Sugar soap

[Cloudscape]

I think most of the oxalic acid is in the leaves and roots. Maybe steeping, or boiling, would result in useable acid. I'd be careful what you boil it in though (and how much of it you breathe in or get onto your skin). Oxalic acid is more powerful than acetic acid. Even the weak concentration in Bar Keeper's Friend will remove thin rust (I've used it for removing rust marks from fabric).

 

[biffvernon]

I think the oxalic acid content of rhubarb stalks increases through the season so it is better to eat it in the spring rather than in the summer.

I expect the oxalic acid that one can buy was made without harming any rhubarb.

 

[Penners]

I expect the oxalic acid that one can buy was made without harming any rhubarb.

I'm afraid that cruelty to rhubarb is widespread, and has been going on for centuries. It's kept in the dark and fed bull***t, before being hauled unceremoniously out of its bed.

Hang on - I spent over 40 years suffering the same treatment, before I retired.