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Roger Weller, geology instructor

wellerr@cochise.edu

Gold Alloys
by Ivonne Suarez-Denton
Physical Geology
Fall 2009
   

Different Alloys of

*GOLD*

A Brief History of Gold
 

     The history of gold has been dated back to the Egyptians. Gold was a representation of wealth and power. Gold has destroyed nations because of it has always been considered precious quality. For centuries now gold has been the obsession of mankind, regardless of the era.
 

            Archeological evidence suggests gold was first used in the Middle East where the first known civilizations began. On fact, the oldest piece of gold jewelry is Egyptian jewelry that had been found in the tomb of Queen Zer. These gold pieces are the oldest examples found of any type of jewelry from the third millennium BCE. Throughout the Egyptian era, tombs were raided in search of gold and other fine treasures. One however, was left undisturbed, the tomb of Tutankhamen. Inside was the largest collection of gold and jewelry in the world to include a gold coffin that had the intricate detail of Egyptian
craftsmanship and gold working.



 
            

            Pictures provided by Heart of Egypt.com

           

     From the Persians to the Romans, gold was used in weaponry, artwork, furniture and jewelry. Gold became a part of every human culture. Its brilliance, natural beauty, luster, and its great resistance to tarnish made it enjoyable to work and play with.
 

            “During the frontier days of the United States news of the discovery of gold in a region could result in thousands of new settlers, many risking their lives to find gold. Gold rushes occurred in many of the Western States, the most famous occuring in California at Sutter’s Mill in 1848. Elsewhere, gold rushes happened in Australia in 1851, South Africa in 1884 and in Canada in 1897 (Northwest Territorial Mint, 2009).”

 

            “Gold was the first metal widely known to our species. When thinking about the historical progress of technology, we consider the development of iron and copper-working as the greatest contributions to our species' economic and cultural progress - but gold came first. Gold is the easiest of the metals to work. It occurs in a virtually pure and workable state, whereas most other metals tend to be found in ore-bodies that pose some difficulty in smelting (History of Gold, 2009).”
 

                Gold is recognized by its yellowish cast. Gold’s aesthetic properties when combined with its physical properties have long made it a valuable metal. The largest deposit of gold can be found in South Africa. This deposit is hundreds of miles across and more than two miles deep! It is estimated that two- thirds of the gold mined comes from South Africa. Other major contributors of gold include the United States, Australia, and the former Soviet Union.
 

            “About 65 percent of processed gold is used in the arts industry, mainly to make jewelry. Besides jewelry, gold is also used in the electrical, electronic, and ceramics industries. These industrial applications have grown in recent years and now occupy an estimated 25 percent of the gold market. The remaining percentage of mined gold is used to make a type of ruby colored glass called purple of Cassius, which is applied to office building windows to reduce the heat in the summer, and to mirrors used in space and in electroscopy so that they reflect the infrared spectrum (Gold, 2009).”
 

Physical Characteristics of Gold
 

     The chemical symbol for gold is Au. Gold is easily shaped, able to be drawn or shaped, and its high resistance to heat and electrical conductivity makes its uses endless. Due to gold’s malleability, we are able to press thin sheets of paper from the gold and use it for delicate needs such as a filling for a tooth or even a coating for parts. Gold’s ability to be flattened so thin is a contributing factor to today’s technologies. We are able to use gold wire for integrated electrical connections for electrical and mechanical wiring. When it comes to industrial uses, the only con about using gold is how soft the metal actually is. To strengthen gold, it is usually alloyed with another type of metal such as nickel, copper, silver, or platinum. In order to measure the alloys of gold, we use karats or carats. A karat is a unit equal to 1/24 part of pure gold in an alloy. Twenty- four karat is the most high quality, softest, and pure gold while 14 karat gold is 14 parts pure gold mixed with 10 parts other metals. “The legal limit in the United States for jewelry to be considered gold is 10 karat. Everything within the legal range is known as Plumb Gold (Gold Chain, 2009).” 

