The  Conservation of Cannons

Do you want to restore an historic cannon?    Please think again!

According to one expert, good intentions can cause additional problems to aging cannons.  If you have are thinking you would like to restore a cannon (and haven't been trained), you would do a better service by raising the funds to have the gun professionally conserved. We suggests that you contact the American Institute for Conservation to locate a company or person in your area to do the job.  

The Conservation of Spanish 12 Pounder

Cannon before conservation treatment.

Conservator removing dirt and surface corrosion with low-pressure water jet.

 Conservation treatment of cannon completed.




A thorough examination of the cannon was the first part of the conservation process. Photographs, drawings and exact measurements were made to document existing conditions. Recording and examining the corrosion and degradation of the cannon was particularly important as it helped determine the cause of decay.

The polluted urban atmosphere in Washington, DC, was determined to be the main cause of the powdery light green corrosion found on the cannon El Alano. Many outdoor sculptures and monuments undergo similar corrosion in an urban environment.

The cannon was power-washed with a low-pressure water jet to remove loosely adhering corrosive elements and bird droppings, and then air-abraded with walnut shells. This removed the loosely adhering corrosion without damaging the protective patina.

After walnut shell blasting, the surface finish became more uniform; streaks and run-marks were not as easily detected once the bright green corrosion was removed. In order to protect the cannon from future environmental risks, it was coated with a high melting point wax. Even though the wax is very stable, the protective coating will not last forever - annual maintenance will be necessary.

The cannon is scheduled to be returned to the Washington Navy Yard in the Fall of 2001.


Note: Conservation was carried out with the support of Department of Defense Legacy funding at the Maryland Archaeological Conservation Laboratory.

 September 18, 2001


What is the preferred method of displaying iron cannon outdoors?
Jason M. Burns  writes

What is the preferred method of displaying iron cannon outdoors
today?  I have been asked to look at a few cannons here locally
displayed.  Concrete plugs installed in the barrels of two 18th
Century iron cannons are obviously not working and allowing water to
get into the barrels.  What materials are being used to seal the
barrels for display?

Of course is it is best not to conserve iron cannons outdoors.
However, when we conserve cannons that are to be displayed outdoors,
we simply stand the cannon upright, plug the touch hole and fill the
bore completely with melted microcrystalline wax.  After the wax
sets, we concave the surface at the bore to give the impression of a
concavity and it is done.  The wax forms an impervious seal that is
easily removed by placing in vat of water and melting it out or it
can be drilled out. It eliminates the bore from holding water and
acting as a repository for cigarette butts and apple cores. It is
simple, fool proof, effective, and easily reversible

For that matter the technique can be applied to cannon in place on a
display stand outdoors.  Just pour the wax in the bore in a series
of pours and complete the last bit in small pours with the bore
partially blocked to fill to the top of the bore.

Donny L. Hamilton
President, Institute of Nautical Archaeology
Professor and Head
Nautical Archaeology Program
Director, Conservation Research Laboratory
Texas A&M University  



Lifting of the cannon

Transportation to the laboratory  

Ulla Klemela, conservator at the Maritime Museum of Finland filling the tank where the cannon will be stored before treatment  

After being lifted from sea, the cannon was immersed in tap water in a temporary wooden storage tank and immediately transported to Evtek.

Before beginning the conservation treatment, further corrosion was inhibited by increasing the pH of the water with potassium hydroxide (see Pourbaix Diagrams).

The conservation plan is to use cathodic polarization at constant potential to remove concretions from the surface (cleaning process) and to extract chlorides which have penetrated into the metal (stabilization process) (see Application for Conservation Purposes).

One cannon from the same wreck was treated previously with hydrogen reduction. This treatment was unsatisfactory as one of the trunnions fell apart and there was a noticeable decrease in the hardness of the cannon's surface. By using another conservation procedure, it will be interesting to compare the results of the two different methods.

The treatment began in January 2001 when the cannon and the new stainless steel tank were moved into the school's laboratory. Stainless steel was chosen for the treatment tank as it can be easily cleaned and reused. Because of the large size of the cannon and the tank, and the small size of the door to the marine conservation laboratory, the lifting operation had to be carefully planned in order to move them indoors safely.

Mechanical removal of large concretions to speed up the treatment process

The cannon was heavily concreted in some areas with several large stones attached. In order to speed up the treatment process, some of the concretions were mechanically removed. During the cleaning process, a piece of waterlogged wood was found in the concretion. The analysis of the wood is in progress, but it is thought that it may be part of the cannon's carriage.

