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I have this piece of cherry (American) heartwood. It had a sticker on it. When I peeled it off there was a lighter area underneath. It is not residue from the sticker (I scrubbed it with xylene), the wood is discolored. I am not sure how deep it goes but I was unable to affect it with heavy sanding.

enter image description here

My question is: What caused that? Was it some chemical in the glue? The paper? Trapped moisture? No oxygen, or blocking sunlight?

I'm not concerned about getting rid of it (that piece lives in the scrap bin now) but I'm curious about the chemistry behind that discoration.

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I would guess that the color difference is from the sticker blocking sunlight -- cherry can darken pretty quickly in the sun:

Cherry with one spot covered

Half of a sheet of plywood covered

Since you don't care what happens to this particular piece of cherry, I'd leave it out in the sun with a different spot masked and watch the effect.

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My question is: What caused that? Was it some chemical in the glue? The paper? Trapped moisture? No oxygen, or blocking sunlight?

Black cherry is well known to darken due to sunlight exposure. In fact, this is made use of by furniture makers to darken the wood and artificially "age" it.

Consider the image below of a cherry door panel.

door panel

You can see the diagonal stripe where the door (for whatever reason) received less sunlight. If this door is left in the sun for a few more days, fully exposed, the colors will even out.

I'm curious about the chemistry behind that discoration.

It's a simple reaction between the ultraviolet rays of the sun and the wood itself. Most woods undergo a similar reaction (purpleheart turns from purple to brown, osage orange turns from bright yellow to dull orange/brown, yellow pine turns from yellow/tan to a rich brown, and many others). If you're looking for a chemical equation, I'm afraid I can't help you there.

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  • I found this book. It looks promising but intimidating. I couldn't look through it in detail yet, though, as Google books is frustratingly impossible to use in Google's mobile browser. – Jason C Nov 4 '15 at 5:15
  • @JasonC, yeah, that looks like a good one. CRC publishes a very well-known Handbook of Chemistry, so I imagine that that book is probably of good quality. – grfrazee Nov 4 '15 at 14:17
  • I wonder how much would need to be planed of of his board to even out the color (assuming he didn't want to darken it more and even it out in the sun)? 1/64th"? 1/32"? – JPhi1618 Nov 4 '15 at 18:43
  • @JPhi1618, I haven't tried, personally, so I really don't know. My gut feeling is 1/16" should take care of it, but that's just a guess. – grfrazee Nov 4 '15 at 18:47

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