Im sure there is a chemist/lab tech that knows a simple solution to my problem, as I cant seem to find a solution. When trying to strip and re-season cast iron cookware, I occasionally have problems with rust. I live near the ocean, so there is a lot of humidity in the air. I strip the cast iron by putting in the oven on the self-cleaning setting for an hour. This normally works perfectly. However, sometimes at the end of the hour, I open the door and see the pan has developed surface rust. Obviously, heat + iron + water = rust. I am sure there is a way to absorb the moisture in the oven, but I cant find anything. I looked at desiccants, but they release moisture in the oven when heated - at least the ones I saw. Is there a desiccant that works at oven temperatures? Or is there another solution? Remember, food will be cooked on the cast iron, so the solution has to be food safe in the end.

  • $\begingroup$ Could you include more details about how you handle the cookware? Exactly what do you do to it before you put it in the oven? Also, what kind of oven is it? This site explains that "[GEs] innovative P7 model introduced the pyrolytic cleaning method ... a fancy name for heating the residue until it carbonized and turned to ash. The oven included extra thermodynamic controls to allow it to reach the high temperatures needed. Today, another option is the steam-cleaning oven, which loosens dirt without resorting to extremes of heat." $\endgroup$ – Buck Thorn Jul 8 '20 at 6:08
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    $\begingroup$ What is the problem with that rust? You should be able to wipe it off easily. $\endgroup$ – Karl Jul 8 '20 at 6:13
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    $\begingroup$ @BuckThorn I dont do anything. I am simply putting it into the oven to strip off the seasoning and any burnt on carbon. The self-cleaning setting is the super high heating, which does turn everything to ash. $\endgroup$ – Keltari Jul 8 '20 at 22:12
  • $\begingroup$ @Karl Yes, I can remove the rust after if forms. However, I was hoping to find a way to prevent that step. $\endgroup$ – Keltari Jul 8 '20 at 22:13
  • $\begingroup$ If the rust is hard to get off, it was on there before, hidden by your "seasoning". Otherwise I´m sure it´s not worth the trouble to get the oven interior super dry. $\endgroup$ – Karl Jul 9 '20 at 5:53

I like the suggested treatment suggested here which is to basically clean/scrub the pan, dry and coat with vegetable oil, invert the pan, place over aluminum foil on an oven rack and bake. To quote:

Now that you’ve returned your skillet to its base layer, you’ll need to season it for use. Drop a tablespoon of vegetable oil on the pan and spread it around with your fingers or a paper towel. (Take care not to burn yourself on a warm skillet!). Oil the outside and the handle, too. Next, take a folded napkin and wipe off the excess oil. You’ll want the skillet to be lightly greased. Set your oven to 350 degrees and put some aluminum foil on the bottom rack. Then pop the skillet in, upside down, on the upper rack for about 1 hour. Turn the oven off and leave the pan inside until it’s cool. The oil will bake into the pores of the pan, providing a non-stick finish. Repeat this step a second time and you’ll have an almost glassy surface of seasoning. Ta-da! Your rusted cast-iron skillet is good as new.

The created carbonized layer is safe, chemically inert and durable.

  • $\begingroup$ Reread the question. I am not asking how to season a skillet. $\endgroup$ – Keltari Jul 8 '20 at 22:14

At the high temperature used to burn off the fatty acids on the surface of the cast iron cookware (pyrolysis) dessicants lose their dessicating ability (at best) and dehydrate (releasing water, which is the opposite of you want to do: capturing water).

Fatty acids oxidize already at low temperatures when highly unsaturated (components of lin seed oil for instance, which can in certain circumstances even catch fire near room temperature), but most commonly need temperature of 300-500 degree celcius to start oxidizing at a sufficient rate to suit cleaning purposes. At these temperatures: CaCl2, Na2SO4, SiO2, MgCO3, CaSO4, MgSO4, ZnSO4, and K2CO3, which are commonly used dessicants, will all have released the water they can capture, and won't be able to rehydrate until being brought back to lower temperatures.

One exception might be CaO, though it's not able to capture as much water per molecule as others. If pure, minute amounts of CaO on the surface of your cookware are perfectly compatible with the large amount of food you'll put in the pots (it will become calcium hydroxide when it comes in contact with enough water). The downside of a dessicant that works at high temperatures is the problem of recycling (i.e. dehydrating it), for CaO you might have to heat it above 1000 degree celcius before it can be used again as dessicant, which certainly is not the practical recycling you'd want to routinely carry out for household purposes.

The good side is that with increasing temperatures the density of water/steam in your oven too decreases (unless it's a pressure chamber and the pressure strictly accompanies variations of temperature). This means that if you put the cookware in the oven when it's already hot you shouldn't have a lot of rust forming, and thus, as Karl writes, it shouldn't be as big of a problem to remove it as it would be to prevent it's formation.

A solution of dilute oxalic acid at 50-65 degree celcius might help remove the rust (it's often used to remove rust stains on metals as iron oxalate is slightly soluble in water unlike most metal oxalates). It would be worth trying and is in any case compatible with your food as small amounts of oxalates are contained in many edible plants (spinach, most notably).

You can rinse off most of it with hot water. If you were to drink too much of a dilute solution of oxalic acid however, you'd develop kidney stones as it would precipitate in your kidneys mainly as Calcium Oxalate crystals, so if you do this often, definitely have the habit of rinsing the pot with hot water before use.

Citric Acid is another tool that you may try to remove the rust as it too is compatible with food.

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    $\begingroup$ Im reading your answer. First I want to say removing rust is not the problem. I can scrub it off easily. My goal is to prevent it from forming. You are saying calcium oxide will work at oven temperatures and to sprinkle it on the iron? Im reading up on CaO, its cheap, but it definitely sounds like something I dont want to ingest. $\endgroup$ – Keltari Jul 8 '20 at 22:30
  • $\begingroup$ You should not sprinkle it (nor any dessicant) directly on the iron. A dessicant needs to have it's own container allowing it a fair contact surface with the humid air without itself escaping the container. Tiny little traces of the dessicant you will use might though escape the container and end up ont the surface of the pot, which is the worst case scenario you'd face if your setting has been well designed. Very small amounts of Calcium Hydroxide can be very safely used in food (E526 is the code in Europe). Don't handle any chemicals at all if you don't know how to safely use them. $\endgroup$ – Hans Jul 9 '20 at 5:50
  • $\begingroup$ Tiny traces of calcium hydroxide on the surface of your pots not only is compatible with foods, but will also further protect the iron from further rust by passivating its surface. $\endgroup$ – Hans Jul 9 '20 at 5:51
  • $\begingroup$ Don't handle Calcium Oxide, nor the resulting hydroxide without proper chemical protection equipment (skin, eyes, airways, and preventing contamination of the room). $\endgroup$ – Hans Jul 9 '20 at 5:53
  • $\begingroup$ If you want to use it directly on the pot you could do so if isolating the pot from the rest of the oven in another, bigger, oven-safe container (to prevent CaO from attacking non-ferrous metal parts of the oven). You'd then have to rinse the pot with a profuse amount of water before heating it again for drying before applying the fat for the ensuing seasoning step. $\endgroup$ – Hans Jul 9 '20 at 6:02

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