Historical Ceramics and Electrolysis: An Experiment

A few weeks ago, I conducted an experiment using electrolytic cleaning to rid stubborn chunks of iron corrosion from ceramic fragments. These ceramic sherds were recovered by Tim Bennett and his family at the historic Warner Pioneer Homestead (20LV334), from Feature 19. Due to post-depositional processes (presumably from being in the ground for a hundred years with oxidizing soils and iron artifacts), these potsherds were encrusted with ferric (iron) concretions, which made it impossible to mend the sherds together with others from the same vessels (i.e. cross-mending). Cross-mending ceramic sherds is useful for determining the forms of vessels and their full decorations, which can be functionally and temporally diagnostic. The ferric concretions that were encrusted on these sherds were impossible to detach with typical artifact cleaning procedures.

For my purposes here, I’ll very briefly summarize the electrolysis process (I posted a lab manual for the process on my blog, located here). In short, electrolysis is the process by which ferrous artifacts are cleared of ferric corrosion via electrical current. Electrolysis is useful for determining an iron artifact’s past function. Since iron oxidizes, electrolysis also helps conserve artifacts by preventing further corrosion. The artifact (or in chemist’s terms, the cathode) is connected to a battery’s negative charge. To ensure an efficient electrical current, the iron artifact is Dremeled in the places where the negatively charged clips will be connected. The battery’s positive charge is connected to a steel rod or mesh, which is called the anode. The cathode and anode are placed in a bath with distilled water and baking soda (which acts as an electrolyte); the battery’s electrical charge essentially attracts the oxidation from the artifact to the anode, aided by the baking soda electrolytes. The end result is an iron artifact that no longer has ferric corrosion on its surface. In order for the electrolytic cleaning process to work, there needs to be a strong connection from the iron artifact’s intact/corrosion-free surface to the negative charge, to facilitate a strong electrical current.

At the beginning of this electrolysis experiment, I decided that I would test three hypotheses. The first hypothesis is that the battery’s electrical current would not be able to flow through the ceramic well enough. According to the American Ceramic Society, there are trace amounts of iron in all earthenware and stoneware clay bodies (I can’t seem to find a source that can tell me exactly how much is in each paste category, but there seems to be many variables that alter the amounts). Some ceramic glazes also contain amounts of iron, all of which vary depending on the glaze’s chemical composition and intended color. Being that the sherds were all plain whitewares and hard-paste porcelains with lead-based glazes, I feared that there wouldn’t be enough iron in the pastes and glazes to ensure a strong enough connection through which the current could flow. My second hypothesis was that the sherds would not be able to withstand the electrical current at all, and that they would fall apart soon after the voltage was increased. Only one of the sherds exhibited any kind of decoration (gilded bands running along the rim); I was concerned that any overglaze decoration would flake away during the process. Finally, my third hypothesis was that the electrolysis process would be successful, and that the electrolysis process could be used by historical archaeologists in the future to clear ceramics of ferric concretions.

Before I began the process, I gave each sherd an identification number. Giving them arbitrary numbers helped me monitor each sherd’s overall progress in the electrolysis bath more closely. I also kept track of which cathode clips were attached to which sherd, and I kept detailed notes during the whole experiment. Sherd #s 1-5 are all from lead-glazed whiteware vessels, and sherd # 6 was made from hard-paste porcelain. Sherd # 2 has two overglaze gilt bands that run along the rim, which unfortunately can’t be seen very well in the photo above (for descriptions of what these paste and glaze categories mean, the Maryland Archaeological Conservation Lab does a nicejob). Since our electrolysis bath at CMU is only equipped with four cathode wires, I put only four sherds in the bath at a time. As I stated previously, the iron artifacts that I clean electrolytically are always Dremeled first in spots where the cathode wires can be connected to the artifact’s original surface under the corrosion, which facilitates a strong electrical connection. Since the sherds were only covered with ferric concretions in small areas, the Dremeling step was not necessary. It did occur to me that a gentle Dremeling on the concretions themselves would efficiently get rid of them, but a grinding stone bit or a steel brush bit of a Dremel would certainly scratch the paste and glaze surfaces, thus furthering the damage on the sherds.

