"Small Entropy," by Bill Luce. This is one of a series studying how a piece can get energy from sitting at an angle, and where the distortion is most
This heavy oak bowl took18 months to air-dry and stabilize.
"Frangipani Bowl," by Gordon Pembridge. It warped slightly, adding more character to the undulating bark rim.
"Pin-Oak Bowl." To obtain an even distortion, the pith of this bowl is across the middle of the top and the wood is of even density. The beads emphasize the distortion, also making this large bowl easier to pick up.
"Oak Wavy Bowl." Despite the dramatic distortion of the thin wall, the foot of this 16-in.-diameter bowl became somewhat oval but stayed near flat.
Distortion is dictated partly by the wood and partly by the form. Burl, like that used by Andrew Gittoes for his Tasmanian myrtle bowl, goes frilly.
Not all distortion is esthetically pleasing. Having the pith off-center to the right in the footed bowl caused it to lurch that way. The bowl on the right suffered from having two densities of wood, the lighter sapwood shrinking more than the darker, denser heartwood.
If you turn a bowl from unseasoned wood, chances are that it will warp and it might also split. However, generations of turners over at least 1,500 years circumvented these irritating traits by choosing and cutting their wood carefully. These generations of woodturners used woods known for their stability and not prone to warp excessively. In Europe these included alder, birch, and the maples, and to those I would add cherry and ash, which I have turned by the tonne with few problems over many years in three continents. Bowls turned from these woods barely distort if you keep away from the pith of the tree and choose straight-grained, knot-free blanks. Select the right wood, and you can turn and finish a bowl using unseasoned wood in the expectation that it will distort very little.
For dramatic distortion you need a wood known for its volatility, a wood that when thin likes to warp but not split, like Tasmanian myrtle, most eucalypts, most burls, most oaks, or madrone burl. To make the most of what will happen when the wood dries out, you need even-grained material aligned precisely within the blank and on the lathe. Even then, the hardest part is to come. After the bowl has distorted, you must cast a critical eye over the result and destroy inferior pieces. Not all distortion is aesthetically pleasing. A reputation for quality is built upon rigorous culling-rather like animal husbandry.
Green-turned bowls should never be finished on the lathe but rather after they have dried. Sealing the surface impedes moisture leaving the wood.
The time required to season a green-turned bowl is, as with all wood, dependent in part on how thick it is.
To ascertain how a wood is likely to distort as it dries, turn a thin test bowl about 4 in. in diameter. Leave the bowl in the sun for a few hours, or cook it in a microwave oven for a minute. If it hardly moves, try another species. If it twists dramatically, it could be just what you're looking for. If the blank was quartersawn, the bowl will go oval. It's worth testing how easily it might bend, break, or split. If it's strong, use it.
The way a bowl warps is dictated by the grain alignment within it, its shape, and how the two relate. How far it warps depends on the species and how green-that is, unseasoned-it is when it is turned. Trees felled in summer contain more water than they do in winter, so their wood will take longer to season.
To get any bowl to warp symmetrically, you need the grain evenly balanced in the blank, the center of the tree aligned with the center of the blank and thus with the center of the bowl. The oak log for the "Pin-Oak Bowl," seen at the left, was nearly ideal; it was straight-grained, of even density, with the pith in the middle. I cut a length from the log equal to its diameter and then cut it along the pith. That cut face became the top of the bowl and the surface to which I attached a faceplate.
The tightness of the growth rings in the center of the rim reveals that little was lost from the top face. The nearer the rim is to the pith, the more dramatic the distortion is likely to be. This is because the distance between the growth rings, and therefore the volume of material between the growth rings, increases the further you are from the pith. Most shrinkage occurs tangential to the growth rings.