Understanding the Chemistry of Black Gold: Causes and Solutions

Gold is known for its lustrous appearance and resistance to wear and tear, but under certain conditions, it can appear dull or even black. This phenomenon can be caused by various chemical reactions, contaminants, and layers of plating. Let's delve into the reasons behind gold turning black and explore the methods jewelers use to address this issue.

Gold's Natural Resilience

Gold, as a noble metal, is naturally resistant to oxidation and corrosion. However, there are specific scenarios where gold can take on a black appearance:

Gold Sulfide Formation

One significant factor leading to gold turning black is the formation of gold sulfide, specifically Au2S. This occurs when gold comes into contact with sulfur compounds found in certain environments or materials. This reaction can result in a blackish or darkened appearance of the gold.

Surface Contaminants

Gemstones and gold jewelry can accumulate contaminants such as dirt, oils, or other substances that can give an overall darkened or dull appearance. These contaminants can be easily removed through thorough cleaning processes, restoring the original shine of the gold.

Alloying with Other Metals

When gold is alloyed with other metals, such as silver or copper, and placed in environments conducive to chemical reactions, it can develop a tarnished appearance. This is often due to the interaction between the alloyed metals and the surrounding environment, leading to a darkened or blackened look.

Oxidation of Alloyed Metals

In cases where the gold is alloyed with other metals, like copper, the oxidation of these elements can lead to a tarnished or blackened appearance. This is more common in silver ornaments compared to gold, which is less reactive with oxygen.

Electrochemical Processes

Electrochemical reactions can also cause a darkening of gold, particularly in environments where it is exposed to certain chemicals. These reactions involve the transfer of electrons between metals, leading to changes in color and texture.

Black Rhodium Plating

A common method jewelers use to create a black appearance on gold is through the process of electroplating. Black rhodium or ruthenium are used as layers to coat the gold, creating a black finish. Over time, this plating can wear off, revealing the original gold underneath.

Preventing and Reversing Black Gold

While pure gold naturally resists oxidation and corrosion, the aforementioned factors can cause it to appear darker or blackened. Jewelers address this issue through various methods:

Electroplating and Laser Treatments

Electroplating involves adding a dark metal layer to the gold, often through the use of rhodium or ruthenium, which gives a black finish. Laser treatments use concentrated energy to darken the gold without adding an additional layer.

The Process of Electroplating

Electroplating is a straightforward process that involves plating the gold with either black rhodium or ruthenium. This creates a protective layer that enhances the shine and appearance of the gold. The name may sound complex, but the process itself is relatively simple.

Environmental Factors

The moisture and oxygen present in the air can affect gold and silver ornaments just like they affect iron. Silver ornaments are more sensitive to oxygen and can turn black more easily compared to gold. Proper storage and handling are crucial to maintaining the shine and lustre of these ornaments. It is recommended to store them in air-locked packets and avoid keeping them uncovered in wardrobes or velvet purses, as such conditions can lead to oxidation and discoloration.

Conclusion

While pure gold remains a lustrous and durable metal, various chemical reactions, contaminants, and layers of plating can cause it to appear darker or blackened. Understanding these causes and employing appropriate care and maintenance techniques can help preserve the appearance of gold and silver ornaments for years to come.

To protect and restore the beauty of your gold and silver pieces, consult your trusted jeweler for expert advice and solutions.