is the 600 °Cisothermal boundary of the immiscibility field asdetermined by McMullin and Norton (13)
copper ternary system, upon which the developmentand manufacture of jewellery alloys is based. Briefly,66 flat polished samples of compositions 10 per centby weight apart were examined by spectro-ellipsometry and the dispersion of their optical con-stants were calculated. These in turn were used todetermine reflection characteristics and Comité Inter-national de l'Eclairage (CIE) colour co-ordinates.
The Metallurgy of the Gold-Silver-CopperTernary System
The gold-silver-copper metallurgical system con-sists of two binary complete solid solutions, gold-silver and gold-copper, and one eutectic system,silver-copper, the latter with very restrictedsolubilities in the terminal solid solutions, especiallyat room temperature. As a consequence of thesefeatures, at any temperature in the solid range, theternary system shows either a two-phase region, basedupon the phases present in the silver-copper eutecticsystem, or a single-phase ternary solid solution. Theequilibrium two-phase region extends towards thegold-rich corner of the ternary system as theequilibrium temperature falls. Finally, the order-disorder reactions associated with the gold-copperbinary alloys intrude into the ternary alloy field closeto the binary compositions and the gold-silver systemalso displays some short-range ordering. A detaileddiscussion of the system is available elsewhere (12).
The 66 alloys examined in this study were made upfrom fine gold, fine silver and spectrographically purecopper at 10 per cent by weight composition inter-vals. Alloy melting was carried out in a carbon cruci-ble in a r.f. induction furnace under an inert gasatmosphere. After casting under oil, the resultingbead was worked into flat sheet with intermediatehomogenizing and fully softening anneals. Finalshaping produced flat rectangular plaquettes whichwere again annealed at 600°C and water quenched.Subsequently, these samples were mounted andmetallographically polished down to 0.25 µm dia-mond paste. The alloys were assayed and examinedby X-ray diffraction (Table II). Examinations forflatness, roughness and phase composition were alsoconducted. Figure 1 compares the single- and duplex-phase regions for the 600°C isothermal section asdetermined by X-ray diffraction after water quen-ching in a previous investigation (13) and in thecourse of the present work. The effect of the waterquench is to retain the 600°C equilibrium structureand to prevent ordering transformations.
Basic Opties of SolidsElementary opties (14, 15) demonstrates that a
monochromatic ray of light meeting a surface at some
10
Table Ii
Composition and Lattice Parameters of the Binary and Ternary Alloys in the Gold-Silver-CopperSystem used in This Study. All the Alloys were Quenched in Water from 600°C
Alloy Au Ag Cu Lattice Alloy Au Ag Cu Latticeparameter ($) parameter ($)
No. (*) Composition, weight per cent (t,t) A No. (*) Composition, weight per cent (t,#) A
3.81240 50.2 (50) 9.9 (10) 39.9 (40) 3.789 * R corresponds to repeat casts41 39.3 (40) 51.2 (50) 9.5 (10) 4.018 tThe figures in parenthesis are nominal compositions
3.829 t The figures in italics were determined by difference
42 40.4 (40)40 39.0 (40)( ) 20.6 20( ) 4.021^ The
Alloyslattice parameters
12 to 20, 38 togiven here are
39, 41 to 44 andthe average of
46 to 66 were foundthree calculations.
to consist of two3.801 phases. Accordingly, two lattice parameters were determined for these
allovs.