A stainless steel mesh cage was constructed to enclose the cannon, this will act as the anode during the electrolytic process. The cage must not touch the cannon or the tank and is separated from the latter using sheets of high density polyethylene. V-shaped supports for the cannon were also constructed from high density polyethylene. The cannon was placed into the tank upside down to prevent damaging any markings on the top surface during the first polarization. The cannon will be turned over later in the treatment when all concretion will be removed.



Drilling through the graphitized layer

Checking the conductivity between the two contacts  

Two holes were drilled through the corrosion layer (approx. 1 cm deep), to obtain good electrical contact with the remaining metal surface. The conductivity between the two contacts was checked with a multi-meter and the cage was then closed. Steel rods were placed into the holes drilled in the cannon and connected to the negative output of the power supply. The cage was connected to the positive output of the power supply, the contact point is kept outside the electrolyte to prevent it being oxidized by galvanic corrosion (the nuts and bolts are stainless steel but copper wire is used to make the connection to the power supply).

Schematic diagram of the electrolytic treatment

A 1% potassium hydroxide electrolyte was prepared and poured into the tank. Potassium hydroxide was only available in pellet form in Finland however, for practical reasons, it is preferable to purchase it as a 30% w/v solution in 30L drums. Safety protection is required when working with caustic solutions. The Ecorr of the cannon was measured before beginning the polarization to check the contact between the cannon and the steel rods.

A catholic potential (Ec ) of -0,95V/Ag-AgCl will be applied during the electrolytical stabilization of the cannon (see Electrolytic Processes). This value was chosen as it favors the extraction of chlorides and avoids hydrogen bubbling. Because most of the concretion was already removed, strong hydrogen bubbling must be avoided as it may fracture the graphitic corrosion layer holding the archaeological information (inscriptions). As the corrosion layers are not highly conductive, it was necessary to initially increase the current by adjusting the power supply to obtain the desired potential at both contact points. The current must be increased slowly to prevent strong bubbling of hydrogen. For the same reason, it was necessary to reduce the current once -0,95V/Ag-AgCl was obtained. The plot below indicates how the cathodic potential could be adjusted by changing the current:

Ec is monitored daily during the first week, then every other day the following week, and once a week after that. Ec is measured by connecting a reference electrode and the cannon to a multi-meter (see Measuring Ecorr ). Samples of the electrolyte solution are also taken weekly to measure the quantity of chlorides extracted using a titrator.

Bronze Cannon restoration and preservation procedures.

Bronze is copper alloy. In modern times, bronze is an alloy of copper and any metal except zinc. It is generally more expensive than brass and more corrosion resistant. Bronze forms a patina (green color) which is protective to the metal and is often seen on artwork. Reproduced it is called Verde. Bronze will deteriorate rapidly if exposed to moisture and chlorides or sulfides.

Weathered bronze usually darkens; however, this is natural and does not harm the piece.

"Bronze disease" is one of the most serious hazards of bronze. This disease is caused when chlorides and oxygen combine in a damp environment, also attacks brass and pewter. The disease takes the form of a sudden outbreak of small patches of corrosion and is distinguished by rough, light green spots. "Bronze disease" usually can be stopped by washing the piece in repeated changes of boiling hot, distilled water. You may have to soak the object for a week or more in distilled water. If this treatment does not work, consult a museum expert about using a strong solution of sodium sesqui-carbonate or have your piece treated by a professional.

Bronze Artifacts or gunmetal?

The bronze commonly used in artifacts and statues is a blend or alloy of two main metals, copper and tin. Very small amounts of other metals may also be present. The tin content is usually between 5 and 20% of the whole. Bronze is harder than copper and can be sharpened to a cutting edge, so our ancestors were able to make better tools and weapons from it (Bronze Age for superior armament). Bronze melts at a lower temperature than copper because of the presence of tin, and the melted material flows well and is easy to cast. The bronze used for casting statues contains about 8 to 10 percent tin.

There are a limited number of major chemical changes that happen to bronze as it deteriorates, depending on age, soil conditions and a few other things that are collectively referred to as 'the conditions of preservation.' The visible results of the changes in the bronze are collectively referred to as the patination, patina, encrustation, or verdigris. Pretty much all of the changes that occur in bronze over time are the result of interactions of the copper in the alloy with the environment. Tin is relatively inert and is stable in alloy with copper.

