When I’m preparing two pieces of wood for a glue joint, before any glue goes on, I go back and forth for a while, making small adjustments to the two surfaces and sliding them together to check the fit. I’m looking for a certain feel. When it’s there, I can feel a suction start to take hold. There’s a sound to it, and a kind of softness to the feel. When I try to pull them apart, they seem to hesitate, ever so slightly. It’s very subtle, of course, but when it’s right, I can feel it.

At first this might seem unrelated, or even counterintuitive, but the way that joint fits and feels has a lot to do with humidity, and it’s one of the reasons I wanted to write this series in the first place.

Last week I said that once you understand how wood responds to water, whether it comes from the air or is applied directly, you can do much more than manage the humidity in your shop to protect your guitars and tonewood.

You can actually put that understanding to work at the bench.

Here’s one example of what I meant…

When I was starting out, I would prepare a glue joint by getting two pieces of wood as flat as I could, putting glue on, and clamping them together.

That’s a pretty good start, but getting two surfaces truly flat is harder than it sounds. And even when you get them just right, it’s easy to assume they’ll stay that way once the glue goes on.

So let’s take a moment and think about what’s in the glues most of us use.

Hide glue and Titebond are full of water.

And over the last two weeks we’ve taken a close look at what happens to wood when it meets water.

Years ago, Eugene Clark opened my eyes to what that means for a glue joint. We took two blocks of mahogany, about an inch thick, the kind you’d stack to make the heel of a classical guitar, and got them perfectly flat.

Then the hide glue went on.

Checked with a straightedge, they weren’t flat anymore. The faces had swelled, and the blocks had gone slightly convex. To close that joint, you’d have to clamp hard on all four corners and force it together.

A lot of builders never think about this, and then they wonder why their glue seams show, or don’t look as good as they hoped.

So let’s go back to that glue joint with what we know now.

We know the wood is going to swell when the water in the glue touches it, and we can plan for it.

Once I have the two faces flat, I use a scraper and put the slightest bit of hollow into them. After a while you begin to have a sense of how the wood is going to move when the water is introduced, and these subtle adjustments become second nature. That feel I described at the beginning is what I’m looking for. When it feels just right, I know it’s ready. But I can’t stress enough how slight these adjustments might be.

Sometimes it’s so little you could barely measure it. It’s less about a number than about how the two pieces feel and look when they come together, which is why I always recommend trying it on scrap first.

The better you know a material, the less its behavior surprises you.

It’s a little like knowing someone well enough to anticipate how they’re going to respond.

When the glue goes on this time, the wood swells to flat instead of convex. Then one clamp in the center is all it takes to close the joint, and the wood itself pushes the edges tight. As the glue dries and the water leaves, the wood pulls back the other way and draws the whole joint together.

The glue is part of the clamping.
The wood is part of the clamping.

You’re not fighting what it wants to do.
You’re working with it, because you understand it in a deeper way.

How much hollow to add? That depends on the wood: its thickness, how it was cut, how dense it is. Some joints don’t need it at all, and some benefit from it quite a bit. It takes some trial and error, and over time you just start to know. You eventually get a sense for how much, and for not overdoing it, because there’s a sweet spot in each case.

This isn’t a new idea.

Furniture makers have a cousin of it they call a sprung joint, and I’m not the only luthier who thinks and works this way.

Not long ago I watched a cello maker trained in Cremona, Italy, glue the center joint of a cello plate. Once the joint was closed and clamped, he brushed water onto the wood on both sides of it, a couple of inches out, staying off the glue line itself. The water made the wood swell, and with the plate held in the clamps, the only way that swelling could go was toward the joint.

He was using the wood’s own movement across the grain as one more clamp.

I thought that was brilliant.

He understood how that wood was going to respond to the water, and he put it to work. Those are the same fundamentals we’ve been talking about for the last two weeks, applied in a different shop by a luthier trained in a different tradition.

All of this reminds me of something that’s changed in how I think about my tools and materials in general. I used to think about them in binary terms, perfect and imperfect.

I think about that less and less these days…

There’s no such thing as a perfectly straight line in a real shop.

Even my Lie-Nielsen No. 7 jointer, the best plane I’ve ever owned, has a tolerance of about .0015″. But that’s OK. I don’t need to fight it. What I think about now is getting to know the nature of each thing: the quirks of a tool, the way a piece of wood moves, what a glue does to a joint. Once you know those things, you stop searching for “perfect,” and you start paying attention on a deeper level and working with them, quirks and all.

That’s why I’ve built my whole practice on first principles: the fundamental forces at work beyond the steps, the tricks, the tips, and the popular techniques. Instead of collecting more techniques, I started asking what was actually going on underneath them.

That one shift changed the way I approach almost everything at the bench.

If you want to feel this for yourself, try it on a couple of scraps of wood, one pair flat and one with the slightest hollow. If you do, hit reply and tell me what you discovered.

So that’s the humidity series. We started with three backs cracking on a shelf, we looked closer at the inside of a single wood cell to learn more about how it’s made and its relationship to water, and finally ended up here at the bench, with two pieces of wood subtly adjusted for how they’ll respond to the water in the glue.

That’s a better glue joint, made through understanding instead of brute-force clamping. Carry that kind of thinking through a whole build and you get a better guitar. More stable, better looking, and even better sounding, because sound has to travel through every one of those joints, too.

Hopefully now this all makes sense, and you can see why I wanted to write about humidity, and why I thought it was important enough to build a whole course around it.

The relationship between wood and water is so much more than a number on a hygrometer.

To me, this kind of learning, thinking deeply, knowing your materials, and trusting your intuition, is the Art of lutherie.

Sapere Aude · Creare Aude

Tom

P.S. This glue joint is just one of what I call the “Advanced Applications” in my Humidity Fundamentals & Advanced Applications course, along with things like swelling out a dent and building a dome into a top and back. In the course I go into each one in depth and show you how I do it. If you’d like a look first, the course trailer is here. It’s included with the Luthier’s EDGE, where we spend a lot of time on the why behind the steps, because that’s where the real growth happens, and where you can bring me questions about your own shop and your own wood as you go.