This blog post is based on various resources referred to below.
If you have a coin currently affected by bronze disease but are not yet ready to begin treatment, submerge it in distilled water to slow down the corrosion process. Change the water frequently to prevent chloride buildup.
Bronze disease is an active form of corrosion that affects copper-based alloys, such as bronze (alloy of copper and tin), commonly found in ancient coins and artifacts. Conceptually, this is similar to rust (where iron turns into iron oxide). Unlike stable patinas, which form protective layers over time, bronze disease is a self-perpetuating chemical reaction that, if left untreated, can completely destroy a coin. It is characterized by the appearance of powdery, light green corrosion spots that continuously spread.
Although bronze is a relatively noble metal capable of enduring prolonged exposure to seawater and soil, it is highly reactive with chloride ions (found in salts and quite common in soil). As a result, bronze coins that have been exposed to chlorides require treatment to ensure long-term stability. The term bronze disease is used because cuprous chloride from an affected coin can spread to an unexposed coin if its patina fails to provide adequate protection.
Chloride ions infiltrate bronze artifacts primarily through prolonged exposure to chloride-rich environments. This commonly occurs when coins are submerged in seawater, such as those recovered from shipwrecks, or buried in chloride-laden soils, particularly in coastal or arid regions. Middle Eastern soils, for example, often contain high chloride concentrations due to their proximity to the sea and arid climate, which may contribute to the prevalence of bronze disease in artifacts from these areas. The presence of chlorides is a critical factor in the development of bronze disease; a coin buried in non-chloride soils is unlikely to be affected, whereas one exposed to chlorides may deteriorate rapidly upon exposure to air.
From a numismatic perspective, bronze disease can lead to loss of detail, surface pitting, and complete disintegration of historically significant coins. Unlike stable patinas, which can enhance a coin’s historical and aesthetic value, bronze disease actively destroys the metal, reducing both the monetary and historical worth of a coin. Proper conservation is necessary to preserve the integrity of ancient bronze coins and prevent irreversible damage.
In this blog post, we will explore a professional and effective method to treat bronze disease with Sodium Sesquicarbonate, ensuring long-term stability while preserving the coin’s original patina and historical character.
Bronze disease is driven by the presence of chloride ions (\(\text{Cl}^-\)), moisture (\(\text{H}_2\text{O}\)), and oxygen (\(\text{O}_2\)). The reaction begins when metallic copper (\(\text{Cu}\)) is exposed to chloride-containing environments, forming cuprous chloride (CuCl):
\begin{equation} \text{Cu} + \text{Cl}^- \rightarrow \text{CuCl} \end{equation}
Cuprous chloride is unstable in humid air and undergoes hydrolysis in the presence of water and oxygen, producing hydrochloric acid (HCl) and basic cupric chloride (\(\text{CuCl}_2 \cdot 3\text{Cu(OH)}_2\)):
\begin{equation} 4\text{CuCl} + 4\text{H}_2\text{O} + O_2 \rightarrow \text{CuCl}_2 \cdot 3\text{Cu(OH)}_2 + 2\text{HCl} \end{equation}
The hydrochloric acid produced in this reaction reacts with any remaining uncorroded metal, forming more cuprous chloride and perpetuating the destructive cycle:
\begin{equation} 2\text{Cu} + 2\text{HCl} \rightarrow 2\text{CuCl} + \text{H}_2 \end{equation}
This cycle continues as long as chlorides, moisture, and oxygen are present, leading to progressive damage that can eventually destroy the coin. The reaction does not stop on its own and requires intervention to remove or neutralize the chloride contamination.
Not all green corrosion on bronze coins is problematic. Many ancient coins develop a natural patina consisting of cuprous oxide (\(\text{Cu}_2\text{O}\)), cupric oxide (CuO), malachite (\(\text{Cu}_2\text{(OH)}_2\text{CO}_3\)), and azurite (\(\text{Cu}_3\text{(CO}_3\text{)}_2\text{(OH)}_2\), which forms a protective barrier against further oxidation. Unlike bronze disease, these corrosion products are stable and do not actively spread.
