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moisture, heat and lignin11/15
My question has to do with heat and it's effect on lignin.
Background: I make Windsor chairs. I split out a green piece of wood and use heat in a steambox to bend it (my understanding is that heat is the primary factor in steam-bending, correct?). I air-dry it, then put it in a kiln and use heat to set the bend and keep it from changing shape. If I don't like the shape of the bend I can use a heat gun to change the bend slightly.
So I'm using heat to do two different things: make wood pliable and then set the wood to it's shape.
Is there a moisture content of the wood above which the lignin doesn't set?
If the lignin has been set, why does re-heating the wood with a heat gun change how my bend's shape?
I would love to find a book that explains this stuff. I have Hoadley and have read it many times, but he only goes so far. Any suggestions?
WOOD AND CELLULOSE SCIENCE by A J Stamm, written in 1964, is a good starting place although research done in the 1970s added to our knowledge.
Moisture makes the wood more bendable, along with heat. Both are very important. Your concept on setting lignin when bending is not quite accurate. For example... The heat does indeed help plasticize wood under stress, but this is not an irreversible reaction. Set is not irreversible either. Very complex.
Click on "bent wood" in the first paragraph of our home page and you will see bent wood extreme. I can't tell you how our customer does it but I will say they avoid heat.
Extreme bending is possible with amp is treated wood. After treatment there is no odor and the wood is hard.
Should say "possible wiith ammonia"
No ammonia used in bending those pieces shown at our website. LOL
Thanks for the info Gene, I've got the book on it's way here. Elia
Good chance the bent wood has been compressed longitudinally. Warm (likely steamed), moist wood is compressed parallel to the grain while constrained in a fixture to prevent buckling. Small "wrinkles" occur in the cells which subsequently allow the piece to be bent into tight curves even after cooling. Information on equipment and suppliers of this material can be found via web search.