Removing tannin stains (from bowls) - Oxalic acid?

[ToneWood]

I have some persistent tannin stains on an oak bowl - I soaked the original bowl blank & lots of tannin leached out. The outside of the bowl is unaffected - just the inside of the bowl. I have re-gouged & then scraped the inside yet it still persists - perhaps fresh tannin is still leaching out (it has been drying for more than a month now tho')?

I have tried the following:
- washing with soap & water,
- washing with dishwashing detergent,
- quick spray of white vinegar* (didn't want to leave vinegar on long in case it made things worse).

I tried Googling for ideas and kept coming up with just one: Oxalic Acid (the acid in rhubarb), which is used both to remove tannin &/or to bleach wood, apparently.
e.g. Video: http://www.youtube.com/watch?v=YTHiqtdZMMI

http://oxalicacidinfo.com/

Oxalic acid is inherently a strong acid: it is about 3,000 times stronger than acetic acid, which is the chemical name for the acid in ordinary vinegar... Oxalic acid is so strong that it is widely used industrially for bleaching and heavy-duty cleaning, notably for rust removal. If oxalic acid is not heavily diluted--as it is in plants--it is quite dangerous to humans, being both toxic and corrosive.

I am loath to use strong chemicals. Might vinegar*, if left on longer, or sunlight work instead? Anybody used Oxalic acid or have an opinion on it - is it reasonably "mild"?

*Apparently vinegar & iron are sometimes reacted together and then applied to wood as a stain/"ebonizer".

[ToneWood]

This is the bowl in question

Second oak bowl - tannin.jpg (77.63 KiB) Viewed 4629 times

Strangely, the underside of the bowl is clean as a whistle:

Bottom bowl: the clean bottom of the oak bowl.

Leylandii reverse bowl on top of upside down oak bowl.jpg (32.85 KiB) Viewed 4629 times

[SeanHellman]

Oxalic acid works a treat on iron stains and I have removed decades old blue stain from oak and douglas fir. Would I use it on a bowl for food, I do not know, maybe with lots of rinsing with plenty of water. Oxalic acid is water soluble.

[Bob_Fleet]

It's found naturally in quite a few things including rhubarb.
I like the taste of wood sorrel and that's partly the oxalic acid.
The LD50 in rats is reported as 375mg/kg which roughly means it would need about 20g for a human and then will kill about 50%. Assuming we're similar to rats.
Not nice but that's a lot more than you're talking about using.
A few grams in water to soak it with and then washed off subsequently.
After drying and oiling that should lessen the amount able to leach out in use.
Still, don't use it as a food bowl if you're worried and take the appropriate precautions when using the acid.

Otherwise, live with the bowl as it is or colour the whole top surface using iron salts.

[ToneWood]

... Oxalic acid is water soluble.

To avoid raising the wood grain, I tried an alternative suggested by a manufacturer: using methylated spirits (alcohol) instead of water.

HOW: I added 3 level 5ml-teaspoons of crystalline oxalic acid to 100ml of methylated spirit, shaken in an old screw-top jar. That is more oxalic crystals than could be dissolved in cold conditions - this is normal & ensures the solution is saturated. BUT it does result in some crystals being left on the wood surface, so flush well afterwards.

I applied it with a 1" housepainting brush, scrubbing it well into the inside of the bowl, concentrating on the darkest areas. Left it half an hour, applied more acid and then left it an hour.

To remove the acid, I first used meths and they washed it quickly but thoroughly with water and immediately dried it with a cloth to avoid raising the grain (which worked).
IMPORTANT: Follow the safety instructions (gloves, glasses, etc.)

RESULTS: I wasn't sure at first if the bleaching had worked but looking at the above image now it clearly has made a huge difference. It worked gradually. So far, it has not completely removed all of the staining, however it has taken off most of that dreadful all-over matt darkening and reduced the darkness of the 2 ends & the size of most dark patches. Next day I scraped the inside of the bowl & repeated the process but that didn't make a significant different.

CONCLUSION: It works and at a reasonable pace & price Far better to avoid staining in the first place though*. Failing that, it would be better to use oxalic acid with water (rather than meths) before final finishing: cheaper, safer & less work. But meths offers an interesting alternative at later stages.

*My big round oak bowl had no such staining, so perhaps this was caused/exacerbated by soaking the blank before & while I worked on it (as it had dried out somewhat).