Mastering the art of moisture control is essential for creating durable and tight joints in woodworking. By understanding the process of drying rungs to a bone-dry state and maintaining this condition until assembly, you can ensure a stronger bond. Discover the techniques and secrets to achieving the perfect dry-super dry joinery.
Transcript
The third component of a good joint is moisture control.
00:05 Dry and Bone Dry
So the normal moisture content of a leg, let's say that I'm working on, will be about 8% rear leg, front leg. It doesn't matter. I try to get them all down to what I would consider normal moisture content in my shop. So that's, I'm just guessing, about 8%.
The rung stock, so rungs like this, these are rung blanks, when I mill them will also be, will be 8% or higher. So what I do is I take the rung, I mill the rungs up into octagons like this, and I put them in a light bulb kiln. To dry the rungs, the rung blanks, to super dry. We want them to get bone dry.
I have no way really of measuring the moisture content of a part this small, so everything I've read in my experience, a week. If you're starting… as long as these are not green when you put them in, if you're starting with normal moisture content, even in the 15 to 20 percent in my age, a week is usually enough to get it down to bone dry.
Mark: You can measure the moisture content while it's still in a wide board and know what you're starting with?
Jeff: Yes.
And then I mill them up. A always cutting, cut the side, cut the knock off the corners. I think that helps with the drying and saves a little bit of time at turning. And then in the kiln for a week, and I don't touch them until… I won't turn the rung or turn the tenons to the final dimension until they're bone dry.
Mark: If you're planning on shaping these tenons by hand, would you leave them pretty much square if you had to?
Jeff: Shaping the rungs by hand?
Mark: Yes, sorry, the rungs. You don't normally, normally you're always on the lathe, but right. It would be really hard to...
Jeff: Yeah, I think I would leave them to the square.
Mark: So it might take a little an extra day or two to dry them?
02:35 How To Know Your Stock Is Bone Dry
Jeff: Maybe. - There is a way to measure it, and I have a student that didn't believe me. (laughs)
Mark: Let's hear that.
Jeff: So I say a week. I always say a week. That's everything I've heard is a week. So I had a student that said, "I don't trust that." And so what he did was he took his… The way you measure it is you take all your rungs, and you put them in a wire basket, and you weigh them with a scale that's fairly sensitive. Actually, it doesn't really even need to be sensitive. - Just the same scale. - You just need the same scale. Put them in a wine basket, then put them in the kiln. Every day, weigh them. When they stop losing weight, they’re as dry as they're gonna get. They won't ever get any drier than that, never.
And he did that. My student did that. And I asked him “How long did it take?” And he said, “It took a week”. So I'm pretty comfortable with a week.
03:28 Turn The Bone Dry Rungs And Keep Them Dry
So we get the rungs to bone dry. And then I turn the whole rung and the tenon to finish size. At that point, I put the rung-- then you need to put the rung back in the kiln and keep it bone dry until assembly.
Mark: So what happens if the rungs moisture-- if you leave them there-- Yeah, if you don't leave the kiln that absorbs moisture, from the air?
Jeff: If you don't keep them in the kiln, they'll absorb moisture and the... The only part that we really care about are the tenons, because that's the component of the joint that we're talking about. The tenons will expand. They can expand and contract fairly rapidly, depending on the conditions in the shop. My shop is normally not too humid. It's often dry. But it's normally not too humid. But in the summer I’m more careful, it gets more humid in here unless I have the air conditioning on.
But anyway, the point is, you need to keep them bone dry until assembly. So if you're gonna turn them and not gonna work for a month or two, turn the kiln off. But a week before you wanna assemble, turn the light bulb kiln and back on so that again, they're in there for a week, getting back down to bone dry moisture content.
05:00 Why Pay So Much Attention To Moisture Control?
The purpose of doing this now is that we have a leg that's at, let's say, 8% and a tenon that's at a rung and a tenon that's at 4% at assembly. And they're already a very tight joint. I mean, they're already as precise as we can get them, which we'll talk about next. After assembly, over time, the rung and the tenon will take on moisture and expand within the joint and get even tighter. So that's the whole point of doing this is to make a tight joint even tighter.
05:50 Dry-Super Dry Joinery Vs. Wet-Dry Joinery
So that's... And so you can call this super dry-dry. So the legs are dry, the rungs are super dry. So this would be super dry-dry joinery. And greenwood chair, they use wet-dry, yeah, wet and dry. Wet-dry. But same thing, same principle. That the tenon is always drier than the leg. And ultimately, the tenon will expand. In wet-dry joinery, the tenon will expand or the leg will shrink. One or the other. So same principle.
Okay. So we're doing dry-super dry.