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Darkening of Oak by Sodium Bicarbonate.

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MJP

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I was interested to read the recent postings regarding the darkening of oak when wiped over with a Sodium Bicarbonate solution - sorry, I can't find the original thread.

So I did a few tests - in each case, a level teaspoonful of chemical dissolved in roughly 100ml of lukewarm water, sample dipped in for a couple of seconds then allowed to air dry at room temperature.

Oak/Sodium Bicarbonate - nicely browned to an attractive slighty pale walnut colour.

Oak/Sodium Carbonate - exactly the same result, indistinguishable from above.

Oak/Sodium Choride - no effect whatsoever.

Pine/Sodium Bicarbonate - very slight yellowing.

Then a dig in Wiki under "Tannin" revealed the cause:

"......adding baking soda to the water to raise its pH level will accelerate the process of leaching, as the more alkaline solution can draw out Tannic Acid from the wood faster than the ph-neutral water"

So what we are seeing is not a chemical reaction between the Bicarbonate and the Tannin but simply an accelerated leaching out of the Tannin onto the surface of the oak.

In which case it sounds as if sanding the discoloration off isn't a practical solution, the only real cure being some chemical reaction to turn the Tannin into a colourless compound.

Martin.
 

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