angle of incidence 0, is generally partly reflected andpartly transmitted. Furthermore, the transmittedbeam is refracted or deviated from its original direc-tion so that it makes some angle B r with the surface.Bearing in mind the electromagnetic=wave nature of
light it is not surprising that this effect of an interfaceon light is predicted by Maxwell's electromagnetictheory and was empirically established by Snell.Basically, the behaviour is described by the set ofequations:
11
Fig. 2 This schematic diagram of the reflection andrefraction of a polarised light beam at a metal sur-face shows selective absorption and phase retarda-tion A hetween components parallel (p) and perpen.dicular (s) to the plane of incidence
sin 0i ri zc/v2 _ (£Zµ2)
sin 0i n ic/v, (E M (1^
where:n is the refractive indexe is the dielectric constantµ is the magnetic permeabilityc is the velocity of light in vacuumv is the velocity of light propagation in the parti-
cular mediumthe subscripts 1 and 2 refer to the incident andrefracting media respectively.The manner in which light is reflected at an inter-
face depends upon the state of polarisation of thelight, the angles 0, and 0, and upon 1 1 and v 2 . Theequations expressing this phenomenon were derivedby the French mathematician Fresnel in 1823. Impor-tantly, besides a change in amplitude occuring uponreflection, a phase change A is also introduced(Figure 2). For light polarised parallel to the plane ofincidence (p-polarisation) and perpendicular to theplane of incidence (s-polarisation) and with incidentamplitudes respectively A and A, the relevant equa-tions are:
v^ 2 tos0 ; -11 1 tosOr A
RP r^ 2 tos 0 ; + fl 1 tos 0, P (2a)
fl 1 tose ; -1,tos0r A
RS = re i tos i + fl 2 tos r g(2b)
For normal incidence, 0^ = 0, consequently 0 = 0 and
fl - 1RP = r^ + AP (3a)
11-1
RG =---l AP (3b)
where:r is the ratio fl 2/f 1 •The reflectance R is the ratio of the light flux J r
reflected from a surface to the light flux J i incidentupon that surface:
R= 1- (4)
While R depends upon the p- and s-polarisation, atnormal incidente the distinction disappears and thenormal reflectance becomes
R= j-1 1 2 (5)\fl+1/
where:the subscript n refers to normal incidence.It will be appreciated that if the refractive index is a
function of wavelength then so too will be the reflec-tance. This wavelength-dependent function of thereflectance is called the reflectivity R (..).
It may be shown that the basic equations presentedabove apply to a conducting medium (such as a metal)if the real quantities of dielectric constant e andrefractive index 1 are replaced by the complexquantities s and . For metals which have complexrefractive indices, – ik, where i is the refrac-tive index and k is the absorption coefficient. Thus,
q2 +k2 + 1 —2r^R^ = 12 +k2 + 1+2n (6)
A generalisation for oblique incidence is also easilyestablished. In particular, the boundary conditionsfor propagation across a surface discontinuity remainvalid and so do the laws of reflection and refraction. Itbecomes then a relatively simple matter to determineti and k for a clean metal surface by measuringamplitude and phase quantities for the polarisationcomponents of polarised light reflected from it.
Metals, it will be readily appreciated from theirexcellent electrical conductivities, possess electronsin energy states which are affected by applied electricfields. These electrons are the conduction electronsbut other electrons are also susceptible to such fieldsand this is particularly so in metals where electronband overlapping occurs. The conduction electronsact very much like free electrons and consequentlytheir optical effects dominate in parts of the spectrumwhere reflectance is high. The electrons capable of in-terband transition super-pose their high absorptionon the spectrum.
Colorimetry and its Application to MetalsThe perception of an object colour by the eye is
determined by the nature and attributes of the light
12
entering the eye and the nature of the observer.Translating instrumental readings into a form thatcorresponds to a visual perception demands at thisstage consideration of the observer, and hence moregenerally the standard observing or colour detectingcharacteristics of the average eye. Once a set ofcharacteristics has been agreed upon, colour deter-minations may be made which are independent of thecolour vision of any particular individual and can becompared in independent laboratories. This involvessetting up agreements on two issues — the response oftristimulus colour perception of a normal observerand the specific reference stimuli, or illuminants. Thedistribution coefficients relative to these standardsand giving the amounts of three stimuli required tomatch colours through the spectrum can then be ex-pressed. Agreement in these areas is now interna-tional and is expressed through the body and worksof the CIE (16).
The Standard Eye and Eye EffectsIt has been found that most people (about 90 per
cent) perceive varied light stimuli in a similar way.These people have so-called `normai' vision.Statistical studies have enabled an `average' eye (or`standard' observer) to be defined which is onlyslightly different from that of people with normal col-our vision. The properties of this average eye, theCIE 1931 Standard Eye, have been embodied innumerical tables. Thus, calculations of colour can beperformed in a reproducible way by differentobservers. The Standard Eye has been necessarilydefined for a simplified and ideal configuration, inwhich samples having the same colour co-ordinates(see the paragraph entitled `Calculation of Colour Co-ordinates') are viewed simultaneously in a 2° com-parison field against a black background.
IlluminationThe light with which a sample is illuminated has a
definite, though not simple, effect on the sample'sperceived colour. Thus, in daylight an object'sperceived colour may appear to be different from thesame object's appearance when observed in artificiallight. It is an advantage to use standard illuminationconditions when viewing samples or when perform-ing colour calculations. The calculations reportedhere have been accomplished using the CIE StandardIlluminant C which represents blue sky light (notsunlight). When viewing conditions are altered, thenfresh calculations taking the change into account caneasily be made. In general, objects may appear to bethe same colour in one light and yet possess differentcolours in a different light. This should be less of aproblem with gold alloys than, for example, withgemstones or organic dyes as the spectral reflectivities
of metals do not show irregularities but seem rather tofall into groups with generally smooth distributions.
