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  Corrosion and "bronze disease"

After casting and cooloing down, any bronze artefact begins to corrode as soon as it comes into contact with air.

 A very thin layer of copper oxidizes on the surface to form a layer of copper oxide (cuprite). Numismatists sometimes refer to this process as "toning." This "dull-

looking" patina usually protects the object from further 0xidation but though impurities from the air, ground, and sea can cause further corrosion.


Usually  the corrosion process often does not stop with toning, especially with objects buried in the ground.

 Acids and salts present in the ground and sea attack the metal.The buried piece's patina then thickens and its cuprous oxide compacts into purplish-red cuprite. Basic green (due to malachite) or blue carbonates  (due to azurite - but only in the presence of very high concentrations of thehydrogen carbonate ion HCO3) can then become imbedded in the cuprite  Other chemicals can also cause further corrosion. Sulphide turns the patina brown or black, oxide turns it red, and sulphates turn it green.


Most of these reactions would not occur without its presence.

Moisture also causes soluble salts in the soil to dissolve and form an electrolyte that conducts electricity, resulting in electrolytic corrosion. If metals such as gold or silver are located nearby, the problem becomes more serious. A natural electric cell is formed which saves the more noble metal (gold or silver) by corroding the less noble one (the copper). However, the presence of other less noble metals will save the copper at their expense. Of course, this process can cause particular problems when the artefact's copper is alloyed with other metals.


What is it and what  does it look like?


The most damaging corrosion occurs when chlorides and moisture come into contact with a coin, and chlorides are unfortunately quite common in the ground. The copper and chlorides react to form cuprous chloride, which causes progressive corrosion in the presence of moisture. It leaves the surface with patches of pale green, powdery material. This is commonly referred to as bronze disease and must be taken care of by removing the chlorides before the whole coin eventually disintegrates.


Bronze disease is a type of corrsion which gives antiquities collectors nightmares!


This is what can happen in just a few weeks!







20th October 2005


The rest of this page was created in June.

But thought it would be intereting and potentially useful to place this sort of update here.


This is the follow up  email I sent to some who bought whom I sent an urget email through eBay a short while after the sale to say......don't pay, contact me!.....all will be explained.





You noted in my listing I said:




I think I know why it didn't sell....because you saw it's twin who was the patient with "bronze disease " on the conservation pages on my website!:o)


There is a tiny speck of verdigris there but no dreaded chloride corrosion.


Tell you what, if it seems to be ailing, we can treat it with sesquicarbonate and then also place the cured patient along side the ailing one on my website!



Well the pic of it on eBay was about 6 months old.

Before packing it , of course I looked at it again.

Hmm, I thought......... Is that patch of verdigris actually slightly larger?

Looked with a x20 loupe. Didn't look like bronze disease.

Poked it gently with a fine probe...

..........and it went straight through!




Before and after.......!


So there is hole in it now.

No bronze disease as far as I can see...but ......

Hence my urgent email to you.

You can have it, but I can't really sell it...that is, you can have it for free.

Let me know.




After  which I  said I pould post all this here with the simple message.


"He who pokes  patina to hard will probably produce a hole"


To which truism, in-so-far as thin pieces of ancient bronze are concerned, I would add that thin and slender ancient bronze (and silver) pieces can very easily break.  They may look fine from the outside, but the inner substance can be crystalised and otherwise altered and extremely fragile. 




What looks like a 'fungus'  of some sort actually has no biological component at all.


People used to think that bronze disease was caused by bacteria because heat seemed to stop it. Bronze disease looks like light-green powdery stuff on the surface, which sometimes simply falls off or is easy to brush off. If you remove it, it will grow again in a few weeks .


It is a reaction of cuprous chloride, which may be present in any ancient bronze artifacts, and water.


The water required to start the reaction is not much, a normal humid day is quite enough. Once the reaction starts, cuprous chloride turns to cupric chloride and hydrochloric acid. And, of course, hydrochloric acid reacts readily with metal.


