I’ve talked to a lot of people who were having issues with steam bending, and troubleshooting steam bending. At a distance, there is a lot of guesswork, because everything in the process of steam bending, much like we were talking about everything in the process of cutting a piece of wood, creates a new variable. And what that variable is is often not measured. So the exact specimen of wood chosen is critical. I’ve had the best luck steam bending wood when the wood going into the steamer has been dried down to 18 percent, rather than being kiln dried, and brought up to 18 percent, not ever having gone into a heated drying process, but just air dried, slowly, down to 18 percent moisture content. That way, it’s got … its fibers are still mostly swollen, but it’s not full of water. It’s like a celery stick that’s stale, or that’s wilted. It’s not got its full trigger pressure in there, so it’s easy to bend that, so I don’t think that’ll flop over if you pick it up, but a totally fresh piece of celery will break if you bend it.
Wood’s not celery, but I think it’s a useful analogy. If the wood that you’re trying to bend is too green, you’re more likely to rupture the cells on the compression side. So bringing it down to 15 to 18 percent, sometimes 20 percent’s fine, helps your success rate. Making sure that you’ve got a steam saturation in the tank, in whatever you’re … whether it’s a plywood box, or a stainless steel insulated cylinder, I don’t think makes any difference at all, in terms of the performance of the steam. It makes a difference in the efficiency and the safety of the process.
But if your wood’s going on the steamer at 18 percent moisture content, you’re getting full saturation of the steam. Another thing that can help your steam bending success is at the end of the cycle, say if you’re bending an inch square piece, and you’ve steamed it for 15 minutes, spray the inside chamber of the steam area and the wood with cold water. Just saturate that wood really well. Close it back up, and let it reheat for about 20 minutes. What that does is it really saturates the surface fibers. They’re not gonna go back up to 25 percent, or 40 percent moisture content, but they’re gonna saturate them, and make them more easily deformable. I’ve seen a huge difference in this, particularly with walnut, when I’m getting a lot of compression wrinkling on the inside of the bend, and I’m almost always bending with a strap, so I’m doing a compression bend. That means the strap, there’s a steel strap on the outside of the bend, woodblocks on the end, and as I bend that around, those woodblocks are pressing against the end grain, putting the piece into compression.
What that does is it eliminates a lot of the expansion on the outside of the part. Expansion is where the fibers are more likely to separate. And it’s that expansion, rather than the bending, or the flexing, that causes failure. The flexing is what causes expansion, because your … the wood’s not compressing all the way. But in a strap bend, where you’ve got those blocks on the end, and they’re pressing against the end grain of that part, you’re putting most of the wood into compression, and eliminating a lot of that expansion, on the outside. So when you do that, you need to have the wood plasticized enough that it can deform in a way that doesn’t create sudden wrinkling, or sudden compression failure.
Another that I’ve learned recently, that helps a lot, is if you can support the sides of the parts being bent, in addition to having a strap on the outside, and a solid form on the bottom, but if you have the form, and the part itself, the same dimension … the dry wood going into the steamer should be the same dimension as the form, and then clamp sides to your form, the wood will actually get squeezed from the sides, because it’s swelled up in the steamer, so it’s gonna be fat going in between those blocks. It’s gonna get squeezed from the sides, which helps hold the bulging of the sides in, and it’ll give you a much more even, uniform compression, and a much smoother part, and it seems to increase your success rate, decrease your failure rate.
So those are some of the things, and then sometimes it’s just the wood itself, and I’ve bent walnut that looks like perfectly good walnut, and it just completely failed in the bending process. And sometimes wood just looks really good, and it ain’t. You just have to figure that there’s gonna be at least a 10 percent failure rate in your steam bending. Make some extra parts. But even with that failure rate, steam bending is … once you’re set up with a steamer, is almost always less expensive than bent lamination. With steam bending, you’ve gotta make one part. With bent lamination, you’ve gotta make 15 parts, and glue them together. So I prefer steam bending, whenever I can make that work, for the design I’m doing.
So hope that’s all helpful.
–Transcribed as spoken.