The Calculation of Colour Co-OrdinatesThe tristimulus values X, Y and Z of an object col-
our stimulus are defined by the equations:
X = k f R(.l)S( ) x( ).d),
Y = k R(i)SQL) y(ij.d), (8)
Z = k C R(^)S(7^) z(ij.dÂ
where:S(^) is the spectral intensity distribution of the
illuminationR(2) is the spectral reflectivity of the objectx O,), ) and z () are the colour matchingfunctions of the CIE 1931 Standard Eye and corres-pond respectively to the spectral sensitivities of thered, green and blue receptors of the human eyek = { f XS(^)Y(ij.dij ' -The computed quantity Y is the luminance, that is
defined in the introduction. The chromaticity co-ordinates x and y complete the three quantities (x, y,Y) required to define the colour stimulus.
The definitions:
X Y Z
xyX+Y+Z X+Y+Z z X+Y+Z
are such that:
x + y + z = 1
and so it is not necessary to tabulate z.These functions are defined numerically, R(7^) by a
particular experiment or calculation and the othersare standards normally obtained from tables. The in-tegrals must be solved by numerical methods. TheCIE recommends the weighted ordinate methodwhich uses approximations of the form:
X = k E R (^,)S(^) X(^).A^
where:AJ is a small, constant wavelength interval.In the present work, the complex refractive index
function vfO) was determined using a high precisionautomatic nulling spectro-ellipsometer (17) and thereflectivity R(,l) was computed. The CIE x and ytristimulus co-ordinates discussed above were thendetermined using standard tables for the standardobserver tristimulus coefficients and spectral distribu-tion of the C illuminant (18). Spectral colours of 100per cent saturation lie on the periphery of thechromaticity diagram, any point inside the colourspace represents a colour that lies between the spec-
13
trally pure colours and the white (or achromatic)point inside it (Figure 3).
The reflectivity curves used to determine the CIEchromaticity co-ordinates were derived from the ex-perimentally measured jO,) values since it was con-sidered that in the ultimate, the colour characteristicswould depend essentially upon the optical constantsof the metal.
Optical Measurements onGold -Silver-Copper Alloys
It has been pointed out, in the above section, that astudy of the specular reflection over the visible rangecould lead to colour specification on the basis of theCIE representation. Furthermore, it was suggestedthat spectro-ellipsometry would provide accurate op-tical constants which in turn would provide a basis forassessing the effect of changes such as roughening,scratches or films at the surface of samples. It is perti-nent to describe briefly the ellipsometer and its ap-plication to the present studies of gold-silver-topperalloys.
Plane-polarised monochromatic light undergoeschanges in its polarisation characteristics upon reflec-tion at an interface. Generally, reflection at a metalsurface produces elliptically polarised light. Ellip-sometry is the measurement of the polarisationcharacteristics of elliptically polarised light and thetechnique, especially when applied to clean surfaces,provides a sensitive and tractable way of determiningoptical constants. The parameters measured are therelative phase change A and relative amplitude change1i suffered by components perpendicular and parallelto the plane of incidence of the plane-polarised inci-dent beam. For a clean substrate these values dependupon the complex refractive index, and hence uponwavelength: i- = f(.1) as indicated above. The deter-mination of. and iJi as a function of wavelength thusenables the calculation of the optical constants. It hasbeen shown that the normal reflectivity or reflectionat normal incidente R, is determined by ij, so thatthis quantity is also easily accessible. This routemakes colour calculations very flexible indeed.
The apparatus used for the present investigation(Figure 4) shines a collimated polychromatic or`white' light beam onto the sample at an angle of in-cidence of 75°. The beam is initially made plane-polarised and, by passage through an achromaticquarter-wave device, is made elliptically polarised. Itis reflection at the metal surface that restores the lightto plane polarisation. The polariser and analyserelements in the system are adjusted, in the presentcase by a servo device, maintaining the system at a nullor balance point. The detection system depends upona photomultiplier-generated signal derived, via amonochromator, from the reflected light.