Once started, a chain reaction can occur with the hydrochloric acid eating into the metal and possibly exposing more cuprous chloride which in turn reacts with humidity to form more of the cupric chloride/hydrochloric acid pair, and so on. If not properly removed,it will "eat" up the entire piece!


 Bronze disease looks like a bright green or blue-green, fuzzy or powdery patch.


While the exact colour may vary, the fuzziness is the key to determining. It is however usualy very bright green. If it is fuzzy and flakey, it is bronze disease. If it is hard, then it may be safely ignored.


You may notice that in the early stages you can wipe the fuzz off with your fingers, but doing so will not help because the cause of the problem is below the surface  If you remove the bronze disease, you would see severe pitting in the areas where it existed. 



Treatments for bronze disease


Treating bronze disease can be a long process. There is no quick solution to its removal.

Heat will temporarily neautralize bronze disease simply by removing the water which is needed for the reaction. Simply bake the piece in the oven at 250 degrees for 30 minutes to an hour.

This is not a cure since as soon as the thing cools down, the moisture in the air may start the reaction again. At best, heating  will temporarily stop or slow the advance of the disease. Note: Heat may change the patina color.


Mild Cases


The distilled water method



  • Using a magnifying glass and a sharp needle to remove any obvious green spots still visible
  • Soak in distilled water. This may seem to be counter-intuitive since moisture is needed to start the reaction in the first place, but it does work.
  • Do not use tap water. The chemicals in tap water could actually increase the bronze disease.
  • Over a period of time, distilled water will attract the chloride ions. As the water becomes saturated with chloride ions, the effectiveness will be reduced.
  • Change the water periodically. Every two or three days at least.
  • Keep on for two weeks.


In addition to  thorough drying, many reccommend placing the piece in an oven at 250 degrees for an hour.





Serious Cases:


Sodium Sesquicarbonate solution.


  • Using a magnifying glass and a sharp needle to remove any obvious green spots still visible
  • Place the piece in a glass container and fill with a 5% solution of sodium sesquicarbonate.  
  • Let it  soak for about fourteen days, replace the solution, and soak for another fourteen days.
  • Then, place the picece in distilled water for about a week.    



  • Some people say it helps to heat the mix to boiling to start with for a few minutes.


Some people reccommend a light scrubbing each day or so to help remove surface contamination.


 Making up sodium sesquicorbonate

You can make it with equal molar amounts of sodium carbonate (Na2CO3 -also called soda ash) and sodium bicarbonate (NaHCO3 - baking soda  ).

For example, a 5% solution would then be 10.6g of carbonate and 8.4g of bicarbonate per 100ml of water.

Use de-mineralized or de-chlorinated water for better results




You may go by weight or simple dry measurement, i.e. tablespoons full.

Store the mix in an air-tight container.. 

Prepare a bath of  8 parts sodium carbonate to 5 parts sodium bi-carbonate.

A 5% solution will remove any patina!!


If there is an exceptionally aesthetic patina to be preserved, try a 1% or 2% solution.

Be warned, though, that it will take three times as long, and has a risk of being ineffective.

After the last soak and scrub, dry the artifact  and soak in 100% isopropyl alcohol, for about 20 minutes. This will help draw out more water from the  fabric. Dry thoroughly, and seal with a paste wax, well rubbed in.


With very prolonged soaking the metal fabric can become weakened.

Prolonged soaking is months rather than days or weeks., though for

a very small object this "risk" period should be considerd  to be shorter.





This is used by restorators and museum preservers.


What is?where do I get it? How do I use it?




ATTENTION !!! Benzotriazole is suspected of being a carcinogen. Don't inhale it. Wear gloves and eye-protection.

Only use it in an AIRED room. Wash your hands carefully. Wipe off your artefact carefully to remove all excessed benzotriazole.


