Seven tips for kiln drying oak plus more FAQs
July 21, 2022 | 1:50 pm CDT
doctor-gene-wengertsq_0.jpg

Question: We have been tasked to kiln dry mixed red and white oak 1-inch lumber from approximately 75 percent down to 6 to 9 percent moisture content. We need to drop this lumber at 2.4 percent or better moisture loss per day. Can you give me some advice?

Answer: Let me present some basics that will help you.

#1. Freshly sawn red oak is considered to be 75% MC, but some pieces will be 90% MC or even higher. White oak runs about 65% MC, with some considerably higher. You need to consider these wetter pieces, as it is the wetter pieces that determine drying time. That is, when the load will achieve the final MC limits.

#2. The final moisture content is also unusual, as many people want 6.8% MC with plus or minus 1% MC variation, but no greater wet or dry pieces.

#3. The maximum drying daily moisture loss rate for 4/4 red oak (with rings spaced less than ¼ inch apart) is 3.8% MC loss per day. Of course, thicker is slower. With this maximum, the average speed will be 3.0% MC loss per day. This will be achieved with a good kiln, with quality sawn lumber, and a good kiln schedule. For white oak, it would be 2.5% MC loss per day maximum, with an average close to 2.0% per day.

#4. These drying rates are applied most carefully at higher MCs, as that is when all drying defects, except cup, are initiated. In other words, above 45% MC is the time we control quality. At MCs under 15% MC, drying will slow below this maximum as moisture at low MCs does not move very fast, even with higher temperatures (160 F) and lower humidities (5.0% EMC).

#5. A standard dry kiln should be able to make about $25 per day per MBF. This means, a 35 to 40-day cycle, such as you might end up with, should make about $1,000 per MBF. Are you getting that much income or value for drying in the kiln? Maybe for higher grades on a good day, but not for the lower grades. I expect you will only make half of this amount, gross, and after expenses, be close to a non-profit operation. Stated another way, you could dry three loads of air-dried, upper-grade oak in the same time and make, in today’s market as much as $2,000 gross profit.

#6.  If you decide to do this anyway, use the appropriate kiln schedule in “Drying Hardwood Lumber.” In the last twenty years, slight modifications have been made to these schedules. First, start at 105F instead of 110 F. Second, never let the kiln get drier than 5.0% EMC, even if the kiln wants drier. These two changes improve quality.

#7. The NHLA has five drying seminars (and several others about yield and gluing) that were each given as two-hour webinars. They can be viewed for free at nhla.com/education.

Q: A local landscape operation wants to buy my sawdust for their use as mulch or compost. Anything I need to know?

A: Contact your local county extension office or public library for a report on how to use wood for landscape mulch or compost. Wood is actually a great way to amend the soil, especially for clay soils, sandy soils, or other less-than-perfect soils. Here are a few key points so you will be conversant with using wood in landscaping.

MULCH. Mulch is basically a weed inhibitor that blocks sunlight to the weed sprouts, and that also slows water evaporation from the soil. Wood has a few trace elements that can be helpful to the growth of some plants, but wood does not contain nitrogen. However, the wood mulch needs nitrogen to decay. Without enough nitrogen, plant leaves turn yellow, and growth can stunt. On the other hand, in a few years, as the mulch decays, it slowly releases the nitrogen it had sequestered, making plants happy again, until a new layer of mulch is put on top. To offset the “yellows,” add extra 30-0-0 fertilizer to the new mulch.

Because small particles of wood form a matt, decay quickly (a waterproof coating will tie-up nitrogen efficiently, and can easily be blown away), wood mulch is typically going to use larger pieces of wood and bark. A creative option for mulch from partly dried, or fully dried, wood is to add a coloring agent so the mulch is green, brown, red and other colors. Wood mulch is much better than stones, as wood does not nick and damage lawn mower blades.

COMPOST. Small pieces of wood, including sawdust, can be decayed before applying them to the soil. Wood compost will have a nice supply of nitrogen, plus wood compost will improve the tilth of the soil. The lack of enough nitrogen will slow decomposition, so adding some nitrogen (33-0-0 fertilizer) is beneficial. Further, lack of oxygen results in anaerobic (no air) decomposition, which means acid compost that will hurt many plants, so stirring the compost pile frequently (daily) will help to speed composting and improve quality.

Also, moisture is necessary for composting; this is really important when using dry wood waste. On the other hand, if compost is too wet, this will limit oxygen and increase undesired acidity. Finally, the more surface area per volume of wood chip is important; small particles go faster than chunks. It is common to mix some soil and even green leaves and grass with the wood to enhance compost formation.

Of course, some species of wood (for example, white oak, cedar, locust) have natural chemicals that inhibit composting. Be aware that some plastic bags that hold composted wood deteriorate quickly due to internal heating and light; bulk loads are easier to handle.

Q: We have some hard maple with what our grader calls mineral. In our finishing room, we are noticing that the finishing is cracking these spots which often makes a serious defect. What can we do?

A: The so-called mineral streaks are developed in the tree. Often there are fungi and bacteria in this defect. The wood in this streak is closed off from the rest of the “normal” wood. It is common that they have a small crack in them, as they dry and shrink prior to the wood around them shrinking. However, as the wood around the streak subsequently dries and shrinks, it pulls the crack closed so that it is usually invisible during manufacturing until the finishing room. The heat and low MC cause this streak to shrink, and that reopens the crack.

The bottom line is that this wood is prone to develop cracks, and there is little that you can do.

Q: We are considering investing in an electric dehumidification dry kiln.  Can this dry lumber okay?

A: Your question is very general without information on species and thickness, and no indication of air drying prior to the kiln.  So, it is hard to provide a specific answer for your specific company.

There are many types of DH kilns.  Some go only to 115 F, others can easily reach 160 F.  Some have less than 1 hp (horsepower) per 1000 BF capacity; others have 2hp per 100 BF.  Larger power is better for faster drying species and white colored woods,.  Some have very poor control systems. Like all drying systems, the skill of the operator is critically important as well. 

In summary, a DH kiln, properly designed and operated, can dry any species.  Like steam kilns, DH kilns are more economical to operate with properly air dried lumber for all species, except whiter woods which need kiln drying green-from-the-saw.

Compared to steam kilns, smaller sized drying operations (under 1 million BF annually) are often more economical with DH than with steam kilns.

Q: When fastening hinges for a door, which is less likely to pull out, doubling the screw length or doubling the diameter?

A: This applies for a screw threaded its entire length and when screwing into solid wood, not MDF or particleboard.  In general, the withdrawal strength doubles if the screw is twice as long or is twice the diameter.

A key to maximizing strength is to use a pilot hole.  Without a pilot hole, the screw will act like a wedge and create a small split or two radiating out from the hole.  This splitting significantly reduces the strength.  Splits can be larger when near the edge of a piece, so pilot holes become more important in this case. What we want is for the threads to cut into the sides of the pilot hole.  For softwoods, the hole should be 70 percent of the root diameter of the screw.  The root diameter is the diameter of the screw shaft not counting the threads.  For hardwoods, 90 percent is optimal.  Pilot holes help considerably when screwing into end grain as well.

Have something to say? Share your thoughts with us in the comments below.

Profile picture for user genewengert
About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.