Since I really did not want my second hypothesis to occur, I altered the typical electrolysis process for cleaning iron artifacts to be slightly gentler on these ceramic fragments. As a general rule of thumb, the amount of voltage/amperage needed to clean iron artifacts in an efficient amount of time is calculated as one amperage per every two square centimeters of the artifact. Additionally, the higher the voltage/amperage, the faster the ferric corrosion repels from the artifact’s core. Since I didn’t want to zap these sherds into oblivion, I refused to follow my own guidelines. Instead, I kept the amperage low at first (5V/0.5A), and I increased the amperage gradually once I knew that it was safe to do so. Since I never leave the electrolysis running overnight (the process needs to be monitored closely), I needed to disconnect the battery wires and take the sherds out of the bath until I could tend to them again. After successfully cleaning iron artifacts, they need to be “stabilized” by letting them simmer in distilled water for 2-3 hours, and then baked in an oven at 200 degrees Fahrenheit for an additional 2-3 hours. After they’re baked and cooled down, they are covered with a microcrystalline wax to ensure that they don’t oxidize again. Since the ceramics are obviously not totally made out of iron, I skipped these last steps.

Fortunately, my first and second hypotheses didn’t work out, and my third hypothesis was deemed successful. Collectively, after about 18 hours in the electrolysis bath at 10V/2A, the ferric concretions could easily be wiped away from the glazed surfaces and hard-paste porcelain. The concretions on the whiteware clay bodies were slightly more stubborn, although I could still persuade them to come off by gently picking at them with a dental pick. The gilt decoration on sherd # 2, fortunately, did not disintegrate. Furthermore, unlike full iron artifacts, leaving the sherds out to dry overnight did not make the corrosion any worse. The ferric concretions flaked away during this process, but the ferrous staining remained. To remedy this, I soaked them in white vinegar for a few hours while periodically giving them a gentle scrub with a toothbrush; however, no changes were made.

Before electrolysis

After electrolysis

To conclude, these results suggest that electrolysis can be used to clean ferric concretions from historical ceramics. The process is especially useful for vessel fragments, since the absence of concretions make the cross-mending and reconstruction of full vessels possible. Furthermore, the electrolysis process did not damage the surface treatment or decoration on the sherds in the sample. Before using electrolysis as a standardized practice, however, more testing needs to be done on ceramic fragments with other forms of decoration to make sure that the electrolysis process does not harm other types of surface treatment. Additionally, the samples need to be monitored to make sure that the ferric staining does not grow worse (especially since the sherds were not “stabilized” afterwards).

For more background information about the Warner Pioneer Homestead, implore you to check out Tim’s blog. I am indebted to both Tim Bennett and Sarah Surface-Evans, who originally came up with the idea and let me take control of the experiment.

Electrolysis: a lab manual

Electrolysis Manual – Introduction

The main goal of this laboratory manual was to help students understand and properly conduct electrolysis procedures for the cleaning and conservation of iron archaeological artifacts. The process of electrolysis is used widely among archaeological laboratories, thus becoming a standard of professional iron conservation methodology. Depending on what you plan on cleaning, the actual processes themselves vary; additionally, the processes will vary from lab-to-lab. At any rate, I felt the need to make my own electrolysis knowledge accessible. Texas A&M University published a handy electrolysis manual through their website; that entire manual is here: https://nautarch.tamu.edu/CRL/conservationmanual/File10a.htm. However, their document is jargon-heavy and densely written. If you’re like me and haven’t taken any chemistry classes since your tenth grade of high school, the chemistry jargon they use is, at best, confusing; their steps might be hard to follow at first glance. As I started doing electrolysis on my own, I made this step-by-step manual, following the processes that I found most practical. Within this manual are also video links and the notes that I made on them; they were useful to me while I was learning the electrolysis process, so I included them, in case they might be useful to you. As I implied already, this manual is written with one goal in mind: to help laboratory practitioners, at all levels, understand and properly use electrolysis as a tool to clean and conserve iron artifacts.

I should note that the following process is ONLY for material culture made of IRON. 