Benzotriazole is a solid. Only use benzotriazole in the cleanest form. This is expensive, but has the best results.

Store the benzotriazole - ethanole solution in a light-tight vessel. Otherwise it will reduce.


Benzotriazole is available from   in the States. 

I don't know where to get it in the UK. If you know, please tell me


   From David

28th May 06

Scroll down the page and you will find Sodium Sesquicarbonate, Benzotriazole

and other goodies for the metal detecting crowd trying to prevent their

finds from disintegrating.

Thanks David!


Dissolve benzotriazole in ETHANOL (3-5% solution) and soak the artefact from 1 hour to 2 days. 

There is no need to remove all the light-green powdery stuff.

But sone people feel it's better to remove most of the disease with a pin or a plastic stick or something similar. The solution penetrates better into the affected areas. 


One can ake a solution of benzotriazole in water also and this has the advantage that it doesn't roughen the patina as ethanol does.

But, it has been reported that it does not dissolve well and easily.


The method


The treatment consists of immersing an object in a solution of 1-3 percent BTA dissolved in ethanol or water.

 In general, the best results are achieved if the specimen is impregnated with the solution under a vacuum for 24 hours.

 If the artifact is left in the solution for at least 24 hours, 1 percent BTA mixed with de-ionized water works as well as more concentrated solutions.

 For shorter treatments, 3 percent BTA mixed in either water or ethanol in recommended.

 In some cases, ethanol is preferred when the BTA treatment is of short duration. The main advantage of using ethanol in the solution is that it penetrates cracks and crevices better than does water.

After the artifact is removed from the solution, it should be wiped off with a rag saturated in ethanol to remove excess BTA and rinsed in water and then thouroughly dried..


If you discover new bronze disease after some time simply repeat the benzotriazole process. 



How it works


Treatment with BTA does not remove the cuprous chloride from the artifact; rather, it forms a barrier between the cuprous chloride and moisture of the atmosphere. In this method of cleaning.  The benzotriazole forms an insoluble, complex compound with cupric ions. The precipitation of this insoluble complex over the cuprous chloride forms a barrier against any moisture that could activate the cuprous chloride and cause bronze disease. Tests at the British Museum (Plenderleith and Werner 1971:254) indicate that if active bronze disease is present, all attempts to stabilize the object with BTA may fail due to the widespread distribution of cuprous chloride in the corrosion layers.


Therefore, for artifacts heavily contaminated with chloride, such as marine-recovered cupreous objects, BTA treatment should follow the sodium sesquicarbonate or sodium carbonate treatment to ensure long-term artifact stability.



Sodium Carbonate Rinses and patination colour changes


The sodium sesquicarbonate treatment has been the standard treatment for fragile cupreous artifacts with chloride contamination and for artifacts that have a patina which is desirable to preserve.

In practice, however, conservators find that the treatment often enhances the patina, making it much bluer in appearance. In other examples, it has considerably darkened or blackened the patina.


With regard to the sodium sesquicarbonate treatment, Weisser (1987:106) states:

Although initially the sodium sesquicarbonate treatment seems to be ideal, since you do not need to remove the outer corrosion layers while the cuprous chloride is removed, it has been found to have a number of disadvantages. First, the treatment may require well over a year before all the cuprous chloride has been converted. This fact makes other drawbacks more serious. It has been shown that sodium sesquicarbonate (a double carbonate) forms a complex ion with copper and therefore preferentially removes copper from the remaining metal (Weisser 1975).

This can be potentially structurally damaging over a prolonged period. It has also been shown that a mixture of carbonates, including chalconatronite, a blue-green hydrated sodium copper carbonate forms over the patina and also seems to replace other copper salts within the patina (Horie and Vint 1982) This creates a color change from malachite green to blue-green, which in many cases is undesirable. In the objects the author has examined the blue-green color can be found in cross section from the outer corrosion crust extending down to the metal substratum.



