Conservation of iron artifacts is an important aspect of the archaeological process at Montpelier, and allows for detailed study of excavated objects far beyond their normal lifespan. Iron artifacts in the ground tend to be more stable and are happy with their natural chemical environment; however, once they are removed from that environment they generally deteriorate rapidly. In general, excavated iron artifacts look more or less like stale Cheetos. The goal of conservation is to preserve these objects so we can learn more about the people who used and made them.
Lets Talk About Rust.
The rapid deterioration of the iron artifact is because of exposure to moisture and oxygen in the surrounding atmosphere, both of which are needed to form rust. In a way it is best to think of the rusting of iron as a chemical reaction, specifically oxidation, where the iron reacts with oxygen in the presence of water. If you take a trip down memory lane, back to high school chemistry, you get the following formulas for the oxidation of iron:
Fe + O2 → FeO
Fe + O2 → Fe2O3
Conservation is the process of reversing these chemical reactions, and stopping them from occurring again, or at least at a slower rate. This happens in two flavors: chemical cleaning (where chemicals or chemical processes are used to remove the rust), or mechanical cleaning (where the rust is removed manually through brushing, grinding, poking, or probing).
One method of chemical cleaning is through electrolytic reduction, which is the reverse of oxidation, and essentially flips the above formulas around. Fortunately, electrolytic reduction is a super easy process and does not require much. Here is our set up at the lab looks like:
See, not too difficult. Just a battery charger, wires, alligator clips, and good old fashioned sodium bicarbonate solution (baking soda in water). The process is as simple as removing a small portion of rust to expose bare metal, attaching it to an alligator clip, and waiting for the rust to flake off. Artifacts run through electrolysis generally take a week or two to fully process, but it’s highly variable and depends on the amount of rust, the thickness of rust, and the quality of the iron.
After going through electrolysis there is often residual rust left over, but running it further in the tanks can start to damage the base metal. Instead we mechanically clean the artifact by brushing and air abrading (with glass instead of sand) until the rust is gone.
Unfortunately, for all you would be at home conservationists, the sandblasting cabinet is a much more expensive approach. But, it sure beats scrubbing rust off an artifact with a steel toothbrush. In general, a combination of different mechanical cleaning methods are needed to get the bulk of the rust removed.
Boiling and Baking
While artifacts are happy to remain the ground, the chemical environment surrounding them is full of dangers. Even though they remain more stable in the ground and do not have as much exposure to moisture and oxygen, they hang out with an unsavory lot: salts and ions. The most heinous of these is chloride, which is very reactive. You might be familiar with this effect on your car in the winter, where the salt they put on the roads, while helpful to not skid out, can start to corrode the exposed metal of your car.
Thankfully, science and chemistry can help us out once again, but this time through osmosis and diffusion. If two systems have a difference in the concentrations of ions, they will naturally try to balance out this disparity, until both of the systems have an equal concentration. By putting the artifact in pure water and boiling it, the ions in the artifact will naturally diffuse out.
I know what you’re thinking. Isn’t water bad for artifacts? Didn’t you JUST say that you need both oxygen and water for oxidation to occur? Short answer: yes. Spoiler alert: we bake the water off. If you’ve ever been inside the Archaeology lab and wondered about the oven in the corner, mystery solved.
After the water is driven off, some residual rust forms (as expected), but we give the artifacts another pass of cleaning before coating. We apply two different solutions to our artifacts to help preserve them for future study: tannic acid and B-72. Tannic acid is a brown powder that is, unsurprisingly, a type of tannin. Tannins occur naturally in a variety of substances, but you might be most familiar with it in red wine or black tea. B-72 is a thermoplastic resin dissolved in acetone and smells and looks just like nail polish. Think of it as Montpelier Archaeology’s duct tape; we use it for coating, mending, labeling, and a variety of other things.
The tannic acid serves as a rust inhibitor and keeps oxidation from occurring by reacting with the base iron and forming a complex. When water is reintroduced into the artifact, it hydrates the tannic acid instead of the iron. Additionally, it has a cosmetic effect and colors the artifact back to black. B-72 is a moisture and oxygen barrier and prevents any of those pesky corrosion makers from coming back into contact with the artifact.
While the conservation process is time consuming, with each artifact taking on average about 2-3 weeks to fully process, it is crucial in keeping our artifacts looking fresh. It also aids greatly in our understanding and analysis of the artifact.
For example, remember the threshing machine teeth shown at the beginning of this blog? Nothing too different between each one, right? Well we thought so too because we couldn’t see beneath the rust surface. Here’s how they look after they went through the conservation process. Can you spot which one doesn’t belong?
What Lies Beneath: The People
It’s a little hard to see because the B-72 is reflective (our photography volunteer hates it), but here’s a photo with the coating stripped:
It’s a maker’s mark! It looks like a “B” superimposed onto an “S”…or maybe an “S” superimposed onto a “B”? Regardless, we can use that information and potentially figure out who and where that particular threshing machine was made.
This gives the artifact much more meaning and a story behind it. No longer is it just a boring old threshing machine tooth, now we can think about the person who made it. What was their life like? Where and when did they live? Did they make other threshing machines? Were they successful, or always struggling?
That is but one example of the power of the conservation process. Even if we don’t currently have the ability to track down this information, maybe a researcher later on will have the time and knowledge to do so. Additionally, our fictitious future researcher will have the actual threshing machine tooth to look at, instead of a photograph of a corroded lump of metal. And, I think, there’s just something about actually holding an artifact, rather than seeing it in a photo, that truly allows you to appreciate its history and the people who held it.
Additionally, if YOU are curious to “hold” history and experience our iron conservation process first hand, sign up for our freshly-minted Iron Conservation Expedition! You’ll work with me and the rest of the Montpelier Archaeology staff to conserve various objects in our collection. And, if iron isn’t your style, we also offer a ceramic-focused expedition!
Ben Kirby, MA
Archaeology Lab Manager
Ben Kirby oversees the daily operations of the Archaeology Lab. He has a BA in Anthropology and BS in Chemistry from the College of William & Mary, and an MA in Historical Archaeology also from William & Mary.