Like Marc St. Louis said there seems to be quite a bit of confusion regarding heat treating wood. One of things that I believe some are confusing is that there are two common reasons why people heat treat wood.
The first reason why people would heat wood is to form it to a shape. This can be done with dry (heat gun), moist (steam) or wet (boiling) methods. For bowyers, all three methods can successfully soften the cells of the wood enough to allow for bending / shaping. There are advantages and disadvantages to each of these methods. One rule of thumb for bending wood states that you should use dry heat for dry wood and moist heat for wet or green wood. But one also must bear in mind other factors like: Material thickness, Wood species and the Degree of bend needed when choosing the most appropriate method to use. It is also very important to remind everyone that whatever method you choose to use to bend the wood, it is the critical that the core of the material reaches the magical temperature (approx. 140' F) before the bend will maintain in the material after the clamps have been removed and all of the material has returned to the relative humidity of the environment it is in. A perfect example of failing to reach the proper core temperature and/or not waiting long enough for the material to return to the relative humidity is when you clamp a piece onto the form, wait till it cools, remove the clamps and the wood cracks and/or springs back close to it's former shape.
The second reason people heat treat wood is to alter / improve some of the natural characteristics of the wood. Certain wood species can gain shear strength, improve the elasticity and harden with dry heat. Many of these improvements do not occur if you use moist (steam) or if you boil the wood. Ash, Junipers, Hickory, Maple, Bamboo all are woods that respond positively to dry heat treating when done properly. A good primitive example of heat treating to improve hardness exists with the native American Indians who would heat the tips of their arrows in the fire. Again, reaching the proper temperature for this process is quite critical for consistency.
I have been asked if one can re-heat treat the wood once it has been treated before. The answer is Yes if you are trying to re-form the material but No if you are trying to add strength to the material. I have also been asked if you can boil or steam a piece of wood to form it and then use dry heat later to hardness. The answer is Yes but one must be extremely patient to do so successfully. The potential problem here lies in the time needed for the wood to return to the relative humidity. If things are rushed, the material will surely crack.
