Case hardened if not loosing moisture over 18 months
About a year and half ago, I bought some eight quarter saw white oak (QSWO) to build Stump Nubs version of the hand saw bench with some hold fast holes to serve as a low/Roman workbench. I chose QSWO to jazz it up a bit as these benches will also likely get some use a seating for some of the family gathers. One likely to live in the shop and one likely in the hallway of the house for sitting when putting on shoes.
When I brought the supposedly kiln dried QSWO home, the moisture meter showed a lot of it was in the mid 30% moisture content range. I now bring my moisture meter with me when shopping for lumber. Rather than take it back, I cut it to oversized lengths and have left it to air dry. Most of the boards have equilibrated down to low teen moisture content which is what I normally get where I live. A few pieces however remain unchanged in moisture content and are still in the 30% range. Since most is at equilibrium MC, I’m a bit puzzled. Do you think those pieces had some case hardening issues? I will likely avoid them.
I elected not to return the wood because I didn’t want to go through the hassle and knew it was going to be a year or more before I got to this piece so I thought it would probably be dried out by the time I need it (and it is). From listening to Shannon’s Lumber Industry Update podcast (which is really excellent), I’ve since discovered QSWO is hard to kiln dry. As such, I probably should have returned it because the MC is likely a tell tale sign of problems to come. We shall see. Won’t happen again as I always now bring my MC meter. I did ask the lumber yard what do others do/say about the high MC kiln dried wood. They told me I was the first person they saw bring a MC meter.
Replies
Casehardening is a stress state in the wood. Measuring MC does not tell you anything about casehardening. Casehardening does not impact permeability or the diffusion coefficient of wood so it will not directly impact drying rate.
KD oak should be at 7-8%. The store should have a climate-controlled environment, so the wood does not gain moisture. Even then, letting it sit in your shop for a while is a good idea.
I assume you are using a capacitance-type moisture meter. These are affected by wood specific gravity. You need to set the meter or correct the meter reading using a paper table. The error is about 1% MC for each 0.04 change in specific gravity. Meters are often calibrated for a SG of 0.48 (Douglas-fir at 12%). Without a correction, a oak board (assuming SG=0.68) at 8% MC would read about 13% on the meter.
Your few pieces that have not lost much moisture might be explained by "wetwood". This referred to bacterially infected wood that loses moisture very slowly. It's described at https://forestpathology.org/bacterial-viral-diseases/wetwood/ .
Thanks for the follow up. What I did to "calibrate" my meter was to measure the MC in a thick short (like 12") piece of oak that has been in the shop for 7 years. The meter shows it reading in the low teens. For many of the pieces of oak that I purchase, I have quite a few that I measured every 6 months and wrote the date and value and then went from low 30% range, to low 20% range, to low teen percent range. It is only a few boards of oak that were purchased at the same time as the others that haven't lowered in 18 months. I will look up the link. The most obvious thing is to avoid using these odd behaving. Given that this more of a shop tool for sawing and occasional seating, I'm not stressing too much about it. My biggest lesson learned is I won't but supposed S3S kiln dried wood without first checking the values with my moisture meter (keeping calibration of different species in mind). I am planning to get a more sophisticated moisture meter in 2024.
It's not cheap, but there might be a "calibration verification block" available for a more expensive meter. Wagner sells one for their meters. Most large sawmills use one, but they have several meters and take a lot of abuse.
Your meter might have an internal calibration check. All the higher end meters I've seen in recent years have that feature.