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Bronze Disease

Bronze disease is a corrosion process which (if left untreated) can destroy a coin. It erodes copper from the coin, leaving pits covered by masses of corrosion products which may grow to resemble malignant tumors.

Coins become susceptible to bronze disease after absorbing chloride ions during ages of  immersion in salt water or burial in salty soil.

The reactions that sustain this process are well understood, and it can be stopped.

This discussion  describes the nature of bronze disease, how it attacks  coins, diagnostic signs, and treating a case of bronze disease.

To avoid bronze disease, keep coins in a dry environment.

  What is Bronze Disease? 

“Bronze Disease” is a progressive, self sustaining, destructive process in which hydrochloric or hydrosulfuric acid forms – on the surface of or internally within - a bronze, brass or copper artifact. The acid eats away the copper alloy from which the artifact was made. This generates corrosion products, usually ugly bright bluish green or brown “growths” covering pitting areas where corrosive attack is active. In severe cases, such growths may resemble malignant tumors.  

Before causative chemical reactions were understood, these growths were thought to be caused by bacteria and for that reason, this corrosion process became known as “bronze disease.” Early numismatists desperately soaked prized specimens infected by bronze disease in strong chemical solutions, hoping to kill the "bacteria" and prevent progression of the disease – which in most cases only made matters worse!  

Does a Specimen have a Green Patina, or Bronze Disease? 

This is the first thing to determine. Most specimens with green surface coloration actually have stable green patinas, which are safe and numismatically desirable.  

Bronze disease presents itself to the eye as one or more bright green to blue-green, fuzzy or powdery patch(es) on a bronze, brass or copper coin. Collectors of modern copper coins (e. g. early US large cents) refer to such greenish corrosion products as “verdigris.” Collectors of ancient coins instead may refer to them as “malignant patina,” an apt description since untreated bronze disease behaves very much like a cancer that relentlessly destroys its host.

Although the exact color may vary, fuzzy or powdery texture of the corrosion products is a key factor in diagnosing bronze disease. Fuzziness however is not present in every case, particularly when a previous owner has concealed or altered the diseased area in order to sell the coin.

Bronze disease will not spread to other coins, unless an infected coin is in direct contact with them.

Diagnosis

In diagnosing bronze disease, one looks for these distinguishing signs: 

Does the greenish coloration cover all or nearly all of the coin, or does it occur only in one or two places? 

Can the greenish substance on the coin’s surface be removed with a toothbrush, toothpick or fingernail? 

Is pitting occurring upon the coin’s surfaces or edge? 

If the coin is soaked in distilled water for a day, and the water is subjected to a pH test, is it acidic (pH below 5) ?  

If the answer to any of these questions is “yes,” the coin may have bronze disease.  

What takes place during the corrosion process 

Copper chlorides on (or within) the coin are hydrated by water vapor in the air, beginning a destructive chemical reaction: 

4CuCl + 4H2O + O2 --> CuCl2O3Cu(OH)2 + 2HCl  (phase one)

 

Hydrochloric acid formed in this phase now attacks exposed copper to form more cuprous chloride: 

2Cu + 2HCl --> 2CuCl + H2  (phase two) 

In the presence of oxygen, the reagents of phase one have now been recreated. The result is a progressive self-sustaining reaction which continues until stopped by conservation, or until the specimen is destroyed.  Sulfurous impurities on or within a specimen may lead to variations in which hydrosulfuric acid is the corroding agent rather than hydrochloric acid.  

Practical Effects

Bronze disease is to copper alloys what rust is to ferrous metals. The matrix of copper and tin comprising antique bronze is decomposed by hydrochloric acid (HCl) into stannous chloride and copper, after which the copper is corrosively attacked as described above.

Cuprous chlorides are very unstable compounds. When artifacts made from copper alloys containing entrained cuprous chlorides are excavated and exposed to air, they inevitably begin (and continue) to corrode chemically by the process described above.

 

Conservation of chloride-pervaded copper-alloy objects requires that chemical activity of entrained cuprous chlorides be inhibited, by their removal or their conversion to cuprous oxide. If this chemical activity is not inhibited, every copper-ally artifact with entrained cuprous chlorides will self-destruct over time.