Bronze is valued in art for its strength and durability, its attractive vibrant color, and the quality of surface which can range from smooth and mirror-like to coarse and rough, as the subject demands. Bronze statues, being hollow, are light in weight (small statuettes however are often solid cast). In casting, the liquid bronze can flow to form intricate folds of drapery, fine detail, slender features such as fingers which, if carved in marble would be in danger of snapping off.

Bronze disease

Bronze disease may be defined as the process of interaction of chloride-containing species within the bronze patina with moisture and air, often accompanied by corrosion of the copper alloy itself, a process which has been more or less understood for the last 100 years. The products of the reaction are light green, powdery, voluminous basic chlorides of copper, which disrupt the surface and may disfigure the object. Several corrosion processes of copper are also enhanced by visible light. The chemical examination of the corrosion of copper and bronze artifacts has been the subject of study for at least 150 years. As long ago as 1826, Davy carried out an examination of a bronze helmet found in the sea near Corfu. Among the incrustations he was able to identify:

the ruby-red protoxide of copper (cuprous oxide or cuprite)

the green rust of the carbonate (basic copper carbonate or malachite)

submuriate of copper (basic copper chloride: probably paratacamite or atacamite)

crystals of metallic copper that had been re-deposited a dirty white material identified as tin oxide.

On a nail from a tomb in Ithaca that was analyzed and found to be a tin bronze with 6% of tin, Davy again found the protoxide, carbonate, and submuriate of copper, as well as tin oxide, although in this case there were no shiny crystals of re-deposited copper present. The first scientific investigations of the aeruginous deposits on antiquities date from this period of the early 19th century and express the same curiosity about the nature and formation of these deposits we have today.

Famous objects in bronze

Greece, 3rd century BC: The Colossus of Rhodes was one of the Wonders of the Ancient World. It was a bronze statue of the Sun God, 105 feet high, which took 12 years to manufacture and erect. It stood for only 56 years before being overthrown by an earthquake in 224 BC.

Japan, 13th century AD: The Great Buddha of Kamakura is a magnificent seated figure 11.4 meters high, cast in bronze in 1252 and weighing 93 tons. It is the most perfect Buddha statue in Japan.

Great Britain, 13th century AD: The earliest large figures to be cast in bronze were life-size effigies of kings and queens made by a London goldsmith, William Torel. His effigies of Queen Eleanor and King Edward 1 can be seen in Westminster Abbey.

Italy, 15th century AD Florence: The East Door of the Baptistery of St John, by Lorenzo Ghiberti. 'Nowhere else has a sculptor expressed himself in bronze as perfectly as in this door'. There are ten panels depicting Biblical scenes with wonderful detail and perspective - Michelangelo was so impressed he named the work the Gate of Paradise (Porta del Paradiso).

West Africa, 16th century AD: Beautiful Benin bronzes produced in regions of Nigeria rank with the masterpieces of world sculpture. Cast heads of outstanding artistic and technical mastery are the most valued pieces. Some can be seen in the Museum of Mankind in London.

Great Britain, 19th and 20th centuries: The Fountains in London's Trafalgar Square, the figure of Justice at the Old Bailey, and the statue of Winston Churchill in the House of Commons are typical of the bronzes we see about us in our daily lives.

What is gunmetal?
Gunmetal is a type of bronze, being an alloy of copper and tin with some zinc added, often 2%. It is no longer used for guns but it does have many industrial uses, and is also used for statues, inexpensive watches, buttons etc. The hour hands on each face of Big Ben are made from gunmetal.


In colloquial terms the patina, consisting of green copper salts, is often described as "verdigris". This is inaccurate. Verdigris is only caused by the chemical reaction of copper materials with acetic acid and is a mixture of basic copper acetates. In contrast to copper salts which form a natural patina, verdigris is water soluble. Visually it may be recognized by its strikingly green color. 

Method For Treatment of 'Bronze Disease'!

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).

Tools: Magnifying glass, pin-vice and needles, round artists' brush, watch glass. Material: Zinc dust, ethanol, water, polyvinyl acetate, toluene, copper pigments.

Procedure: 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 grayish powder may be seen on the treated spot. Remove the powder gently and match the color of the treated spots by using copper pigments mixed in 4% polyvinyl acetate in toluene.

1. The procedure is simple and does not require elaborate laboratory set up.
2. The reagents required are inexpensive and commonly available.
3. The method is successful even at high humidity.
1. Do not expose the untreated objects to high humidity.
2. Keep the objects under observation for reappearance of the disease at new spots.
Limitation 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 crystallized 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.




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Last up-dated on 10/23/2014