preparing the woodThis discussion is mainly about handling Arbutus, as many woodworkers struggle to dry this temperamental wood without it cracking. Most of the following tips, however, can successfully be applied to other woods as well.
It's usually not too difficult to decide how to cut, say, a two-foot diameter Arbutus tree. I think about the diameter and the depth of the salad bowl I want to create. Experience has shown that most people prefer salad bowls in the range of 12"-16" diameter. Arbutus will go out-of-round while drying, so it's best to have rough-turned bowls at least two inches larger than the desired finished diameter. So, if you want a 15" Arbutus bowl, you should cut the rounds off the tree-trunk at least 17"-18" long.
With maple, the extra size is not so crucial, so a 16" round can often yield a 15" bowl.
Most cracking associated with wood drying originates from the pith or living centre of the tree. Usually the turner tries to "find" two bowls in each round of wood, one on either side of center. The first decision on where to cut involves avoiding imperfections such as knots, cracks, bark inclusions, nails or other factors, and finding where the best two bowls lie.
After establishing how the bowls lie within the round, I turn the round so that the potential bowl rims are vertical to the ground, then wedge the wood so it won't roll. My first cuts are to vertically remove the sapwood on either side, perhaps cutting two inches off each side. Dealing with the sapwood is far more critical with fruitwoods and with Arbutus than it is with maple, as the sapwood in these trees is very unstable and if it cracks it will carry the cracks right into the good wood. Cutting off the sapwood also gives the block a flat base for when it is bandsawn later.
My final cut on each round is to rip it in half down the middle, parallel to the outside cuts and right through the pith. It helps to use a lumber crayon to first draw a vertical line at each end of the round through the pith and then draw a connecting line on top of the round. This gives me a line to follow when cutting. This final chainsaw cut results in the round falling open on the ground, the two piths facing up. These are the faces where compass circles are later drawn that represent the rims of the bowls.
When working with a very large diameter trunk, there is an opportunity to cut several bowl blocks from one round of wood. In this case, instead of making just one cut down the middle, there will be several parallel vertical cuts down the end of the round. The distance between the parallel lines will determine the depth of the bowls.
Once a number of bowl blocks are created and transported to the workshop area, the circles are drawn on them with the centers accurately marked. Using a chainsaw or bandsaw, the edges are cut off the block outside the circle to make the piece lighter and more balanced. With a very well-balanced piece of wood, a screw-chuck can be used for mounting on the lathe, in which case a small hole is drilled in the center of each circle to receive the screw-chuck.
However, for most of the larger, more unbalanced blocks, it is preferable to use a face-plate for lathe-mounting. Rather than drilling a hole in the center of the circle, I set the point of the compass at the center and draw a 4" diameter circle, which helps me center the 4" diameter faceplate. Once the block is mounted on the lathe, a tailstock is used to support the opposite side, which will be the base of the bowl.
Now it is time to rough-turn the bowl. Once the profile is obtained by chiseling, the block should be better balanced, speed can be reduced, the tailstock can be removed and the chuck-mounting recess or stub can be created on the base of the bowl. The faceplate is then removed, the block is turned around and remounted on the lathe using the chuck and the tailstock is reattached, this time in the center of the bowl's rim-end.
Now is the time to decide whether or not there is potential to create a second (and maybe a third) bowl from the wood to be removed during the hollowing-out of the bowl, using a wood-saving system such as the McNaughton System. For Arbutus bowls in the range of 16"-18" diameter, I would generally try to leave them 1-1/2" to 2" thick on the rim and a little thinner toward the bottom of the bowl.
I put all Arbutus bowls in plastic bags at this stage to slow the drying down. Cracks are caused by moisture trying to escape so, by slowing down the drying process, most cracking can be prevented. Downed Arbutus that has been lying in the sun is almost certainly useful only as firewood.
During the initial drying of Arbutus rough-turned bowls, they will change shape and have to be remounted on the lathe to be made round again, sometimes twice during drying. Care must be taken to leave more than an inch of wood at the rim so there will be enough thickness left for the final turning. It is important to watch for cracking during this time and use Hot Stuff glue in any cracks to prevent them from travelling deeper into the wood.
It's a balancing act for every piece of Arbutus because they will change shape, going oval instead of round, while they're drying. Leaving too little wood results in not enough thickness to final turn; too much wood results in cracking as the moisture rushes to escape.
In conclusion, although Arbutus trunks are rather homogeneous and often lack the imperfections of other woods, thus reducing the difficulty regarding cutting decisions, their tendency to crack and to change shape while drying still make them a challenge to deal with. Time and experience, though, can reduce losses to only about two percent!
With maple, there is generally far less change-of-shape during drying and the drying occurs much quicker, sometimes in two or three months. However, there are more time-consuming decisions about how to get the most beauty out of the wood, especially when it is figured or spalted.
Imperfections such as bark inclusions, knots, holes, insect channels and so on, can either be key features in a bowl or they can ruin it. It's helpful to decide this while initially cutting the wood - in time, experience will make this less of a gamble. Your decision may turn a proposed functional salad bowl into an artistic creation or a piece of firewood. It's all a great learning experience!