Bronze disease is a term used to describe a type of corrosion that affects bronze artifacts, particularly those made from copper tin alloys. It’s a chemical reaction that causes the formation of copper chloride and other compounds, leading to the degradation of the bronze surface.
Bronze disease typically appears as greenish or bluish-green spots, powdery patches, or a general discoloration on the surface of the bronze. If left untreated, it can spread and cause significant damage, including pitting, flaking and structural weakening.
The disease is often triggered by exposure to moisture, salt and pollutants, which react with the copper in the bronze to form corrosive compounds. It’s a common concern for museums, conservators and collectors of bronze artifacts, as it can irreparably damage valuable historical pieces.
Treatments for bronze disease usually involve careful cleaning, stabilization, and protection to prevent further corrosion. In some cases, affected areas may be removed or consolidated to prevent further damage.
Causes of bronze disease
The main causes of bronze disease are: (1) Moisture: High humidity or direct exposure to water accelerates corrosion; (2) Chlorides: Presence of chlorides, often from sea water, cleaning products, or pollution, triggers the formation of copper chlorides; (3) Sulphur compounds: Exposure to sulphur dioxide, hydrogen sulphides or other sulphur pollutants contributes to corrosion; (4) Acidic environment: Low PH levels, such as those found in acidic soils or polluted air, promote corrosion; (5) Temperature fluctuation: Changes in temperature can cause condensation, leading to moisture accumulation; (6) Poor storage or display: Inadequate housing, inadequate ventilation or direct sunlight exposure can contribute to bronze disease; (7) Cleaning products: Using inappropriate cleaning products or solutions/methods can damage the bronze surface; (8) Contamination: Dirt, grime, or other substances on the surface can react with the bronze; (9) Inherent material flaws: Impurities or inconsistencies in the bronze alloy can increase susceptibility to corrosion; (10) Lack of maintenance: Failure to regularly inspect and maintain bronze artifact can allow corrosion to progress unchecked.
By understanding these causes, collectors, conservators, and museums can take preventive measures to minimize the risk of bronze disease and ensure the long-term preservation of bronze artifacts.
Chemical equation for bronze disease, which involves the corrosion of copper-tin alloys (bronze) in the presence of chlorides and moisture, can be represented as follows: Cu (copper) + 2cl (Chloride ions)n Cu Cl2 (copper (ii) chloride).
CuCl2+ H20 (water)n Cu (OH) Cl (copper (ii) hydroxide Chloride + HCl (Hydrochloric acid). CU (OH) Cln CUO (copper (ii) oxide) + HCl. CUO + CO2 (Carbon dioxide) + H2On Cu2 Co2 (OH)2 (copper (ii) carbonate hydroxide).
This series of reactions leads to the formation of copper chloride, copper hydroxide chloride, and ultimately, copper carbonate hydroxide, which is the greenish or bluish-green compound characteristics of bronze disease.
Additionally, the presence of tin in the bronze alloy can lead to the formation of tin oxides and chlorides, which can also contribute to the corrosion process.
Treatment
Treating bronze disease requires careful consideration and a multi-step approach to stabilize and conserve the affected bronze artifact. Here is a general outline of the treatment process.
Assessment: Evaluate the extent and severity of the bronze disease considering factors like the type of bronze, degree of corrosion and overall condition.
Cleaning: Gently remove dirt, grime and other substances using soft brushes, distilled water and mild detergent. Avoid abrasive materials or harsh chemicals.
Stabilization: Apply a stabilizing agent like benzotriazole (BTA) or a similar corrosive inhibitor to halt the corrosion process.
Desalination: If chlorides are present, use a desalination treatment to remove them. This may involve soaking the bronze in distilled water or specialized solution.
Rinsing and drying: Thoroughly rinse the bronze with distilled water and dry it with soft material to prevent water spots.
Consolidation: If the bronze is fragile or flaking, apply a consolidant to strengthen the surface.
Protection: Apply a protective coating, such as wax, varnish, or a synthetic polymer, to shield the bronze from further corrosion.
Monitoring and maintenance: Regularly inspect the bronze and perform maintenance tasks like cleaning and reapplying protective coatings to prevent fortune corrosion.
Its essential to note that treatment should only be carried out by trained conservators or professionals with experience in bronze conservation as improper treatment can cause damage or irreversible harm.
Is bronze disease the same as patina?
No, bronze disease and patina are not the same, though they can sometimes be confused due to their similar appearances.
Patina is a natural layer that forms on the surface of bronze over time, resulting from exposure to air, moisture and environmental factors. Typically, a stable, harmless layer that can enhance the bronze’s appearance, often with attractive colours and textures. Can be uniform or uneven, ranging from subtle to vibrant colours like green, brown or reddish-brown.
Patina is generally considered desirable, as it adds character and historical significance to the bronze.
Bronze disease, on the other hand, is an active corrosion process that causes damage to the bronze leading to structural weakening and potentially irreversible harm. It is characterized by greenish or bluish-green spots, powdery patches or a general discolouration, often accompanied by pitting, flaking or crumbling. It is caused by exposure to moisture, chlorides, and other pollutants which trigger chemical reactions that degrade the bronze.
While patina is a natural, stable and often desirable aspect of bronze ageing, bronze disease is an active, harmful process that requires treatment and conservation to prevent further damage.
Chemicals used for treating bronze disease
(i) Benzotriazole (BTA), a corrosion inhibitor to stabilize and prevent further bronze disease; (ii) Ethanolamine, a complexing agent to remove chlorides and stabilize the bronze; (iii) Sodium sesquicarbonate, a desalination agent to remove chloride and other salts; (iv) Distilled water, used for cleaning, rinsing and desalination processes; (v) Mild detergents like sodium dodecylsulphate (SDS) used for gentle cleaning; (vi) Acetic acid, in dilute solutions, used to remove corrosion products and stabilize the bronze; (vii) Alkaline solution, like sodium hydroxide or ammonia, used to neutralize acidic corrosion products; (viii) Consolidant, like acrylic polymers (e.g., paranoid B.72), used to strengthen fragile or flaking bronze surface; (ix) Waxes or varnishes, applied as protective coatings to shield the bronze from further corrosion.
It is crucial to note that chemicals treatment should only be carried out by trained professionals. The specific treatment and chemicals used depend on the bronze alloy, severity of the disease and environmental condition.
Always test a small, inconspicuous area first to ensure the treatment does not damage the bronze. Follow established conservation guidelines and safety protocols when handling chemicals. Remember, preventive and regular maintenance are key to minimizing the risk of bronze disease.
Recommendations
Regular inspection: Regularly examine bronze artifacts for signs of bronze diseases, such as greenish or bluish-green spots or powdery patches.
Controlled environment: Store or display bronze artifacts in a controlled environment with stable temperature (around 18-20oC; 64-68oF) and relative humidity (around 40-50%).
Avoid exposure to pollutants: Keep bronze artifacts away from pollutants like dust, moisture and chemicals.
Proper cleaning: Clean bronze artifacts gently with soft brushes, distilled water and mild detergents. Avoid abrasive materials or harsh chemicals.
Conservation treatment: If bronze disease is detected, consult a professional conservator for treatment.
Preventive conservation: Apply a protective coating like wax or varnish to shield the bronze from corrosion.
Documentation: Keep records of conservation treatments, environmental conditions and any changes in the artifacts condition.
Collaboration: Share knowledge and best practices with other conservators, curators and collectors to advance the understanding and treatment of bronze disease.
By following these recommendations, you can help prevent, detect and treat bronze disease, ensuring the long-term preservation of bronze artifacts.
