Pen blank moisture content

[terry q]

Hello All

I have some air dried lumber and want to make some pen blanks from the crotch figure. I know the mositure content isn't there yet and I can cook them in the microwave to get there. My question is what is a good mositure content to shoot for so my pens don't end up splitting?

Terry

 

[Andrew]

The ambient moisture content where you live. You want your wood to be at the same level of moisture content as it's surrounding environment ... having an equilibrium moisture content (ECM).

For example, if you live in the hot, dry Arizona area, you'll want it dryer than if you're in the wetter Florida area. Make it too wet for your area and you might have problems with cracking or warping due to moisture loss. Make it too dry for your area and you'll potentially have problems with warping and machining problems due to uneven moisture uptake. Yes, you can have them too dry.

So, it all depends on where you live and/or where you plan to sell these pen blanks. For a very very general number here in the USA, get it down to about 4-6% EMC for desert conditions and 9-15% EMC for here in the midwest. This all assumes you have a good, accurate moisture meter. A different, albeit less costly and not nearly as accurate, method of seeing if the wood is close to EMC for your location is to just let it cool after microwaving (or other drying treatment) and touch it. If it feels cool to the touch, then it still has too much moisture in it.

 

[terry q]

Thanks Andrew. Thats exactly the answer I needed.

Terry

 

[DennisM]

Using a moisture meter

A follow up question on moisture content. Has anyond had experience using a moisture meter to guage when green wood is no longer green. Wouldn't a electronic moisture meter take much of the guess work out of microwaving, paper baging, etc. Would appreciate the groups thoughts and experiences - even so far as the correct moisture meter to buy.
Thanks

 

[arbud]

a soggy mess

Dennis, I have the ever popular and inexpensive Harbor Freight moisture meter. I think it was $24.99. The amazing thing is that I checked it against a lumber mill's $400.00 meter and mine was 2% higher than theirs, i.e., theirs read 12% mine read a blinking 13-14%.

You have to remember that you're only measuring the moisture about 2mm below the surface.

 

[Andrew]

I use moisture meters all of the time. Most of it is with my kilns and kiln designs though. Not so much in a woodturning sense anymore. Moisture meters, even the kinds we use in the kilns, are only going to get you relatively close and not exactly to what the moisture content is in that stick of wood.

You use them in a relative manner instead of a specific reading manner, frankly. It's all in how you use them (as with most things, eh?) and how good you are at doing it that determines how well many of these meters work too. We've never found 2 meters of the exact same model and calibration that'll give the same direct, specific reading.

For many turnings, you don't really need a specific, exact reading anyway. If it's within 5% of actual, then you're doing pretty good. The main thing is just taking a series of readings (and practicing helps so you can do it consistently) and noting the relative dropping of the moisture content. When you're within a few % of your target AND the relative readings start to level off, you're good enough.

Unless you're wanting to target a specific % that's either much lower or higher than your environment then simple weighing of the wood (after it's allowed to "live" in it's environment for awhile after the drying method is applied) is going to give you a reasonably accurate indicator of whether it's at the correct "dryness" for your environment.

Of course, wood can be too dry as well. This isn't usually a problem with commercial drying operations because they're trying to shortcut it and leave it a bit wet if anything, usually. <g> But it is a problem otherwise.

So, yes, a moisture meter is going to work but it takes practice and looking at it in a relative manner. Just don't expect it to be terribly accurate or tell you what the reading should be for a given piece of wood to be labeled as "dry" or drying enough for what you're going to do with it.

 

[DennisM]

Moisture metering again

Thanks for your responses. As I think this through, I really don't care what the moisture content of the wood is. What I care about is doing the final turning on a piece of wood that won't warp out of shape when I am done. I realize that is somewhat dependent upon the species of wood, but I am looking for a way to tell when the wrap is out of the wood.

I turned some green locus to a rough blank and then microwaved it for 15 minutes. The final form was stable, but it was as hard as steel and not much fun to turn. I later noticed some "micro cracking" which leads me to believe that I had over cooked it.

