Here are 10 wood technology factoids every woodworker should learn.

1. In my 45+ years working with wood flooring, at least three out of four problems or issues have moisture content as an important factor.

2. Moisture content is the only reason why wood products move in use; that is, shrink, swell, warp, or make other size/shape changes.

3. VERY ROUGH Rule of Thumb: 1% size change across the grain (width and thickness of lumber requires a 4% MC change (about 20% relative humidity change). 
There is no change in length except for special, unusual cases (tension wood, juvenile wood).

4. Be very skeptical of people who talk about MC without using a decimal point. For example: Saying “6 to 9% MC” actually means 5.5% to 9.5% MC, which is usually too wide a span. If you mean 6.0% MC to 9.0% MC, then be precise.

5. The ideal MC for woodworking in almost all cases (because the humidity in a home or office on average is between 30% RH to 50% RH) is 6.8% MC with maximum variation from piece to piece of plus or minus 0.9% MC. (Moisture variation is a Standard Deviation = 0.3 or smaller.)

6. If wood is drying and shrinking, but then is exposed to higher humidity, it swells, but there is about a 5% RH gain before any MC change and size change occurs. 
There is a similar dead zone when the humidity drops after a gain. Also, this swelling is less than expected due to hysteresis. Overall, this is why the 6.8% MC target will produce more stable wood when in service.

7. Many moisture meter MC checks are incorrect because the operator did not follow the proper procedures. When properly operated, the MC values are within 0.5% MC of the true reading. Thickness and correct species’ density are extremely important for precise MC readings with a pin-less meter, as well as having air underneath the sample. The pin meter needs a temperature correction.

8. All warp is fundamentally caused by the grain angle variations in the tree, by the way the sawmill sawed the lumber, or by the way the manufacturing plant cut the lumber (with one uncommon exception called casehardening). 
Even with these “errors” in the lumber before we begin processing, it still takes a MC change to cause a size change.

9. It is virtually impossible to create NEW checks in dry wood in use. Any checks seen are old checks that have re-opened.

10. The creation of NEW fine dust (called frass) and 1/16” holes in dry wood indicate lyctid powderpost beetle activity. To establish how woodworking got infected with lytic powderpost beetles, you should ask yourself “Where was the lumber 10 to 12 months prior to the first evidence of the PPB ?”

Q: From time to time, but not very often, we run into small white areas that do not stain (or finish) very well when working with light colored woods. Often they are smaller than a quarter. However, this non-stained white appearance looks pretty bad. Can you give me any insight?

A: Indeed, we do see this behavior in wood on occasion. I am sure that there are a number of reasons why this impermeable wood is created by the tree. Here are a few.

One reason is that spring comes very early and the tree begins to put in new wood cells right under the bark. Then winter returns and so growth stops. Then when the “normal” springtime comes, normal wood is created. The wood that was created during the early spring is abnormal in the sense that it doesn’t conduct fluids. Sometimes this abnormal wood is called a “false growth ring. “
Trees, unlike humans, do not have antibodies to fight off fungi, bacteria, or other infections. 

As a result, so the tree doesn’t die from such infections, many species of trees create a barrier or encapsulate the infected area, so that the infection cannot spread. 

This encapsulated area is typically quite impermeable and can be rather small or extend to a much larger area. 

In any case this impermeable wood looks pretty much like regular sapwood in that area, but the impermeable nature is certainly unique. 

Often these injured areas will form on the inside of the encapsulated region. These encapsulated areas account for brown streaks that we often see in hard maple.

Some species, especially southern pine, have permeable and impermeable wood within the same annual growth ring. 

The tree creates more permeable wood when it first starts growing in the spring; and as summer comes, the cells are created to become more dense and more impermeable. Finishing and uniform staining can indeed be an issue.

Bottom line: These impermeable areas as you indicated, do not stain or finish very well. In addition, gluing and machining can be issues. As far as I know, there is little that we can do to compensate for these tree growth “defects.”

Q: Can you comment about damaged trees in bad weather?

A: Pine trees have a root system like a carrot. In a strong wind, yard trees can fall after suffering some decay. This is mainly an issue with yard, open-grown trees versus closely spaced forest trees. 

Forest trees with other trees around will fall into each other in a strong wind, and the break tends to be a long split. 
And, there goes any commercial sawing value, not to mention, the tangle makes logging with conventional equipment almost impossible. 

Certainly forest land owners with mature pines need to consider the risks of letting trees mature and get larger versus the potential weather damage. 

Without decay, a long split can develop in very high winds. 

Lightning strikes can also cause the stem to explode due to instant steam created by the heat. 

In warm weather, any saw logs need prompt harvest to avoid blue stain.

Leaf trees, called hardwoods, have a root ball at and near the surface instead of the deep carrot-like root. 

Witt a lot of rain, the soil loses consistency, so a strong wind can actually cause the tree to fall and create a large hole from the root ball. 

Yard trees seem more susceptible to this damage, but it happens in the forest as well, especially after a partial harvest. 

These root balls and their holes make transportation of logs out of the woods tricky indeed, and expensive. 

Often prompt harvest after blowdown is required to preserve lumber value.

It is natural for most trees to lose the lower branches as the lower branches do not get enough sunlight. 

So, the tree grows upward as needed to get sun, and the lower branches die and drop, encouraged by wind. 

Branches at 90 degrees to the stem are stronger than branches at a small angle.

A typical yard tree has a 30-year life, but it varies by species.