In 1981, Andr� L�pine, an archaeologist from the David M. Stewart Museum in Montreal, asked the Canadian Conservation Institute for help in conserving a selection of objects that he wanted to raise from an unidentified wreck located in Baie de Gasp�. An initial survey of the site indicated that the vessel probably dated to the late 17th or early 18th century. To get a more precise date and determine the origen of the ship, L�pine needed to examine a representative sample of material from the site, including at least one cannon. In August 1982, CCI staff went to the site and helped retrieve and pack 168 bits of ceramic, fragments of bone, lead weights, fragments of an anchor, cannon balls and a 2.27-metre cannon.

To determine the extent of corrosion and to see if the cannon was loaded, the Canadian Steel Foundries Division of Hawker-Siddeley Canada Limited radiographed it at its Montreal facility. The radiographs showed that the cannon was extensively corroded and very porous. Since the cannon lay for many years in salt water, many chloride-containing salts were found in the corrosion layers. Chlorides are extremely damaging to iron. They react with iron to form akaganeite, a type of rust that is much larger in volume than ordinary rust. If this type of corrosion forms, the object literally falls apart.

To conserve the cannon, the concretion and the salts were removed using a technique called electrolysis. In electrolysis, iron corrosion products are converted to magnetite when a small electric current is passed through the object and conducted through an alkaline electrolyte to stainless steel plates. Because of the electric current, the chloride ions, which are negatively charged, migrate from the object to the positively charged steel plates. Magnetite is a stable iron compound, so the treatment not only removes substances that would cause the iron to corrode, but also converts unstable compounds to ones that do not change once the object dries out.

As electrolysis progressed, the corrosion holding the concretion on the cannon gradually softened. It was then possible to chip off the concretion without damaging the surface. The cannon was in electrolysis for more than four years. Besides removing the concretion and converting the corrosion products to magnetite, the treatment removed a large quantity of chloride. The amount of chloride removed was measured regularly and the sodium hydroxide solution was changed several times.

After electrolysis, the cannon was washed to remove the sodium hydroxide electrolyte and any remaining soluble material. This took another 21 months. The cannon was then dried and treated with tannic acid. Tannic acid reacts with iron to protect it from further rusting; it turns the iron a dark bluish-black. After a final cleaning the cannon was sent back to the museum for display.

Figure 3. Radiograph of the cannon

Figure 4. Wrought iron artifact disintegrating due to the formation of chloride containing corrosion (orange crystals).

Figure 5. Diagram of an electrolysis tank.

Figure 6. Cleaning the cannon during treatment.