23 July 2019

Gold World: Colored Gold

Pure gold is slightly reddish yellow in color, but it can be produced in different colors. 
Colored ones can be classified into three different groups:

Alloys with silver and copper, produce white, yellow, green and red golds. These are typically malleable alloys. 
Intermetallic compounds, producing blue and purple golds, and others. These are typically brittle.
Surface treatments, producing black gold or blue gold.

Alloys

White Gold
White gold is an alloy of gold and at least one white metal  (usually nickel, silver, or palladium). The purity of white gold is given in karats.


Yellow gold 
While we often think of yellow gold as being purer than white one, but yellow gold used for jewelry is an alloy made by combining pure gold with metals 
The purity is also given in karats, so the higher the karat amount, the higher the actual gold content. 


Rose, red, and pink gold
Rose gold is a gold-copper alloy used for specialized jewelry. It 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


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


In this group, we can also find SpangoldGreen Gold  and Grey Gold. 



Intermetallic

The intermetallic compounds tend to have poor corrosion resistance. 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 aluminum rich in gold–aluminum intermetallic  (AuAl2).

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 AuIn2. 



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.


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 450–600 °C.
A rich sapphire blue colored gold of 20–23K can also be obtained by alloying with rutheniumrhodium and three other elements and heat-treating at 1800 °C, to form the 3–6 micrometers thick colored surface oxide layer.

Are you considering a jewel purchase? Be sure that you know gold fineness.

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