Comment by kragen

I don't know how he got it, but if I were faced with that problem myself, I'd try this:

1. dissolve a bunch of rust in hardware-store hydrochloric acid,

2. dilute it in a lot of water,

3. into a similar quantity of water, mix an large excess of baking soda to neutralize the acid,

4. rapidly mix the two solutions together to precipitate a very fine iron hydroxide powder,

5. decant the powder and/or filter it with coffee filters,

6. rinse it to remove the remaining salt and sodium carbonate,

7. heat it to convert it to Fe₂O₃, and

8. heat the Fe₂O₃ in a sealed container with enough carbon to reduce it to Fe₃O₄.

I don't know if this would actually work, because my entire education in chemistry consists of watching NileRed videos in which the primary lesson is that nothing works the way you think it will. Wikipedia has some more-promising-sounding approaches that require materials I don't have: https://en.wikipedia.org/wiki/Iron(II,III)_oxide#Preparation

> use ammonia to promote chemical co-precipitation from the iron chlorides: first mix solutions of 0.1 M FeCl₃·6H₂O and FeCl₂·4H₂O with vigorous stirring at about 2000 rpm. The molar ratio of the FeCl₃:FeCl₂ should be about 2:1. Heat the mix to 70 °C, then raise the speed of stirring to about 7500 rpm and quickly add a solution of NH₄OH (10 volume %). A dark precipitate of nanoparticles of magnetite forms immediately.[9]

You can also buy it as a pottery pigment or as a black "ferrite" pigment for mixing into whitewash to make black paint, but if the particles are too coarse, you probably can't mechanically grind them down to be small enough.

You can get ferrous sulfate from the garden store as a fertilizer, and if you get it wet it likes to oxidize to ferric sulfate with the air. Or you can encourage it with hydrogen peroxide. I wouldn't be surprised if that would work as a replacement for the ferrous and ferric chloride mix in the Wikipedia recipe.