Colored gold
Colored gold is the name given to any gold that has been treated using techniques to change its natural color. Pure gold is slightly reddish yellow in color,[1] but colored gold can come in a variety of different colors by alloying it with different elements.
Colored golds can be classified in three groups:[2]
- Alloys with silver and copper in various proportions, producing white, yellow, green and red golds. These are typically malleable alloys.
- Intermetallic compounds, producing blue and purple golds, as well as other colors. These are typically brittle, but can be used as gems and inlays.
- Surface treatments, such as oxide layers.
Pure 100% (in practice, 99.9% or better) gold is 24 karat by definition, so all colored golds are less pure than this, commonly 18K (75%), 14K (58.5%), 10K (41.6%), or 9K (37.5%).[3]
Alloys
White gold
The word white covers a broad range of colors that borders or overlaps pale yellow, tinted brown, and even very pale rose. White gold is an alloy of gold and at least one white metal (usually nickel, silver, platinum or palladium).[4] Like yellow gold, the purity of white gold is given in karats.
White gold's properties vary depending on the metals used and their proportions. A common white gold formulation consists of 90% wt. gold and 10% wt. nickel.[3] Copper can be added to increase malleability.[2] The alloys used in the jewelry industry are gold-palladium-silver and gold-nickel-copper-zinc. Palladium and nickel act as primary bleaching agents for gold; zinc acts as a secondary bleaching agent to attenuate the color of copper.[citation needed] As a result, white gold alloys can be used for many different purposes. Nickel alloys are hard and strong, and therefore good for rings and pins. Gold-palladium alloys are soft, pliable, and good for white-gold gemstone settings. The strength of gold-nickel-copper alloys is caused by formation of two phases: a gold-rich Au-Cu, and a nickel-rich Ni-Cu, and the resulting hardening of the material.[2]
The nickel used in some white gold alloys can cause an
Yellow gold
Gold is rarely pure gold, even before another metal is added to make a white gold alloy, and can contain mercury from its production that can cause an allergic reaction.[6]
Examples of the common alloys for 18K yellow gold include:[citation needed]
- 18K yellow gold: 75% gold, 12.5% copper, 12.5% silver
- 18K yellow (darker) gold: 75% gold, 15% copper, 10% silver
Rose, red, and pink gold
Rose gold is a gold-copper alloy
Although the names are often used interchangeably, the difference between red, rose, and pink gold is the copper content: the higher the copper content, the stronger the red coloration. Pink gold uses the least copper, followed by rose gold, with red gold having the highest copper content. Examples of the common alloys for 18K rose gold, 18K red gold, 18K pink gold, and 12K red gold include:[3]
- 18K red gold: 75% gold, 25% copper
- 18K rose gold: 75% gold, 22.25% copper, 2.75% silver
- 18K pink gold: 75% gold, 20% copper, 5% silver
- 12K red gold: 50% gold and 50% copper
Up to 15% zinc can be added to copper-rich alloys to change their color to reddish yellow or dark yellow.[2] 14K red gold, often found in the Middle East, contains 41.67% copper.
The highest karat version of rose gold, also known as crown gold, is 22 karat. Amongst the alloys made of gold, silver, and copper, the hardest is the 18.1 K pink gold (75.7% gold and 24.3% copper). An alloy with only gold and silver is the hardest at 15.5 K (64.5% gold and 35.5% silver).
During ancient times, due to impurities in the smelting process, gold frequently turned a reddish color. This is why many Greek and Roman texts, and some texts from the Middle Ages, describe gold as "red".[citation needed]
Spangold
Some gold-copper-
The alloy of 76% gold, 19% copper, and 5% aluminium yields a yellow color; the alloy of 76% gold, 18% copper, and 6% aluminium is pink.[2]
Green gold
Green gold was known to the
Cadmium can also be added to gold alloys to create a green color, but there are health concerns regarding its use, as cadmium is highly toxic.[8] The alloy of 75% gold, 15% silver, 6% copper, and 4% cadmium yields a dark-green alloy.
