Addressing the Precipitation of Hydrated Carbonates on a Bronze Cannon from the Alamo

Following their defeat in the Texas Revolution of 1836, the Mexican Army disabled and buried cannons used in the defense of the Alamo. Rediscovered in 1852, 13 of these cannons have since journeyed through private collections and public exhibits before arriving at the Alamo. Among them is a bronze 4-pounder cannon, thought to have seen action during the battle itself. In 2017, the gun was conserved using electrolytic reduction with sodium hydroxide. White powder later appeared around the breech, identified as thermonatrite (Na₂CO₃·H₂O), trona (Na₃H-(CO₃)₂·2H₂O), and spertiniite (Cu-(OH)₂). The precipitate is likely the side effect of conservation treatment, and the cannon was retreated and boiled in deionized water in an attempt to remove all carbonates. This was ineffective, and an experiment was conducted to determine and effective way to neutralize the carbonates. Formic and citric acids were found to have the least negative effects on the experimental ingots, while phosphoric, acetic, and sulfuric acids were ruled out as too problematic. Formic acid was the most effective at preventing recurring precipitation, and was chosen for application to the artifact. It successfully kept the precipitate at bay for three months before reapplication was required, which was expected based on the experiment. Time to reapplication is expected to lengthen as the carbonate reacts with the formic acid and is removed.