In contrast, the key characteristic of bronze disease is continued activity. It spreads over time, causing pitting and structural degradation. If new powdery green spots appear even after cleaning, the coin is still affected by bronze disease and requires further treatment.
Sodium sesquicarbonate (\(\text{Na}_2\text{CO}_3 \cdot \text{NaHCO}_3 \cdot 2\text{H}_2\text{O}\)) is one of the most effective and widely used chemical treatments for removing chloride contamination from bronze artifacts. It works by leaching out chloride ions (\(\text{Cl}^-\)) from the metal while maintaining a stable, mildly alkaline environment that prevents further acid formation. This method is favored by professional conservators because it does not strip the patina more than necessary and allows for gradual decontamination.
Note: sodium sesquicarbonate can also be written in its’ anhydrous state as \(\text{Na}_3\text{H(CO}_3\text{)}_2\).
Several other methods exist for treating bronze disease, but each has limitations compared to sodium sesquicarbonate. Electrolytic reduction can be used to remove chlorides and restore some metallic integrity, but it is highly aggressive and can strip patinas, making it unsuitable for numismatic preservation. Alkaline dithionite is a rapid method that reduces corrosion products back to metal, but it also removes stable patinas and can alter the coin’s appearance. Sodium carbonate rinses are sometimes used as an alternative to sodium sesquicarbonate, but they are less effective at extracting deeply embedded chlorides. Benzotriazole (BTA) treatment alone does not remove chlorides but instead forms a barrier around them, meaning it must be used in combination with other methods for long-term stability.
Sodium sesquicarbonate is preferred by professional conservators because it is non-destructive, effectively removes chlorides without stripping the patina, and maintains a stable, alkaline environment that prevents further acid formation. While the treatment requires weeks to months of monitoring, it is the most reliable method for ensuring that bronze disease does not return, making it the standard choice in museum conservation.
Bronze disease is driven by the formation of cuprous chloride (CuCl), which hydrolyzes in humid air to form hydrochloric acid (\(\text{HCl}\)), perpetuating corrosion. Sodium sesquicarbonate neutralizes this process in two ways:
\begin{equation} \text{CuCl} + \text{OH}^- \rightarrow \text{Cu}_2\text{O} + \text{Cl}^- \end{equation}
This reaction prevents further formation of hydrochloric acid. Cuprous oxide can appear either yellow or red, depending on the size of the particles.
Sodium sesquicarbonate is available in, e.g., the Museum Services Corporation web shop referred to under resources at the beginning of the blog post. However, with some care you can also create it yourself, which I explain in a section below.
The presence of chlorides can be detected using the silver nitrate test (Plenderleith and Werner 1971). To perform the test, the artifact is first soaked in distilled or deionized water for several hours or overnight. A 10–20 ml sample of the solution is then taken and acidified with a few drops of dilute nitric acid (∼10%). After mixing, five drops of 0.2 N silver nitrate solution (prepared by dissolving 17 g of \(\text{AgNO}_3\) in 1 liter of \(\text{H}_2\text{O}\)) are added.
To observe the results, the test tube should be held against a black background with good side lighting. If chlorides are present, a white opalescence or precipitate of silver chloride (AgCl) will form:
\begin{equation} \text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl} \downarrow. \end{equation}
Under ideal conditions, using clean glassware and uncontaminated reagents, this test provides a reliable qualitative indicator of chloride contamination.
Note that this is a qualitative test and quantitative tests are more reliable but also not very available to perform at home. The same goes for conductivity tests.
Most people seems to stop the sodium sesquicarbonate soaking when the coin no longer corrode and the solution no longer turns blue. A silver nitrate test might be overkill for home study. However, a pH test is a good simple way to test if any chlorides are present which creates hydrochloric acid and lowers the pH.
