I don't think this is what they meant by wet, green wood. But it works.

Most of my chairs are made with kiln-dried hardwood from the lumberyard. Its moisture content is usually at equilibrium with my shop – just like a good flat woodworker would want.

But I know and respect the wet/dry construction methods used by Jennie Alexander and other chairmakers who use moisture content to strengthen mortise-and-tenon joints. (An oversimplified explanation: A wettish leg mortise shrinks on a dry tenon rung to lock the joint.)

After much flailing about and experimentation, here is how I now imitate that process with wood that is kiln-dried.

Let's say the joint has a 5/8" (.625") mortise, which I drilled with a spade bit. I then make the tenon about 15 thou oversized (.640"). This size tenon is too big to enter the mortise.

The oversized tenon, before compression.
Compressing the tenon.
The same tenon after compression with soft-jaw pliers.

So I compress the tenon with soft-jaw pliers. By using a firm grip on the pliers and rotating the tenon, I can compress it to just under 5/8" in diameter (.620"). This tenon will easily fit into the mortise.

Then, when I add hot, wet glue and put the joint together, the tenon swells up to (nearly) its original size, locking the joint.

I know this is true because I've done a lot of experiments where I have cut apart sample joints. Plus, I have made a lot of chairs this way. But here is a simple experiment you can try, which is more decimal.

First, some facts. The common woodworking glues (both PVA and hide glue) are about 40 percent water. I use liquid hide glue for almost all of my joinery, plus a little PVA here and there. (Note that there are waterless glues out there, especially polyurethane glue. I haven't experimented with these glues, so I have no opinion on whether they would work.)

So when I take the compressed (.620") tenon and dip it in some warm water for a second or two, it swells. (This is much like steaming out dents in pieces of wood with a wet rag and steaming clothes iron.)

After two minutes, the tenon has swollen to almost its original size.

After a couple minutes, the tenon swells to .635". After an hour or two, it swells to almost its full size (.640"). 

When I assemble a chair, I can feel this process at work. The tenons enter the mortises with a little effort. The hot, wet glue hits the mortise and within 10 seconds or so, the joint becomes difficult to rotate. After a few minutes the joint is impossible to move.

If this seems like a lot of work, it's not. It might add 10 minutes to the entire construction time involved in a chair. But I suspect those 10 minutes of extra work might add years to the lifespan of a chair.

Try it before you decry it.

— Christopher Schwarz