{{Short description|Various colors of gold obtained by alloying gold with other elements}} [[File:Ag-Au-Cu-colours-english.svg|thumb|right|350px|Ternary plot of different colors of Ag–Au–Cu alloys<ref name=Carpenter1986>{{Cite journal |title=Metals Suitable for Enameling |author=Woodrow Carpenter |journal=Glass on Metal Magazine |date=June 1986 |access-date=17 November 2007 |url=https://www.ganoksin.com/article/metals-suitable-for-enameling/ |archive-url=https://web.archive.org/web/20170205182301/https://www.ganoksin.com/article/metals-suitable-for-enameling/ |archive-date=5 February 2017}}</ref>]]

'''Colored gold''' is any gold alloy that has a distinctive color. Pure gold is slightly reddish yellow in color,<ref name="chem">{{cite book|title=Encyclopædia of Chemistry, theoretical, Practical, and Analytical: As Applied to the Arts and Manufactures|url=https://books.google.com/books?id=o-FYAAAAYAAJ&pg=PA70|year=1880|publisher=J. B. Lippincott & Company|pages=70–}}</ref> 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:<ref name="cretu">{{cite journal |last1=Cretu |first1=Cristian |last2=van der Lingen |first2=Elma |title=Coloured gold alloys |journal=Gold Bulletin |date=December 1999 |volume=32 |issue=4 |pages=115–126 |doi=10.1007/BF03214796 |url=https://link.springer.com/content/pdf/10.1007/BF03214796.pdf |archive-url=https://web.archive.org/web/20180730063422/https://link.springer.com/content/pdf/10.1007/BF03214796.pdf |archive-date=2018-07-30}}</ref>{{rp|118}} * 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 (58.3%), 10K (41.7%), or 9K (37.5%).<ref name="natbuild"/>

==Alloys== ===White gold=== {{Main|White gold}} [[File:White-gold--rhodium-plated.jpg|thumb|Rhodium-plated white gold wedding ring]] 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).<ref name=AssayOffice>{{cite web |title=White gold |publisher=Assay Office |place=Birmingham, UK |url=https://theassayoffice.com/white_gold_labexpert |access-date=7 January 2019}}</ref> Like yellow gold, the purity of white gold is given in karats.

===Rose, red, and pink gold=== {{Redirect|Red gold}} {{redirect|Rose Gold|the album by Shekhinah|Rose Gold (album)}} {{redirect|Rosegold|the album by Ashley Monroe|Rosegold (album)}} {{redirect|Rose gold|the color|Shades of rose#Rose gold}} thumb|Rose gold diamond engagement ring Rose gold is a gold-copper alloy<ref>{{Cite book|url=https://books.google.com/books?id=SUXRAwAAQBAJ&pg=PA23|title=Handcrafting Chain and Bead Jewelry: Techniques for Creating Dimensional Necklaces and Bracelets|last=Plumlee|first=Scott David|date=2014|publisher=Potter/TenSpeed/Harmony|isbn=978-0-7704-3469-4}}</ref> widely used for specialized jewelry. Rose gold, also known as pink gold and red gold, was popular in Russia at the beginning of the 19th century, and was also known as Russian gold.<ref>{{Cite web |last=Paris |first=Calin Van |date=2016-11-09 |title=Why Rose Gold's Romantic Glow Is Here to Stay |url=https://www.allure.com/story/rose-gold-obsession |access-date=2024-07-09 |website=Allure |language=en-US}}</ref> Rose gold jewelry is becoming more popular in the 21st century, and is commonly used for wedding rings, bracelets, and other jewelry.

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:<ref name="natbuild"/>

* 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.<ref name="cretu"/> 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|date=May 2011}}

===Spangold=== {{Main article|Spangold}} {{One source section|date=July 2024}}

Some gold-copper-aluminium alloys form a fine surface texture at heat treatment, yielding a spangling effect. At cooling, they undergo a quasi-martensitic transformation from body-centered cubic to body-centered tetragonal phase; the transformation does not depend on the cooling rate. A polished object is heated in hot oil to 150–200&nbsp;°C for 10 minutes then cooled below 20&nbsp;°C, forming a sparkly surface covered with tiny facets.<ref name="cretu"/>

