Vacuum Chucking gummy boundary

[GuyO]

The other night I had a piece of really dry cherry vacuum chucked so that I could finish off he bottom. After a couple of minutes I noticed a couple of sticky spots developing on the outside of the bowl. I seemed to be at the cross grain and at the same location on the outside that the outer rim of the vacuum chuck was on the inside..

The first time this happended and not realizing that the gummy spots were at the vacuum - normal pressure boundary, I tried to clean it up with some DNA.

I finally got it cleaned up, and standed - so I put a pad over the tailcenter point, and snugged everything up and killed the vacuum. After a couple of minutes the gumminess disapperared and the bowl was removed from the lathe.

Then tonight I was performing the same operation on a Willow bowl and that exact same thing happened...

I have a theory as to what is happening. ---

There is some moisture remaining in the wood or the moisture is being pulled through the open endgrain of the wood towards the low pressure area behind the gasket of the vacuum drum. The moisture is being pulled totally out of the wood once it is on the low pressure side of the gasket... -- so the moisture sort of piles up right at the boundary....

 

[Steve Worcester]

What I think is happening is that (dry or wet) within the vacuum boundary, and fine particles get sucked back into the areas of end grain and start to clog those tubes. The fine sawdust is sucked back into those areas. Once the vacuum is shut off they tend to spin away. You can sort of seal those areas by going over it with lacquer or whatever the final finish is, but the vacuum may also suck it into those tubes also, creating a ring of finish on the inside.

 

[MichaelMouse]

If it's dry, water would not be the problem. The water left below the FSP (fiber saturation point) or ~30% by weight is bound to the sugars in the cellulose. Don't think your vacuum is going to break those hydrogen bonds.

The picture you show looks typical of what happens when unbound water is thrown centrifugally along the short grain. You get the wet spots forming, your paper gums up and you get some major drag. Of course you're pulling the moisture the other way where you have the vacuum. I often use compressed air to push unbound water out from the inside, especially on light-colored woods which don't improve with mildew. Without any precise science to back up the assertion, I think it may take a couple days off the time to <18%, where mildew seldom starts. Brought a lot of maple through last summer with no black spots, anyway, while the cherry stored beside had the beginnings in evidence.

Other source of gum is the sugars in the wood. You can warm them and make caramel spots if you're not careful. Sticks to your paper just like it does to your molars.

Could be what Steve suggests if the wood is really dry, though there it would be a case of burnish reflection versus particle scatter, and that's not what your picture looks like.

 

[GuyO]

Thanks for the suggestions -- the piece of cherry had been turned to final thickness < 1/4" for about 3 weeks and was already dry when I started. If I ever get this again I'm going to dig out the moisture meter an get a reading on either side of the vacuum boundary.

I may go find another piece from the same log and run an experiment with some dye so that I can watch the moisture movement when the wood is under vacuum...