 

Conservation Measures  

Time: An Essential Ingredient 

The chlorides causing bronze disease pervaded the specimen over hundreds or thousands of years, and they won’t leave instantly. Treating bronze disease is a time-consuming process – no quick "solution" will leave the specimen relatively intact. Electrolysis (like other “quick fixes) inevitably leads to recurrence of corrosion and ultimate disaster. To do the job right and prevent recurrence, one must be vigilant, knowledgeable and patient - in some cases, it may take a year or more to stabilize the specimen.

Dessication

The least invasive countermeasure to halt progression of  bronze disease is dessication -  removing water needed to sustain the corrosion reaction. Baking at 250 degrees for at least 30 minutes does that. Once the specimen cools, water vapor in the air may be absorbed and act to restart the reaction. If the specimen is instead placed in an airtight container with a moisture absorbent such as silica gel, it will be safe until further conservation can begin.

It has been reported that such heating may tend to darken a coin’s patina. Thus, it is best not to exceed 250o F during dessication.

Treating Incipient Cases

In cases where bronze disease is beginning to appear, soaking the specimen in distilled water (NOT chlorine-containing tap water) may suffice. Since moisture is needed to start the reaction, soaking in water may seem counter-intuitive. However, distilled water attracts chloride ions resident on or within a coin into solution, gradually removing them from the coin. As the water becomes saturated with chloride ions its effectiveness in removing chlorides diminishes, so changing the water periodically is necessary. The water should be changed every few days at first, and then weekly. Periodically check progress by drying the coin and examining it. Repeat the soaking as needed. In many cases, this is all that will be required to stabilize the specimen.

Treating More Advanced Cases

Sodium Sesquicarbonate Immersion  

1) Open up every pore site where bronze disease is progressing with a toothpick or (very carefully) with a steel needle, so that the solution can penetrate to the corrosion sites. It is permissible to remove corrosion products for appearance reasons, although that does not additionally contribute to halting bronze disease. Dessicate the specimen.

2) During dessication, if made-up solution is not already on hand, prepare a 2% (by weight) aqueous solution of sodium sesquicarbonate in distilled water.  If one doesn't have easy access to sodium sesquicarbonate, it can be made up from equal molar quantities of sodium carbonate (or soda ash - Na2CO3) and sodium bicarbonate (NaHCO3). A 2% solution would dissolve 4.24 g of sodium carbonate and 3.36 g of sodium bicarbonate in 100 ml of distilled water. 

Although many conservation authorities recommend a 5% sesquicarbonate solution, this strength will strip the patina from a specimen. The weaker 2% solution takes three times as long (and may be ineffective), but the reaction is slowed enough to allow monitoring and ending the soak before the patina is stripped. Use a 5% solution only if the 2% solution fails to stabilize, and if the conservator is prepared to accept a stripped specimen. Otherwise, repeat the distilled water soak and 2% sesquicarbonate solution immersion as many times as is required to stabilize the specimen.

3) Place the specimen in a covered glass container (a Petri dish is suitable, as is the 8 oz. “jelly jar” used by home canners), and fill with a 2% solution of sodium sesquicarbonate until the specimen is well covered. Soak for two weeks, replace the solution, then soak the specimen for another two weeks or until a greenish color begins to appear in the solution - indicating that the patina is beginning to dissolve. Rinse the coin thoroughly in tap water. 

4) Soak the specimen in distilled water for 24 hours. Dry, then perform a silver nitrate test per  Testing For Chlorides With Silver Nitrate . Use this procedure: “To Test for Chlorides in Objects Not Yet in Treatment.” 

5) If necessary, repeat steps 3) and 4) until the silver nitrate test is negative.  Dry the specimen thoroughly. Do NOT seal its surface with lacquer or wax. 

Chlorine ions have now been removed from its surface. If the specimen is kept in a dry environment, bronze disease should not recur. 

Treating Serious Cases 

Additional steps for preventing recurrence of bronze disease normally are only necessary for objects that cannot be kept in a dry environment, or are internally pervaded by chloride ions Do not attempt such treatment unless safety precautions for handling toxic materials are understood.

IMPORTANT OBSERVATIONS

Specimens  subjected to conservation measures that improve their appearance (e.g. coins pitted by bronze disease) may not ethically be sold without full disclosure to the buyer. Classical Coins will not acquire or sell any coin whose appearance has been improved by such measures.

The information below is reported to assist qualified numismatists or curators who need to halt a serious case of bronze disease. Classical Coins is not responsible for any problems which may result from unsatisfactory conservation efforts or exposure to hazardous chemicals.

