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Gold Jewellery AlloysPure (24 carat) gold is a deep yellow colour (an orange shade of yellow) and is soft and very malleable. The coloured carat gold alloys range in gold content from 8 to 22 carats (33.3% - 91.6% gold) and can be obtained in a range of colour shades: green (actually a green shade of yellow), pale yellow, yellow, deep yellow, pink/rose and red. There are also white golds and even unusual coloured golds such as 'purple gold'. They all have different mechanical properties such as strength, hardness and malleability (ductility) and some alloys can be heat treated to maximise strength and hardness. There are gold alloys that are optimised for different manufacturing routes such as lost wax (investment) casting and stamping.
How can colour be varied and why do different gold alloys (an alloy is a mixture of two or more pure metals) have different mechanical and other properties? To answer these questions in depth requires a good technical knowledge of metallurgy. However, it is possible to give some simplified answers.
The Coloured Carat Golds
Almost all conventional, coloured carat golds are based on gold-silver-copper alloys, often with minor alloying additions. All three metals have the same crystal structure (face centred cubic, FCC) and so are compatible with each other over a large range of compositions. Typical minor additions include deoxidisers such as zinc and silicon, grain refiners such as iridium and cobalt and possibly metals such as nickel to strengthen the alloy. Larger zinc additions (about 1-2%) can improve melt fluidity and hence 'castability' in lost wax casting, as can silicon, resulting in better filling of the mould and better reproduction of surface detail. Even larger zinc additions (up to 10%) can improve malleability of certain carat golds, particularly 14 carat and lower, used for making jewellery by stamping from sheet. Additions of low melting point metals such as zinc, tin, cadmium and indium lower melting ranges and hence are used to make carat gold solders.
Gold is yellow and copper is red, the only two coloured pure metals. All other metals are white or grey in colour. The addition of a red colour to yellow, as every school child knows, makes the yellow pinker and eventually red. The addition of a white makes the yellow colour paler and eventually white. This principle of mixing colours is the same in carat golds. Adding copper to gold makes it redder and adding silver, zinc and any other metal makes gold paler. Thus, we can understand that lower carat golds, because we can add more alloying metals, can have a wider range of colours than the higher carat golds.
Thus at 22 carat (91.6% gold), we can only add a maximum of 8.4% of alloying metals and hence can only obtain yellow to pink/rose shades. At 18 carat (75.0% gold) and lower, we can add 25% or more alloying metals and hence get colours ranging from green through yellow to red, depending on the copper: silver plus zinc ratio. Thus at any given caratage we can vary the colour by varying the copper: silver plus zinc ratio. This can be demonstrated in the following table:
Alloying additions affect other physical properties as seen in the next table:
Physical Properties of Typical Gold AlloysCarat Composition % Colour Density
g/cm3 Melting Range
°C Silver Copper 24 - - Yellow 19.32 1064 22 5.5 2.8 Yellow 17.9 995-1020 3.2 5.1 Dark yellow 17.8 964-982 21 4.5 8.0 Yellow-pink 16.8 940-964 1.75 10.75 Pink 16.8 928-952 - 12.5 Red 16.7 926-940 18 16.0 9.0 Pale yellow 15.6 895-920 12.5 12.5 Yellow 15.45 885-895 9.0 16.0 Pink 15.3 880-885 4.5 20.0 Red 15.15 890-895
As caratage reduces, the melting range and alloy density are lowered. But at any given caratage (gold content), the actual values vary according to the relative silver and copper contents.
As well as affecting physical properties, alloying additions to gold generally increase the strength and hardness, with some reduction in malleability / ductility. The silver atom is slightly larger than that of gold, so alloying gold with silver gives a moderate improvement in strength and hardness. The copper atom is significantly smaller than that of gold and so it has a greater effect on strengthening gold than silver, as it distorts the gold crystal lattice more. Thus reducing caratage from 24 carats through 22 ct and 21 ct down to 18 carat gold results in stronger and harder alloys, as can be seen in Table 3. Beyond 18 ct down to 10, 9 and 8 carats does not have much further effect.
