Black streaks in birch
by Professor Gene Wengert
To clean this up, I remove the knives and wire brush them and then resharpen. I've experimented with some solvents and not resharpen. This works temporarily. I also notice this black streak first on the heart wood. Have you heard of this problem? What should I try to change or try to address (knife angle, head speed, tool steel, MC, all of the above?)?
I do not believe that birch has much tannic acid, so that eliminates iron tannate (like we see with oak); but just to be sure, put a little oxalic acid on the stain and see if it instantly goes away. If it does, then iron tannate. You can get a small quantity of nearly saturated oxalic acid from the drug store (or high school chemistry teacher).
I assume it isn't burning, as you would smell that and it would be brown in color. It does seem to be related to rubbing, as you say that when you resharpen the problem goes away. A dull knife rubs a lot more than a sharp knife.
The more I think about it, the more iron tannate sounds like a good possibility. What is the MC? If it is really low (under 6%) then we better look at burning. In fact, I would check the clearance angle--maybe the heel is rubbing. I would be glad to look at a sample or two if you want to send me one.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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