Q I have seen your articles where you say that heating the air drops the humidity. Can you tell me what is going on and what this means?

A Great question. Unfortunately, the answer requires some technical explanation, so I will use examples to try to help.
Let me make sure that you know I am not a chemist. So, this drop in RELATIVE humidity as air is heated has something to do with hydrogen bonding of the water molecules, and something about the rapid movement of molecules when heated. The simple explanation is the hotter the air, the more moisture air can hold per cubic foot.

Simple example
A cubic foot of air at 40F and 100% RELATIVE humidity (foggy) will contain 3 grains of moisture (a grain is 1/7000 pound); a very small amount indeed (for comparison, a $1 bill weighs 15 grains). The weight of water per volume is called the ABSOLUTE humidity. The amount of moisture in the cubic foot will stay the same when the air is heated; that is, no water is added or subtracted from the cubic foot.

So, when this air is heated to 70F, we still have 3 grains per cubic foot of moisture. However, at 100% RELATIVE humidity, air at 70F will have an ABSOLUTE humidity or will hold 8 grains per cubic foot. 

RELATIVE humidity is the actual ABSOLUTE humidity compared to the maximum ABSOLUTE humidity that the air can hold at a given temperature. So, 3 grains per cubic foot divided by 8 grains per cubic foot times 100 (to get percentage) is 38% RELATIVE humidity.

EMC
Of course, moisture in the air can be expressed by a more useful number (sometimes) —  the EMC, Equilibrium Moisture Content. Note: It is incorrect to say that wood has an EMC; EMC refers to moisture in the air. 

The EMC is defined as the moisture content that wood will attain at a given humidity and temperature, when first losing moisture with a thin piece of wood (hand-plane shaving) and waiting at the constant humidity and temperature until weight loss stops.

As a rough estimate, at 99.9% RH, wood will dry to an estimated 30% MC, 28% MC, or as low as 22% MC, depending on species. It is common to see 30% MC used, which means in the example, the air’s EM at 100% RH and 70F is 30% EMC. Now for air at 100% RH and 40F that is heated to 70F and achieves 38% RELATIVE humidity, the air’s EMC is 7% EMC. The value is obtained from a table or chart (Drying Hardwood Lumber, page 19, Internet availability) and some calculation. 

Practical guidance
Appreciate that green wood at 30% MC will dry when exposed to air at 7% EMC. Low shrinking woods (we call them stable; low weight usually) may shrink only 3%, but a high shrinking wood like oak could shrink 6 to 8%.

Let’s say that we have outside air at 40F, and there is dew on the grass and car windshield (parked outside). We can assume the morning’s low temperature is at or close to 100% RH almost every morning. We heat this air as we bring the air into a warehouse or into our shop (or into a customer’s dwelling). We heat without using a humidifier to add moisture. Here is what will happen to the EMC.

Outside air at 40F

So, heating air with a morning’s low temperature of 40F is likely not an issue. Now, let’s bring in 30F, 100% RH into the warehouse, shop or customer’s facility.

Outside air at 30F

Conclusion: Overall, when the outside morning’s low temperature drops below freezing, we need to use less heat (good luck) or begin to humidify the air. Stated another way, 6.8% EMC is ideal for hardwoods; 9.0% EMC for softwoods. Being drier causes manufacturing issues with warp, gluing, machining, and finishing.

Q We are getting into some bending and need guidance. We have been told to dry the wood to 15% MC, then steam it, then bend it, and then dry it further. Does this seem correct? Can we use a dehumidifier kiln?

A Appreciate that bending is indeed an involved process affected by the wood’s grain angle, the pressure of the steam, and much more. I cannot cover all the details here. Here is a quick review.

First, 15% MC is appropriate for gentle bends, sometimes called pan bends. More severe bends need wood at 25% MC. At wood moistures over 28% MC, there will be liquid water inside the wood and when bending, you create hydraulic pressure that can damage the wood. Appreciate that when bending, the majority of the stress created is in the outer layers of the wood. So, when steaming before bending, you raise the MC in the outer layers, and heat them. (Heat and moisture make wood more bendable.)

After the wood has been bent, it must be held in this bent position while it is dried to its final, in-use MC (maybe 7% MC). Then, when dry and the restraint is removed, the wood will straighten a small amount, but then will stay bent and not try to straighten further; you could say that the wood lost its memory of being straight and flat. Of course, like all wood, changes in the humidity around the bent wood will cause some MC change and movement.

Drying initially from green to 15% MC is the same no matter what conventional hardware and conventional drying operation is used. The hard part is avoiding under or over drying. This avoidance is done by using a high humidity in the kiln, which can sometimes deteriorate the building or equipment inside.

Once bent, the type of properly running, subsequent drying  equipment is not important. But not all systems can reach all required drying conditions. A kiln designed to dry pine 2x4s at 230F cannot dry oak at 110F with the controlled humidity required. In your case, the dehumidifier can dry as you require if the control system is appropriate and run correctly. In the “old days,” a heated room with venting was often used for final drying.

Q How do I measure both the shell and core MC of lumber?

A You can use the shell and corer test using the oven technique. This is in every drying book. So, I believe your question is asking about electronic moisture meters. 

We have two types of moisture meters: pin and pinless. 

It is important to realize that the meters do not actually measure MC, but measure an electrical property that is then related to the actual MC. Meter MCs are estimates.

The pinless meter does a quick and accurate reading, and leaves no holes behind. 

However, it measures the AVERAGE MC of a piece. 

Such meters can include species and temperature corrections along with memories and other features. 

No question that these are really the top choice when many readings will be taken of the AVERAGE MC.

The pin meter measures different MCs depending on the needles used. Pin meters, like pinless have many worthwhile features.
The wettest spot along shiny, uninsulated needles is what these provide. 

Obviously, on a foggy day with more temporary surface MC, the readings are likely higher than they would be on a drier day. 
The reading is NOT the AVERAGE MC, but the wettest spot.

With insulated needles, only the tips are exposed to the wood, so that is where MC is being measured. 

For example, an insulated needle driven into the wood 1/16-inch deep is measuring the SHELL MC. As the machining is done in the shell, the shell MC is helpful in analyzing machining defects. 

If the needles are driven about 1/4 of the thickness of the lumber (example: with 8/4, driven 1/2-inch deep), you obtain an estimate of the AVERAGE MC. 

If the insulated needles are driven half the thickness, you will obtain the CORE MC. 

The core MC is quite often the MC when the lumber left the kiln; core MC is slow to change.
Hopefully, this provides the basic info that you need to choose the correct meter for your operation.

Q  I am new to working with red oak.  Sometimes, there seems to be little red color at all.  Please open my eyes.
A There are two groups of oak lumber, red and white.  There are about twenty commercial individual p species that make up each group.  

Although we can identify individual species if we see the acorn or leaf, once the log is produced. Identification of the species becomes almost possible.  It is easy to separate logs into red or white.

In my experience,  northern red oak, cherrybark red oak and Southern rd oak trees produce the reddish lumber.  Of course, every oak log has an inch to three or four inches of white sapwood.  

It s rare to eliminate the sapwood when sawing lumber, except export grades, which get special prices.
Bottom line: Find a sawmill that uses mainly a reddish oak species.