Sunday, July 21, 2024

Why does ivory turn green?

Ivory has long been prized for its beauty, durability, and historical significance. However, over time, many ivory artifacts, jewelry pieces, and collectibles develop a curious greenish hue. This phenomenon can be perplexing and concerning for collectors and historians alike. In this article, we will explore the reasons behind ivory turning green, delving into the scientific, environmental, and historical factors that contribute to this transformation. By understanding these factors, we can better appreciate and preserve ivory artifacts for future generations.

The Composition of Ivory

To understand why ivory turns green, it’s essential first to comprehend what ivory is made of. Ivory primarily consists of dentin, a dense, bony tissue found in the teeth and tusks of animals such as elephants, walruses, and narwhals. Dentin is composed of hydroxyapatite crystals, collagen fibers, and water. This unique composition gives ivory its characteristic hardness and smooth texture, making it highly valued for various artistic and functional purposes throughout history.

Chemical Reactions: The Role of Copper and Other Metals

One of the primary reasons ivory turns green is due to chemical reactions involving copper or other metals. When ivory comes into contact with copper or copper alloys, a chemical reaction occurs, resulting in the formation of copper acetate or other copper compounds. These compounds can produce a greenish patina on the surface of the ivory. This reaction is often seen in ivory items mounted with or stored near copper or bronze fittings, such as handles, jewelry settings, or decorative mounts.

The green patina is a result of the ivory absorbing copper ions from the metal. Over time, these ions penetrate the porous structure of the ivory, leading to discoloration. This process is accelerated in environments with high humidity or acidic conditions, which can enhance the mobility of copper ions and promote their absorption by the ivory.

Environmental Factors: Humidity and Pollution

Environmental factors also play a significant role in the green discoloration of ivory. High humidity levels can cause ivory to absorb moisture from the air, which in turn facilitates chemical reactions with pollutants or metal ions present in the environment. Additionally, airborne pollutants, such as sulfur compounds from industrial emissions or household chemicals, can interact with the surface of ivory and contribute to the formation of greenish compounds.

Polluted air can contain various substances that, when combined with moisture, form acids. These acids can etch the surface of ivory, creating a more porous structure that is susceptible to absorbing metals and other pollutants. Over time, this can lead to a noticeable green discoloration.

Historical and Preservation Issues

The historical context of ivory artifacts can provide insights into why some pieces turn green. Many ancient ivory objects have been buried in the ground for centuries, where they were exposed to soil minerals and metals. For instance, ivory objects buried with bronze or copper artifacts can develop a greenish hue due to prolonged exposure to these metals in the soil.

Moreover, preservation methods used throughout history can influence the color of ivory. Early conservation techniques often involved the use of various substances that might react with ivory over time. For example, coatings or adhesives containing metal compounds could lead to green discoloration. Understanding the history of an ivory artifact, including its burial environment and preservation treatments, is crucial in diagnosing and addressing green discoloration.

Biological Factors: Mold and Algae

In some cases, biological factors such as mold or algae can contribute to the green coloration of ivory. Mold spores present in the environment can settle on the surface of ivory and, under suitable conditions of humidity and warmth, grow and proliferate. Certain types of mold can produce green pigments, which can stain the ivory. Similarly, algae, which thrive in moist environments, can colonize the surface of ivory and impart a greenish tint.

The presence of mold or algae on ivory not only affects its appearance but can also cause further deterioration. Mold can produce organic acids that degrade the surface of the ivory, while algae can create a slippery, biofilm layer that attracts more moisture and pollutants. Proper environmental control and regular cleaning are essential to prevent biological growth on ivory artifacts.

Preventive Measures and Restoration

Preventing and addressing the green discoloration of ivory requires a multifaceted approach. Here are some key measures to consider:

Environmental Control: Maintaining a stable, low-humidity environment is crucial for preserving ivory. Using dehumidifiers and climate control systems can help prevent moisture absorption and subsequent chemical reactions. Additionally, keeping ivory art ifacts away from pollutants and metal objects can reduce the risk of green discoloration.

Regular Cleaning: Regular cleaning of ivory with appropriate, non-abrasive methods can remove surface contaminants and biological growth. It is essential to use cleaning agents that do not contain harsh chemicals or acids that might react with the ivory.

Protective Coatings: Applying protective coatings, such as microcrystalline wax or specialized conservation-grade sealants, can provide a barrier against moisture and pollutants. These coatings should be carefully chosen to avoid any adverse chemical reactions with the ivory.

Professional Conservation: In cases where ivory has already developed significant green discoloration, seeking the expertise of professional conservators is recommended. Conservators can employ advanced techniques to remove green patina, stabilize the ivory, and restore its original appearance.

See Also: Can Ivory Be Appraised?

Case Studies and Examples

Examining specific case studies can provide further insights into the causes and treatments of green discoloration in ivory. For instance, an ancient Egyptian ivory carving found in a tomb exhibited a greenish hue due to prolonged contact with bronze artifacts. Conservation efforts involved carefully cleaning the surface and applying a protective coating to prevent future discoloration.

Another example is a 19th-century ivory piano key that turned green due to exposure to humid conditions and copper alloy fittings. Conservators used a combination of mechanical cleaning and chemical treatments to remove the green patina and restore the key to its original state.

Cultural and Ethical Considerations

While understanding the scientific reasons behind ivory turning green is essential, it’s also important to consider the cultural and ethical dimensions of ivory conservation. Ivory artifacts often hold significant cultural and historical value, representing the artistic and technological achievements of past civilizations. Therefore, preserving these artifacts in their authentic state is crucial for maintaining our cultural heritage.

Ethically, the use of ivory has become increasingly controversial due to the impact on endangered species such as elephants. Modern conservation efforts must balance the need to preserve historical ivory artifacts with the imperative to protect living species. This involves adhering to international regulations and guidelines, such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which governs the trade and handling of ivory.


The phenomenon of ivory turning green is a complex interplay of chemical, environmental, biological, and historical factors. By understanding these factors, we can develop effective strategies for preventing and addressing green discoloration, thereby preserving ivory artifacts for future generations. Maintaining proper environmental conditions, conducting regular cleaning, applying protective coatings, and seeking professional conservation services are all crucial steps in this process.

Ultimately, the preservation of ivory artifacts requires a holistic approach that considers not only the scientific aspects but also the cultural and ethical dimensions. By doing so, we honor the legacy of past civilizations while ensuring the protection of our natural heritage.

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