BTA does not remove cuprous chloride but instead forms an insoluble complex with cupric ions, precipitating over the chloride and blocking moisture. Therefore, BTA treatment should follow sodium sesquicarbonate treatment to ensure lasting stability.
Please note that BTA is a suspected carcinogen. Avoid inhalation and wear gloves and eye protection.
Hamilton (1999) adds that 3 percent BTA can be added to the drying alcohol, combining steps 7 and 8.
Hamilton (1999) recommends the spray “KrylonClear Acrylic Spray No. 1301” and the British Museum uses Renaissance Wax.
Note that experts are divided on whether sealing a piece after treating bronze disease is advisable. The main concern is that if the removal was incomplete, the disease could resurface at any time, from days to years later. Sealing the artifact could then make future treatment more challenging.
Sodium sesquicarbonate treatment is a scientifically proven, museum-grade method for removing the chloride contamination that causes bronze disease. While it requires patience and careful monitoring, it ensures long-term stability of ancient bronze coins. By combining it with proper drying, BTA protection, and low-humidity storage, bronze disease can be permanently eliminated.
If sodium sesquicarbonate (\(\text{Na}_2\text{CO}_3 \cdot \text{NaHCO}_3 \cdot 2\text{H}_2\text{O}\)) is unavailable for purchase, it can be prepared by mixing sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and sodium bicarbonate (\(\text{NaHCO}_3\)) in the correct proportions.
Sodium sesquicarbonate is a double salt of sodium carbonate and sodium bicarbonate. It maintains a mildly alkaline pH, making it effective for chloride removal without being overly aggressive on patinas. The reaction in water helps buffer pH and stabilize the solution:
\begin{equation} \text{Na}_2\text{CO}_3 + \text{NaHCO}_3 + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3 \cdot \text{NaHCO}_3 \cdot 2\text{H}_2\text{O} \end{equation}
The key to making an accurate 5% sodium sesquicarbonate solution is to:
Maintain the correct molar ratio of 1:1 between sodium carbonate and sodium bicarbonate. This ensures the proper buffering effect in solution.
Ensure that you are using anhydrous sodium carbonate (\(\text{Na}_2\text{CO}_3\)). Sodium carbonate exists in multiple forms with different molecular weights depending on how many water molecules are attached to each \(\text{Na}_2\text{CO}_3\): anhydrous (no water), monohydrate, dihydrate, pentahydrate, and decahydrate. This means that we should use anhydrous (pure) sodium carbonate and not just “washing soda” that is in the hydrated form, and therefore in the form of, e.g., decahydrate sodium carbonate (\(\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{0}\)). The reason is that the molar ratio that we are calculating below is based on the anhydrous form. Thus, when you buy the salts make sure you buy chemical grade in the anhydrous form.
Since we need equal moles of both compounds:
\begin{equation} \frac{\text{Mass of Na}_2\text{CO}_3}{\text{Molar Mass of Na}_2\text{CO}_3} = \frac{\text{Mass of NaHCO}_3}{\text{Molar Mass of NaHCO}_3} \end{equation}
Let’s assume we use 1 mole of sodium carbonate which is 105.99 g, then we need to have 84.01 g of sodium bicarbonate to match in moles.
\begin{equation} \frac{84.01}{105.99 + 84.01} = 0.442 \quad \text{(for NaHCO₃)} \end{equation}
\begin{equation} 5 \times 0.558 = 2.79 \text{ g Na}_2\text{CO}_3 \end{equation}
\begin{equation} 5 \times 0.442 = 2.21 \text{ g NaHCO}_3 \end{equation}
By following this molar ratio, the solution effectively leaches out chloride ions from bronze artifacts without altering their patina.
On a practical note, it might just be easier to by sodium sesquicarbonate from a museum shop and that will also assure that you have a correct concentration in your final solution.