Turning green holly...huh?

[Doug Rasmussen]

6" diameter bowl burned from green holly. This wood is really wet, it was given to me a year ago and stored outside unprotected Very little end checking. What minor checking there was only was only about an inch deep into the ends. The blank was sawed off about 6 inches from the end. Note the blank is a 6-1/2" cross section of the tree's trunk.

I have great plans for the holly because of its pure white coloring.

As the turning progressed (actually not turned, but done on the CNC) everything looked good until the last light pass hollowing out the inside and a 1/4" crack appeared at the outer edge. All the turning was done in about a half hour to a little over 1/8" wall thickness, maybe an hour elapsed since sawing the blank off the main log..

In the past I've turned green wood without having cracking. Is there something about holly that makes it different? My thought was using a cross section of the tree stresses might be fairly even around the piece's perimeter.

The pictures show what happened by this morning.

How should holly be handled in green turning?

[Reed Gray]

Wood that size, coming in from outside is no where near equilibrium/dry, and opening it up/turning/removing bulk, will expose a lot of green wood at a rapid pace to drying stresses. Turning end grain is always problematic with the pith still in the wood. Having the thick foot puts a lot of difference in thicknesses, and adds to stress. Hollowing out the base/foot might have helped. Not sure if Holly is more prone to this than other woods. I would not attempt a piece like that without it being totally dry, which would mean 2 different pieces, one for the base and one for the bowl. Like the idea of the piece though.

robo hippy

[John K Jordan]

Doug, I cut most of my holly into spindle squares (1" to 4" square) for end grain turning and air dry it before turning. (I prefer to turn dry wood.) I can't help with the green bowl question but I have discovered that SOME holly turning squares can very quickly warp and crack like crazy. I've had some that are very stable and others that I almost think are worse than dogwood!

For example a couple of days ago I pulled out some about 3" square I cut a couple of months ago and sealed the ends - a few are terribly warped and some have split down the length. (I like to check drying blanks after the first few months so I can recut into smaller blanks if needed to salvage any that have split.)

I suspect some of how holly reacts (and some other species too) may depend on the specific tree and where the blank is cut in the log. Some larger blocks on my shelves (maybe 4"x4" and 3"x8") cut from a much larger tree did not crack at all and barely even warped, drying now for years (maybe 6 or 8). These were from 18"-20" holly logs while the less stable wood was probably from 10"-12" logs.

JKJ

[Doug Rasmussen]

Reed and John, thanks for the replies.

I'm thinking there's something odd about this material. In my limited experience with green wood it's always seemed you could do most anything with it prior to its beginning drying. This cracking happened way before any significant drying, well within a half hour of my cutting from the tree trunk log. The relatively thick base was going to be cut off right after the last finishing pass in the hollow so I don't see that as a factor.

I'm curious enough to try this same design with another orientation of the grain. I'll split the trunk down the middle using a half circle orientation, instead of end grain I'd be working across the grain.

[Michael Mills]

I think your best bet is to go cross grain as you last indicated.
I have not had a lot of luck with holly; either splitting/cracking or warping a lot.
The pic is of a piece of holly about 7" diameter. If you follow one of the flats with your minds eye you can see how much it shrank (of course double that for the full diameter log).
I would try to go with a fairly long tenon (1/2") and then hollow the tenon. Maybe there will be enough length to allow you to re-flatten the base of the tenon for it to sit flat on a table.
346935

[John K Jordan]

I'm curious enough to try this same design with another orientation of the grain. I'll split the trunk down the middle using a half circle orientation, instead of end grain I'd be working across the grain.

That may well work better. A bowl cut with the grain going across will generally warp into an oval shape instead of develop radial cracks.

The T/R ratio for American Holly is pretty high, over 2.0. This means as a slice of a holly tree dries, the tangential shrinkage, along the circumference (around the rings) is then twice that of the radial shrinkage, from the outside directly towards the pith. When you machined away the bulk of the end grain the water could escape from the thin sections even more rapidly through the end grain. Since the circumference will shrink faster than the radius, radial cracks develop to relieve the stress of tension. This is typical in most woods but some are worse than others depending on the shrinkage ratios, the grain structure, and the strength of the wood.

