Flat Earth Catalogue

2018-05-24

Cast iron chemistry

So, treating your cast iron. The virtues of flaxseed oil for seasoning cast iron have been much discussed, and by experiment I confirm that the stuff is awesome. This post explores some ways to make it more awesome, which I have recently tried.

Washing your flaxseed oil. Food-grade flaxseed oil will contain the lonnnnng polyunsaturated triglycerides that we want, but also other stuff: mucilage, phospholipids, flavorful short-chain fatty acids. They smoke easily but don't help with polymerization. In fact, they retard it and create gumminess, skins, and such. They conduce to rancidity in the unused oil, too. We want them out.

There are refined flaxseed oils manufactured for painters and woodshops, but I wouldn't trust my food to them, so we're going to have to do this ourselves.

Ethanol washing is very effective. 1 part each oil and 40% ethanol will emulsify beautifully and the ethanol will dissolve so much crap out of the oil. To break the emulsion, add 4 parts water, agitate, and settle. The aqueous phase separates overnight, but there's a particulate phase suspended in the oil which separates rather slowly.

You can give it a rinse or two with water after the ethanol wash. This is better than aqueous-only washing in that it's less laborious and it does not pre-polymerize the oil as much, so it spreads thinner and doesn't cure as fast.

Washing with brine pre-polymerizes more than plain water does. Extended contact with the dissolved short-chain fatty acids pre-polymerizes too. Painters seek these. Ironmongers should not. Thin coats, baby. Thin coats.

To help with the mixing, you can add inert ceramic chips or clean coarse silica (pool filter sand). After separating layers you may need to knock or sharply twist the container to shake loose any trapped globules. If the oil is turbid at the end because of suspended water droplets, freeze/thaw, time, or CAUTIOUS heating will clarify it. Cautious because you don't want water to pool on the bottom of a hot pan under oil.

Prepping your iron. Scrub off whatever will scrub off. Electrolysis would be awesome if you can arrange it, but I was unable to get a suitable current source. If the outside of your pan retains some scaly bits, that's fine, but you may want to mix a little seasoning oil with limonene to thin it so it will penetrate the little crevices around there. Food-grade limonene is great to have around anyway, for certain kinds of cleaning.

Your main problem is red rust, which is ferric oxide or iron III oxide, because it hydrates, expands, crumbles, and falls off, exposing more surface to oxidation. Black oxide or magnetite is a mixture of iron II and iron III oxides, and it's a sweet deal. Dimensionally stable, and a great surface for your seasoning oil to bind to. In fact, a better substrate than bare iron is; smooth bare iron doesn't offer much for the seasoning to hold onto.

Unfortunately, converting red oxide to black oxide is typically done with reducing agents like hydrogen gas or carbon monoxide, neither of which I care to dick around with, and oxygen tends to snap up these reducing agents anyway unless you exclude it, which is not often an option for the humble kitchen chemist. If by some chance you can exclude oxygen, even water is reductive enough to do the job (well, steam anyway; it has to be pretty hot).

All that said, I'll stick with the recommendation to have the iron hot when it receives the first coat of oil. Below the oil's ignition point, please, but hot. It just might reduce some red oxide to black, and it just might drive some water of hydration out of the ferric oxide that's left, so sure, go for it.

But another attractive option is to phosphatize the iron. Very good anti-rust coating and an amazing substrate for the seasoning oil. Here we do want iron III, in solution with phosphoric acid. When the acid hits bare iron, electrons from the metal form gaseous hydrogen with protium from the acid, carrying both away. So locally the pH rises. This causes FePO3 to come out of solution and coat the metal.

1M phosphoric acid works very nicely on bare iron. You have to sweep off the hydrogen bubbles (I used the top part of a bendy straw as broom, with the bottom part as handle) so they don't keep the acid from touching the iron. Later I hope to add an oxygen donor like nitrate to prevent gaseous hydrogen from evolving. Anyway, when it's done there's a nice grippy coating on the metal (wash & wipe it to remove loose particles, heat it to dry thoroughly) that drinks oil up thirstily. (So thirstily. If you've previously oiled bare iron or existing seasoning, oiling this stuff is going to astonish you.) It would also be nice to have extra iron III ions in the acid solution.

Oxygen donor: I've tried hydrogen peroxide (at drugstore concentrations), and it suppresses bubbling, but it also dilutes the acid. The reaction still happens, but it's much slower, which I don't like. But there's a potential oxygen donor I've been overlooking: red rust! One mole of ferric oxide + two moles of phosphoric acid yields two moles ferric phosphate and 3 moles water. The evolved water raises the local pH, precipitating the ferric phosphate. Nice! Rusting the bare iron with peracetic acid (add H peroxide to vinegar) will probably prep it quick. Maybe in a fine mist? This will also produce some ferrous (black) oxide, which will not dissolve sufficiently to participate in the reaction. It stays attached to the metal. That's fine. The red oxide will dissolve in the aqueous acid, giving us just the iron III ions we need.

OK, but can we add additional food grade iron III ions? Iron supplements usually contain iron II sulfate. We can bring it up to III with hydrogen peroxide, plus an acid (phosphoric would be the logical choice) to take up hydroxide ions. How to get rid of the sulfate? Maybe we can salt it out with calcium, but I'm not sure. Food-grade calcium carbonate (clean roasted eggshells) will dissolve in an excess of phosphoric acid to give dissolved calcium phosphate. Calcium sulfate will not dissolve in water. But is there a pH where calcium phosphate is soluble but calcium sulfate is not? My chem is too long ago to tell me. You can get dissolved calcium in plain water with acetate, nitrate, or chloride, but that's just trading one anion for another. "Monocalcium phosphate" (calcium biphosphate IIRC) is soluble (2 g solute / 100 ml solvent) and is used as a food additive, so that's a possibility.

05:29