Surprising as it may seem, when someone purchases kiln-dried hardwood lumber, there is not a standard or rule that defines what the term "kiln-dried" actually means. There is no specification of the average moisture content of the lumber and no specification of the variation of moisture content around an average. Further, there is no specification concerning the quality of the kiln-dried lumber, other than the general specifications in the grading rules, which ignore most drying quality factors.
The bottom line is that, as a purchaser of KD lumber, you need to specify the drying quality that you need. As a consultant in this wood manufacturing industry, I would estimate that 75 percent of the calls I receive are related to incorrect MC in the lumber. Many yield issues in plants are related to poor drying quality.
Do you want to learn a little more about drying? The best text on the detailed drying procedures is Drying Hardwood Lumber, which is available free from the U.S. Forest Service's Wood Education & Resource Center, 301 Hardwood Lane, Princeton, WV 24740, phone 304/487-1510.
The correct MC
The best kiln-dried MC for lumber is identical to the moisture that the lumber will have in service. If there is not a change in MC, then there is no shrinking or swelling of the wood.
In North America, the in-service moisture content for wood products ranges from a low of 6 percent MC in the wintertime to 8 or 9 percent MC in the summertime. This is equivalent to 30 percent relative humidity in the wintertime and 45 to 50 percent RH in the summertime. Certainly, it does get a bit drier in some heated locations when it is really cold outside in the winter and it might get more humid in coastal locations. The desert Southwest United States certainly is drier in the summertime, too.
In general, if the wood product is going to be cycling between 6 and 9 percent MC, it is easier to tolerate a slight expansion than it is to tolerate shrinkage. Shrinkage often results in cracks and splits. Therefore, the correct moisture specification for most products most of the time is 6.0 to 7.0 percent MC. Note that any finish used only slows down the MC change and does not prevent it.
If you asked your lumber supplier to give you 100 percent of the piece of lumber at a moisture content between 6.0 and 7.0 percent MC, he or she would laugh. It is impossible. Wood is too variable, and the cost of smoothing out this variability is too high. So, as a workable compromise, we would like for 90 to 95 percent of the pieces of lumber between these 6.0 percent to 7.0 percent limits. Certainly, if the product you are making is not sensitive to moisture changes, these specifications are too tight.
Note: We generally will specify the method used to measure the moisture, as different techniques and equipment will get different answers at times. Check the article I wrote in the February 2005 issue of CabinetMaker for more details.
Second note: If you dry lumber and make cabinets or furniture in a humid, off-shore location, there is no way that you can provide the customer with as high a quality product, moisture-wise and stability-wise, as we can provide here in America. Don't forget to tell your customer this important good news! If you don't, who will?
Other quality issues
Some of the other quality concerns that we have when purchasing KD lumber include
- amount of warp
- amount of surface checking and internal checking
- amount of end checks and splits
- color and discoloration
- amount of drying stress
The hardwood grading rules are supposed to deduct for warp that will affect the yield of clear cuttings. However, in common practice, many graders will overlook a little bit of warp, especially side bend and cup. (With side bend, the rules require all the clear cuttings to be parallel to each other; they cannot curl around with the lumber. With cup, the piece is supposed to still have enough wood so it could be planed fully to standard thickness without skip.)
So, what really happens is the person kiln-drying the lumber does not take a loss, but the person using the lumber certainly does. My suggestion is that you return any pieces that you believe do not meet the warp limits in the grading rules. If you use them, you may be paying 30 percent too much for your lumber. With lumber costs often being 60 to 70 percent of the cabinet value, we cannot afford this higher cost and still beat the competition. Our customers are certainly very picky; as users of lumber, we have to learn to be equally picky.
Checking and splits
If certain species, such as oak and hickory, are dried too quickly very early in drying, they will develop small cracks on the surface that are called checks. These checks will never heal, but they may be tightly closed until the wood gets to the finishing room or sometimes until it gets to the customer. Always check incoming lumber for the presence of surface checks by cutting a few surface check samples. Basically, we cut a piece that is full thickness and full width and about 1 inch along the grain from a flatsawn piece of lumber. Then we saw a very thin wafer of the face of this piece. If there are checks, then this wafer will end up in two or more pieces and not one complete, check-free piece.
Internal checking is hard to detect until the lumber is planed and/or cut into smaller pieces. If you do discover checking at this point, you should be entitled to a refund for the defective pieces. Do not let your supplier try to fool you by saying you had to discover such a defect within 10 days, or similar; that is not true unless specifically written into the purchase contract.
End checks and splits do not automatically drop the grade of lumber. If there is still enough clear area in the lumber, then the grade is maintained. Yet, these defects do cause yield loss. With end coatings available today for preventing most end checking, you should insist on perfect ends. Your competition probably already does.
Do you need a certain color? Can you accept sticker shadow (not a grading defect usually) or sticker stain? Can you accept a "white wood" species that is actually a little dingy or pink colored? If the color is not acceptable, make sure your contract allows you to return the shipment intact or at least return the problem pieces.
Drying stress causes a piece of wood to warp immediately when you machine it. If you need straight, flat pieces after you rip, mold, plane or otherwise machine, you need to specify that the lumber will be free of drying stress (or free of casehardening same thing).
Check your own operation. Determine if you are losing any yield or incurring any additional expense due to incorrect MC or other drying defects. Indeed, much of the lumber today is dried perfectly. But having a few specifications in a purchase order would be good insurance.
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