Before electrolysis. Not from a provenience - it's an iron cultivator sweep that dates to circa. 1984. 

What You’ll Need:

•      A Dremel
•     Face masks (optional)
•    FUME HOOD (or, the great outdoors)
•    Baking soda
•   Distilled water
•    Steel object(s) (to use as the anode)
•    Measuring cups
•    Safety glasses
•    Rubber gloves
•  Plastic tub – polypropylene, or one of the other chemical-resistant plastics – to use as a vat
•   Field tape (to help you keep track of multiple proveniences if necessary)
•   Wooden dowel
•   Copper wire
•   Electrical tape
•   Battery/electrical power source
•   Iron artifacts
•   Access to the electrolysis worksheets in the lab
•   Paper/shop towels
•   Access to an oven and stove; pots and pans that you don’t intend on using for food
•   Renaissance™ micro-crystalline wax polish

The Electrolysis Process

                In its simplest terms, electrolysis is a chemical process by which ferric corrosion is broken down, or reduced, via electric current. More specifically: during electrolysis, the corrosion on the outside of a negatively charged cathode is being repelled onto the surface of a positively charged anode by electric current and electrolytes. Electrolysis is simply shorthand for electrolytic reduction cleaning. In our case, the cathode is the artifact you’re interested in cleaning. The cathode will be negatively charged when it gets hooked up to the battery. The anode – referred to by scientists as the “sacrificial anode” – typically consists of a rod, sheet, or mesh made from steel. Most steel products are “pure”; “pure” metals have better electrical potential, i.e. an ability to maintain a higher electric current. Other such metals include zinc, magnesium and aluminum. In our case here at CMU, we’ve been using stainless steel rods that have been flattened; they work pretty well and they take a long time to become unusable. The anodes will be connected to the positive battery charge. The electrolytes, in our case, will be the particles of baking soda (sodium bicarbonate, NaHCO₃). Some labs use sodium carbonate (Na₂CO₃), which is more basic and is slightly stronger than NaHCO₃ (it will also burn your skin). Others use different chemical formulas, according to how acidic or basic they need the solution to be, which depends on the contexts from which the artifacts were recovered (e.g. saltwater versus soils), and how large the artifacts are. Remember that most of the time, we work with ferrous artifacts that were found from within the ground, and most of them are relatively small or medium-sized. For our purposes, baking soda is enough.

The Steps:

