Steam Bending, Part V

 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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