Drying a green bowl

[Scott Brandstetter]

Okay folks. I have been turning everything I can trying to work with various tools and to really understand their use, and effect on the different wood blanks. Up to this point I haven't worried so much about the finished product rather can I get this curve, or this bottom, or this universal thickness. I'm proud to say I must be doing something right because I am now starting to have people ask me to turn various things for them. Of course it's not show room stuff or anything I could sell, but still, proud to be asked.

So I am turning (no pun intended) my focus to the drying of the final product. As usual, I have read and watched many video's on this and like most things, there are varying thoughts My instructor says to turn the finish bowl, put it in a paper bag with moist shavings from the turn, and a week or so later (not exactly sure of the time) finish it by sanding and polish. Others say to turn a piece to 1 inch or 1/2 inch, put it in only a brown bag, then turn months later. I watched a video today that many use a bath of denatured alcohol to dry and then finish.

I would appreciate your thoughts on this. I do realize there is no one answer that will fit all turnings. Thanks in advance

****side note**** I have turned several end grain cedar bowls and for some reason, they don't shrink or warp at all. Needless to say I went back to the same cedar tree and cut out 15 more blanks that are 12 inches round by 20 inches long thinking I could get 30 blanks out of them.

 

[hockenbery]

Scott,

I live in a humid area so I don't put wet shavings in with bowls because they encourage mold.

I turn hollow forms and natural edge bowls to finished thickness. I rinse them with water, dry with a towel then put them in a bag or a box for a day or two, open the bag for a day, put them on a shelf for a day all,in a room with a dehumidifier set at 50%. Then sand and finish. These bowls and hollow forms warp in an expected manner.
If I turn a bowl that is 1/4" thick I will leave it in the bag a full week.

I turn "round" bowls by drying the bowl and returning it round. These I rough turn with a wall thickness 10% of diameter. I put them in a paper bag and change the bag daily for 4-5 days until the bags are not damp. Then leave it in the bag on a shelf for 4 months. Take it out of the bag check with a moisture meter every couple of months. When it reads 10% the bowl goes on my ready to turn shelf. Most bowls take 6-8 months to be ready to finish turn. A few make it in 4.

Hope this helps.
Al

 

[john lucas]

I also do like Al. I don't like the shavings in the bowl for the same reason Al doesn't. The idea is to slow down the drying because wood looses moisture at different rates through end grain vs side grain. I think the reason people use shavings is they contain moisture and hopefully even out the drying. In my experience they just cause more problems. Sealing the bowl in a paper sack or inside a box with the lid closed works well for these green thin bowls turned to completion.
For twice turned bowls which is what we call bowls that we rough turn and then finish turn after they dry, I usually seal the end grain areas with end grain sealer and stack them together. I used to weight them and when they stop losing weight they are dry enough to re-turn. Most of the time now I don't even bother with weighing them because I have enough other projects that by the time I get back to the rough turned bowl it's been 6 months or more and it's dry.

 

[R Henrickson]

I live in central KY. For turned to finish, I use paper bags, changing them if they get damp. I have never included shavings in the bag; my local mentors were definite on not doing that. I work in my basement, so the 'climate' might be somewhat more controlled than a garage or separate shop. I used to rough-turn bowls for later second turning, but it was some time ago. Can't remember if I bagged those or not -- some were too large for that but I think I bagged smaller ones.

 

[Odie]

The standard rule of thumb for rough turned bowls is 1/10th of the diameter......or 5" diameter=1/2" thick, 7.5" diameter=3/4"thick, 10" diameter=1" thick......etc.

This formula is only generic, and you can rough thicker if the time element isn't a rush. The more time you take, the slower the seasoning process will be......and the better the resulting warpage to cracking ratio will be realized. Some species and the associated grain patterns with that particular piece of wood will react differently than others, and it's not difficult to misjudge what results you will get......my personal rule is to never go less than the general rule of thumb, but thicker is better, because the results will be more dependable. No matter what the diameter, about 1.5" is the maximum thickness for a roughed bowl. (My maximum diameter is about 14.5")

I always use a wax-based solution (commercially available) to coat the entire bowl. This effectively slows down the rate of moisture release, and believe it also contributes to a more evenly distributed release throughout the exposed surface area.

