Q: I have run into several people who are claiming that the problems we see in manufacturing are due, in part, to juvenile wood. I hope you can tell me what this wood is and how it affects us.
A: Indeed, I'm also hearing more people use this term to explain a problem, but they usually don't know what they're talking about. To answer your question, I do have to get a bit technical, but I'll try to keep it interesting.
When a stem of a tree first starts to grow, this stem must be quite flexible compared to the stem at larger diameters. Therefore, for the first five to 10 years of growth (five to 10 annual rings) the tree creates a special type of wood called juvenile wood. For most hardwoods that grow about five to 10 rings per inch, this means that the juvenile core is under two inches, and often closer to one inch, in diameter.
When examining this juvenile wood under a microscope, it is seen that the wood fibers (or cells) are shorter than normal. This shortness results in weaker and more flexible wood than normal. (The term "normal" refers to more mature wood that's further than five to 10 rings from the center.)
It's rare to find juvenile wood in lumber we use for furniture, as the juvenile core will also contain a preponderance of knots; that is, lumber containing juvenile wood will be low-grade lumber. Stain or discoloration will also be likely in this wood right around the pith, making such wood a defect. (The pith is the exact center of the tree or the first year's growth.) As hardwood lumber grades are based on the lumber's clearness, knottiness would be considered a defect.
Pallets and flooring
In other words, any lumber containing juvenile wood would be low-grade and most often used for pallets or sometimes flooring. If such low-grade lumber were put into a furniture or cabinet plant, juvenile wood would be eliminated when the lumber was cut into clear, knot-free parts. Further, the pith itself is a dark streak and would not be included within pieces of lumber cut into furniture or cabinet parts, especially if the designation is for white maple.
It should also be appreciated that juvenile core wood is an extremely small part of the total tree. In a log that's 20 inches in diameter, the juvenile core, if 2 inches in diameter, would be less than 2 percent of the total wood volume.
What all this means is that, even if juvenile wood was included in commercial lumber, it would be rare to find juvenile wood in a load of furniture-grade lumber. It would be even rarer to find juvenile wood in parts of lumber used for furniture or cabinets. If juvenile wood did get into a furniture part, it would only be in one part out of tens of thousands.
Longitudinal (lengthwise) shrinkage of normal wood (shrinkage parallel to the grain) is generally quite small. Juvenile wood in softwoods, and sometimes in hardwoods, shrinks lengthwise more than normal wood. Even though the juvenile wood shrinkage is very small, any long lumber (12 feet long or longer) that has juvenile wood on one edge, but not the other, can develop non-uniform shrinkage (called crook or side bend) in drying of hardwoods, bending an inch or two over 12 feet.
This is important for lumber, but would not be an issue for hardwood parts, as they are so much shorter and the change in MC in parts is very small (4 percent MC perhaps) compared to the change in lumber's MC when it is dried (60 percent MC perhaps).
Juvenile wood can result in side bend in drying. As most juvenile wood in hardwoods is also associated with the presence of knots, it would be rare to find any juvenile wood in furniture or cabinets. Anyone who blames juvenile wood for their manufacturing problems is "barking up the wrong tree."
Q: We're having a problem with shrinkage. We make furniture, but someone else sells and delivers it. This person claims he did everything correctly, including opening the furniture wrapping (we had wrapped the furniture with shrink-wrap and it was fairly well sealed) and letting it acclimate to the house climate.
When the customers moved in, they said the furniture looked really wonderful, but within a week it started to warp, open joints and crack in a few places. We're so careful to keep our plant at 40 percent RH and check the MC of the lumber. This is frustrating. Can you help?
A: This seems to be an age-old problem for some people. Let's review the basics. First, wood doesn't shrink or swell unless its moisture content changes. You're already aware of this, apparently, from the information in your letter. Your plant is just a little bit more humid than I'd like; you're running at about 7.5 percent EMC. Can you lower it to 7.0 percent EMC or 35 percent RH? Or maybe even 30 percent RH and 6.0 percent EMC? In the wintertime, this will be just a bit more reasonable.
Your customer's home is oftentimes going to be 6.0 percent EMC (30 percent RH) in the wintertime, so running your plant a bit drier will make less of a change between your plant and the customer's home.
This plant RH change will not be very effective, however, if the main problem is that the incoming lumber is higher than 7.0 percent MC, on the average, with 95 percent of the pieces between 6.0 percent MC and 8.0 percent MC. How closely do you check the incoming lumber's MC? Remember, that in a kiln, there are perhaps 15,000 pieces of lumber and the operator has only checked 10 or 12 of them. It's indeed possible that your lumber is too wet at times for a very dry home.
I do like the fact that you're wrapping the furniture so that it can't change MC in transit. A polybag or shrink-wrap will prevent any significant MC changes, even though the outside conditions vary in temperature and RH.
I do wonder about what the salesperson is doing, however. If he's unwrapping the pieces before the people move in and the heat is not turned on (and maybe the paint, drywall taping and concrete are still drying), the furniture will actually be in a high RH situation and will pick up moisture. Then, when the home owners move in and the heat is turned on, the sudden, large drop in RH and MC will cause a lot of shrinkage, especially compared to a much slower MC change. Try to avoid large, fast MC changes if at all possible.
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