Essential Protective Equipment 

Goggles that seal tightly to the face
Respirator
Nonpermeable rubber laboratory gloves (going as far up the arm as possible)
Vaseline (petroleum jelly)
A bath of clean water and some soap should be kept nearby, so that one can thoroughly rinse and wash the face and arms in case of necessity.  

Materials 

Benzotriazole or BTA  (C6H5N3)
BTA IS A DANGEROUS TOXIC SUBSTANCE THAT MUST BE HANDLED WITH EXTREME CARE  --  it is a suspected carcinogen, and a strong irritant to the skin and mucous membranes. Benzotriazole is available (in solid form) from artcraftchemicals.com, also from chemical laboratory suppliers such as VWR and photographic chemical suppliers. 
Zinc powder 
Shellac
Coloring agents that will dissolve in shellac
Microcrystalline wax (e.g. Renaissance Wax)
Clear lacquer
Denatured alcohol (Ethanol)

Lab Equipment 

Small artist's brushes for applying colored shellac
Small pointed wood sticks, such as large toothpicks or small skewers
Beaker(s) of a convenient size
Brown Glass laboratory bottle(s) of a convenient size
Funnel to fill bottles
Long handled metal laboratory spoon - that holds about 1 gram in its bowl
Shallow jars with a sealing cap (1/2 pint jelly jars used for home canning are suitable)  

Process 

Chelating Solution Preparation 

IN AN EXTREMELY WELL VENTILATED AREA (IDEALLY OUTDOORS): 

Apply a light coating of Vaseline to the skin of the face, arms and hands. It should be as much as can be "rubbed in," leaving a sheen but no wipeable residue. Put on the protective equipment. 

Pour as much ethanol as is needed to treat the specimens to be conserved into a beaker, allowing two ounces (or 60 ml) per specimen. It will be convenient to make up 500 ml (or more) of solution, if many specimens are to be treated.  Dissolve as much BTA in the ethanol as is required to make up a 6-7% solution by weight, adding a little at a time with the laboratory spoon and gently stirring. Be VERY careful not to cause splashes, or get BTA powder on the skin or inhale it - BTA is a powerful irritant that can induce unpleasant allergic reactions. 

Pour the solution into brown laboratory bottle(s) for storage. If the BTA-ethanol solution is not stored in a light-tight container,  it will decompose

Carefully rinse everything that came into contact with BTA in tap water.  

Chelation of Exposed Copper   

Soak the specimen in acetone for 24 hours in a jar with a sealing cap, to draw out remaining H2O molecules.  Remove specimen from acetone and dry thoroughly. Acetone can be reused (but do not mix with fresh acetone)

Immerse specimen in 60 ml of BTA-ethanol chelating solution for 24 hours in a light-tight container. (Note: Benzotriazole does not remove chlorides - instead, it forms an insoluble complex compound with exposed copper ions, preventing them from becoming active and vulnerable to bronze disease.) Then remove the specimen from the BTA solution. Used solution can be stored in a brown glass bottle for reuse (but do not mix used and new solutions)

Rinse specimen in tap water and soak in distilled water for 24 hours. Remove and dry by acetone immersion as above. All copper atoms that could be reached by the BTA solution have now been chelated. These copper atoms are no longer able to combine with chlorine.  

Packing Pores to Stabilize Internal Chlorides 

Many ancient artifacts (during ages of immersion in salt water or burial in salty soil) have been internally pervaded with chlorides. Internal chlorides cannot be neutralized by a brief immersion treatment, and must instead be dealt with by zinc dust packing. 

Prepare a small amount of shellac by coloring it to match the specimen. 

Fill all pits, pores and other apertures in the surfaces of the specimen with zinc dust. Pack zinc dust in tightly (compressing with small wood sticks) in the manner of a dentist filling a cavity in a tooth. Lightly brush after packing to remove any zinc dust that does not adhere. Then seal each zinc-packed pit, pore and other aperture with colored shellac.  

Subsequent Preservation

 

Opinion is divided as to whether it is wise to seal a specimen’s surfaces after conservation for bronze disease. The primary concern is that if chloride removal was not complete, bronze disease may recur. If the specimen has been sealed with anything other than wax  (for example Incralac) it may be much more difficult to treat a recurrence. Whatever sealing material is used (other than Renaissance Wax) should be tested prior to application, to confirm that it will not adversely affect shellac pore seals.  

If the specimen is sealed: after the sealing agent has dried, polish with a soft horsehair shoe brush until the specimen's appearance is as desired.

 


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