Bronze Disease: A Major Challenge in the Conservation of Archaeological Copper-Based Coins

Corrosion is a major threat to metal artifacts in various environments, including museums, outdoor atmosphere, and burial soil. Coins, being small objects with intrinsic value, hold rich information about architecture, culture, and arts of their time. They often bear images of rulers, national or religious symbols, and inscriptions, offering insights into the political and economic history of their period. Many coins show important buildings, temples, and other architectural marvels, serving as small canvases for the artistic styles of their period. The study of coins combines elements of history, archaeology, and art history.

Bronze represents a class of alloys widely used in ancient times, valued for its hardness and greater corrosion-resistant compared to pure metals. Ancient cast bronze typically contains copper with varying amounts of tin, and zinc depending on the desired properties of the alloy. The addition of lead (Pb) improved the fluidity of molten bronze, making it a preferred material for production of decorative coins, particularly during the Greco-Roman ages. The corrosion products of bronze are mainly composed of copper oxides, forming a uniform pale brown to black-brown layer known as the primary patina. Preserving this patina especially the cuprite, along with surface details, is the main goal in the conservation of copper-based coins.

Patina is a thin, natural or artificial layer that forms on bronze due to chemical reactions such as oxidation. It develops over time through exposure to environmental factors, including humidity, pollution, and soil. Patinas are composed of compounds such as copper oxides (e.g., cuprite), carbonates (e.g., malachite), or sulfates. Natural patinas provide protection against corrosion, while artificial patinas are applied for aesthetic or protective purpose. In archaeology, patinas are valuable as they act as a historical record and serve as a protective barrier, making their preservation crucial in conservation efforts.

Surface degradation occurs due to corrosion, which involves not only oxidation but also the formation of copper chlorides (bronze disease) and other corrosion products that vary in thickness and composition. Bronze disease is an active corrosion process in which bronze reacts with chloride ions to form cuprous chloride (CuCl), leading to metal degradation. In humid environments, cuprous chloride reacts with water to produce hydrochloric acid, further accelerating deterioration. This results in pitting and flaking, weakening the structural integrity of artifacts. Bronze disease can compromise the structural, historical, and aesthetic values of archaeological coins. The reaction speed on the bronze surface varies depending on the object’s history and surrounding environment.

Sulfur and chloride ions can penetrate the patina and metal core, causing pits to form on the surface. Aggressive chloride accelerates the dissolution of copper-based alloys, causing the loss of native protective layers in burial environments. The corrosion layers mainly consist of malachite (CuCO3. Cu(OH)2), which disrupts the CuCl/Cu2O mixture and causes exfoliation. Post-excavation cuprous chloride (CuCl) hydrolyzes into hydroxyl chloride polymorphs Cu2OH3Cl in the presence of water. Bronze disease occurs with the formation polymorphous copper hydroxyl chlorides, including atacamite, paratacamite, clinoatacamite, and botallackite.

Lead plays an effective role in influencing the corrosion rate by forming highly stable lead carbonates and insoluble lead chloride, which remain stable after excavation. Protective conservation is the most effective process to prevent or at least delay degradation. However, achieving suitable environmental conditions is challenging, making it necessary to apply a protective system. Protective coatings, including natural patinas or applied treatments, inhibit corrosion by forming a barrier against moisture, oxygen, and pollutants, helping to preserve the artifact and minimize further damage. The coating system is a significant topic to stop corrosion and ensure the long-term stability of archaeological coins. There is a continuous study to provide better protection for objects by preventing the influence of humidity and contaminations with special requirements for conservation-restoration ethics such as ease of application, transparency, good appearance, long-term stability, reversibility, and safety for conservators-restorers and the environment.

Through ancient conservation, natural resins were used to coat metal surfaces, but they often curled and flaked away. In contrast, methyl acrylate/ ethyl methacrylate 30:70% (Paraloid B-72®) is considered the most widely used coating for archaeological objects in Egypt due to its economic efficiency, transparency, and reversibility. It can be applied on both clean metal surfaces and those covered with patina.

The study of archeological coins is essential for both the preservation of cultural heritage and advancements in corrosion science. Analyzing the corrosion products of bronze archeological coins provides insights into the surface morphology, which aids in the preservation of cultural heritage.