This the second part of a very long explanation and description of steam bending. I cannot run the whole part at once, and too much goes on around here to run it over consecutive weeks. Therefore, I am posting it as I can. If you are only starting to read my blog, you may want to search for Part I and start there. Mike Dunbar.
Trees that are cut in the summer or late spring are more likely to decay quickly. At that time of the year the tree is in its growth cycle and the sap is up. The weather is also much warmer. A tree dropped in July when temperatures are in the 90s can begin to decay in a week. A tree dropped in October when the tree is dormant and the weather cool, will remain fresh much longer. In fact, we like to put in a large supply of logs in the late fall, as they remain frozen from December through March. In the warm weather we buy small numbers of logs, and more frequently.
The problem is that other than in the sapwood, you cannot always see the early stages of decay, a break down of the wood that makes it brittle and incapable of bending. Although over the phone or via email, I cannot diagnose why wood will not bend, I suspect that decay is most often the culprit. Your best protection is to know a tree’s history — when was dropped and where it has been in the meanwhile.
No matter how fresh your log, it will not remain that way. This means you need to get to work on it right away. There is no difference between wood taken from a log that has been at the mill for six months and one that has been lying in your back yard for the same amount of time.
This the analogy I use when describing wood selection during a class. Think of yourself as a farmer putting down a cow for meat. You will not take the cow out into the field, drop it, walk away, and return three months later to cut off a steak. The meat needs to be processed right away. Once it is cut up you have two choices – freeze it or dry it into jerky.
Treat a tree the same. Split it up right away. Then, you have two options, freeze it or dry it. Here at The Institute, we have a large 6 foot chest freezer which we fill with riven wood for our classes and for sale.
Unfortunately, back in the 1970s working wood that has been split from the log was been dubbed “green woodworking.” As a result many people think the wood needs to be kept wet. This is wrong, and results in a lot of ruined wood. Some people try keeping the billets submerged in water. This is unnecessary. Others wrap it in plastic. Still others wax the ends These steps only promotes decay.
If you cannot freeze your wood, allow the billets to air dry. Although successful steam bending requires the wood to be wet, the steam box will take care of that. Treat the billets like you would any other wood you buy. Keep it dry and off the ground. I remember one fellow who called because his bends were breaking. It turned out he had stored his billets on the ground under his back porch. Of course, the wood began to rot just as would a board stored on the ground.
The best place to store billets is in a garage or other unheated building. Unless you live in a desert, it will not air dry much below 10% - 14%. Wood stored this way will be good for years. I have successfully bent air dried wood I split into billets a decade earlier.
Here at the Institute we split our logs with a log splitter. Every couple of months we have what we call a “spilling party.” Fred, Don, and I, along with a farmer who lives down the road, split enough wood for our upcoming classes and for sales. The farmer Kevin, drives his tractor down here with a four-foot splitter mounted on rear. Splitting the logs this way saves us a great deal of back breaking labor.
If you are a chairmaker working on a smaller scale you will most likely split your logs by hand. Using a maul and splitting wedges, split the log into halves. This is called riving. Use a hatchet to snip any wood that is tearing from the two halves and holding them together. Otherwise, these tears may lengthen and waste good wood. Next, split the halves into quarters and then, the quarters into eighths. These eighths - called billets - have a cross section that looks like a slice of pie.
With a maul and wedge split away the pointed piece of the pie. This is the tree’s juvenile wood, and it is seldom useful. Next, use a drawknife to peel the bark off each billet. Remember, the tree’s living growth layer is right under the bark. It is wet and rich with nutrients. If left this way, boring insects will quickly make your riven billets their home.
After our splitting parties we take these billets and carefully following the grain, cut them on our Hitachi band resaw into arm and bow blanks. We use this big saw because we are cutting enough stock for as many as six classes at a time. You are not likely to place these demands on your equipment and so, can use your shop band saw. To make the stock more manageable, you might want to split your billets one more time, into sixteenths.
Once the oak has been sawn into bending stock it is ready to be worked. In our experience, stock that has been set aside for even a couple of days and has lost a bit of water will bend better than wood that is dead fresh. As I explained last issue, the use of the term “green woodworking” leads many people to think that wetter is better. However, as long as wood is not heated by kiln drying, being placed near a stove, or stored in a hot attac, moisture content is largely irrelevant. The steam box will provide the necessary moisture.
The type of chair you are making determines the stock’s shape and dimensions. When sawing, the goal is to keep the blade in one layer of growth as much as possible, as doing this perfectly results in stock with no grain direction. Following one layer of growth may result in stock that is not perfectly straight and that has a slight bow. This is not a problem, as the part is going to be bent anyway.
Like most other human endeavors sawing bending stock is not always possible to do perfectly. As a result, when shaping the wood into chair backs there will sometimes be places in the stock where you will be cutting with the grain and other times, against it.
When a tool begins to dive or choke, it is necessary to cut in the opposite direction. The greatest risk occurs when using the draw knife. This tool’s open blade can dive as it follows the stock’s grain and ruin the part. A light test cut is always best.
Do not be concerned by the light colored sapwood. In our experience it bends well. However, it does best when in compression. Therefore, when it is present, we plan our work so that it will be on the inside of the bend. In other words, so it will be placed against the bending form.
Pin knots are a real hazard, as they create weak spots. It is best to plan your work so they are removed while shaping the part. If this is not possible, we again prefer to place them on the inside of the bend so they are in compression.
When either sapwood, pin knots, or some other risk is present, our habit is to mark the area with large, dark Xs made with a Sharpie permanent marker. When the part comes out of the steam box, this reminds us that when making the Xs we had determined a preferred placement on the form.
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