Tetsuaiga":12srxyiy said:
I'm wondering if the kiln drying process makes the wood harder to work with hand tools? I think I remember reading about it having an irreversible effect on the cells and lignin but can't remember exactly what it said.
There is some truth in what you recall, and it's an example of a hysteresis effect. One form of hysteresis is defined as: the current behaviour of a material is dependent upon the conditions it experienced in its history. Drying wood has a number of consequences, two of which are that it strengthens it and hardens it. In this case, wood kilned to the US standard of 7% MC is stiffer and harder than wood dried to only, say, 20% MC, about as low as wood ever air dries in the UK, unless supplementary drying takes place, e.g., in a dry shed, workshop, kiln, or habitable building, etc.
Wood cells that have become very dry can never again quite return to the suppleness and softness they exhibited when the wood was at Fibre Saturation Point (FSP) or green (wetter than FSP, and just felled for instance). The stiffness and strength imparted by drying has a permanent effect, although this effect may be quite small. So, yes, a piece of kiln dried wood is somewhat harder to plane, chisel and saw than the next board from the same tree where this piece was air dried to only, say, 20% MC, even if the piece kilned to 7% MC is subsequently brought back to 20% MC.
I disagree somewhat with Matthew in that I don't think air dried wood is necessarily "vastly superior" to kiln dried wood. I understand what he's getting at, but there are times where kiln dried wood that has been kept very dry after kilning will almost always be a better choice than air dried, e.g., where the end use is in some kind of hot and/or dry atmosphere.
Picking up on another comment, kiln dried wood can be steam bent, but it's generally easier, and a better compromise to steam bend air dried wood at perhaps 18 or 22% MC. In part this is because of the inherent suppleness of the wetter wood, but because some drying has taken place some of the distortion that can occur during drying has already happened, so there should be fewer surprises with the end result. The easiest wood to steam bend is wood at FSP or above, and this is a good choice in some circumstances, e.g., for many parts destined for exterior locations.
Lastly, on this subject, I think it's worth mentioning that very straight (continuous) grained wood is best for bending, especially steam bending, because run-out of the grain or short grain in the part to be bent is more likely to suffer failure (split, snap, etc). Cleaved or riven wood generally produces pieces with continuous grain, and white oaks are some of easiest wood species to rive or cleave, making it a good choice for this procedure, planking for Viking longboats in long ago times being an example. Slainte.
