Gold is a precious metal that has been valued throughout history for its beauty, rarity, and resistance to tarnishing. Unlike many other metals, pure gold does not oxidize or corrode when exposed to air or moisture. This quality makes it very attractive for jewelry, coins, and other applications where a lustrous, untarnished surface is desired. However, under certain conditions, gold can take on a reddish or copper-like discoloration that gives the appearance of tarnishing. So does gold actually tarnish to a copper color?
What is Tarnish?
Tarnish refers to a surface discoloration or loss of luster on a metal. It is caused by a chemical reaction between the metal and compounds in the surrounding environment, such as sulfur or oxygen. This reaction results in the formation of a new compound on the metal’s surface, which obscures its original color and shine.
Tarnishing is commonly seen on metals like silver, copper, and brass. These base metals readily react with sulfur-containing compounds in the air to form a dark silver sulfide layer on their surface. This tarnished layer can range from pale yellow to nearly black in color depending on the metal and extent of sulfidation.
Does Pure Gold Tarnish?
Pure gold, or 24 karat gold, does not naturally tarnish, corrode, or oxidize under ordinary conditions. This is because gold is a noble metal that is resistant to chemical reaction with oxygen and moisture. The filled d-orbitals of gold atoms bond strongly with other gold atoms, but do not readily form bonds with other elements. This makes 24 karat gold very stable and gives it excellent corrosion resistance.
However, while pure gold does not tarnish, it can appear to develop a tarnish-like film if certain impurities are present:
- Copper: Gold alloys with copper can develop a copper-oxide surface layer that imparts a reddish, rosy, or coppery hue to the gold. This can make it appear as if the gold is tarnishing.
- Silver: Gold alloys containing silver may develop a grayish-black tarnish of silver sulfide.
- Oils or organic films: Handling gold can transfer oils and organic materials from skin that can give a stained appearance if not cleaned away.
So while pure 24 karat gold does not actually tarnish, lower karat gold alloys with other metals can take on tarnish-like discolorations.
Common Karat Grades of Gold
Gold is commonly alloyed with other metals like copper and silver to increase its hardness, durability, and color variation. Here are some typical karat grades of gold and their properties:
| Karat | % Gold Content | Properties |
|---|---|---|
| 24 karat | 100% | Pure gold, very soft, easily scratched |
| 22 karat | 91.7% | Soft, easily scratched |
| 18 karat | 75% | More durable, yellow color |
| 14 karat | 58.3% | Harder, more copper alloying |
| 10 karat | 41.7% | Significant copper content, more rose colored |
Lower karat gold with more copper and silver alloyed will be more prone to developing tarnish-like surface discoloration.
Why Does Gold Appear to Tarnish?
There are a few key reasons why gold jewelry or other gold items may appear to tarnish over time:
- Copper oxidation – Lower karat gold alloys containing copper can experience surface oxidation of the copper metal, resulting in a pink, rose, or reddish discoloration.
- Silver sulfidation – Gold alloys with silver can undergo silver sulfidation, creating a grayish-black tarnish layer of silver sulfide.
- Reactions with skin oils and lotions – Organic substances transferred from skin to gold jewelry can give a dirty, stained appearance if not cleaned away.
- Tarnish layer diffusion – Copper and silver tarnish layers can gradually diffuse into the bulk gold alloy over time, giving the surface a discolored look.
While pure gold does not tarnish, the small amounts of other metals in karat gold alloys can react with the environment to produce tarnish-like films on the surface. Copper oxidation is the most common cause of gold’s tarnished appearance.
Does Gold Plating Tarnish?
Gold plating involves applying a thin layer of gold, usually 1-2 microns thick, onto the surface of another metal through electroplating. This outer layer of gold provides the attractive shine and luster of gold, while the underlying base metal provides strength and saves cost compared to solid gold.
Because gold plating only contains a thin layer of gold, the base metal underneath can still undergo oxidation and sulfidation to form tarnish. Over time, this tarnish layer can diffuse through the thin gold coating, leading to an overall discolored appearance.
Common base metals like silver, copper, and brass will gradually tarnish over years of use. Scratches and wear on gold plated jewelry can allow the base metal to be directly exposed to air, accelerating local tarnishing.
The gold plating may also be too thin in certain areas, allowing the base metal’s tarnish to show through more easily. Higher quality gold plating is typically thicker and more durable, providing longer lasting shine before any tarnishing becomes visible.
Does Gold Vermeil Tarnish?
Gold vermeil describes jewelry that has a gold coating over sterling silver. It typically consists of a thick layer of karat gold (usually 18k or 22k) electroplated onto a sterling silver base. The gold layer is much thicker than regular gold plating, ranging from 2-10 microns.
The thicker gold coating helps protect the sterling silver base from tarnishing. However, sterling silver consists of 92.5% silver and 7.5% copper. Over time, the copper atoms in the silver alloy can migrate to the vermeil surface and oxidize, creating areas of coppery-rose discoloration.
Humidity and sulfur compounds accelerate silver tarnishing, so gold vermeil jewelry is more prone to developing tarnish in certain environments. Scratches and wear of the gold coating can also expose the sterling silver and allow localized tarnishing.
Overall, gold vermeil is less prone to tarnishing than regular gold plating, but can still exhibit discoloration over time where the base metal is exposed.
How to Clean Tarnished Gold
If your gold jewelry, watch, or other item appears tarnished or discolored, there are methods to remove the tarnish and restore its original shine and luster:
- Use a mild soap and soft brush to gently scrub away any dirt, skin oils, or lotion residues on the gold. Rinse thoroughly.
- Soak briefly in jewelry cleaner or mild soap solution. These can dissolve oils and organic films.
- For copper oxide tarnish, use a reducing agent like vinegar or lemon juice containing citric acid. This reverses the oxidation reaction and removes copper tarnish.
- Silver tarnish (silver sulfide) requires a jewelry dip or polish with sulfide-removing chemicals like thiourea or sodium thiosulfate.
- Avoid abrasives like steel wool or certain jewelry cleaners that can damage gold coatings.
- Use a soft jewelry polishing cloth to buff away any remaining tarnish and restore luster after cleaning.
Be cautious when cleaning antique or fragile gold items, and avoid harsh chemicals that may damage the gold. For valuable gold jewelry, professional jewelers have best knowledge for properly restoring original shine and color.
Conclusion
In summary, pure 24 karat gold does not naturally tarnish or corrode. However, lower karat gold alloys containing copper, silver, and other metals can undergo chemical reactions resulting in tarnish-like discoloration or dullness on the surface.
Common causes of gold’s tarnished appearance include copper oxidation, silver sulfidation, organic films from handling, and diffusion of tarnish layers into karat gold alloys over time. While not considered true tarnish, these effects create undesirable discoloration and loss of luster.
Proper care and occasional cleaning can minimize gold’s tarnished look. But some darkening of intricate gold jewelry is expected, especially for lower karat gold, gold platings, and gold vermeil.
Pure 24 karat gold remains the most untarnishable form of the metal. But for jewelry and decorative use, lower gold karat alloys are often preferred for better durability, affordability, and color – despite their increased tendency towards tarnish.
