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Thread: Moisture content

  1. #1

    Moisture content

    Hello all. This is my first post, so please bear with me. I am pretty new to wood turning, been doing other types of woodworking for years though. I bought my first lathe (a Nova Comet 2) and I love it. I have been turning everything I can get my hands on. I had my first bad experience, and I am trying to learn from my mistake. I bought a really nice piece of Bloodwood from Woodcraft, took it home, scraped off the wax, and turned a gorgeous bowl. Next morning, the thing was just about cracked into a hundred pieces!!! Ouch. So I learned all I could about drying wood slowly, using chemicals vs. twice turning and so on. Well I went out and got myself a moisture meter to help figure out how dry the woods I want to use are. It works great, but one problem...I have no idea what MC the wood should be at before I put it on the lathe! I got a bunch of exotic blanks that I am starting to dry out, but how do I know if they are dry enough to turn?? Unfortunately I only have about a year left before I wont be able to do this anymore, so I want to be able to enjoy it as much as possible while I still can. Any advice would be really appreciated.

    Thanks,
    Bob

  2. #2
    Join Date
    Aug 2008
    Location
    Reno, Nv
    Posts
    3,632
    Welcome Bob!!!
    Since you are in a hurry, I'd get right to DNA or De-natured alcohol. About $18 a gallon at most stores. In a 5 gal bucket, I'd use 3 gallons. Turn your bowl to about 10% of thickness. Toss it in the DNA...5 days...3 weeks, doesn't really matter. Pull it out, let it flash dry on the floor for about 30 minutes, this way the surface DNA evaps off. Wrap in newspaper and weight it...cheap food scale from Wal-Mart or Target is great. Note the date and weight and set aside. Weight it every few days and when it stops losing weight...turn it to a cool bowl!!! 7-12% is normal. DNA is extremely flammable so take precautions. It also evaporates really fast so cover it up. Others will chime in with details. If you note where you live, you maybe near someone who can help you out. Again...Welcome!!
    Your Respiratory Therapist wears Combat boots

  3. #3
    Join Date
    Oct 2006
    Location
    ABQ NM
    Posts
    30,008
    I'll also add my welcome. Sorry to see you ran into problems with your bloodwood blank.

    In general, it is better to turn a bowl blank to rough size and shape while it's still wet, then dry it and re-turn it to the final dimensions. The reason for this is because wood will almost always move when it dries. If it's thick (like a solid blank), it tends to crack. If it's thin (like a rough-turned bowl) it tends to warp instead of crack. As a general rule of thumb, most folks rough-turn the wall thickness to about 10% of the bowl diameter. In other words, for a 10" bowl, most leave the walls about 1" thick. Also, the wood will dry much faster if it is rough-turned first. (It takes much longer to dry a 5" thick blank than it does to dry a rough-turned bowl with 1" thick sides and bottom.) A couple of caveats that only experience and blind luck will help with: First, if you leave the walls too thick, there is still a good chance they will crack, because they will not be flexible enough. Second, if you turn them too thin, there's a good chance there will not be enough wood left to re-turn the bowl and make it round again after it warps.

    Jim's suggestion for the DNA bath is a popular one. I've used it a lot, although now I tend to only do it for wood that's real fresh. I've not seen any evidence that it helps much on wood that's partially dry. Jim is right that it's not critical how long the wood stays in the DNA. Most folks shoot for at least 24 hours, but longer doesn't hurt. DNA or not, the critical point is that the wood does not dry too quickly. That's when cracks really tend to happen. For this reason, most folks will wrap the rough-turned blank in paper, or pack it in wood shavings, to regulate the speed of drying.

    The actual moisture content (in percentage) is not really important, because that number will change depending on your location and atmospheric conditions. Jim's 7% to 12% might apply in his location (Fresno), but if you're in South Florida those numbers will likely be higher. The goal is for the wood to reach equilibrium...the point where it is no longer losing moisture to the air. That's the reason for Jim's suggestion to weigh the blanks periodically as they dry. Once you see the weight is staying the same for a few days, the wood is as dry as it's going to get, and it'll be ready to finish turn. If you don't have a scale, there is also the "cheek test". You simply place the room-temperature wood against your cheek (no, not THAT cheek, the one on your face) and if it feels cool on your skin, it still needs to dry some more. (That's what I typically do, even though I've got a scale.)

    Years ago I bought an inexpensive moisture meter, but once I saw that it never gave the same reading twice, I put it on the shelf, where it has stayed ever since.

    I hope this helps, and don't hesitate to ask more questions. We all did at one point or another.
    When the going gets weird, the weird turn pro. - Hunter S. Thompson
    When the weird get going, they start their own forum. - Vaughn McMillan

    workingwoods.com

  4. #4
    Join Date
    Jul 2009
    Location
    Amherst, New Hampshire
    Posts
    10,604
    Sorry, I'm no help at all. Just wanted to say Welcome
    Faith, Hope & Charity

  5. #5
    Join Date
    Jun 2008
    Location
    GTA Ontario Canada
    Posts
    12,251
    I too am no help Bob, so like Bob G I will just say a big Welcome to the Family. You may not realize it now but its great to have new guys ask these questions. For a guy like me that's only a weekend warrior and dabbles in all sorts of woodworking, I like the refresher course I get by reading replies to your questions. Thanks Jim and Vaughn for the refresher some day it will all sink in and stay. There is a nice huge maple being cut down the road and I was contemplating getting a piece but my brain strained at the thought of getting back on top of the whole drying process, now I might just saunter over and see if I can get a bowl blank from the guy.
    cheers

  6. #6
    Quote Originally Posted by Rob Keeble View Post
    I too am no help Bob, so like Bob G I will just say a big Welcome to the Family. You may not realize it now but its great to have new guys ask these questions. For a guy like me that's only a weekend warrior and dabbles in all sorts of woodworking, I like the refresher course I get by reading replies to your questions. Thanks Jim and Vaughn for the refresher some day it will all sink in and stay. There is a nice huge maple being cut down the road and I was contemplating getting a piece but my brain strained at the thought of getting back on top of the whole drying process, now I might just saunter over and see if I can get a bowl blank from the guy.


    Thanks for the warm welcome, I look forward to being a part of this world!

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