Determining when stabilization occurs is best done by weight, because it is a physical measurement that applies directly to the process. If an accurate determination of moisture stabilization is required (and I believe it is) then monthly weighings are a good way to observe the progress. Usually, three months of unchanged weight is the indicator of stabilization, but dense woods and seasons of the year will mean a longer benchmark might be more desirable. Sometimes, depending on these things, 4 or 5 months of unchanged monthly weights, might be a better indicator with dense woods during fall/winter months. The best "game plan" is to have more roughed bowls in process than you will need to prevent your waiting for them......that way, you won't be tempted to finish turn before the process will safely allow it.

Scott.....You are correct that there is no one answer that covers all turnings, and I'll add that there is no one method that turners will agree on, as well. I'm simply giving information that works for me, and I've been doing this for over 30 years with a specific application to bowls with a high MC. My methods work, and are demonstrable.

ooc

 

[Bill Boehme]

The largest bowl that I have turned so far is 18" diameter. The roughed out thickness is about 1½" at the rim and close to 2" at the foot. The amount of warping at the rim was less than a half inch so I will have plenty of leeway in the final shape. Some wood will warp a bit more than this and some less, but I agree with Odie that 1½" may be plenty of thickness for any reasonably sized bowl.

After five months, the 18" bowl went from sopping wet to completely dry so I plan to start the final turning this weekend. After rough turning, the bowl was covered with Anchorseal. I would advise against using wood shavings. The object of drying is to slow down the process to minimize cracking and warping. My experience is that packing in wood shavings does just the opposite.

 

[hockenbery]

Warp is a combination of three things
1 Wood does not shrink in the vertical direction
2 Shrinks around the growth ring (tangential)
3 Shrinks from the center of the tree to the bark (radial)

Radial is about half tangential for most woods. This is why whole logs split open.
I made two crude diagrams of dried bowl cross sections for what you would get from laurel oak.
This is for a 10" diameter hemisphere bowl rim to tree center with 1" thick walls and the grain nicely balanced.
I rounded radial shrinkage to .4% and tangential shrinkage to 10%
On the left is a cross section cut parallel to the grain on the right perpendicular to the grain.
I drew the grain lines on the cross sections.

The key thing here is the one cross section shrinks to inside diameter of 7.2"
Opening this inside diameter to the 8" of the other cross section leaves a wall thickness a bit over 1/2 an inch. So the 1" thickness works. Also this is a conservative estimate as the table values are usually for oven dry from fresh cut.
The bowl blank will have dried a little before I turn and I will return before oven dry.

Below is a table like you will find in Hoadly and other sources.
Big numbers shrink more and when the ratio of the two gets above 2.5 a thicker wall is needed and the bowls don't dry as successfully.

Species Radial Tangential Species Radial Tangential
(%) (%) (%) (%)
Camphor 2.6 4.6 Maple, Red 4 8.2
Cherry, Black 3.7 7.1 Oak, Laurel 4 9.9
Elm, American 4.2 7.2 Oak, Water 4.4 9.8
Elm, Winged 5.3 11.6 Oak, Live 6.6 9.5
Hackberry 4.8 8.9 Persimmon 7.9 11.2
Pecan. 4.9 8.9 Sweetgum 5.3 10.2
Holly, American 4.8 9.9 Sycamore 5 8.4
Madrone, Pacific 5.6 12.4 Tupelo, Black 5.1 8.7
Magnolia, Southern 5.4 6.6 Bald cypress 3.8 6.2
Magnolia, Sweetbay 4.7 8.3 Cedar, Eastern Red 3.1 4.7

 

[Pete Blair]

Drying green bowls

When I started turning I followed this protocol http://www.woodturningvideosplus.com/boiling-green-wood.html of Steve's for boiling. Works really well but I now do all sorts depending on the wood, time of year and how much of a rush I'm in to turn.
I use all the above methods as well as this one and a few of my own.