1. Take a photo of the artifact before using electrolysis to clean it. Record its provenience information as well. If you’re cleaning multiple artifacts at once (I suggest limiting to no more than three), please make sure you can track which provenience goes to which artifact. If you cannot do so, then only clean artifacts from the same provenience together.
2. If the artifact is significantly corroded, you’ll need to use a Dremel to sand down the portion of the artifact you plan on attaching to the negative charge. This ensures that the electric current can flow through. Only use electrolysis on artifacts that still have an intact metal core. If the artifact is already flaking away, the electrolysis will actually eat away the whole artifact. Use a face mask while using the Dremel on anything.
3. ONLY CONDUCT ELECTROLYSIS INSIDE OF THE FUME HOOD. The electrolysis process emits fumes and particulates into the air; said fumes and/or particulates might cause a fire or explosion if exposed to electrical sparking from the battery. Besides, you do not want to inhale the electrolysis byproducts. If you don’t have access to a fume hood, then please conduct electrolysis outdoors.
4. Pour five gallons of distilled water into the plastic tub. Remember that electrolysis is a chemical process; not all plastics can withstand it. Make sure that the tub is made out of polypropylene, or polytetrafluoroethylene, or polyvinylidene. Measure out a ½ cup of baking soda and pour that into the tub of water. The baking soda acts as the electrolyte.
5. We use a plug-in battery as the main power source for this process – a tan Tekpower (TP1503C) battery. The battery comes with one red alligator clip and one black alligator clip. [INSERT PHOTO] Remember that your anode is the object to which the rust corrosion will be attracted. Notice that the red wires are connected to the steel shelving components on the inside of the plastic vat. Connect the exposed end of the red wire to the red alligator clip that is connected to the battery.
6. Hook the black wires – they have alligator clips on the ends – to the artifact. The clips should be attached to the portions of the artifact that you Dremeled. Alternatively, if the artifact is too thick and the clip won’t stay on, you can use copper wire to attach the clips (this method doesn’t work quite as well – it just takes a little longer to work).
7. Once the wires are hooked up, gently place the artifact into the baking soda/water solution. ENSURE THAT THE ARTIFACT AND THE WIRES CONNECTED TO IT DO NOT TOUCH THE ANODES OR THE WIRES CONNECTED TO THEM. Additionally, make sure that the red and black clips connecting to the battery itself DO NOT TOUCH. A bit about electrical safety: our steel anodes are covered in chrome; as the electrolysis process is happening, the chrome breaks down in the air and turns into carbon dioxide. Also, there are other particulates that let loose into the air as soon as the electrolysis process starts. For these reasons, we only conduct electrolysis in a well-ventilated area such as the fume vent hood. This is also why we can’t let the wires touch; if they bump into each other they have the chance to spark. Should that spark hit the particulates in the air, there’s a chance that it could cause an explosion.
8. Plug in the battery and turn it on. The gauge on top is for voltage; the gauge on the bottom is for amperage. 15V/0.70A works well for heavier/thicker artifacts; for smaller/thinner artifacts, 12V/0.45A works. You may increase or decrease the voltage/amperage as you monitor the process. A good hint that the electrolysis is working is seeing the artifact “fizz”. The “fizzing”, “bubbling”, or whichever adjective you want to use to describe it, is the visual demonstration of the electrolytic reaction in action. Supposedly, the more the artifact is fizzing, the more effective the electrolysis. Be mindful of how high the voltage is; monitor the electrolysis closely. After a few hours, the water will turn get cloudy and orange-brown; this is a good sign that the electrolysis is working, but it keeps you from looking at the artifact. You can check the artifact’s progress by turning the battery off and taking the artifact out of the water. Use common sense – remember to not let the clips, wires, cathodes or anodes touch each other. Wear the rubber gloves that are provided when you’re playing around in the electrolysis bath water. 

   

   Electrolysis in action. Note the fizzing. 

9. The electrolysis process takes anywhere between a few hours to a couple of full days, depending on the size of the artifact and how many you’ve got in the bath at once. The iron artifacts are ready to be taken out once they have turned a slick black or dull silver color. When you take the artifacts out, turn off the battery first and unhook everything. Rinse the artifact(s) in a bit of distilled water. Pat dry with a towel.
10. You may notice bits of stubborn rust still attached to the artifact. It’s a good idea to use the Dremel again to get rid of those tough spots. You might need to rinse it again in the distilled water after that.
11. Next, you’ll need to boil the artifact in distilled water. The application of heat and repeated rinsing of distilled water helps the artifact become “stabilized”; i.e., it’ll lose the rest of the oxidized bits that are still embedded within the metal. Full the pot with distilled water, enough to cover the artifact completely. Put the artifact inside, and bring the water to a boil. Let the pot simmer for 3-4 hours.
12. Once the artifact has been stabilized in the water, it should be a nice shiny black or shiny silver color. At this point, you need to wear gloves or use tongs when handling the artifacts; the oil on your hands can cause the metal to start rusting again. It is imperative to begin the baking process immediately after taking the artifact out of the water. Set your oven to 200, and let the artifact bake for 3-4 hours. If you can’t finish it all at once, let it bake for at least an hour; you can finish the baking the next day. When the baking is done, wrap the artifact in tin foil and set in an air-tight container. [INSERT PICTURES]
13. At this point, you can let the artifact(s) hang out in the air-tight container. This might allow you to work on several artifacts at once. Alter the process according to the amount of artifacts that you have to work on, in a way that makes sense.
14. Coating the artifacts is the final step. We use micro-crystalline wax polish made by Renaissance™ to coat our newly cleaned artifacts. The purpose for this is to help keep the stabilized artifacts from getting oxidized again. Apply a conservative amount of the wax to a paper/shop towel, and buff the artifact gently with the wax. The wax should dry almost instantly. Note: The wax smells bad. Like, REALLY bad. Do the waxing process from within a well-ventilated area.