Mechanical Properties of Typical Gold AlloysCarat Composition %, wt. Condition Hardness HV Tensile Strength N/mm2 Silver Copper 24 - - Annealed 20 45 Worked 55 200
225.5 2.8 Annealed 52 220 Worked 138 390 3.2 5.1 Annealed 70 275 Worked 142 463 21 4.5 8.0 Annealed 100 363 Worked 190 650 1.75 10.75 Annealed 123 396 Worked 197 728 18 12.5 0 12.5 Annealed 150 520 Worked 212 810 4.5 20.5 Annealed 165 550 Worked 227 880
Table 3.2: Mechanical Properties of 18 Carat GoldsComposition, wt% Hardness, HV Elongation, % Gold Silver Copper Annealed Cold worked Annealed c.w. 75 25 - 36 98 36.1 2.6 75 21.4 3.6 68 144 39.3 3.0 75 16.7 8.3 102 184 42.5 3.2 75 12.5 12.5 110 192 44.8 3.3 75 8.3 16.7 129 206 47.0 2.6 75 3.6 21.4 132 216 42.0 1.5 75 - 25 115 214 41.5 1.4
c.w. = cold worked
However, copper-containing carat golds in the range of 8-18 carats can be hardened even further because of their metallurgy. Hard second phases can be precipitated out in the solid state as they cool below about 400°C, making the carat gold less ductile. Because of this, such alloys must be quenched in water after annealing to retain the single phase, ductile state if further working is required. This can be seen in the next table, Table 4.1
Effect of Cooling Rate on 18 Carat Golds after Annealing at 650°CComposition, wt% Hardness, HV Gold Silver Copper Slow cooled in air Water quenched 75 25 - 56 56 75
223 90 88 75 17 8 138 136 75 12.5 12.5 160 160160 75 8 17 170 165 75 3 22 196 177 75 - 25 242 188
Special low temperature (ageing) heat treatments (typically 3-4 hours at 280 -300°C) can later be employed to give substantial hardening to such annealed and quenched alloys. This is known as age-hardening. In 18 ct red golds, the hardness can be doubled, as shown in Table 4.2!
Effect of Heat Treatment on 18 Carat AlloysComposition %, wt Colour Condition
N/mm2 Silver Copper 12.5 12.5 Yellow Annealed, quenched 150 520 Aged 230 750 4.5
20.5 Red Annealed, quenched 165 550 Aged 325 950
As all goldsmiths know, working a metal makes it harder and stronger, as we can see in the previous tables, but if it is overworked, it will eventually fracture. So, they know that worked carat golds must be annealed to restore the soft ductile condition. Typical annealing temperatures for carat golds are given in the following table:Alloy Annealing temperature
°C Colour Pure gold, 24 carat 200 Black heat 21 - 22 carat 550 - 600 Very dark red 18 carat 550 - 600 Very dark red 14 carat 650 Dark red White gold (palladium) 650 - 700 Dull cherry red White gold (nickel) 700 - 750 Cherry red Sterling silver 600 - 650 Dark red
Apart from copper, all other alloying metals to gold will tend to whiten the colour and so it is possible to make carat golds that are white in colour. White golds for jewellery were developed in the 1920's as a substitute for platinum.
Additions of any white metal to gold will tend to bleach it's colour. In practice, nickel and palladium (and platinum) are strong 'bleachers ' of gold ; silver and zinc are moderate bleachers and all others are moderate to weak in effect.
This has given rise to 2 basic classes of white golds - the Nickel whites and the Palladium whites. At the 9 carat (37.5% gold) level, a gold-silver alloy is quite white, ductile although soft and is used for jewellery purposes. White golds are available up to 21 carat.
