Unknown substance growing on Alamo cannon removed

Texas A&M University researchers have been combatting a conservation threat on an 18th century bronze cannon that was used at the Battle of the Alamo on March 6th, 1836. An unknown white chalky substance has been growing on the surface of the bronze four-pound cannon. It has not been found on any other bronze or iron cannons at the Alamo Museum.

The cannon was in central Mexico and was then deployed to the frontier where it was used in several battles, including the 1813 Battle of Medina, the largest land battle in Texas history. At some point, perhaps during the Battle of Concepción in 1835, it was captured by Texas and brought to the Alamo. and It was made in a sandcasting technique, and is unusual in its metal composition. The research team found lead and other atypical metals in the mix, and there are areas where defects in the casting and air bubbles are clearly visible. Given the foundry’s limited skills, it’s remarkable that this cannon saw decades of battlefield action and never blew itself up.

“We’ve spent the better part of a year and a half trying to work out why this substance is growing,” Dostal revealed. “It turns out, this might be a byproduct of the chemicals used in the cannon’s conservation process back in 2008 and again in 2019. The substance is a precipitate of these chemicals starting to crop up on the outside of the cannon.”

To replicate the results, Dostal and Breyfogle created their own bronze blocks with a composition resembling that of the cannon. They then applied identical methods and treatments to these blocks.

“This is a rare occurrence, and we spent a large amount of time figuring out the exact cause,” Breyfogle said.

They aren’t sure exactly what the white material is even now, but it’s a precipitate of the electrolysis process used to clean the cannon previously. Instead of cleaning it, somehow the combination of caustic chemicals in the electrolysis tank combined with the non-standard metal composition in this cannon to create carbonate blooms that are now marring the surface.

To remove the white substance and prevent it from reoccurring, researchers have enlisted another chemical, a 5% solution of formic acid and deionized water. It removed the chalky deposits without harming the bronze underneath. They applied the solution with cloths on the outside of the cannon and a tennis ball on a stick to apply it down the full length of the bore. The white substance disappears on contact, but conservators had to apply the solution in several coats over the course of months. This process was done in view of the public. Now that it is complete, the cleaned cannon is back on display at the Alamo Museum.

One hopes this chemical doesn’t suffer from the law of unintended consequences like the last one did, but ultimately the conservation team preferred to roll the dice with a dilute organic acid the cannon might have brushed against on its own rolling over an anthill or stinging nettles or a pineapple rather than allow the white substance to proliferate with potentially damaging long-term side-effects.

Because the team’s work on the cannon occurred during regular museum hours, enabling numerous school groups to observe the process, Lanham says their preservation efforts also had an invaluable educational impact, inspiring the next generation of historians and conservationists.

* This article was originally published here