Q: We've been having a finishing problem. We're seeing differences that aren't acceptable in the way that yellow birch and white birch finish; that is, they don't look the same after finishing. Can you shed some light on this?
A: White birch and yellow birch are in the same genus, but are different species. White birch is about 12 percent lighter than yellow birch. When a wood is lighter weight, it's generally because the wood is more porous, which will be noted especially on the end grain. But this slight increase in porosity would not greatly affect finishing.
White birch sapwood is creamy white colored while yellow birch has some pale yellow tones in the white sapwood. Perhaps the difference you see is a result of this slight color difference.
However, it's my guess that the differences you're seeing are because of processing differences, not because of inherent differences within the wood itself. Here are three items to check.
If the yellow birch is at a higher moisture content than the white birch, the yellow birch will not be as absorptive. (If yellow birch and white birch are dried in the same kiln load, it would be likely that the white birch would be over-dried unless special precautions were taken.)
If dried by two different suppliers, it's possible that one supplier is using higher kiln temperatures than the other. Heat reduces absorptivity and darkens the color of the wood. Such color changes are usually not seen until the wood is finished.
Because of the higher density, the yellow birch may be burnished more by the planer knives, sandpaper or other tools, especially if dull knives, dull sandpaper or carbide knives are used. The heat generated with dull tools and the burnishing both reduce absorptivity.
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