I rough turned some sycamore and then nuked it for two sessions of three minutes each, brown bagged it, and let it sit for 6 weeks. Close, but wasn't quite long

Point is how can I tell when the moisture content is low enough that the likelihood of warping and cracking are pretty much over?

 

[Andrew]

1) There are no guarantees with wood. And especially with forms like bowls and hollowforms. You're setting up geometric stresses (and stabilities) beyond just the normal stresses in the wood.

2) Some woods are simply more stable than others. Sycamore doesn't tend to crack as much as other woods but it does warp. Mesquite is quite stable and you can do anything to it at any time and you'll not have too much trouble. Red Oak cracks terribly. These are just generalizations and each stick is going to behave differently depending on so many other factors.

3) Moisture content is only one variable having to do with cracking and warping (stresses). This just happens to be one that we can, somewhat, control and, mostly, measure. The other forces/stresses within a piece of wood can only be guessed at most of the time and are completely not under our control except to simply not use that piece of wood (which would be after the fact, unfortunately).

Using a moisture meter is going to help a bit. Without a doubt it will. You'll get close to what you want with it but don't expect exact accuracy between readings and pieces of wood. Ideally, it'll measure down below the environmental MC (moisture content) which the other, simpler methods can't. Those other, simpler methods are simply touch (when it no longer feels cool to the touch) and weight measurements (when it no longer loses weight). Both of these measurements should be taken after the drying method of choice has been done and the wood allowed to be brought back to equilibrium with the environment. These two methods are fine for drying bowls, for example, because all you need is to get it "in tune" with the environmental MC. It'll take on and release moisture along with the environment it's in relatively equally at that stage.


In kilning we shoot for about 6% MC (Moisture Content) in hardwood lumber that's going to be used for milling. It has to be stable. But, these are thinner pieces of wood and it can be done fairly easily. For this area, especially lately with the amount of moisture we've had here, if I take my rough turnings down to that MC, final turn them and apply a finish rather quickly, I'm setting myself up for problems later. Yes, wood can be TOO dry. However, I do want my pen blanks and trembleur, most spindle and many turned box blanks to be that dry.

You have a choice of whether to ...

1) dry your wood (or have it done for you, buy it that way, etc.) before you start working it

2) rough it, dry it, final turn it

3) final turn it and then possibly dry it.

The first option is going to be wasteful, expensive and hard to find for large blanks like bowls, hollowforms, or big boxes. That's because most kiln operators aren't going to do thick pieces of wood. You get high losses. I sell my kiln dried thick blanks for at least double the green stuff. The second option, depending on how you chose to dry it, can be done very well but can take some time. Time, unfortunately in some cases, is going to be your friend. ANY shortcuts are going to have their pitfalls. The third option, the one that I most often take for free-form pieces, is going to get you done quicker but demands more attention to wall thickness but, most especially, even-ness throughout the piece so that it doesn't crack. Warpage is something I don't mind in these pieces. If it's a piece that I can't have that in, I take either option #1 or #2 and give it more time.


Did you microwave that locust for 15 minutes on high? Surely not. Must have been on low. Either way, that's just too much at once. Locust is a hard wood when dry. That and Osage Orange aren't something I don't turn dry. If I can't drive a nail into it, I don't turn it. Yep, lots of micro-cracking is a good sign that you've gone too far, too fast.

The un-comforting answer to your last question is, simply, that you won't know. You can increase your odds by getting the moisture content down to the 10% range AND picking a species of wood that is more stable AND picking a piece of wood that is straight grained, uniform, boring AND picking a piece of wood that was from the main trunk of the tree about 8 feet off the ground AND picking a piece of wood that was from a tree that was grown on flat ground, without other trees around it, that wasn't subjected to too much wind or drought or flood AND AND AND. Otherwise, it's going to be a guess. And I'm most thankful that it is. Otherwise, I might as well be turning epoxy.