The samples, mounted and polished rectangularplaquettes, were always fitted on the specimen tableof the ellipsometer and measured immediately after afinal rinse with de-ionized water. Organic solventswere avoided for washing and rinsing since they areknown to leave very tenacious films on gold (19).Optical alignment of the angle of incidence was ac-complished by the autocollimation of a laser beamand angular positioning of the specimen was within0.025°. The angular settings of the optical axis andellipsometer moveable arm were reproducible tobetter than 0.005°.
The true optical constants of a surface are indepen-dent of the angle of incidence and can therefore beused to determine the normal reflectivity R.Readings of the analyser and polariser azimuths weretaken after the system had reached a balance at Bachwavelength typically of resolution of 0.01°. A total of121 wavelengths was used in each run. Calculations ofall the relevant quantities were then carried out on acomputer using the input data of angle of incidence 0,wavelength ) and analyser and polariser settings.Thus the quantities determined were:
A the relative phase change0 the relative amplitude changei l) the complex refractive index functions( 2 ) the complex dielectric functionR@) the normal reflectivity function
from which the following were derived:the CIE colour co-ordinates x and ythe luminance, per centthe saturation, per centthe dominant wavelength 2dIt is also possible to develop differente curves and
first- and second-derivative functions of these majorfunctions.
The results of this study are best presented by dis-tinguishing between single-phase solid solution alloysand duplex-phase alloys. All the alloys are bulk and re-present the system after water quenching from 600°C.
The reflectivity curves of the gold-silver and gold-copper binary alloys are displayed in Figure 5. It canbe seen that in both cases the gold spectrumprogressively transforms into that of its appropriatepartner. Particularly, in the gold-silver system the dif-ference between the gold and silver 'absorption edges— corresponding to interband transitions — is veryconsiderable. The gold edge is shifted by alloyingwith silver until it passes out of the visible and intothe ultra-violet spectrum. In the case of gold-topper,however, the respective absorption edges are veryclose together and their mixing by alloying occursessentially within the visible spectrum. Other changes
14
0.8
510
0.7
0.6
500
Y 0.5
OA
0.3
0.2
0.1
0
Fig. 3 On the 1931 CIE chromacity chart,the co-ordinates x and y define thechromacity and Y, which is not shown onthis two-dimensional representation,defines the luminance. The three values x,y and Y define the colour. The C illumi-nant (x = 0.3100, y = 0.3162) is marked onthe diagram. The dominant wavelength ^ D
of a test point T is given by the intersec-tion of the C—T line and the spectrumlocus. The saturation is the distante TC asa percentage of CD; it is 0 per cent whenT falls at C and 100 per cent when T falls at^D. The area covered by Figure 7 is in-dicated by the dotted line, hence the state-ment that metals are highly desaturated.After (18)Reproduced by permission of N.V. Philips'Gloeilampenfabrieken, Eindhoven, Netherlands
0.1 0.2 0.3 0.4 0.5 0.6 0.7
S
occur on ordering in this system but are not discussedhere since the structure after final heat treatment wasin the disordered range. It is fair to say, therefore, thata concomitant feature of solid solutions is the smoothmixing of electron populations from each atomicspecies as indicated by the optical spectra.
The gold-silver and gold-topper alloy systems dif-fer in one very important aspect — the lattice spacing(Table II). The former shows little variation withcomposition while the latter does and consequently,effects due to the change of lattice spacing dominate.Furthermore, the variation of CIE co-ordinates with
composition (Table III) emphasises that opticalphenomena are atomic effects and hence alloysdesignated in weight percentages will tend to shownon-linear variations of 'colour' with composition.
For the silver-topper system (also on Figure 5), theeffects of immiscibility are encountered. With thesealloys the individual spectra of the components,which are dilute silver and topper terminal solid solu-tions, remain separate. During alloying, only relativechanges in spectral intensities, depending upon theproportion of phases present in the alloy, aredistinguishable.
