This the fifth and final part of a very long explanation and description of steam bending. I could not run the complete text at once, and too much goes on around here to run it over consecutive weeks. Therefore, I have posted it as I could. If you are only starting to read my blog, you may want to search for Parts I- IV and start there. Mike Dunbar.
Second, when bending, it is common for students pulling a part, towards themselves to actually twist the part forward.
We suggest students be conscious of this and counter it by purposefully twisting the part away from themselves. If in spite of these efforts a roll up begins, we can minimize it by allowing the bend to ramp up the side of the block. When it is cool we drive it back flat against the back board.
Once a part has been bent it has to dry before it can be used in a chair. For single plane bends such as sack back and bow back side chair, we wait until the part is cool. We then tie the bending with a string and remove it from the form. The c-arm is bent in two planes and has to dry on its form. Crests are bent in a press and have to stay clamped in this device until dry.
At The Institute we dry bendings in the furnace room, which we use as a kiln. During the heating season the furnace keeps the temperature around 100 degrees. Because humidity is very low in the winter, the warm room will dry bendings in several days. In the summer we maintain the same temperature with a heat lamp. We lower the summer humidity with a dehumidifier.
At home, you can allow a part to dry on its own, if you have the time to wait. In the winter you can speed it up by placing a bending on a heating duct, or above a radiator. In the summer, a part will dry in about a week if left in the sun. When I taught on the road, we would dry summer bendings by placing them in the backs of pick up trucks with black bed liners.
If you want to dry your parts more quickly build a small kiln out of foam core board. In a pinch you can also use the kitchen oven. Set it to its lowest possible temperature and crack the door to allow the moisture to escape.
Anyone who has ever read an article on steam bending knows that it is necessary to over bend to allow for spring back. While everyone knows this, it is flat wrong. It seems to be one of those things that having made its way into print, just keeps getting repeated. When wood is sufficiently dry it compresses further. We can tell with a glance whether or not a bending is dry. As it comes off the form, the string is taut. Once the part is dry, the string droops. Obviously, the part did not spring back, but moved in exactly the opposite direction. When it dries, a c-arm will compress so that the wedges that hold it on its form will loosen and fall away. A fully-dried crest will fall out of its press.
In fact, this extra compression presents problems for a chairmaker who bends a long time before being ready to put the part in the chair. The compression set makes the amount of curve too extreme for the chair. We have had good luck correcting over compression by filling the sink with hot water and soaking one side of the bend. Once it has become wet, we can force the bend back open. We then repeat the process on the other side. Once you have restored the part to the desired shape, use it in a chair immediately. Otherwise, it will compress again as it dries. As I pointed out earlier in the series, most of the problems people have bending chair backs is caused by decay. However, other things can go wrong and result in repeated breaks. This is an example of a problem with the equipment. During a class a couple of summers ago one of the other instructors called me out to the bending area. On the ground were four or five broken arm and bows. Over a little less than half its length, each part was a strange purple-brown color, and each had broken within this discolored area. However, the other end of each part had bent well.
First, we examined the broken parts then, the steam box. I discovered that instead of being level, one end of the box was significantly lower than the other. The landscapers had recently replenished the crushed stone in the bending area, and one end of the saw buck that supports the steam box had sunk into the fresh stone. The Institute’s steam boxes have vent holes on both ends. These are drilled through the lower surface, just before the end caps. Since these vents are the only escape, the steam flows evenly through the tube in both directions. This two-way flow plasticizes the entire chair back uniformly.
Because the box was at an angle with one end lower than the other, the steam entering the tube in the middle rose and exited only out the higher end. None was flowing down through the lower end. However, the jet of steam entering the middle of the tube and rising did heat the air in the lower part of the tube without wetting it. This hot dry air not only failed to plasticize the wood, it began to toast it. That explained the discoloration we had observed. The parts on the ground had all broken in the end that was lowest in the tube.
As I described earlier, in order to bend wood it has to be both hot and wet. This wood was only hot and wet on one end, and hot and dry on the other. In fact, it was so dry and so hot, it had begun to char. We leveled the steam box and every part after that bent as it should.
We quickly made replacements for the students whose parts had broken, and these too, bent without trouble. I saved one of the discolored parts and hung it on the shop wall. I tell this story to each class and use it to illustrate my tale.
During a writing arm class one year we also experienced a series of breaks. I went out to the bending area to watch and help. As I assisted a student bending his arm I commented that the wood seemed too cool. Sure enough, it broke. Polling the staff and students, I learned that all the breaks were coming from one box. Those from the other box were bending properly. This ruled out bad wood. As I studied the box, I observed that very little steam was coming out the vent holes. Ordinarily, plumes of water vapor blow down to the ground from each end of the tube. Assuming the boiler was running dry, I took it by its handle to shake it. I expected it to be nearly empty and thus, light in weight.
All I remember is a moment of surprise at how heavy the boiler was, because as I began to agitate the boiler a geyser of hot water and steam erupted from the filler spout. Next, I was running across the lawn with the skin of my face stinging. After lots of cold water and aloe I was sufficiently recovered to look for the cause of the accident. We use utility cans as our boilers. About six months earlier the old boiler on the problem steam box had rusted through its bottom. Unable to immediately find a replacement boiler of the type we prefer, we had bought another brand. This brand of utility can had a fine mesh screen at the base of the spout, which was intended to act as a filter.
Over time, the steel screen began to rust and the tiny holes became more and more constricted. Eventually, this constriction cut down the flow of steam up the spout, creating back pressure in the boiler.
In the filler spout we have a wooden plug with a funnel though its center. This allows us to maintain the water level in the boiler without shutting down. As the water begins to boil the wooden plug becomes wet and tightens. When I agitated the can the plug let go. The water and steam trapped in the boiler by the constricted mesh filter erupted out the filler spout.
Fortunately, my face was not directly over the hole. Most of the hot spray passed by with only some of it landing on my right cheek and neck. For several days I looked like I had a very oddly shaped sunburn. However, I healed quickly.
Like most accidents, this one could have been avoided if I had exercised plain old, common sense safety. The lesson — always shut down a steam box before working on it.
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I know the date is close, but I still have space in the September 22 c-arm class. If you miss this one, you will have to wait until Aug. 2009.
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