Solving stickers & color changes when drying wood, plus more FAQs
October 13, 2023 | 3:40 pm CDT

Q Three questions about drying wood. 1. We are drying western cedar, and we notice that wherever there was a sticker, we have a light colored streak, but between the stickers is the more normal reddish coloration. Stickers were very dry to avoid staining. We air dried the wood carefully in our Northwest climate. What has happened? 2. We had some black walnut, unstemmed and after drying, we had light marks where the stickers were located on the lumber. Help. 3. Same, but with red oak.

A The key to color development or color changes when drying is: “Differences in drying rate at high moistures will result in differences in color after drying.” This is especially true (that is, the RISK is higher) for almost all species of lumber sawn from logs that have been in the wood or log yard for several months or longer at temperatures above freezing before sawing.

Let’s apply this axiom to your particular situation, and these comments here apply to most all species. You stacked the lumber with very dry stickers. So, the wood in direct contact with the stickers dried quite rapidly at cool temperatures. This results in little oxidation of the color components in wood (starches and sugars mainly), so the wood in these locations develops a light color, or we could say that the expected color development does not happen. On the other hand, the wood between the stickers dried slowly, which allowed plenty of time for oxidation, developing darker colors. In brief, we have two different drying rates when comparing the rate under the sticker with the rate between the stickers.

The solution, assuming we want the dark colors is in these situations, is to use stickers at perhaps 12% MC initially. We can often achieve this higher MC — stickers stored in small packs outside with some air flow and exposure to outside humidity, but protected from wetting from rain.

Special note for oak drying: Very dry stickers can result in drying under the sticker that is so fast that surface checks and honeycomb can occur over and under the sticker.

Special for maple, ash and white wood drying: As we want the whitest color, we need to avoid slow drying by using very dry stickers and low relative humidities with good air flow in the kiln. Usually this also means that we must avoid any air drying, as the oxidation risk is highest in the first few days after sawing in warmer weather. Hence, in warm weather, we target having the white woods to be in the kiln and under correct humidity within hours after sawing, if possible, and if the whitest color is desired.

Q We normally dry hardwood lumber but are currently drying some 8/4 southern pine. We have some surface checks, but I thought that SYP did not check. We are only drying green from the saw (100% MC) at 130 F and a wet-bulb of 120 F.  This 73% RH would seem high enough to prevent checking, from what I read about pine. Can you help?

A Certainly, it would be best if I were on-site to make an inspection, but we do know that surface checking is caused by rapid drying of the exterior of the wood during the beginning days of drying. In other words, the wood on the exterior of a piece (sometimes called the shell) dries a lot faster than the interior wood (sometimes called the core). This shell-to-core moisture difference results in a shrinkage difference (the shell is shrinking while the core is not), which results in stress in the shell that can be large enough to crack the wetter wood. Dry wood under 20% MC is too strong to initiate new checks.

So, what causes wood to dry fast? The speed of drying results from the temperature, humidity and velocity of the air going over the lumber’s surface. From my experience, it is unlikely that your velocity in a hardwood kiln is too high. Further, it is unlikely that the temperature indication is wrong or off by more than a degree or two. So, the fast drying is caused by low humidity. (A few pieces in a load might check due to weakening of the wood by bacteria that are in the living tree. Usually, shake and/or a foul odor are indicators of bacteria.) So, something is wrong with your wet-bulb temperature reading. Maybe the wick is dry, maybe the bulb is too far from the water, maybe there is low air flow over the bulb, maybe the wick is contaminated from the sap exudation from SYP, maybe the bulb is in the wrong position, maybe the water is too hot, maybe the water reservoir is too large and blocks air, and so on. In short, your conditions in the kiln are much drier than the instrument indicates.

Note that when drying air-dried hardwoods, errors in the wet-bulb reading usually have a smaller effect on drying than when drying very wet wood. So, any errors would not have been noticed.

Q We have finally decided to predrill our screw holes to allow us to screw close to the end of a piece of wood. Please clarify the acceptable hole diameter. Thanks.

A A screw is labeled by its “thread diameter,” which means the threads are included in the diameter measurement. So, as review, a #4 screw is 7/64” diameter; #5, 8/64” or 1/8”; #6, 9/64; #8, 10/64” or 5/32”.

The following recommendation applies to holes in the face or side grain, but not end grain. When predrilling a hole, the hole size is determined by the screw’s root diameter, which is the stem diameter, not including the threads. Sometimes the thread diameter is called the major diameter and the root diameter is called the minor diameter.)

The USDA Wood Handbook recommends the hole diameter of about 70% of the root diameter in softwoods (needle trees), and about 90% in hardwoods (leaf trees). I do believe that for lower-density hardwoods, we should follow the softwood recommendation. There is no loss of strength when predrilling. In fact, because a split weakens the screw’s strength substantially, predrilling often offers a strong joint as well.

I could find no recommendation for end grain holes. We do know screws into end grain likely have 3/4 of the strength of a screw into side or face grain.

Q When we use a light colored paint finish on our doors, every so often we see, months later, a dingy look going through the finish.  Is this from the finish, the glue, the wood or what?

I suspect that it is some of the chemicals within the wood, some of which can be water soluble, moving out of the wood and up through the finish. 

As a quick test, soak a piece of the wood in hot water for a few days and see of the water discolors.  If so, then my suspicion increases.  Further analysis is required by a chemist; many finishing manufacturers have chemists that might help.  I am not a finishing expert, so I cannot advise you further.

Q I have to glue up a laminate involving 9 to 12 layers. When I spread the glue, I am worried that the first few layers may have set too long before I get the pressure on it. Should I be worried?

A It depends a little on your operation and how rapidly you assemble the layers. In general, we would prefer making about five layers at a time, and then after a day, gluing the two five layer pieces together to get ten layers.
I think it would be prudent to resurface lightly the two surfaces to be joined in the five-layer pieces to assure a strong joint. 

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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.