Fig. 4 Diagramatic representation of the spectro-ellipsometer used for this investigation
PHOTO -MULTIPLIER
MONO -CHROMATOR
15
100
Rn
80
60
40
20
0
100
Rn1
80
60
40
20
/ / — -- 100 Au / 0 Ag
/ / // •••..•••••• 80 Au / 20 Ag
•// / / — -- 60 Au / 40 Ag
/ / // — — 40 Au / 60 Ag
' % /
— 20 Au / 80 AgI
/ / / 0 Au / 100 Ag
_
80
i
S-,
100 Au / 0 Cu
-- 80 Au / 20 Cu
---- 60 Au! 40 Cu 60
— — 40 Au! 60 Cu
--- 20 Au/80Cu
- -"' 0 Au / 100 Cu
40
0 1 n
200 300 400 500 600 700 800 x, nm
Fig. 5 Normal reflectivity curves Rn ) of gold-silver, gold-topper and silver-copper binary alloys water quenchedfrom 600°C
100
Rn
16
The reflectivity curves of the ter-nary alloys exhibit the same basictype of behaviour although, aswould be expected here, the silver-rich and copper-rich phases derivedfrom the silver-copper system arenow alloyed with gold and showshifts in their absorption edges asdescribed for the binary solid solu-tions. Where the ternary two-phaseboundary is crossed,there are ap-propriate changes in the spectra ofsingle-phase to duplex-phase alloys.Figure 6 shows reflectivity spectrafor constant gold contents close to8, 12 and 18 carat. The CIE co-ordinates of these alloys are shownin Figure 7. The luminosity is notshown since this would require theCIE colour space to be three-dimensional; the values werehowever determined. It is seen thatthe alloy system still occupies atriangular area but that the triangle,essentially of equal weight percen-tages, is now severely distorted.The silver-rich alloys all tend to lietogether, close to the illuminantwhite point; a similar effect is notedamong the copper-rich alloys. Theaddition of silver or copper to gold,however, produces profound effectsboth in hue - which may beregarded as variations circumferen-tially around the white point - andof saturation - which changesalong the radius through the whitepoint. The dominant wavelength ofa particular co-ordinate is thewavelength at the intersection ofthe spectral locus with the line fromthe white point through the co-ordinate. It can be clearly seen thatthe addition of silver to gold makesthe resulting alloys progressivelymore yellow and then green butthat the saturation gets progressive-ly lower, that is the colours aremore diluted with white. Thechange when copper is added togold is similar but in the directionof red and is accompanied by verymarked falls in saturation. Thesilver-copper alloys show effectsdue to phase mixtures but the col-ours are very de-saturated.
Table III
CIE (Illuminant C 1931) Colour Co-Ordinates of Binaryand Ternary Alloys in the Gold -Silver -Copper System
Fig. 6 Normal reflectivity curves Rn ) of ternary gold-silver-copper alloys at the constant gold contents of 30, 50and 80 weight per cent. The alloys were all water quenched from 600°C
100
Rn
80
60
40
20
100
Rn1
80
60
18
Fig. 7 CIE co-ordinates x and y X39(chromacity) of binary andternary alloys in the gold-silver-topper system after water quen-ching from 600°C. The Bots cor- 3.38respond to actual measurementson the 66 alloys prepared for thisinvestigation while the dottedlines are extrapolated )37
1.36
035
3.34
033
0.32
0.310.32 0.33
Conclnsions
ier/ I^ \\1
f ^Í Au
/ \
/ ` / 1 /
/ f ^
✓ ^1 \ at l /^ Il //
BINARY ALLOYS
• Au - Ag
• Ag - Cu
• Cu - Au
TERNARY ALLOYS
--- CONST.Au-Ag-Cu
--- Au-CONST. Ag -Cu
- - Au-Ag-CONSTCu
• Au-Ag-Cu
0.34 0.35 0.36 0.37 038 x
The mail conclusions arising from the aforegoingresearch are as follows:(1) The optical characterisation of all the alloys from
within the gold-silver-copper system is possiblethrough the determination of the dispersion of theoptical constants ij) and k)
(2) Spectro-ellipsometry forms an appropriate basisfor experimental determinations and has high sen-sitivity as well as sufficient reproducibility
(3) Colour characterisation on the basis of CIE stan-dards is easily accomplished through the deter-mination of the reflectivity curves R 0(.l)
(4) The effect of any assumed illumination and anyobserver specification can be predicted by usingthe determined optical constants
(5) A11 the alloys measured show low saturation, alimited range of dominant wavelengths and highluminance. Such features are typical of metallicreflections and limit the range of colours requiredfor normai assessment of jewellery alloy colours.
AcknowledgementsThe authors gratefully acknowledge the support given to them
during this werk by the Worshipful Company of GoldsmithsResearch Foundation and the International Gold CorporationLimited. They would also like to thank Messrs. Reuben Cohenand Robert Hunt for their painstaking preparation and measure-ment of alloys. Finally, they wish to express their gratitude to theILEA, City of London Polytechnic and Dr. L. L. Shreir of theDepartment of Metallurgy for the use of extensive computing andlaboratory facilities without which this contribution to the goldindustry would not have been possible.
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