 

[KellyDunn]

Scott, I am a production turner. Mostly double turn bowls. Rough out and coat the outside with wax. I live in the tropics so if I coated the whole piece I stand a good chance of it rotting. Very dry climates perhaps the whole piece. Some woods I just air dry under my house. They will never get below 14% here. So in most cases I kiln dry the pieces. And I pop them back in a kiln a day or so before finishing to pop them back down to under 10%. Its been very wet. Bowls on the shelves are 18% right now. Point blank they move to much during finishing. If the piece goes to a dry climate the warp and twist stays. Since my name is on the bottom my drying methods work for me. You will need over time to come up with what works for you. I now work by myself so a kiln load of bowls I can dry real slow. I made a kiln out of an old fridge and for slow have a 40 watt bulb in the bottom. But I have many hundreds of roughed out bowls. All that said some woods are a gas to do green and let them move. Some woods are very stable and can be done while some moisture is still there with little movement. Some woods crack when you look at it. Try a few things and see what suits you. I do have a moisture meter. I have a mini ligno. i have heard of folks getting used meters on ebay cheap.

 

[Christopher Martin]

So I have read some of your processes an I will be changing my process too but I have bowls that need either help or to the "burn pile" This wood is just poplar so if I need to pitch it no big deal...

So I dried some bowls in shavings and found mold on them would you discard them for safety reasons or would you try turning the mold out??
And this type of mold very dangerous?... I'm little confused what I should do at this point. My shop is in my basement and don't want to get sick or
make anyone else sick either...and I don't have an air cleaner but I don't think that would help either.

 

[Owen Lowe]

So I dried some bowls in shavings and found mold on them would you discard them for safety reasons or would you try turning the mold out??
And this type of mold very dangerous?... I'm little confused what I should do at this point. My shop is in my basement and don't want to get sick or
make anyone else sick either...and I don't have an air cleaner but I don't think that would help either.

The mold is very common when the drying is slower than optimum but it’s nothing to be concerned about. You’ll end up turning or sanding it off anyway. If it still worries you, spritz it with diluted bleach to kill it.

Your next step is to modify your drying process so that initially the surface dries more quickly, to ensure the mold doesn’t develop, and then slows down so that the wood doesn’t crack while the internal moisture evaporates.

 

[hockenbery]

So I have read some of your processes an I will be changing my process too but I have bowls that need either help or to the "burn pile" This wood is just poplar so if I need to pitch it no big deal... So I dried some bowls in shavings and found mold on them would you discard them for safety reasons or would you try turning the mold out?? And this type of mold very dangerous?... I'm little confused what I should do at this point. My shop is in my basement and don't want to get sick or make anyone else sick either...and I don't have an air cleaner but I don't think that would help either.

The major issue I have with mold is that it discolors the wood usually in a bad way.
I kill any mold with Clorox

What I try to do is catch the mold early and wipe it off with Clorox.
Caught early it does not go deep,and is turned off in bringing the dry bowl back to round like it was never there.
I never worry about the mold I have treated with Clorox.

I do wear a respirator when turning dry wood for the dust.

If you have lots of mold the wood may be colored in a bad way.
Great opportunity to experiment with color, painting, gilding.......

Have fun,
Al

 

[Christopher Martin]

The major issue I have with mold is that it discolors the wood usually in a bad way.
I kill any mold with Clorox

What I try to do is catch the mold early and wipe it off with Clorox.
Caught early it does not go deep,and is turned off in bringing the dry bowl back to round like it was never there.
I never worry about the mold I have treated with Clorox.

I do wear a respirator when turning dry wood for the dust.

If you have lots of mold the wood may be colored in a bad way.
Great opportunity to experiment with color, painting, gilding.......

Have fun,
Al

Well I'm glad some clorox can take fix my mistakes... Now far as color yeah some don't look to pretty and one or two aren't to bad. Thank you guys for the help. Thinking back I sprayed some of my bowls down with water thinking they might be to dry thinking this would give them some type of even drying ( I don't know what I was thinking...) But I remember what i did and not doing that again.