 

         

Pictures provided by Ivonne M. Suarez- Denton. 22 karat gold necklaces with 22k gold charms.



14k gold compared with 22k gold
 

Process of Making Gold
 

            Gold can generally be found in two types of deposits, lode (vein) or placer deposits. The method used to extract this gold from lode deposits is by drilling. The method used to extract gold from placer deposits is by power shoveling or hydraulic mining. Once the gold has been extracted the gold ore is grinded into a powder in order to prepare it for refining. There are four different types of refining methods: floatation, amalgamation, cyanidation, and carbon- in- pulp. With each process relying on the initial grinding of the ore, more than one process can be used on the same batch of gold.

Mining

1 “In lode or vein deposits, the gold is mixed with another mineral, often quartz, in a vein that has filled a split in the surrounding rocks.

2 Placer deposits contain large pieces of gold ore (nuggets) and grains of gold. The three main methods used to mine placer deposits are hydraulic mining, dredging, and power shoveling.

Grinding

            3 Once the gold ore has been mined, it usually is washed and filtered at the mine as a preliminary refinement technique. It is then shipped to mills, where it is first combined   with water and ground into smaller chunks (Made How, 2009).”

Separating the gold from the ore

            4 The gold is then separated from the ore using the four different methods previously stated (made How, 2009).”

            5 “If the gold is still not pure enough, it can be smelted. Smelting involves heating the gold with a chemical substance called flux. The flux bonds with the contaminants and floats on top of the melted gold. The gold is then cooled and allowed to harden in molds, and the flux-contaminant mixture (slag) is hauled away as a solid waste (Made How, 2009).”
  

  

Picture provided by Open 2 Net

 

 


                              

Pictures provided by Geekology.com
 

Gold Alloys By Weight and Volume

Percentages by Weight

 

 

 

 

 

 

Alloy

Gold

Silver

Copper

Zinc

Nickel

Palladium

9 Yellow

37.5

10

45

7.5

-

-

9 White

37.8

0

40

10.4

11.8

-

14 Yellow

58.5

4

31.2

6.3

-

-

14 White

58.5

0.5

27

7

7

-

18 Yellow

75

16

9

-

-

-

18 White

75

4

4

-

-

17

22 Yellow

91.7

5.5

2.8

-

-

-

 

 

 

Percentages by Volume

 

 

 

 

 

 

Alloy

Gold

Silver

Copper

Zinc

Nickel

Palladium

9 Yellow

22

11

56

12

-

-

9 White

21

0

48

16

14

-

14 Yellow

39

5

45

11

-

-

14 White

39

1

38

12

10

-

18 Yellow

61

24

16

-

-

-

18 White

63

6

7

-

-

23

22 Yellow

84

5

10

-

-

-

 

 

Color Chart
 

Type

Gold % wt

Silver %

Copper %

Colour

22 ct

91.6

8.4

-

Yellow

 

91.6

5.5

2.8

Yellow

 

91.6

3.2

5.1

Deep yellow

 

91.6

-

8.4

Pink/rose

18 ct

75.0

25.0

-

Green-yellow

 

75.0

16.0

9.0

Pale yellow, 2N

 

75.0

12.5

12.5

Yellow, 3N

 

75.0

9.0

16.0

Pink, 4N

 

75.0

4.5

20.5

Red, 5N

14 ct

58.5

41.5

-

Pale green

 

58.5

30.0

11.5

Yellow

 

58.5

9.0

32.5

Red

9 ct

37.5

62.5

-

White

 

37.5

55.0

7.5

Pale yellow

 

37.5

42.5

20.0

Yellow

 

37.5

31.25

31.25

Rich yellow

 

37.5

20.0

42.5

Pink

 

37.5

7.5

55.0

Red

 

 

     The only two colored pure metals are gold which is yellow and copper which is red. All other metals are a gray color.

 

 

Physical Properties of Typical Gold Alloys
 

Carat

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 karatage reduces, the melting range and alloy density are lowered. But at any given karatage (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 Alloys
 

Carat

Composition %, wt.

