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Thread: Hollow-form with face-grain orientation, pith included, green wood

  1. #1
    Join Date
    Jan 2015
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    Lake Burton, Northeast Georgia
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    Hollow-form with face-grain orientation, pith included, green wood

    I took a section of black walnut branch log, about 12" diameter, very green, and turned the outside of it for a hollow-form vase. Here's a picture, after roughing out the outside. The pith is perpendicular to the lathe spindle, through the center of the log. All of the bark and most of the sap wood has been turned away; it is mostly heart wood now.

    I also drilled a hollowing pilot hole, about 1" diameter, 6" deep, using a forstner bit mounted in the tailstock. The vessel is mounted on a face plate.

    IMG_20220105_195216524.jpg

    I'm thinking about how to deal with the drying process. With something like this, I'm not sure the "rough to thick walls, dry, then re-turn to thin walls" approach would work. I think the distortion and cracking would make it difficult to re-turn it after it dries.

    I'm thinking about a different approach. This approach would be intended to make it dry from inside the hollow form, not from the outside. I hope that this would make the outside of the vessel more resistant to cracking.

    To do this, I'm considering applying a multi-coat (2-3) clear epoxy finish to the outside of the vessel, BEFORE hollowing out the inside. Once the inside is hollowed to the desired thickness (say, 1/4"), it would be able to dry, though somewhat slowly since there's not a lot of air circulation through the small opening used for the hollowing. The wood on the outside would not be able to "breathe" because it would be coated with epoxy, which is waterproof and impermeable, so I think the shrinkage and stress on that surface would be less. Surface cracking due to drying of the inside surface wouldn't be a problem to me, unless the cracking extended through the wood to the outside surface. And the epoxy should offer at least SOME resistance to that cracking, acting as a nearly spherical semi-rigid constraint on the outside of the vessel.

    Any thoughts?

  2. #2
    Join Date
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    Turn it thin and do it ASAP. I go closer to 1/8" wall or less. I did that with a red oak vessel and it dried to the shape of a football. It's going to move a lot and if you wait cracks will already be forming. Cracking never stops in the middle of the wood. If it cracks it cracks through. Forget the epoxy, no advantage to that.

  3. #3
    Join Date
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    E TN, near Knoxville
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    I've watched the "other" John Jordan hollow pieces such as that. He turns the outside then inside to final thickness without stopping. He turns thin, not twice, and lets the piece dry and warp as it will (usually not much for an end grain piece). One thing he did I haven't seen elsewhere is after turning he puts a compressed air nozzle in the open end, seals a bit with his fingers, and blows air inside. You can see the water bubbling out of the pores. He said that helps in drying. Makes sense.

    JJ also has some articles about turning hollow forms. The one "How to create the perfect hollow form" might be interesting. It indicates a 1/4" wall. I didn't read it but just looked at some of the pictures, maybe he discusses drying. https://www.johnjordanwoodturning.co...sarticles.html

    You method of sealing the outside might work, it's often done with bowls but with anchor seal, plastic wrap, or something, but I haven't heard of using epoxy. (Will epoxy stick properly to wet wood?) Drying from the inside should put the outside in compression. As you said, the problem might be the air exchange through the small opening. I wonder if you could compensate for the reduced air exchange by inserting a tube inside with low pressure air. Also, what about filling it with desiccant beads with a funnel and changing it when it got saturated? Could be a good experiment.

    JKJ

  4. #4
    What Richard said.

  5. #5
    Join Date
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    Inver Grove Heights, MN
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    I like Robert Marshall's idea and would try it. However, I don't think the epoxy is the right exterior finish. I wonder about it sticking to the wet surface. Also, if it does stick and remains ridged it might promote cracking as the wood will move some and the epoxy might not move as much. I often soak the outside of bowls with shellac before turning the inside when turning wet to final thickness. I think it helps prevent cracks, but don't really have proof that is accurate. Be sure to tell us what you decide to do and how it worked. That is going to be an interesting hollow form if it works.

  6. #6
    Join Date
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    Epoxy can bond to wet wood. West Systems makes one that is made for repairing wet wood on boats. Here is one.

  7. #7
    I'll add my two cents.
    The epoxy will seal the outside from moisture movement. It will not do anything for maintaining shape.
    When you turn away the inside material, you relieve stresses as well as expose the interior wood to the ambient air. Now the inside can "breathe" and the outside cant, it will warp.
    If you want to maintain shape, you can twice turn as mentioned or turn thin and put a form or forms around the vessel as it dries to physically retrain it from moving.
    JMO

  8. #8
    Join Date
    Jan 2015
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    Lake Burton, Northeast Georgia
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    Thanks to all who chimed in with comments or suggestions.

    I decided to hollow it all the way in one operation, then immediately apply thinned epoxy to minimize surface cracking. So the day after I hollowed it (to about 1/4 to 1/2 inch), I mixed up 1.5 ounces of clear epoxy, added in 1/2 ounce alcohol, and applied that to the outside of the hollow-form vessel. The epoxy was sufficiently thinned with the alcohol that it soaked into the wood, so it was not necessary to rotate the vessel while the epoxy cured.

    Looking inside the hollow-form after applying the epoxy, I could see that the epoxy had soaked through to the interior wherever there were hairline cracks starting to form, those cracks having provided pathways for the deeper penetration of the epoxy. However, since applying the epoxy and allowing it to cure, there has been no noticeable further cracking, or enlargement of existing cracks. I believe that having the hairline cracks filled with epoxy, which then cured in place, restrained the wood to some degree.

    Here is a photo of the exterior of the hollow form, after the epoxy was applied and cured. This shows one of the two sides in which the pith is visible, which is the portion most likely to crack during drying. There were hairline cracks in this area, but they seem to have been arrested by the epoxy application.

    Screenshot 2022-01-10 3.57.29 PM.jpg

    The next things I plan to do to complete this vessel are (1) sanding the inside bottom (the only part visible through the hole), (2) sanding the exterior where the thin epoxy was applied, (3) applying 2-3 coats of epoxy at full strength (not thinned) to the outside and the inside bottom and around the hollowing hole, using a rotator to keep the epoxy from dripping or sagging, (4) finishing the bottom, using a vacuum chuck, to remove the wood which was penetrated by the face-plate screws, and (5) applying epoxy to the outside bottom.

    Robert

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