Issues with drying, steaming wood
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Q. We have been asked to kiln-dry some cherry and some maple. The Dry Kiln Operator’s Manual for the US Forest Products Lab suggests starting at 130 F. Likewise for their kiln schedule book. This seems awfully hot. This seems really high if we want white color in maple. Guidance please.

A. The basic kiln schedules for hardwoods were formulated in 1950 by the US Forest Products Lab, relying in part, on industry experience. Back then, the only cherry kiln dried was from large trees and was upper grade lumber. Therefore, the 1950 schedule reflected that high quality. Further, most dry kilns back then could not get to 130 F quickly; many kilns had heating systems that were low-pressure steam pipes without fins, so, heating time would be a day or so. 

Plus, almost 100% of the lumber was air dried before kiln drying. In addition, the white color requirement for maple was often not important then, as the maple would be stained dark or painted.

Today, we have more powerful kilns; we are drying No.2 and 3A, which means more defect-prone lumber; and maple is supposed to be white. So, we need to factor the resource and equipment changes into a modern kiln schedule.  

We need to start 10F or 15F cooler than “in the book.” We need to avoid temperatures over 160F in the main schedule. (Steam has a temperature of at least 212 F, so we do not inject live steam into a kiln until the final conditioning step.) 
We never let the kiln get drier than 5.0% or maybe 5.5% EMC (meaning no low humidities), which results in more uniform final moisture and “softer” lumber when machining as there has been no over-drying.
Plus, it means no equalization is required, saving a day perhaps. If this is confusing, send me the species and thickness and I will send you a schedule printout.

Q. Every so often, I hear about a person who is steaming freshly cut wood before drying. Can you comment about this?

A. When we heat wood, several things happen. When the wood is wet, the temperature rises to the wet-bulb temperature (temperature indicated by a moistened thermometer bulb), not the dry-bulb. 

However, in steam, the wet-bulb temperature is very close to 212 F, especially with low-pressure, wet steaming. This is 100% relative humidity, which means that when steaming, the wood does not dry. As you might imagine, hot air (over 120 F for oak) not at 100% RH can damage most hardwood lumber causing mainly severe cracking and color change.

The heat when steaming, or even with hot air heating, causes water in the wood to expand. But, there is also a lot of air inside wood when first harvested, as the wood substance is 1.5 times heavier than water, and logs would sink in water if there was not a lot of air. The air expands, or tries to expand, substantially. This expansion of water and air forces liquid water (we call it sap):out of the wood, perhaps 20% MC loss, if the wood is permeable enough. If the wood is not permeable enough, then the pressure internally will rise, rupturing some of the cellular structure. 

We can also actually see that the heat over 160 F begins to weaken the wood permanently. Hotter temperatures weaken wood more quickly.

Using heated or steamed wood for legs or other structural uses might be questionable.

The heat also results in the wood cell walls losing moisture and shrinking. This water goes into the air spaces. In fact, believe it or not, a piece of oak in boiling water will shrink 2 to 3 %.

Although cooling restores some of this heating change, there is permanent change, including the relationship between relative humidity and moisture content. Steamed wood is more stable.

One other effect is that with the movement of liquid water quickly to the wood’s surface, chemicals like the sap are also brought to the surface. When the water evaporates quickly, if not at 100% RH, these chemicals are left behind. In many cases, this results in a dark color near the surface of the wood, while underneath, the wood is lighter in color. This color change due to the movement of chemicals is beneficial in woods like walnut and cherry, and even oak, where the white sapwood is at the surface.

Another effect of steaming is that the growth stresses (stresses in the tree that cause immediate warp when sawing as well as subsequent warp when drying) are partially reduced.

Finally, the wood itself begins to char after a few months, as the heat causes this oxidation damage. So, steamed wood is noticeably darker in color. The heat also deactivates any enzymes. The heat also affects the sugars in the wood, causing some chemical changes, just like heating sugar on the stove.

I probably have not included all the effects. However, you should get the idea that steaming or heating wet wood over 200 F is a complex event, depending on the wood species. Some effects are positive, and some could be quite concerning.

Although steaming of sweet gum, walnut, beech and cherry have been practiced for almost a century, be aware that some patents have been issued in the past 20 years that may inhibit steaming or heating some wood species.

Q.   When we were processing yellow-poplar, we ran across a few pieces that were exceptionally white.  We pulled them out.  Any idea what they might be?

A. Congratulations on having an employee that was sharp enough to notice this difference in wood.  Sharp eyes are indeed helpful. It is almost 100% that the white wood is cucumber wood.  This tree grows with yellow poplar and looks so similar in the woods that from time to time, a few logs end up with the yellow poplar. 


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.