 

Condition

Hardness

HV

Tensile Strength

N/mm2

 

Silver

Copper

 

 

 

24

-

-

Annealed

20

45

 

 

 

Worked

55

200

 

 

 

 

 

 

22

5.5

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

 

 

 

Mechanical Properties of 18 Carat Gold
 

Composition, 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

 

 

     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°C
 

Composition, wt%

 

 

Hardness, HV

 

Gold

Silver

Copper

Slow cooled in air

Water quenched

75

25

-

56

56

75

22

3

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.

 

 

Effect of Heat Treatment on 18 Carat Alloys
 

Composition %, wt

 

Colour

Condition

Hardness
HV

Tensile Strength
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

 

 

 

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

 

 

White Golds
 

            Apart from copper, all other alloying metals to gold will tend to whiten the color and so it is possible to make karat gold that are white. White gold for jewelry was developed in the 1920's as a substitute for platinum. Additions of any white metal to gold could potentially bleach it's color. In practice, nickel 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 Platinum white. At the 9 karat (37.5% gold) level, a gold-silver alloy is quite white, ductile although soft and is used for jewelry purposes. White gold is available up to 21 carat.
 

Nickel White Gold
 

            Nickel alloying additions form hard and strong white gold up to 18 karat. They are difficult to work and suffer from so-called 'firecracking'. This copper addition affects color, and white gold alloys are not a good white color - more a slight yellow/ brown tint, particularly if nickel content is also low. As a consequence, white gold jewelry is normally electroplated with rhodium (a platinum metal) which is tarnish resistant and imparts a good white colour.

 

            Unfortunately, many people are allergic to nickel in contact with the skin and this gives rise to a red skin rash or irritation. In Europe, nickel white gold is being phased out and being replaced by platinum white gold. The USA is taking a more relaxed approach, requiring jewelry to be labeled as nickel-containing, and much jewelry in the West is now advertised as 'non-allergenic' or 'nickel-free'.
 

Typical Nickel White Gold
 

 

Gold,
% wt

Copper,
% wt

Nickel,
% wt

Zinc,
% wt

Hardness
Hv

Liquidus
°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 Gold


            Additions of about 10 -12% platinum gold impart a good white color. But platinum is an expensive metal. Jewelry in platinum white gold will be more expensive than identical pieces in nickel whites for 2 reasons: firstly, the cost of the platinum  and secondly, the impact of density - palladium white gold is denser and so such jewelry will be heavier and also contain more gold. It is also more difficult to process as the melting temperatures are substantially higher.
 

     Many commercial platinum white gold only contain about 6-8% platinum  plus silver, zinc and copper. These may also have less than a good white color and so may also be rhodium plated.
 

     Platinum white gold tend to be softer and more ductile compared to nickel whites and so will not wear as well. They are available in all karatages up to 21 carat. It is not possible to have a 22 ct white gold.

 

Typical Palladium Alloys
 

 

Gold

Pd

Ag

Cu

Zn

Ni

Hardn
Hv

Liq,
°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

 

 Charts and information were provided by World Counsel Gold

 

            Finally we can have a better understanding of gold and its different characteristics. It is not only beautiful but useful in out technology and advancements of today. As it has been for centuries, gold will continue to be used as an accessory, as electrical parts, in paintings and drawings, even as such small things like fillings on one’s teeth. Gold and it’s alloys will be apart of man’s history now just as it was centuries ago and will continue to be.
 

Works Cited

Gold Alloys. 2009. http://www.18carat.co.uk/goldalloysbyvolume.html

Gold Chain. 2009 http://www.goldchain.com/index.asp?PageAction=Custom&ID=6

Geek Gold. 2007. http://www.geekologie.com/2007/06/17-week/

King Tut. 2009. http://i236.photobucket.com/albums/ff305/yusefsmum/king_tut1.gif

Northwest Territorial Mint. 2009. http://bullion.nwtmint.com/gold_history.php

World Gold Counsel. 2009. http://www.utilisegold.com/jewellery_technology/colours/colour_alloys