Oak window board plug discolouration

5 Feb 2016
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United Kingdom
Hi folks,

Any ideas why this has gone black like a ring on the pellets?

I have a feeling it's the wood glue to hold the pellets in as I tried a small repair on the edge which had split, so put abit of wood glue on , clamped it down and it turned black but I thought it might have been due to glue being too old! (more like a banana milkshake colour)

The wood is American white oak

Is it possible not using wood glue could stop the ring effect? The bummer part about it is I've done all 8 like this and didn't particularly want to drill them all out! :(



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So the only way was to try it. Turns out it was the glue.

I have glued them in, but instead of putting it on the pellet, I used the teng of a file to put abit on and put it the hole.

Still would be interested to see why it's reacting in this way as it should dry naturally :/


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Oak contains tannin. Tannin in conjunction with the water in PVA glue creates weak tannic acid. Tannic acid in turn attacks iron to form a black oxide (ferric oxide?) which is your black ink like staining, especially if the glue was contaminated by contact with iron (can even happen with tap water 8n some districts). It's why oak items should always be assembled using brass or stainless steel fixings - never mild steel or zinc plated items.

It may be possible to remove the finish, treat the affected area with oxallic acid (made from crystals and water - add the crystals to the water NEVER the other way round) and then washing and refinishing
I actually bought a brand new bottle of glue as I thought this may cure the issue.

Thank you for the explanation

Speaking of zinc plated screws.... having not known about the above if they where used to screw the boards down but pelleted do you think it will turn the top of oak black?
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Possibly. All I can say for certain is that there is iron in there somewhere
I've purchased some of the oxallic acid crystals anyway to see if it can reduce the black ring effect.
I'll have to re oil the top anyway as its gone a bit patchy from sanding the pellets back. At least the this way I've give it a try and I know for future projects!

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