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.<ref name="cretu"/>

===Green gold=== {{Further|Electrum}} Electrum, a naturally occurring alloy of silver and gold, develops a greenish cast with increasing silver content, ranging in color from green-yellow (for proportions of silver between 14% and 29%) to pale green-yellow (for proportions of silver between 29% and 50%).<ref name="Rapson1990">{{cite journal |last1=Rapson |first1=William S. |author-link1=Bill Rapson |title=The metallurgy of the coloured carat gold alloys |journal=Gold Bulletin |date=December 1990 |volume=23 |issue=4 |pages=125–133 |doi=10.1007/BF03214713}}</ref>{{rp|at=Fig. 2}} It was known to the ancient Persians as long ago as 860 BC.<ref name="natbuild">{{cite book |last1=Emsley |first1=John |author-link1=John Emsley |title=Nature's building blocks: an A–Z guide to the elements |date=2003 |publisher=Oxford University Press |location=Oxford |isbn=0-19-850340-7 |edition=Reprinted with corrections, 1st |url=https://archive.org/details/naturesbuildingb0000emsl/page/168 |page=168}}</ref> However, electrum was used even thousands of years before that, by both the Akkadians and Ancient Egyptians (as evidenced by the Royal Cemetery at Ur). Even the tops of some Egyptian pyramids were known to be capped in thin layers of electrum. Fired enamels adhere better to these alloys than to pure gold.

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.<ref>{{Cite journal |pmid=21123140 |pmc=3002210 |year=2010 |last1=Mead |first1=M. N. |title=Cadmium confusion: Do consumers need protection? |journal=Environmental Health Perspectives |volume=118 |issue=12 |pages=a528–34 |doi=10.1289/ehp.118-a528 }}</ref> Adding 2% cadmium to 18K red gold yields a light green color, whereas the alloy of 75% gold, 15% silver, 6% copper, and 4% cadmium is dark green.<ref name="cretu"/>

===Gray gold=== Gray gold alloys are usually made from gold and palladium.{{citation needed|date=April 2024}} A cheaper alternative which does not use palladium is made by adding silver, manganese, and copper to the gold in specific ratios.<ref name="Ribault-2003">{{cite patent|inventor-last1=Ribault |inventor-first1=Laurent |inventor-last2=LeMarchand |inventor-first2=Annie |title=For manufacturing jewels by the disposable wax casting technique; does not cause allergies | issue-date = 2003 | patent-number = 6576187 |country-code = US |url=https://www.freepatentsonline.com/6576187.html }}</ref>

==Intermetallic==

All the AuX<sub>2</sub> intermetallics have the cubic fluorite (CaF<sub>2</sub>) crystal structure, and, therefore, are brittle.<ref name="cretu"/> 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.<ref name="klotz"/>

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.<ref name="klotz"/>

===Purple gold=== [[File:Phasendiagramm Gold-Aluminium.svg|300px|thumb|Gold–aluminium phase diagram]] Purple gold (also called amethyst gold{{citation-needed|date=July 2024}} and violet gold{{citation-needed|date=July 2024}}) is an alloy of gold and aluminium rich in gold–aluminium intermetallic (AuAl<sub>2</sub>). Gold content in AuAl<sub>2</sub> is around 79% and can therefore be referred to as 18 karat gold. Purple gold is more brittle than other gold alloys (called the "purple plague" when it forms and causes serious faults in electronics<ref>[http://std.iec.ch/terms/terms.nsf/9bc7f244dab1a789c12570590045fac8/7533d5855f45dca1c1256eb50047590a?OpenDocument "Purple plague"] {{webarchive|url=https://web.archive.org/web/20140504121626/http://std.iec.ch/terms/terms.nsf/9bc7f244dab1a789c12570590045fac8/7533d5855f45dca1c1256eb50047590a?OpenDocument |date=2014-05-04}}. International Electrotechnical Commission Glossary</ref>), as it is an intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter.<ref name="purple gold">{{cite web |title=Gold In Purple Color, Blue Color And Even Black Gold |url=http://www.newsletter.kaijewels.com/gold-colors.htm |work=kaijewels.com }}</ref> It is therefore usually machined and faceted to be used as a "gem" in conventional jewelry rather than by itself. At a lower content of gold, the material is composed of the intermetallic and an aluminium-rich solid solution phase. At a higher content of gold, the gold-richer intermetallic AuAl forms; the purple color is preserved to about 15% of aluminium. At 88% of gold the material is composed of AuAl and changes color. The actual composition of AuAl<sub>2</sub> is closer to Au<sub>6</sub>Al<sub>11</sub> as the sublattice is incompletely occupied.<ref name="cretu"/>

