An Experiment

In woodworking, wedges are everywhere.

From the very first steps to the last we use wedges both as tools, and as technique.  When our timber is still a tree, we use wedges both to control where a tree falls, and to split it into billets (if we’re not taking it to a mill).  The cutting iron in a wooden plane is secured with a wedge.  And almost any edge tool from an axe to a chisel can, used differently, split wood as a wedge.

Once we’re assembling our piece, we can still use wedges.  Mortise and tenon joints are very commonly wedged to create a mechanical lock on the joint, rather than relying solely on the glue.  One very common joint in pre-IKEA knockdown furniture was the tusk tenon.  It uses a wedge as the main structural component.  And dovetails are a form of wedge, creating a mechanical lock (in one direction).

But what about a wedged dovetail?

No, that isn’t a redundancy.  This form consists of a kerf sawn through the middle of the pins, and a wedge inserted.  I’ve seen this form of joint pretty exclusively on old Pennsylvania Dutch work.  Seeing it, I wondered why it was there.  So, I fell back on the old academic standby of searching “the literature”.

Yeah.  I asked Google.

These days, the only reason people suggest for wedging a dovetail is to cover a gap.  And it will do that.  But it didn’t seem right.  If it was for covering a gap, then why was every pin wedged, so neatly and precisely?  Surely Herr Zimmerman (yes, the Pennsylvania Dutch were Germans) wasn’t so sloppy as to need every dovetail kluged together like that!  The Prussian perfectionist streak would have produced an aneurysm right there.  There wasn’t any primary source I could find that would say why such a joint existed.  There was only one option remaining to me…

SCIENCE!

That’s right, I performed an experiment.  First I made the roughest, gappiest dovetails I could.  I measured nothing, just traced the boards over each over for a baseline.

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I figured that if it was going to work, I’d do it under “field conditions”.  So once I had them cut, I put in a kerf down the middle of the pins and made two wedges.  One was stubby, one was elongate.  I figured that I’d see which worked better.  Maybe you only needed a little wedge?  So I inserted them in and pounded down with my mallet.

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As you can see, the skinny wedge was, like the number five, right out.  But the stubby wedge filled the gaps admirably.  So yes, that works as advertised.  I planed it down to match, and especially after a coat of paint, you’d never notice it was there.

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But it still niggled at me.  I’ve come up with two reasons through reflection.  The first revelation came to me when I was going to disassemble the joint and put the pieces in the woodpile.  I pulled on it like any normal dovetail would come apart.

Nothing.

¿Que?  Obviously it was tighter now, but it was one measly little dovetail.  How tough could it be?  So I pulled harder.  Still nothing.  With both hands I pulled, and finally it yielded.  Now, this was opposite the way a dovetail would normally resist.  Without glue, it should come apart readily.  But with the introduction of a wedge, it resisted quite strongly.  I think this was part of the reason for wedging these dovetails, a sort of belt-and-suspenders method of keeping the piece together.

The other part of the puzzle I had to ruminate on for a while.  It has to do with glue.  While I was working out in the shop, I was thinking about how tacky any overrun of hide glue gets, even when “dry”.  The reason is the heat and humidity of late.  It takes a long time for it to cure under these conditions.  I thought to myself that I should enjoy the long open time while I had it because trying to get everything clamped down before the glue set up in the winter was going to be an interesting exercise—

And it came to me then.  This is a two part explanation.  First, an arsenal of clamps is a rather modern convention.  Historically, they didn’t show up nearly as much because they were expensive to produce.  This is the reason for nailing through the tail board in some period items: to clamp the joint together while the glue goes off.  Second, working in a cold, dry climate (like a German winter), the hide glue used exclusively during that time would set extremely quickly.  Imagine trying to get all the dovetails in a blanket chest glued together in time, and then have nothing to clamp with!

But here is where Herr Zimmerman was clever.  See, each of those kerfed dovetails is, effectively, an individual joint.  So he could dab a little glue in there, put in the wedge, and bang it all together while the glue was hot.  And then, the wedge would clamp the joint together while it cured, and he could move on to the next dovetail, rather than panicking over trying to get tail boards on pin boards before the glue set.  Both problems solved with scraps.  Frugal and effective: very German.

So I imagine that while the English folk settling along the Eastern seaboard might not have seen the necessity for such a practice, in that little region of Pennsylvania, some recent German immigrant, making a blanket chest for his daughter Brünhilde did what he did in the old country.  He used a technique he was familiar with to make a strong joint efficiently that would last for centuries.

That’s my story, and I’m sticking to it.

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