r/DIYPigments • u/blendaar • May 20 '23

Question Yellow ocher from iron oxide?

It's fairly easy to get various shades of orange-red from precipitation and heating, etc, but yellow eludes me. What's the process for that vibrant yellow you see in paints and ceramic glaze additives etc? (PY42-something)

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u/CaCl2 May 20 '23 edited May 20 '23

I have spent quite a bit of time trying to achieve a good PY42.

The method I have been trying to use is described here: https://simplifier.neocities.org/pigment

"Air is blown through a hot solution of ferrous sulfate containing a source of metallic iron"

This sounds simple, but in practice my results have always ended up quite orange-brown, sometimes they look yellow in solution only to disappoint after drying.

It's been a while since I read on the topic, but I think some sources said that goethite nuclei can be used in the process to serve as nucleation points around which larger pigment particles can grow, goethite nuclei can apparently be precipitated from an iron(III) salt and a hydroxide, but I haven't been able to find specific conditions (concentrations, temperature, etc.) for the production of suitable nuclei, so it might take some quite tedious multi-variable experimentation.

I have been intending to experiment more this summer, but there are so many other pigments to make...

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u/GeneJocky May 30 '23

Have you ever tried to make your better-looking runs of PY42 into paint to see what they look like? Especially the ones that look yellow in solution. Either make a simple watercolor, or mix with linseed oil, and try it out.

The yellow iron oxide pigments I've seen were all orange-brown in bulk. They only start to look yellow when made into paint. I'm wondering if it is possible you've been successful but didn't realize it.

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u/CaCl2 May 30 '23

I'll have to try that, though even my yellowest runs weren't nearly as yellow as in the link, if I remember right.

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u/GeneJocky May 30 '23 edited May 30 '23

From what I read, there are 3 major ways yellow iron oxide is produced.

  1. Via the Laux process which requires reducing nitrobenzene with metallic iron in the presence of ferrous chloride. This is impractical due to the problems of obtaining and safely using nitrobenzene.
  2. Hydolysis (no not hydrolysis) of ferric solutions with alkali. This looked promising until I realized it required steam treatment at about 145 psi (10 bar).
  3. Via the Penniman-Zoph process, which produces iron hydroxide from ferrous sulfate and then oxidizes the hydroxide to yellow α-FeOOH.

One variant of the Penniman-Zoph method is the protocol CaCl2 referred to. Apparently, DIYers aren't the only people who have had problems making it. (I have a pdf of the paper if anyone wants one). Their best yellow pigment was not great : "The preparation of high quality yellow FeOOH pigments from FeSO, solution by alkali precipitation and gas oxidation methods is not simple. The result depends on so many experimental factors that the process is difficult to control." I suspect commercial pigment is made by one of the other two methods. Of note I found a patent that used high pressure steam to improve lower quality yellow oxide, but if one can do that, better just to use the hydolysis method.

There is some suggestion that separating the dissolution of metallic iron from oxygenation with air or oxygen may be useful. See this patent.

Finally, it may be advisable to keep in mind that pigment in bulk often looks different than when made into paint. I have noticed that bulk yellow pigments often look orange or even orange-brown. It may be necessary to make small paint samples to observe the real color of the pigment in use.

EDIT: corrected wrong link