Last night as I drifted off to sleep, this image stuck in my head. It reminded me of something, and then it hit me, boy am I a dope. It's the marks that the bandsaw made on the bending form!
Here's the culprit.
I've been known to make some pretty rough and ready bending forms, it's actually one of my favorite parts of chairmaking that I can cut a shape on the bandsaw at a moments notice and bend to it. The white oak that I normally work with never softens to the point that the tiny surface variation creates a problem.
But the walnut is a different story. It gets so soft when steamed that the tiny ridges left by the bandsaw cause a slight crease that is the beginning of the compression failure. It's important to note that the ridges aren't just embossed into the surface of the walnut, which would steam out, but that the failure they start goes deep into the wood.
Smoothing the form and fairing the curve is no problem, I simply run the spokeshave across the surface while holding it skewed, which cuts the high spots.
Here is the finished surface, I even sanded it.
With the air dried wood, I've been using an overbend form to push the wood beyond the final curve before I relax it and kiln it into final shape. The bend went beautifully on the smoothed form which also had less extreme of a bend than the previous attempt. After bending, I let it sit in the form for a few hours and then take it out to sit overnight. Below, you can see the springback after I unclamp the bend. One of the beauties of using air dried wood is that the bends set almost instantly.
You can see in the image below that the bend, when placed against the final form is actually a tighter curve, just what I want.
My friend Andy Jack wrote me with some sage advice about bending. He recommended that if I was having no trouble on the expanding surface yet compression failure, that I might be steaming too long. This makes great sense as a means to developing the steaming times of any wood, the point being that oversteamed wood gets too soft and fails on compression and that there is no reason to steam beyond the point that the expanding surface can stretch to the bend.
So for now, I'll console my feeling like a dope for not refining my form properly with the satisfaction of having learned something.