How conservators got just the right polish on an Alamo cannon

Cleaning an item that people expect to look old means finding a delicate balance.

Although artifacts do need upkeep from time to time, some might look strange with a pristine exterior. Most people don’t expect a bronze cannon used at the Battle of the Alamo to be shiny, for example.

Chris Dostal, assistant professor of nautical archaeology and director of the Conservation Research Laboratory at Texas A&M University, recently worked with preservationists at the Alamo. He spoke with Texas Standard about the process of restoring and preserving the cannon. Listen to the interview above or read the transcript below.

This transcript has been edited lightly for clarity:

Texas Standard: Tell us a little bit more about how this cannon may have been used at the Alamo.

Chris Dostal: Well, I’m not an expert on the provenance of the cannon. I know that it’s been in the Alamo care for a number of years now. When it came into the Alamo’s hands, our lab decided to help out with the conservation of it. And so it’s undergone treatment with us now a couple of times.

Well, what was wrong with it that needed to be fixed?

Probably nothing too serious other than the fact that it didn’t look very good. And we wanted to make sure that there were no compounds on the cannon that would cause the metal to corrode over time.

So what exactly did you do? I think about, like, say, plating as a way that you can change the exterior color of the metal. Did you basically do something similar?

Plating kind of changes the nature of the object. And so we don’t want to do that. We don’t want to misrepresent an archeological object like that.

You could polish them up to make them look brand new. But we tend to not do that because nobody wants to go see a 200-year-old cannon and have it shining like it just came off the factory floor.

So we put it through a process known as electrolytic reduction. We put the cannon into a vat with an electrolyte, and then ran current through it to kind of force any salts or other type of materials that might be in the metal out to stabilize it.

I understand there were some white chalky deposits that seemed to be adhering to the outside of the cannon. What was that? And how did you get there?

Well, so after our most recent retreatment of the cannon, we sent it back to the Alamo and they discovered that it was growing this kind of white precipitate on the outside.

Admittedly, we kind of got tunnel vision on that and sort of just kept running different experiments to see what we could determine about what the make up of the substance was and ways that we could mitigate it. At some point, they reached back out and said, “hey, maybe you guys could work on this remotely and we could have our cannon back. We’d love to put it back on display.” So that’s exactly what we did.

We cast a number of bronze small ingots of similar composition to the cannon, and then sort of ran them through an accelerated process to try and replicate the problem that we had. And then we tested a number of different solutions that might correct that.

Was there any way of knowing what that precipitate actually was?

It was basically carbon that blooms on the surface. What we did is we took samples of it, sent it over to Texas A&M Department of Chemistry, and they did a test on it called X-ray diffraction, which is kind of like looking at the way that X-rays bounce through crystals. So they were able to determine exactly what it was.

And based on that, we saw that it was most likely something to do with our previous treatment. And we didn’t see any indications that it was disturbing the metal, yes, but we wanted to get rid of it in case that changed in the future.

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So what are the risks you’re dealing with trying to remove some of these impurities? I mean, is there much of a risk to the integrity of the underlying metal?

So that is the primary concern, the risk of removing it. We also didn’t want to risk inaction.

So one of the discussions we had to have was “I know it kind of looks a little ugly, but is it harming anything?” And the concern was that through time it may well start to do so. And so that’s why we set up a series of tests to figure out a way that we could remove it without harming the underlying metal.

I don’t want to be the guy that messed up one of the cannons from the Alamo.

What did you end up doing? Did you have to dip it in some kind of chemical or brush some kind of chemical on top of it or what?

Yeah. So with our ingots, what we did is we selected a number of different assets that would neutralize and prevent future carbonate production. And then we just applied those and waited and tested different ways to apply test or different times, different strengths and things like that.

And we ended up on a low-concentration solution of formic acid. So what that did is it went in and it neutralized carbonates, kind of created a layer that should prevent future outbreaks, but it didn’t harm the underlying metal.

Well, I wonder what, if anything, you can learn about the way that these cannons were built or the way that metals were formed back in the early part of Texas history? I mean, is there anything that can be derived from how these cannons were first, I guess, poured?

So we’ve talked about this with Kolby Lanham from the Alamo, and his understanding is that this is a relatively poorly-made cannon. And we did actually fluorescence testing on different parts of the cannon and have determined kind of the direction that it was poured and some of where there’s some higher levels of lead and things like that.

By all indications, this was not made by a particularly experienced foundry.

And so perhaps you’re dealing with a particularly delicate artifact here.

Yeah, I think that’s the case.

Our lab has conserved a lot of different cannons, including the bronze guns from the French ship, La Belle, which is on display in the Bullock Museum and this is the first time we’ve ever seen anything like this. And reaching out to my international colleagues at labs around the world, no one has really seen anything quite like this.

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