Weisser suggests that if previous treatments with BTA have not been successful, the objects can be treated with 5 percent sodium carbonate in distilled water. The sodium carbonate removes the cuprous chlorides and neutralizes the hydrochloric acid in the pits. Sodium carbonate, unlike sodium sesquicarbonate, which is a double carbonate and acts as a complexing agent with copper, reacts relatively slowly with copper metal. Still, in come cases, slight alterations in the color of the patina can




What to do afterwards?





The jury is sharply divided on whether it is a good idea to seal a piece after a treatment for bronze disease. The problem is, if the removal was not complete, bronze disease may re-occur at any time from days to years later. And if the artefact is sealed it may make it that much more difficult to treat the next time.


Stabilize with microcrystalline wax,  Renaissance Wax, or Incralac

Microcrystalline wax is used by the British Museum for cleaning and conservation.

There are other products, such as Incralac that may be used, but they are more difficult. 





When using chemicals, always follow the manufacturers instructions for use and in mixing solutions. Many of the chemicals you will use are harmless, but for safety sake always read and follow instructions.Follow the usual precautions of using gloves, not breathing in the vapours and keeping the liquids away from your eyes.

Wear a thick jumper in cold weather and never talk to strangers.


Small print


Following electrolytic or chemical cleaning, the objects are put through a series of hot rinses in de-ionized water until the pH of the last rinse bath is neutral. Because copper tarnishes in water, Pearson (1974:302) recommends washing the objects in several baths of denatured ethanol. If a water rinse is used, any tarnish can be removed with 5 percent formic acid or by polishing the area with a wet paste of sodium bicarbonate.

After rinsing, copper objects should polished to any degree desired and treated with BTA. The object is then dehydrated in acetone or a water-miscible alcohol and coated with clear acrylic lacquer or microcrystalline wax. The commercially available KrylonClear Acrylic Spray No. 1301 is recommended for ease of application, durability, and availability. For increased corrosion protection, Pearson (1974:302) recommends that 3 percent BTA can be added to the drying alcohol, as well as to the lacquer. Microcrystalline wax can be used, but in most cases, has no special advantage over acrylics.


Of the conservation alternatives electrolytic reduction and alkaline rinses are the only ones which actually remove the cuprous chlorides.


Electrolytic reduction cleaning of copper-alloyed objects, such as brass and bronze, is often avoided because it removes any aesthetically pleasing patina and may change the colour by plating copper from the reduced corrosion compounds onto the surface of the alloyed metal. In the case of cupreous metal recovered from marine environments, however, the chemical stability provided by electrolysis often takes precedence over aesthetics. The history of success in applying electrolytic reduction techniques to cupreous artifacts clearly demonstrates that electrolysis is the quickest, most effective, and enduring means of processing copper, brass, or bronze objects from a salt water environment. This statement is especially true for larger objects, such as cannons.

The extremely long time required for sodium carbonate and sodium sesquicarbonate treatments discourages their use. Preliminary treatment of artifacts with sodium carbonate followed by benzotriazole treatment may provide satisfactory results, but more experiments are needed before a final judgement can be made. Alkaline dithionite treatments have also proven effective for conserving cupreous alloys.

Regardless of the preliminary treatment, an application of BTA should be an inherent step in the conservation of all cupreous metal artifacts. In most cases, if the artifact is effectively treated with any of the treatments discussed above, as well as with BTA, and then sealed and stored in the proper environment, it will remain stable.


Electrolytic cleaning

Either 2 percent sodium hydroxide or 5 percemt sodium carbonate can be used for the electrolyte. The latter is used most often, although acceptable results have also been achieved using 5 percent formic acid as the electrolyte. A mild steel anode can be used, but Type 316 stainless steel or platinized titanium is required if formic acid is used as the electrolyte. The same electrolytic setups described for iron or for silver (below) are used.

