Today’s Glues are Super Strong
When customers stop by to peruse the lumber and slabs I have for sale, they inevitably end up near the back of my shop, where I do my woodworking. They like to see what I am up to and discuss woodworking in general. Lately, I have been making a lot of live-edge tops, so I usually have at least one being glued up, and I can guarantee you that the first question is going to be, “What do you use to join those two pieces of wood together?” They are expecting a dramatic answer full of technical jargon, like tongue and groove or sliding dovetail or dominos or even biscuits, but I always disappoint them and just say, “glue”. I like to say it in a sort of caveman fashion for dramatic effect and a bit of humor, but then I quickly jump in and fill the awkward silence with a more detailed explanation, especially since I can tell that just blurting out the word “glue” isn’t going to be enough.
I use Titebond original wood glue with the red cap. There is Titebond II and III for more water-resistency, but I usually stick to the original unless it is a project that is prone to getting very wet. I like that the original cleans up easily with water and that even dried glue can be soaked and removed from brushes and clothing. I don’t prefer Titebond for any special reason, except that it is widely used and widely available. I would just as confidently use other name-brand wood glues and expect similar results.
The glues available today are strong, super strong, stronger than the wood itself. To prove this, I always save the end cuts from my glue-ups, so I can break them later for demonstration purposes for customers and inspection purposes for myself. If the glue is fully dry (results are not guaranteed if the glue is still wet), the glued-up scraps will always break somewhere in the wood. Even if it does happen to spilt close to the glue line, there is always plenty of wood stuck to the glue to make anyone doubting the strength of the joint to become a believer.
In comparison, I have worked with plenty of reclaimed wood, especially old oak church pews, that have a tendency to split along the glue joints. When closely inspected, it is clear that the old glue had become dry and brittle, and though it stuck to both surfaces, the glue itself broke down, like old plastic that has been outside too long. Most likely, the older glues, while strong at the time, weren’t formulated correctly to stay flexible over time. Current glues are formulated to hold strong and not break down during regular indoor use. Note that I wrote “indoor” use – for outdoor use, all bets are off. From extreme wood movement to glue breakdown, there is simply too much wear and tear outdoors for the glue to hold a jointed edge together on its own without any eventual failures.
So, we know that the glue is strong and is more than capable of holding a joint together, but just how strong is it? There is probably some value on some fancy scale to tell you exactly how strong the joint is, but it doesn’t really matter, as long as you know that it is stronger than the wood. At that point, to know the strength for sure, you would need to know the strength not only of the wood you are working with, but the weakest point in any given spot in a board, which you just can’t know, so I say stop worrying about it. Just know that it is more than strong enough to do the job.
Now, for the glue to work correctly, your machining and joints need to be reasonably good. I say, “reasonably” good because I think there is a lot of wiggle room here. Obviously, if everything is perfectly square and straight, there is no question about your joint integrity. You can simply coat the joint with glue, apply just enough pressure to pull everything together, and you will end up with a strong, wonderfully impressive joint. But, what if your jointed edges are square but the boards are long and have a bit of a bow and they will require a bit of extra clamp pressure to pull them together, is that gonna work? Heck yeah! Did I mention the glue is strong? A little extra clamp pressure is fine.
What about a lot of clamp pressure? Now this is where the “reasonably” good part comes into play. I say if you are doing a glue-up and you feel like you have applied so much pressure to pull things together that it just feels wrong, then you should probably work on the joint some more. But, here’s the kicker. I can tell you that I have been involved in more glue-ups than I should admit to that have required an inordinate amount of clamp pressure, and to this day (knock on wood), I have never had a joint fail. Maybe I have just been lucky, since I have done tons of glue-ups, but I use this as a real world testament to the strength of the current glues.
The problem with needing a lot of clamp pressure to pull joints together is two-fold. The first issue is that there are built-in forces that are always trying open the joint with the same amount of pressure it took to close up the joint, which can be significant. The other issue, and the one that is commonly more worrisome, is that more clamp pressure means less glue in the joint. The concern being that if all of the glue is squeezed out then obviously there is nothing to hold the wood together. As far as I can tell, especially since I have not had a failure yet, is that this isn’t easily accomplished. I am not saying it isn’t possible, but it isn’t easy. Many woods have open pores that will hold glue no matter how much pressure you give them (think oak and walnut), and if you are fighting at all to pull a joint together, that means that somewhere along the line things are loose enough to hold some glue. Sure, it might completely squeeze out in one spot and make the joint a bit weaker, but in other spots the glue will hold like it is supposed to and keep things from coming apart.
With all of this cavalier talk about crappy joints with extra clamp pressure, you still have to show some restraint. There are going to be times when you can’t rely on just the glue, no matter how strong it is, to hold everything together and you will need to rework your joints for a better fit. A couple of instances come to mind. Some woods have very tight grain that is smooth and won’t hold much glue (think hard maple), so it is possible to end up with a joint that has almost no glue in it. The second instance where more jointing work will be required is if the boards are tight in the middle and loose on the ends. The ends are where a top will want to naturally split, so trying to use extra pressure in this case, is inviting an issue down the road. I feel a million times more confident closing up a gap in the middle of a glue-up than I do the ends, knowing that the entire joint is holding things together, not just the glue on the ends.
One last category that requires a little extra attention is exotic wood. Some have oils in them that just won’t glue properly. They need to be cleaned with lacquer thinner before gluing to provide a good surface and they are often extra hard, so they don’t absorb much glue. I have had problems with bloodwood in the past, which fell apart during my initial tests because I had not cleaned the wood enough. To be safe, I cleaned the wood even more and roughed up the surface a bit with sandpaper to give the glue something to grab. Before the sanding, the edges were just too hard and too smooth. Since then, the extra hard and oily exotics scare me, so I would never force a glue joint with them. I trust the current glues a lot, but there are limits.
Assuming that you have decent joints and wood that will accept glue, all you have to do is make sure that both surfaces are coated with wood glue and clamp them together until the seam is tight. That is really all there is to it and all that is done at almost every professional shop I can think of. You don’t need any special tricks at all, just “glue,” I remind you in my caveman voice.