You woodworkers with more experience doubtless know this, but I just learned it and thought I’d share.
I recently bought a slab of walnut for the countertops to go on some office file cabinets I made. I bought it at a local (south-central TX) mill. They buy logs from other states, mill and dry them in a kiln. I found a slab that was perfect. Right length, width – almost no waste. I asked if the slab was “dry” and they said yes. I neglected to inquire about the moisture %. I assumed it was 6-8%.
I then bought some walnut boards for another cabinet at another place. This place just sell boards – they don’t mill their own. I ordered a pinless meter and checked both. The boards are around 7% moisture content and the slab is over 12%. Now I know why the slab is cupping a bit. I know no one will notice but me, but still it irks me.
I won’t leave my moisture meter at home when I buy wood again.
Replies
With slabs, you usually don't have much choice. I'd guess that 12 percent is probably at the low end of what you'll find.
Wow, I was expecting something below 10% OK, I'm learning new things all the time. Thanks.
While moisture meters can be useful they won't prevent all wood movement problems. Wood still needs to be acclimated to your shop which ideally is similar to the final location for the piece.
Can I ask how you handled the slab? Did you acclimate it at all? I like to allow 3-4 weeks for thicker boards to achieve equalibrium. Did you plane it flat after acclimating? Did you finish both sides? Cupping and bowing is often the result of moisture leaving or being absorbed unevenly through the primary faces of the board. This why when you store wood it should be stickered to let moisture leave or enter both sides evenly. Another common problem is applying finish to only one side effectively sealing one side of the board while allowing the other to take on or release moisture as conditions change.
Moisture meters are best used to judge a piece of wood relative to another piece of wood to help determine when it is ready to be milled not as an absolute value. If you bring a load of wood home from the dealer and it reads between 6-7% but lumber that has been in you shop for a month or more is reading 10% there is a high likelihood that the new wood is going to take on moisture and needs to be acclimated or you could be faced with problems
Another factor is different regions of the country will have different equilibrium moisture contents, although I'm not familiar with it I picture central Texas as being a dryer climate where a 6% moisture content is common but in the Pacific Norwest it may be that a 10% moisture content is typical so get to know what is the norm in your area.
How did I handle the slab? Well, probably not the right way considering the info above. I was working it within a week. The slab was about 30" wide and I needed 25" in one section and 20" in another. I also kept the live edge, so took waste off one side only. It was also the perfect thickness (why cut good stuff off?). I don't have a wide planer and did not consider (even think of) working 13' of slab by hand. As it sits, it looks great. But looking closely you will see gaps between it and the cabinet underneath on each end and in front. I finished both sides (I do know that much) with lacquer. The slab is just sitting there, not attached. I can wait a year or so and go back and plane it down if needed. As to my humidity, Austin - 35 miles north of me - has an average relative humidity of 67. Only slightly less than Houston and Chicago, much more than Denver's 52.
From a different perspective, wood that is kiln dried to less than 8% across the board will produce stable indoor furniture. Acclimating dry wood in a shop will bring it closer to the shop equilibrium moisture but will do little to match the final environments equilibrium moisture. Swelling a little is rarely a problem in well designed furniture, drying can lead to cracks in the boards ends. In our area, and most of North America, equilibrium moisture is around 12% if one is in a non climatized environment. In a house that has air conditioning and heat for winter in the North East, we average 30% rh in the winter and 50% rh in the summer at 70F, equating to 6% and 9% equilibrium moisture for the wood, a 3% difference. Your slab was likely air dried, if it had been kiln dried properly, it would have been dryer. I used pin moisture meter in the 80's and whenever the lot would read 9% at receiving, it would crack in the ends going through the factory, and this would become waste.
Case hardening is a big issue with thick lumber it looks good till you cut into it.
"Kiln dried" is like saying "oven baked" it doesn't mean much unless you know long and how slow - most commercial mills can't tie up a kiln that long + the have to move material to make $$.
I think the RH in Denver is more like 5-10%.
One day the slab mania will subside.......
You don't really need a pinless moisture meter. A pinned meter will work just fine for most people. The number isn't really what's important. If you measure a few boards (in your shop) that have been lying around forever, you can get a good baseline. Your baseline will be different depending on your climate, the season, etc. I live in the high desert in Northern Nevada so my baseline is pretty low 6-8%. Someone in the South might not get below 10%.
Basically, whether your moisture meter has pins or not, the number is not that important. How it compares with other known dry material in your shop is what matters.
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