A small inscribed lead plaque discovered during excavations at Sainte-Marie Among the Hurons, identified the burial place of two Jesuit martyrs. (The article describes the history of the mission.) When white corrosion products formed on the plaque, it was sent to the Canadian Conservation Institute (CCI) for treatment. Basic lead carbonate was found, both in the white corrosion product, and in a black material found in some of the incised letters. Lead carbonate can form a protective layer, but can also form a loose powder in the presence of organic acid vapours, which acts as a catalyst, enabling corrosion to continue in the presence of carbon dioxide alone. Thorough documentation was assisted by using a laser scanner belonging to the National Research Council of Canada, and the data stored in a computer. Plaster copies of the plaque were made and engraved, and can be colored to simulate the original. Lead test-pieces were prepared and corroded, then tested, to find a method to convert the lead carbonate back into metallic lead. DTPA (diethylene triamine penta-acetic acid) was selected. The plaque was immersed in this, leaving it until the letters were legible, but still with some corrosion. It was rinsed, dried, and placed in a desiccator.