JKJ

[Dick Strauss]

It is not the holly necessarily. In addition to what has already been said about the T/R shrinkage ratio by JKJ, sharp edges and wet wood usually do not coexist peacefully! To build on what Reed said, the sharp edges tend to dry much faster because of the large ratio of surface area (that loses lots of moisture quickly) versus the overall localized mass (think of mass or wet wood like a reservoir of moisture). In other words, the water pool will dry much faster when it is very shallow and has lots of surface area (like the sharp edges of your piece). The base has just the opposite issue. So the edge of your piece dries extremely quickly while the base doesn't really dry much at all due to its small surface area to mass ratio. This sets up lots of tension due to moisture differentials causing shrinkage that often results in what you are seeing. You usually see this same issue when folks try to dry cookies (or cross section tree slices) as well. This is why it is so important to turn the walls to an even thickness with maybe even a slight thinnning toward the center of the piece because the center will still dry more slowly when the piece is an even thickness (the center can draw moisture from the wood all around it but the edges can only draw moisture from one side which slows drying in the center). In addition, moisture is typically lost even more quickly through the endgrain putting your piece at a further disadvantage in terms of success rates. The scallop wall design makes things worse by making the walls thickness vary between the peak and trough of the scallop.

There are multiple ways to slow moisture loss/displace moisture/help relieve stress that folks use that might help under normal conditions. Even the time of year or recent local humidity can cause you to be more or less successful with the exact same piece of wood/design. Your beautiful piece is at the extremes IMHO and the least likely to be successful based on its endgrain orientation, scallop design, and thickness variations. I would suggest dry wood only for this design in the future.

[Ralph Lindberg]

As for drying.... locally (the pacific north-wet) it's almost impossible to air-dry Holly and keep it white. It gets a grey/green spalting fungus and just looks awful.
One method that works, but is slow, is freeze drying the wood. Takes about the same time as air-drying, but no spalting. Unfortunately it does take a fair amount of freezer space :D
A lumber processor I know likes to get any Holly in the kiln within a day of it being cut down for this reason.

[Bill Boehme]

?.... I'm thinking there's something odd about this material. In my limited experience with green wood it's always seemed you could do most anything with it prior to its beginning drying. This cracking happened way before any significant drying, well within a half hour of my cutting from the tree trunk log. The relatively thick base was going to be cut off right after the last finishing pass in the hollow so I don't see that as a factor....

Actually what happened is exactly what one would expect for the majority of hardwoods and while holly has a greater tangential to radial shrinkage than average, the way it was cut, as Dick Strauss pointed out, is an invitation to what you experienced. As for how quickly it happened, that also is not surprising especially given that the wood was dripping wet. The rate at which wood dries looks like a decaying exponential curve with a lot of moisture loss immediately from moisture near the surface and with the rate of loss decreasing as deeper moisture migrates to the surface and evaporates. If the surface area to volume ratio is high (meaning the wood is thin) then the wood will begin to shrink as soon as most of the free water (in simple terms, free water is what makes the wood feel wet) is gone and the bound water starts to evaporate (bound water is the water in the cell walls). Water evaporates much faster through the exposed pores of end grain than it does through side grain. The very thin outer edge, where the wood dries very quickly, and gradually tapers to a very thick and wet center in conjunction with the end grain orientation that includes the pith Is tempting fate a bit too much.

If you make the same shape using the wood in face grain orientation, you will probably have better luck, but even then you might encounter a similar cracking problem if you use the same profile and you machine it to final dimensions in wet wood. Your best bet would be to rough turn it and let it completely dry before doing the final machining. The roughed out piece should be fairly uniform in thickness with a short tenon (a tenon length of 1/4 to 3/8 inch should be plenty).

I really like the grain figure in the end grain piece and I think it would be very worthwhile to store some of the holly and let it dry before you turn it. One other thought -- if the wood was from a limb, it is certain to have significant internal stresses. Before it is cut, the stresses are in equilibrium and when part of the wood is removed, the stresses are no longer in equilibrium until it moves to get everything back in equilibrium.

[Doug Rasmussen]

Thanks to all who responded, some good info here. I'm temporarily abandoning the holly for now, as the cracks expanded there's twisting to the piece indicating severe stresses in the wood.

I ran another of the same design in sapele (because I had it), with sanding sealer on it. Sapele is not good for this design, I believe woods that are less grainy with white or dark coloring like cocobolo which I'll try next. The flutes are a bit too coarse, maybe increase them from 18 to about 30 for finer detail.

The other picture is an oval design in some unknown tree limb wood. My CNC programs are parametric so changing a parameter here or there can completely change the look. In the oval piece I changed the proportions from symmetrical to oval with one keystroke.