Grey gold
Grey gold alloys are usually made from gold and palladium. A cheaper alternative which does not use palladium is made by adding silver, manganese, and copper to the gold in specific ratios.[9]
Intermetallic
All the AuX2 intermetallics have the fluorite (CaF2) crystal structure, and, therefore, are brittle.[2] Deviation from the stoichiometry results in loss of color. Slightly nonstoichiometric compositions are used, however, to achieve a fine-grained two- or three-phase microstructure with reduced brittleness. Another way of reducing brittleness is to add a small amount of palladium, copper, or silver.[10]
The intermetallic compounds tend to have poor corrosion resistance. The less noble elements are leached to the environment, and a gold-rich surface layer is formed. Direct contact of blue and purple gold elements with skin should be avoided as exposure to sweat may result in metal leaching and discoloration of the metal surface.[10]
Purple gold
Purple gold (also called amethyst gold and violet gold) is an alloy of gold and aluminium rich in
Blue gold
Blue gold is an alloy of gold and either gallium or indium.[12] Gold-indium contains 46% gold (about 11 karat) and 54% indium,[3] forming an intermetallic compound AuIn2. While several sources remark this intermetallic to have "a clear blue color",[2] in fact the effect is slight: AuIn2 has CIE LAB color coordinates of 79, −3.7, −4.2[10] which appears roughly as a greyish color. With gallium, gold forms an intermetallic AuGa2 (58.5% Au, 14ct) which has slighter bluish hue. The melting point of AuIn2 is 541 °C, for AuGa2 it is 492 °C. AuIn2 is less brittle than AuGa2, which itself is less brittle than AuAl2.[10]
A surface plating of blue gold on karat gold or sterling silver can be achieved by a gold plating of the surface, followed by indium plating, with layer thickness matching the 1:2 atomic ratio. A heat treatment then causes interdiffusion of the metals and formation of the required intermetallic compound.
Surface treatments
Black gold
Black gold is a type of gold used in jewelry.[13][14] Black-colored gold can be produced by various methods:
- Patination by applying sulfur- and oxygen-containing compounds.
- Plasma-assisted chemical vapor deposition process involving amorphous carbon
- Controlled ).
A range of colors from brown to black can be achieved on copper-rich alloys by treatment with potassium sulfide.[2]
Cobalt-containing alloys, e.g. 75% gold with 25% cobalt, form a black oxide layer with heat treatment at 700–950 °C. Copper, iron and titanium can be also used for such effect. Gold-cobalt-chromium alloy (75% gold, 15% cobalt, 10% chromium) yields a surface oxide that is olive-tinted because of the chromium(III) oxide content, is about five times thinner than Au-Co and has significantly better wear resistance. The gold-cobalt alloy consists of gold-rich (about 94% Au) and cobalt-rich (about 90% Co) phases; the cobalt-rich phase grains are capable of oxide-layer formation on their surface.[2]
More recently, black gold can be formed by creating
Blue gold
Oxide layers can also be used to obtain blue gold from an alloy of 75% gold, 24.4% iron, and 0.6% nickel; the layer forms on heat treatment in air between 450 and 600 °C.[2]
A rich sapphire blue colored gold of 20–23K can also be obtained by alloying with ruthenium, rhodium, and three other elements and heat-treating at 1800 °C, to form the 3–6 micrometers thick colored surface oxide layer.[2]
See also
- Corinthian bronze
- Crown gold
- Electrum
- Hepatizon
- List of alloys
- Mokume-gane, a mixed-metal laminate
- Orichalcum
- Panchaloha, alloys used for making Hindu temple icons
- Pyrite, often referred to as Fool's Gold
- Shakudō, copper with 4–10% gold
- Tumbaga
References
- ^ Encyclopædia of Chemistry, theoretical, Practical, and Analytical: As Applied to the Arts and Manufactures. J. B. Lippincott & Company. 1880. pp. 70–.
- ^ .
- ^ ISBN 0198503407. p. 168
- ^ "White gold". Birmingham, UK: Assay Office. Retrieved 7 January 2019.
- ^ "White gold". Jewellery technology. Colours of gold. World Gold Council. Archived from the original on 2008-11-22.
- PMID 28223758.
- ISBN 9780770434694.
- PMID 21123140.
- ^ Ribault, Laurent and LeMarchand, Annie (June 10, 2003). "For manufacturing jewels by the disposable wax casting technique; does not cause allergies" U.S. patent 6576187.html
- ^ .
- ^ "Purple plague" Archived 2014-05-04 at the Wayback Machine. International Electrotechnical Commission Glossary
- ^ a b "Gold In Purple Color, Blue Color And Even Black Gold". kaijewels.com.
- ^ "Jewellery Technology". World Gold Council. Archived from the original on March 3, 2006.
- ISBN 978-0-12-581250-4.
- ^ "Ultra-Intense Laser Blast Creates True 'Black Metal'". Retrieved 2007-11-21.
- S2CID 103781835.
External links
- Media related to gold-containing alloys at Wikimedia Commons