There may indeed come a day where you will need how to seriously rehabilitate a rusted cast iron cauldron. Perhaps you come across a fantastic piece at a flea market or thrift store and it’s only a few dollars. Perhaps you left Grandma’s treasure out in the morning dew after a festival bonfire. Esoteric kitchen witchery though this may be, it is entirely within the realm of possibility that having a working knowledge of rust removal will save your bacon someday.
Simply put, there are four basic techniques that could be employed in stripping your cauldron of its rusty shell: mechanical removal, incineration, chemical removal, and electrolysis. Of course, all have their benefits and detractions.
The one most people would think of first is, of course, mechanical removal. This would be the act of scouring the rust away from the intact metal with aid from some frictive substance like sandpaper, steel wool, or perhaps a steel brush attached to a drill. If the rusting is very light, this is probably the best way to go: you’ll only lose a little time and elbow grease. The only real thing to keep in mind is to not scour any one spot too much–especially if using a drill attachment. Too much rubbing in one spot can mean you’ll actually wear away spots of iron and create small depressions in your piece, and that can affect how the pot cooks. (Of course, if the cauldron won’t be used for cooking, this is not a major concern.) When the rust issues are heavier, though, it’s a lot more important to save the elbow and find a better solution.
One technique that’s been around just about as long as we’ve had fire is incineration. In days gone by, people used to put rusted iron pieces in the heart of a great bonfire that would burn for hours. The fire would not only strip the iron of any seasoning or baked-on crud, but could actually turn rust to ash, too. If tending a bonfire isn’t to your speed and you have a self-cleaning oven, you could just clean the oven with the cast iron piece inside. Any residual rust sould be able to be removed with a light scour.
Some people who rehabilitate cast iron as a job or a hobby really prefer this oven method since it is so easy. However, there are some notable detriments, not the least of which is the incredible energy cost. More immediate, though, is the risk of damage to the oven or to the cast iron piece (it is rare, but occasionally a piece will crack with this method). Oven damage can most easily occur if you leave the metal racks inside your oven while running the self clean cycle. The racks themselves will, of course, warp and discolor, and they may become permanently stuck in the oven. You can also do damage to the oven and the cast iron if you put the cast iron piece directly onto the oven floor in the self-clean cycle. A common home hack is shown above where the cast iron is balanced on something ceramic (in this case a brick). If something does go wrong, though, you may void any warranty on your oven, and possibly create a fire hazard. In short, this method can be easy and can work, but if something does go wrong, the stakes are really high.
Of course, there’s always better living through chemistry…and turning to what those of us without self-cleaning ovens do to clean up our baking grime. Plenty of chemical solutions eat away rust, and one of them (lye) happens to be the active ingredient in Easy Off oven cleaner (alternately, you can just use plain lye). All you really need to do is brush the big flakes of rust away, spray the piece down, then put it in a plastic bag, squeeze the air out, and let it sit for a long time: 2 days. The process may have to be repeated for up to a week. Residual rust should be easily scoured away. If not, try soaking the piece in a solution of 50% vinegar and 50% water for 24 hours, and scrub off what rust remains. Of course, you may have to return to step one if the piece is really rusty. You should also be very sparing with soaking the piece in vinegar, as–unlike the lye–it reacts with the iron as well as with the rust.
The negatives with using chemical solutions are obvious. Not only are you using caustic lye, which could cause injury if accidents occur, you’re going to be eating up a lot of time. Unfortunately, the lye solution weakens as it works, and eventually doesn’t do much to affect the rust. That results in constantly re-applying the solution and a lot of babying the piece during the process.
The gentlest of all the methods is electrolysis. Using only electrical current and a mildly basic conducting solution, rust and crud literally lifts right of the pan without causing any damage to the iron itself. Unfortunately, it requires a little bit of equipment and a little bit of care in assembling the rig. You need is your cast iron piece (hereafter referred to as the ‘cathode’), a cheap piece of steel (hereafter referred to as the ‘anode’), a non-conductive container big enough to fit both your cast iron piece, another non-conductive piece to keep the cathode and anode physically separated, a box of either washing soda or baking soda (washing if you can), and a 2 amp or larger automotive battery charger with an ammeter (see if you can borrow one…it’s about $30 to purchase it). Additionally, you may want an extra set of jumper cables if you’re concerned the electrolysis will damage the battery charger cables (it likely will not if you’re only doing this once or twice).
All you do is fill up the tub with water and add about 1 tablespoon of soda for each gallon of water, then submerge the cathode and anode. Make sure that the two pieces cannot touch each other (which would complete the circuit and potentially cause a surge), attach the negative cable to the cast iron cathode and the positive to the steel anode, and turn on the electricity. Hold the current steady, and walk away. In 5 hours to 2 days (depending on how rusted the piece is and how deep of a pot it is) the piece will essentially be free of all rust and gunk after a good regular washing. (Still confused? Check out this YouTube video.)
Of course, you wouldn’t want to touch anything in this set up while the current flows lest you be electrocuted, nor would you want to perform this in a very enclosed space as one byproduct of the electrolysis is hydrogen gas, which is flammable (think Hindenburg!). However, nothing toxic will be produced from the electrolysis, and the soda solution can be poured onto the earth or down the drain afterwards (even if it looks horrendously disgusting). Electrolysis also doesn’t use a terribly large amount of electricity, when all is said and done. It is well worth the time it takes to set up the rig.
Happy cauldron cleaning!