Precise data concerning optimum current densities for cupreous artefacts are not available. Plenderleith and Werner (1971:198) state that the current density should not be allowed to fall below 0.02 amp/cm2 in order to prevent the deposition of a salmon-pink film of copper on the objects. Keel (1963:24) states that a current density above 0.01 amp/cm2 will damage cupreous objects. Along these same lines, Pearson (1974:301-302) correctly observes that care must be taken when electrolytically cleaning marine-recovered mineralized bronze in order to prevent damage to the artifact surface by the evolution of hydrogen gas. Current densities, both within and in excess of the given ranges above, are commonly applied to different cupreous objects. North (1987:238) recommends using the hydrogen evolution voltage techniques described for the treatment of iron. In general, the same procedures regarding current density that are described for the treatment of iron apply to the treatment of cupreous artifacts. The main variations in treatment involve the fact that the duration of electrolysis for chloride- contaminated cupreous objects is significantly shorter than that for comparable iron objects. Small cupreous artifacts, such as coins, require only a couple of hours in electrolysis, while larger cupreous specimens, such as cannons, may require several months.






New Method For Treatment of 'Bronze Disease'


The zinc dust method



V.C. Sharma and U.S. Lal of the NRLC developed a simple and successful

method for the treatment of bronze disease using zinc dust. The

results of the studies have been published in the Studies in

Conservation, 40, 110-119 (1995).



 Magnifying glass, pin-vice and needles, round artists' brush,

watch glass. Material: Zinc dust, ethanol, water, polyvinyl acetate, toluene, copper pigments.




 Place the object under a magnifying glass. Remove the light

green powder completely from the affected areas with the help of needle

held in a pin-vice. Take a small amount of the zinc dust in a watch

glass and moisten it with aqueous ethanol solution (1:10 v/v). Apply

the moistened zinc dust in the excavated spots with the help of a round

artists' brush and press with a small steel spatula so that the zinc

dust goes inside the cavity as much as possible. Keep the treated spots

moist with the aqueous ethanol solution for 3 days, and then allow it

to dry. After drying, a greyish powder may be seen on the treated spot.

Remove the powder gently and match the colour of the treated spots by

using copper pigments mixed in 4% polyvinyl acetate in toluene.






The procedure is simple and does not require elaborate laboratory

set up.

 The reagents required are inexpensive and commonly available.

 The method is successful even at high humidity.




 Do not expose the untreated objects to high humidity. Keep the objects under observation for reappearance of the disease

at new spots.



Each spot of the disease has to be treated individually.


How Zinc Dust Works :



Zinc in presence of water reacts with chlorides of cuprous chloride to

form zinc hydroxy chloride, and cuprous chloride is converted into

cuprous oxide. The zinc hydroxy chloride is a highly insoluble

crystalline compound, formed through a series of complex reactions.

First, the zinc hydroxy chloride is crystallised in irregularly thin

but relatively extensive plates and acts as a moisture barrier. As more

chlorides become available, dense hexagonal plates of 4Zn(OH)2.ZnCl2

are formed on standing. These plates are tough and impervious to

moisture and act as a seal, and, therefore, in absence of moisture

further deterioration process stops. If any chloride diffuses from

underneath and reaches the seal, the excess zinc present in the seal

will react with it and the quality of the seal will further improve.


 From Roger


You might find this slideshow interesting. Just about impossible to put all the information that the argument has on a slide show.  But some basic notions are mentioned.


It was a joint members project of the Antiquities_Science yahoo group.


The project on the slide show is a simulation of mechanical cleaning, BTA (Benzotriazole) treatment, sealing and waxing of a bronze coin.


Roman Coin Project Part 1 Cleaning & BTA Treatment.

Roman Coin Project Part 2 Sealing & Wax Coatings.

The quality still is not  the best but under the viewer there should be an option to view in higher definition. I have tried it and it works very well and streams OK.





This is an interesting article: 



 >>>>More on bronze disease