===Blue gold=== Blue gold is an alloy of gold and either gallium or indium.<ref name ="purple gold" /> Gold-indium contains 46% gold (about 11 karat) and 54% indium,<ref name="natbuild"/> forming an intermetallic compound AuIn<sub>2</sub>. While several sources remark this intermetallic to have "a clear blue color",<ref name="cretu"/> in fact the effect is slight: AuIn<sub>2</sub> has CIE LAB color coordinates of 79, −3.7, −4.2<ref name="klotz">{{Cite journal|doi=10.1007/BF03214961 |title=Metallurgy and processing of coloured gold intermetallics — Part I: Properties and surface processing |journal=Gold Bulletin |volume=43 |pages=4–10 |year=2010 |last1=Klotz |first1=U. E. |doi-access=free }}</ref> which appears roughly as a grayish color. With gallium, gold forms an intermetallic AuGa<sub>2</sub> (58.5% Au, 14ct) which has slighter bluish hue. The melting point of AuIn<sub>2</sub> is 541&nbsp;°C, for AuGa<sub>2</sub> it is 492&nbsp;°C. AuIn<sub>2</sub> is less brittle than AuGa<sub>2</sub>, which itself is less brittle than AuAl<sub>2</sub>.<ref name="klotz"/>

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. <!-- Does anyone have any information on AuB2 or AuTl2? -->

==Surface treatments==

===Black gold=== {{For|the colloquial term ''black gold''|crude oil}} Black gold is a type of gold used in jewelry.<ref>{{cite web|title=Jewellery Technology |url=http://www.gold.org/jewellery/technology/colours/special.html |work=World Gold Council |archive-url=https://web.archive.org/web/20060303151145/http://www.gold.org/jewellery/technology/colours/special.html |archive-date=March 3, 2006}}</ref><ref>{{cite book |author=Rapson, W. S. |publisher=Academic Press |title=Gold Usage |year=1978|isbn=978-0-12-581250-4 }} </ref> 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 oxidation of gold containing chromium or cobalt (e.g. 75% gold, 25% cobalt<ref name="natbuild"/>).

A range of colors from brown to black can be achieved on copper-rich alloys by treatment with potassium sulfide.<ref name="cretu"/>

Cobalt-containing alloys, e.g. 75% gold with 25% cobalt, form a black oxide layer with heat treatment at 700–950&nbsp;°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.<ref name="cretu"/>

More recently, black gold can be formed by creating nanostructures on the surface. A femtosecond laser pulse deforms the surface of the metal, creating an immensely increased surface area which absorbs virtually all the light that falls on it, thus rendering it deep black,<ref>{{cite web |title=Ultra-Intense Laser Blast Creates True 'Black Metal' |url=http://www.rochester.edu/news/show.php?id=2701 |access-date=2007-11-21 }}</ref> but this method is used in high technology applications rather than for appearance in jewelry. The blackness is due to the excitation of localized surface plasmons which creates strong absorption in a broad range in plasmon resonance. The broadness of the plasmon resonance, and absorption wavelength range, depends on the interaction between different gold nanoparticles.<ref>{{Cite journal|last=ElKabbash|first=Mohamed |display-authors=et al|date=2017|title=Tunable Black Gold: Controlling the Near-Field Coupling of Immobilized Au Nanoparticles Embedded in Mesoporous Silica Capsules|journal=Advanced Optical Materials|volume=5|issue=21|article-number=1700617|doi=10.1002/adom.201700617|s2cid=103781835 }}</ref>

===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&nbsp;°C.<ref name="cretu"/>

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&nbsp;°C, to form the 3–6 micrometer thick colored surface oxide layer.<ref name="cretu"/>

==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 * Titanium gold * Tumbaga

==References== {{reflist|30em}}

==External links== *{{commons category-inline|Gold-containing alloys|lcfirst=yes}}

{{Jewellery}}

Category:Gold Category:Precious metal alloys