There is no legal definition of what constitutes a 'white' colour in golds and hence trade description of white gold may not mean 'detergent white'. Many commercial white golds are not a good white colour.
Nickel white golds
Nickel alloying additions form hard and strong white golds up to 18 carat. They are difficult to work and suffer from socalled 'firecracking'. Most commercial alloys are based on gold-nickel-silver-zinc alloys with copper often added to improve malleability. This copper addition, of course, affects colour, and so such white gold alloys are not a good white colour - more a slight yellow/ brown tint, particularly if nickel content is also low. As a consequence, such white gold jewellery is normally electroplated with rhodium (a platinum metal) which is tarnish resistant and imparts a good white colour.
Unfortunately, many people, the female population especially, are allergic to nickel in contact with the skin and this gives rise to a red skin rash or irritation. The European Union countries have enacted legislation valid from the 20th January 2000 that limits nickel release from jewellery. Thus, in Europe, nickel white golds are being phased out and being replaced by palladium white golds. The USA is taking a more relaxed approach, requiring jewellery to be labelled as nickel-containing, and much jewellery in the West is now advertised as 'non-allergenic' or 'nickel-free'. [See Separate Information Sheet, "The European Directive on Nickel ." and the article in Gold Technology, No 28, Spring 2000, "Nickel gets under your skin"]. Some typical nickel white gold compositions are shown in Table 6
Typical Nickel White Golds
% wt Nickel,
% wt Zinc,
% wt Hardness
°C 18ct 75 2.2 17.3 5.5 220 960 75 8.5 13.5 3.0 200 955 75 13.0 8.5 3.5 150 950 14ct 58.5 22.0 12.0 7.4 150 995 10ct 41.7 32.8 17.1 8.4 145 1085 9ct 37.5 40.0 10.5 12.0 130 1040
Palladium white golds
Additions of about 10 -12% palladium to gold impart a good white colour. But palladium is an expensive metal, dearer than gold and it is also a heavy metal. Thus jewellery in such palladium white golds will be more expensive than identical pieces in nickel whites for 2 reasons: firstly, the cost of the palladium and secondly, the impact of density - palladium white golds are denser and so such jewellery will be heavier and also contain more gold. It is also more difficult to process as the melting temperatures are substantially higher.
Many commercial palladium white golds only contain about 6-8% palladium plus silver, zinc and copper. Some may even contain some nickel [so a palladium white gold is not necessarily nickel-free]. These may also have less than a good white colour and so may also be rhodium plated.
Palladium white golds tend to be softer and more ductile compared to nickel whites and so will not wear as well. They are available in all caratages up to 21 carat. It is not possible to have a 22 ct white gold, for example. Some typical compositions are given in Table7.
Typical Palladium AlloysGold Pd Ag Cu Zn Ni Hardn
°C 18ct 75 20 5 - - - 100 1350 75 15 10 - - - 100 1300 75 10 15 - - - 80 1250 75 10 10.5 3.5 0.1 0.9 95 1150 75 6.4 9.9 5.1 3.5 1.1 140 1040 75 15 - 3.0 - 7.0 180 1150 14ct 58.3 20 6 14.5 1 - 160 1095 58.5 5 32.5 3 1 - 100 1100 10ct 41.7 28 8.4 20.5 1.4 - 160 1095 9ct 37.5 - 52 4.9 4.2 1.4 85 940
Pd- palladium; Ag- silver; Cu - copper; Zn - zinc, Ni - nickel. [In wt %]
Alternative white golds
In the European Union especially, there is a demand for cheaper alternatives to white golds than the palladium whites which are nickel-free. Many new alloys are coming to market, most of which rely on manganese additions as the main whitener. Some are palladium-free and others are low palladium alloys. Chromium and iron are also be used as whiteners. They tend to be hard and more difficult to process. Many of these alloys are not a good white colour, requiring rhodium plating, and many suffer cracking problems and tarnishing.ViewRSSHelp
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