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,
Colored golds can be classified in three groups:
-
with silver and copper in various proportions, producing white, yellow, green and red golds. These are typically ductility 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 fineness by definition, so all colored golds are less pure than this, commonly 18K (75%), 14K (58.5%), 10K (41.6%), or 9K (37.5%).
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).
Like yellow gold, the purity of white gold is given in
Fineness.
Rose, red, and pink gold
Rose gold is a gold-copper alloy
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.
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:
-
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.
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".
Spangold
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 °C for 10 minutes then cooled below 20 °C, forming a sparkly surface covered with tiny facets.
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.
Green gold
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%).
It was known to the ancient Persians as long ago as 860 BC.
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.
vitreous enamel 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.
Gray gold
Gray 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.
Intermetallic
All the AuX
2 intermetallics have the cubic
fluorite (
calcium fluoride) crystal structure, and, therefore, are brittle.
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.
The intermetallic compounds tend to have poor corrosion resistance. The less Noble gas 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.
Purple gold
Purple gold (also called amethyst gold and violet gold) is an alloy of gold and aluminium rich in gold–aluminium intermetallic (AuAl
2). Gold content in AuAl
2 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
[ "Purple plague" . International Electrotechnical Commission Glossary]), as it is an
intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter.
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
2 is closer to Au
6Al
11 as the
sublattice is incompletely occupied.
Blue gold
Blue gold is an alloy of gold and either
gallium or
indium.
Gold-indium contains 46% gold (about 11 karat) and 54% indium,
forming an intermetallic compound AuIn
2. While several sources remark this intermetallic to have "a clear blue color",
in fact the effect is slight: AuIn
2 has CIE LAB color coordinates of 79, −3.7, −4.2
which appears roughly as a grayish color. With gallium, gold forms an intermetallic AuGa
2 (58.5% Au, 14ct) which has slighter bluish hue. The melting point of AuIn
2 is 541 °C, for AuGa
2 it is 492 °C. AuIn
2 is less brittle than AuGa
2, which itself is less brittle than AuAl
2.
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.
[
] Black-colored gold can be produced by various methods:
A range of colors from brown to black can be achieved on copper-rich alloys by treatment with potassium sulfide.
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.
More recently, black gold can be formed by creating 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,
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.
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.
See also
External links
