To Trust or Reinforce?
In traditional woodworking, most of the joints we make have some mechanical strength, in addition to being glued. And a well-cut mechanical joint holds together without any adhesive at all—the glue is just there as a bonus.
But edge-jointing and gluing long boards, especially for wide panels, can be an exception. For these joints, we often trust the glue alone to hold the two pieces of wood together. (Glue is one of those miracles, like sending an email, that does its job without leaving any hint of how it happens. That disturbs my simple woodworking mind.) For small glue-ups, such as the top for a side table, I'm happy to appreciate this miracle.
But as the joint gets longer and thicker, I start to lose trust in glue alone. For these joints, I like to add mechanical reinforcement for peace of mind. For instance, on a large table I'm building, I've opted for loose tenons, drawbored into each board, that physically hold the two pieces together. I can see how it works, and that makes me happy.
With modern wood glues this may seem like overkill. And adding these extra tenons makes a lot of work. A good friend said that my joints are outdated because manufacturers claim modern glues are stronger than the wood itself. But I have two problems with this.
Strength is important, but when building furniture we want a balance between strength and flexibility. I look for joinery that sympathizes with the nature of the wood, its elasticity, and its will to move. We don't simply stick the end grain of an apron to the side of a leg with super glue, for example; we use a mortise-and-tenon joint. I don't consider it overkill to take the same approach when edge-jointing long boards. In fact, I find it quite strange that it's so uncommon to take this approach today.
The second problem I have with putting all my faith in a glue is consistency of application. In my testing, good joints would not give, no matter what. But bad joints plopped apart with a mere touch. That's because gluing is very scientific. It's affected by humidity and temperature, surface prep, and whether you've applied too little or too much clamping force. I learned that some glues will not reach their stated strengths if there's any more than 2 percent difference in the moisture content between the pieces of wood being joined. This is easy to avoid by using kiln-dried timbers, but large sections can have more than 2 percent variation within themselves.
Building workbenches for a living gives me more reason than most to obsess about glue lines. For my benchtops, I add tongues between each face-to-face joint. I'm not against glue lines. There is world-class furniture held together with the stuff. It all depends on your application, and because my methods are traditional, I find it suits best to stick, if you will, with what has always worked.
So, yes, modern glues are great. But I'll keep my trust in reinforcements I can see.