    

    After electrolysis and wax.

15. Store the stabilized and polished artifacts in a cool, dry place. It would be ideal to keep them in a place that is temperature-controlled. You’ll need to check on them every now and again, to make sure they aren’t oxidizing again. 

    

    This was found on the underside of the cultivator sweep. Nok-On was a company that made agricultural equipment and machinery in the 1980s. We couldn't see this under all of the rust!

Other Resources on Electrolysis and Iron Artifact Curation – Video Links and Notes

As I learned the electrolytic reduction process, it helped me to watch a couple of relevant videos online, just so that I could see how other folks did their own electrolysis / to see the process in action before I tried it myself. Here are a few video links, including my notes on them, that might help others as they’re learning this lab technique.

Richard Gessford, an antique collector, made a video on using electrolysis for cleaning a cast iron muffin pan that was made circa 1870. His process is really simple; he used a five gallon bucket, 1/c cup of Arm & Hammer baking soda, a portable battery, and a pair of jumper cables. He used a steel strip as the sacrificial anode; the positive jumper cable is attached to the steel strip. The negative jumper cable was attached to the muffin pan, which was suspended in the bucket via a wooden dowel and metal hook. He stressed that you should never let the negative and positive cables nor the sacrificial strip and the muffin pan touch. He let each side of the muffin pan stay in the electrolysis bath for two hours per side. He stresses that good ventilation is important during the electrolysis process.

An important thing to note is that Gessford used water from his garden hose to fill up his bucket (his electrolysis bath). Fun fact – tap water/city water contains chlorides, which help cause metal objects to rust. As indicated in Lauren’s notes, you should use only distilled water for the electrolysis bath. Dry brush or clean artifacts with distilled water before the electrolysis process, if it’s necessary.

He pointed out that sometimes, stubborn oxidation stays within the nooks and crannies of the iron object after the electrolysis process is done; his methodology for getting rid of the stubborn rust was to let the iron object sit inside of his oven, which was set at about 400 degrees Fahrenheit (presumably, to let it dry out), and then using a metal brush to sort of scrub the rust away. I assume that using a Dremel to get rid of the stubborn oxidation should probably do the trick. He also mentioned that putting it inside of the electrolysis bath for a while longer could also help. His video is here:

https://www.youtube.com/watch?v=0XlsNucmbiE

               

                At the archaeological museum at Historic Jamestown, the curators use a variety of different techniques for conserving iron artifacts. Although we do not have access to most of these techniques in our lab here at CMU, it’s kind of cool to watch them in action. Processes include x-ray, air abrading, and special desiccation techniques.

                The part of the video that does cover electrolysis is short and is sort of lacking in terms of its usefulness as a tutorial, but it exhibits a couple of differences from the video above that I think might be good to note here. Both the artifact (the negative cathode) and the sacrificial steel strip (the positive anode) were suspended via metal wire, which was attached to metal rods. The cables were clamped onto the metal rods. The curator of archaeology mentioned that they were inside of a solution that contained 3% sodium carbonate. The video can be located here:

https://www.youtube.com/watch?v=IcFAxXJCxLQ

The archaeology lab at Historic Jamestown came out with another video in 2015 that covers their iron conservation process in a slightly more in-depth way. Their whole conservation process is rather lengthy; it begins with the electrolytic reduction, in which multiple artifacts are suspended in the sodium chloride solution via metal wire, which is clamped onto a wooden dowel that is wrapped in more metal wire. From what I could tell from the video, the negative charge was clamped onto the wire-covered dowel.

After the electrolysis process is done at Historic Jamestown, the artifacts are air-abraded – basically, it’s a miniature sand blaster that gets all of the extra corrosion off of the metal artifact. After the air abrading, the artifacts are soaked in deionized water for 4-6 weeks (or longer, depending on the size of the artifact), to “electrically neutralize” them. Then, the artifacts are placed into an oven at 300 degrees Fahrenheit, in order to rid the artifacts of any extra moisture. The artifacts are then coated and curated inside of their “dry room” – a temperature- and humidity-controlled room at the museum. The video is here:

 https://www.youtube.com/watch?v=JR2GilP7N74

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