DOS and metal (dreaded orange spots and wire racks) ...

[SaltedFig]

Do you have a link please ?

Hopefully that will be a more concise read than The Mechanisms and Factors for Edible Oil Oxidation paper, which did give a lot of information that was interesting, but wasn't so much about listing the metals in order of effect. They too singled out contact with copper as being a cause.

From a research paper I read today, copper tops the list as the Bad Boy of Rancidity followed closely by lead and iron (not saying people would use lead stuff for soaping, just passing info along), then zinc, followed last by tin and aluminum.

Exposing fat to any of these metals increased the rate at which the fat became rancid compared to fat without any metal. That means it's not a good idea to let any metal touch your soap if you want to avoid DOS/rancidity. But aluminum was low on the list of these troublemakers.

While I agree in general, I would add that carefully selected grades of stainless steel can safely be used directly in contact with lye (and soap batter) during the manufacture of soap.

 

[DeeAnna]

...I would add that carefully selected grades of stainless steel can safely be used directly in contact with lye (and soap batter) during the manufacture of soap.

Yes, you are right and I agree. I spoke too generally.

It's my preference, however, to err on the side of caution even with stainless steel. If I have a choice between using a stainless steel utensil or a plastic version that is just as suitable, I'll choose the plastic one.

 

[DeeAnna]

I see today that I didn't provide the reference that @SaltedFig requested. From the "better late than never" department comes my belated followup --

James A. Emery, R. R. Henley. Studies on Rancidity: The Influence of Air, Light, and Metals on the Development of Rancidity. JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY. Vol 14, issue 7. Oct 1922. pg 937-940.

You can get the e-book from Google Books.