Q: I have a defect that we see from time to time in our white pine and even more in red pine. Photo included. What is it caused by?

A: This defect is sometimes called shelling and sometimes raised grain. It is most common in flat sawn pine. When seen in other species, including both hardwoods and softwoods, it is likely related to a bacterial infection that weakened the connection between adjacent wood cells.

Have you ever watched an artisan make woven wooden baskets from ash? How do they get those thin stripes of ash wood? The answer is that they actually pound the daylights out of the wood squares of billets. This pounding separates the wood into one-year thick strips.

In pine, shelling is most common when the wood is over-dried, meaning under 9.0% MC (check the MC with a meter frequently, especially on defective pieces), AND when the knives of the planter or other tool pound the wood excessively.

Basically, the drier the wood, the easier it is for the “normal” pounding of the knives to fracture the wood, causing shelling. Of course, improper knife angles, exacerbated (For 40 years with FDMC, I always wanted to use this word in a Q&A) by slow feed rates and dull knives, increase the risk of this defect. 

Chances are that if your planer machines oak and maple well, the knife angles are not the best for pine.

Q: We are molding some softwood and we have very poor corner (where the edge meets the face) quality; that is, the edge has small splinters that have broken off. Why is this happening?

A: When a knife (or blade or sawtooth) begins to cut into wood, it has four options:

#1. Cutting the wood fibers cleanly (usually what will give us the best surface, with many fibers being torn);
#2. Pushing the fibers out of the way (which occurs mainly on the edge corners, especially wood that has an over-dried surface or dull knives or large cuts per knife, etc.); 
#3. Pushing the wood fibers into the surface (fuzzy or raised grain later); or,
#4. With a deep cut or large cut especially from a high feed rate, the knife becomes somewhat buried in the wood. As the knife exits, it cracks the wood rather than cuts it (a crack along the grain is lower energy than rapidly cutting). The result is that the knife  takes out a large chip that follows the grain rather than cutting the wood. This defect is also more likely with a slender knife, which means a large hook or rake angle.

The option requiring the least energy wins. In general, 10% - 12% MC helps achieve the first option; under 9% MC increases the risk of the second option; and over 15% MC favors the third option. You are experiencing the second option. (For hardwoods, 6.0% to 7.5% MC is ideal, with over 8.5% MC likely to result in fuzzing and under 5.5% MC chipped grain and splitting. The species being machined also has a huge effect which can result in option 4 Slower feed rates mean better cutting action.)

As moisture content is a big factor, let’s talk a minute about practical ways of measuring moisture. It is really important to think about the MC exactly where the knife, blade or sawtooth is working. A softwood piece that has 7% surface MC and 13% core MC will average 10% MC, a good MC for softwoods. But the knives are working with 7% MC wood, which is not good. So, when drying wood, you will have much better machinability if the kiln is controlled to avoid low humidities that will over dry the surfaces and ends of a piece of lumber.  Once over dry, restoring some moisture back does not restore machinability. A softwood kiln should have a lower limit of 9.0% EMC; a hardwood kiln 5.5% or 5.0% EMC. Never go drier than these limits for best machining.

Q: What is the “oven-dry” moisture content? Is it the same as the moisture measured with an electronic pin meter?

A: The moisture content (MC) of lumber and most solid wood is measured by a technique established a century ago. It is a measure of the weight of moisture in wood. It is almost always given as a percentage.

In wood, the MC is related to relative humidity (RH), not temperature. In brief, 30% RH is 6% MC; 50% RH is 9% MC; and 65% RH is 12% MC.

OVEN-DRY. There is a standard for the oven-dry measurement technique.  

Step 1. Weigh the piece of wood that you need to know the moisture content of. This weight is called the “current weight,” “present weight,” or “green weight” (GWT). It does not matter if we use grams (most common), or ounces or pounds. When weighing a 100 gram (about 1/4 pound) piece of wood, we should have two digits to the right of the decimal point in order to get the MC reading accurate to 1/10th percent MC. 

Step 2. Put the wood in an oven heated to 215 degrees F (plus or minus 2 F).  Leave the wood in the oven at 215 F until the wood stops losing weight. This can take 24 hours or so with larger pieces. Weigh the wood and then put it back in the oven and weigh it again an hour later to check for constant weight. When weight loss stops, then the piece is defined as being “oven-dried.” The weight is called the oven-dry weight (ODWT).

Step 3.  Calculate the MC. %MC = [(GWT - ODWT) - 1] x 100

For accuracy, it is best to use the same balance scale to weigh the wood both before and after oven-drying.

HINT 1: The most common error with this test is that the pieces of wood do not reach full dryness. As a result, the calculated MC will be lower than the true MC. The best ovens have a fan with air circulation and do not have too much wood inside at one time.

HINT 2. The oven-dry test can be accelerated by using a microwave oven set on medium low power. A maximum of two or three pieces (spread out) can be dried at one time. The oven must be watched carefully to avoid smoking the samples. A carousel tray is essential. Repeat weighing every few minutes until weight loss stops.

Hint 3. Because the oven-dry test is both a time consuming and a destructive test, most wood manufacturers use electric moisture meters. These meters measure an electrical property that has been related to MC. At the low MCs we use for furniture, cabinets, flooring and so on, these meters, when used properly, will be within 1% MC of the oven-dry MC. Proper usage is essential.

Q: Can I mix air-dried red oak, white oak and hickory in the same kiln load?

A: You can safely mix these species, and even thicknesses, if they are at the same starting moisture content when they are loaded into the kiln- -in this case, all are air-dried prior to going into the kiln. The kiln is run based on the wettest and the slowest drying species and thickness. In this case, white oak is likely the wettest and slowest drying. The other species are “along for a free ride.”

This technique is best when the incoming MC is 30% MC or lower.

To avoid over-drying the faster drying pieces (bad machining and maybe more warp and some gluing issues), it is essential to prevent the wood from getting too dry. In most cases, “too dry” means under 5.5% MC. Therefore, to avoid over-drying, the kiln is never allowed to have an interior humidity condition below 5.5% EMC or 5.0% EMC for hardwoods and 9.0% EMC for softwoods.). Kiln operators will understand this terminology and how to run the kiln. Obviously, proper kiln operating techniques must be used.

Unfortunately, once the wood is over-dried, its desirable properties will not return by adding moisture.

For information on mixing other species, see DRYING HARDWOOD LUMBER, General Technical Report FPL-GTR-118 (on-line only), pages 81-84.