I know some guys will probably jump all over this one but here it goes............ When you "heat treat a piece of wood ,what you are doing is removing the moisture from the cells. The more you do it , the less moisture. Wood is made mostly of water while growing ,and when we cut it ,it starts losing that water immediately. When they talk about a piece of wood reaching equilibrium they are talking about it losing or gaining moisture ,relative to the ambient humidity present in the air.(RH- relative humidity) When a piece of hardwood is brought down to say 10-12% MC(moisture content) the cells have shrunk significantly already. They will never again be able to contain as much water as when fresh cut.Even if submerged in water.If that piece is brought out of the kiln in mid summer when the RH is 80-90% it will absorb some moisture and maybe creep up to 18% or so depending on species.After the winter when the air is drier and the board has come down to 6-8% in some areas ,in the spring and summer of the following year,it will not get back up to the 18% it did the following year.And so it goes through time until the wood has lost more and more ability to hold the moisture that once made it flexible.Old and brittle. Some woods fight this better than others . When we heat treat we are artificially collapsing the cells(shrinking them ) making the wood more compact but in turn ,less able to absorb moisture. The wood turning brown from heat is the sugars in it caramelizing,just like onions in a fryingpan or sugar heated with a torch on Creme Brulé. MMMMMMMMMMM yummy! the more sugar ,the browner it gets. When the wood reaches a point where it no longer contains enoughwater to keep the temperature below the woods flash point, it cumbusts.You don't need a spark or flame to make fire, you just need enough heat. If you leave the heat gun on the same area too long you shrink the cells to the point where they can no longer bind to each other .Too much of a good thing.If you can heat a piece quick enough,you can harden the outside without collapsing the cells all the way through to the back.If its really thin and the wood doesn't contain much sugar, you may wind up drying the whole piece through before you see any toasting happen. the cells on the back have then collapsed and are no longer able to regain enough moisture to become elastic again.You now have a dry brittle piece of wood. Try this for an experiment,it might give you a better idea ,visually of what is going on. Take a piece of wood 3/4"x3/4"x24" long. Straight grain(ish) Square and straight.Heat one side only , in the very center, for about 6" or so .Heat it until it becomes light brown on that side only.Maple would work good for this.The two ends on the side that is heated will draw into reflex because the cells have shrunk.The piece of wood will now be narrower in the middle as well . The side closest to the heat will be narrower than the side farthestThe piece may also twist ,depending on the grain.The cells closest to the outside of the tree will be larger and therefor shrink more. On a larger sample you can shrink one side until it cracks the opposite . This is what happens to your staves when they dry and split. The larger cells shrink more than smaller ones near the center of the tree or around knots.Back to the experiment......... Measure the size at the middle and both ends. Now throw the piece into some water for a few days. Extreme ,I know , but leave it for the amount of time that you would leave a bow after heat treating. Take it out when you are satisfied that it can gain no more moisture . Dry it off with a towel and stand or hang it somewhere to dry so that it dries even all around. When it no longer appears wet on the outside, measure your spots again. The ends should be larger than 3/4" the toasted side is probably the same and the side opposite might be marginally larger but still smaller than the3/4" you started out with. the cells will never be the same even on the side you didn't think you heated.Some guys use oil when they are doing heat correction ,as the oil has a higher evap. temp and keeps the cells from collapsing while bringing the wood to a temp.high enough to bend.Just make sure it's compatible with your finish.Contemporary accounts of native bowyers of the 19th century said they applied grease or oil from animals to the back of the bow to keep it flexible while they were making it and throughout it's working life .Once again ,if you wanted to sinew back it ,this would not be advisable. Hope this helps. Sorry if it was long winded,but there is allot of info for one small aspect of wood.
Cheers, Dave.
