In Chris Schwarz Blog, Raw Materials

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The back page of the upcoming issue of Woodworking Magazine (which mails to subscribers at the end of November) focuses on wood structure. What’s the difference among ring-porous woods, non-porous woods and diffuse-porous woods (not to mention semi-diffuse/semi-ring porous)? What’s a tracheid? A vessel? What’s meant by earlywood and latewood? And most important, what’s it all mean to a woodworker?

While researching the topic (I know far more about parenchyma cells and fusiform rays than my high-school biology teacher would ever credit), I discovered that cherry and maple are diffuse-porous woods, and therefore ought to take up stain fairly evenly according to the basic structural properties they share with all diffuse-porous species. But if you’ve ever worked with cherry and maple, you know that’s not the case. They can get blotchier than Chris in his Clearasil days.

So what’s the explanation? Our money is on elves. R. Bruce Hoadley doesn’t provide an answer in “Understanding Wood” (our wood technology bible). The Forest Products Laboratory doesn’t have an answer. Our finishing expert Bob Flexner doesn’t have an answer…¦and neither do any of the several world-renowned wood technologists he’s asked (though apparently, Bob has a scientist in Switzerland looking into it).

Anecdotal evidence points to stress. The explanation goes like this: In the winter, when snow is piled up on tree limbs, they’re bent down under heavy pressure. Or in windy forests, gusts stress limbs in a constant direction. These areas of stress change the grain pattern, and the irregular grain pattern is where the blotching occurs. Uh huh. This apparently has yet to be scientifically proven. Black walnut (another diffuse-porous wood) doesn’t blotch…¦or when it does, it’s good-looking blotch. Black walnut’s natural range includes western Vermont. I’m pretty sure it snows there.

I still think it’s elves (the fellow pictured above is named Eugene)…¦but I’m willing to entertain other explanations, should you care to comment below.  

– Megan Fitzpatrick


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Showing 7 comments
  • Carl Stammerjohn

    My understanding is that woods with relatively high resin content (pine, poplar, and cherry — ever wondered why hardwoods poplar and cherry are prone to burning when cut on the table saw?) are more likely to blotch because the unequally distributed resin causes uneven absorption.

    As for the maple, my guess is that it’s so dense that even small variations in grain direction make for large variations in finish absorption.

  • Megan

    Hi Justin,
    I’ve found several references that say Walnut straddles the border…it was a poor choice of examples, and I apologize. I should have selected buckeye, as I do, after all, live the buckeye state, and it snows here, too (though not as much as in Vermont). That’s an interesting theory on grain direction changes combined with moisture uptake causing the blotching.
    Megan

  • Justin Tyson

    Just thought I would clarify that black walnut is a semi-ring-porous, not a diffuse-porous wood. It is also my belief that blotchiness in cherry and maple is caused by variations in grain direction, and the extreme differences in moisture absorption between the long grain and short grain. Consider a dovetailed black walnut box. The end grain is not significantly darker than the face grain, because even though the end grain has absorbed more moisture, it has not darkened significantly. However, in a dovetailed cherry or maple box, the dovetails stand out due to fact that the end grain has absorbed a lot more moisture, and it has darkened the wood significantly. When you see blotchiness during finishing, you’re basically seeing a less severe example of that.

  • Chris C.

    Obviously this blotching is caused by the wood gods. These
    are the same folks who cause you to "accidentally" leave
    a piece 1/2" short. You know you measured it three times.
    They are punishing you for past transgressions. Like
    that time you skipped over sanding with 150 grit and went
    right to 220. Didn’t you hear that low, soft rumble of
    thunder off in the distance?

    chris

  • John Cashman

    I still have not seen a definitive explanation for what causes curly or birds-eye maple, which is surprising considering the number of really smart people who have studied wood over many centuries. I have personally felt that "blotch" was just a very weak type of figure — curl or quilting, and are all related. But that’s just a guess. Someday we’ll be genetically engineering 24" wide fiddleback maple, so the question will be moot.

    And it’s the Elf and Gnome footprints that cause blotching. They walk the length of trees, not across. That’s why end grain doesn’t blotch. Duh.

  • Mattias in Durham, NC

    My vote is to hold the gnomes and elves equally responsible. Or maybe gnomes can be responsible for trees felled between October and March, and elves for those felled between April and August?

    Anyway, the new issue sounds like it will be packed with interesting and useful stuff, as usual. But please don’t tell me when to expect the next issue – you know I will be skipping to the mailbox every day through the end of November, and imagine the disappointment every time it’s not there!

  • Wilbur Pan

    Eugene looks more like a gnome than an elf, and I would bet that blotches are more likely the fault of gnomes. You know how they are. :@)

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