Edge joint failure - what is the cause?

Question

I've had a few instances where my glued up panels, breadboards etc. have had gaps open up at the edges. This happens a few days or weeks after gluing. The gaps have all stopped at a certain point and the join remains strong, but it looks terrible. I'm hoping to start selling soon and I can't afford for things to fail like that. It seems that the wood is moving after it gets to its new environment and is able to overcome the strength of the glue.

This has happened with multiple woods and multiple glue bottles so I know they aren't problems. I tried using splines to reinforce the joint but it still failed.

To mate the edges to be joined I start with a straight edge off the jointer, thicknesser, or table saw and then hand plane it smooth and flat. I am pretty certain the quality of the mating surfaces at this stage is not a problem.

I then usually put a very slight concavity in the surface, both across its width and its length. I was taught to do this as it supposedly helps the ends and sides of the joint that are exposed to air and moisture mate properly. I then glue with PVA and clamp.

Possible causes that have occurred to me are:

1 - The moisture content of the wood just isn't close enough to what it will reach in the environment it goes into. This seems like a big issue but one that is hard to avoid. The area I live in is fairly humid and my workshop is not well insulated or heated. The pieces mostly have gone into a very modern house which feels very dry which can't help. It seems unlikely to me that commercial furniture is made in a climate controlled environment though. Do I need to relocate?

2 - The concavity I put into the mating surfaces might need to be more extreme. Since the joint is pulling apart where it should be tightest it seems unlikely to me that the concavity is causing the problem. Rather, it seems that I might be making the concavity too subtle. Thoughts?

3 - I might need to use a different kind of glue. It is often said that any of the major woodworking glues are stronger than the wood itself but that isn't my experience; these failures are right along the glue line.

I've done some testing where I glue two bits of wood together, leave it overnight, then break it apart with a hammer. The break would generally be fairly clean with only a bit of splintered wood. This would suggest a glue issue to me.

Perhaps there is something I am doing wrong but I can't imagine what there is to mess up with PVA. I always leave it to cure overnight. Maybe I need to switch to something stronger, like epoxy?

4 - Clamp strength. I've read about glue starvation from excessive clamping pressure but I've also read that it is a myth. I would describe my clamping as "pretty damn tight". Maybe I need to either increase or decrease my clamping pressure.

5 - Reinforcement. I know a butt joint is supposed to be strong enough on its own but maybe there is something I can do to really pin the joints together at the ends? Splines didn't work but then I suspect they may only help with alignment and may not give the joint any extra strength. Is there some way to mechanically lock the ends of a but joint together that is invisible or that is easy to hide? If there is a method I suspect it would only work on thicker boards.

Answer 1

Professional woodworker here. I have to admit when I first answered this, I was thinking about face to face wood joints (rather than end to face) so most of this answer is talking about that BUT a lot of these points apply to all types of glued joints:

There's a lot to go over here so I'll cover each point in turn:

1 - Moisture content. This can certainly affect glue joints. You are correct that in professional joinery/furniture etc. workshops, humidity is often (tightly) controlled. If the moisture content of the wood or the air is off, or the curing environment is not warm enough then this can lead to problems. Particularly, if it's too cold then PVA will cure as a frosty white colour instead of more clear. This can affect joint strength. If you have two different pieces which you are gluing together and they differ in moisture content (or if they are different species or very different sizes and are likely to lose/gain moisture at very different rates) then this can also cause high relative movement which can cause a joint to fail. If environmental differences are unavoidable then you'll want to let the timber acclimatise as gradually as possible.

2 - Concave faces. I've never heard of this and it shouldn't be necessary. I've got an oak breadboard sat on my desk right now that was glued up straight off of our thicknesser (jointer) and the joints are fine, no gaps have formed. This article from popular woodworking seems to agree.

3 - Glue. If you can break a joint at the glue line then something isn't right - unless it's an end-to-end butt joint or something which doesn't really work anyway. It's not just a thing that people say - a properly processed glue joint will be stronger than the wood surrounding it. You could try different glues but if there's something wrong with your process it may not help. Also bear in mind that the glue must be stored correctly - if it's been stored in particularly high/low humidity or has ever been allowed to freeze then this can affect joint strength and cause joints to fail.

4 - Clamps. I'm pretty sure that "glue starvation" is a myth as you say. When I'm gluing up, I use heavy duty T-bar sash clamps like this:

The second image there is something that's in our workshop right now. I've tightened these up to the point where they've started to crush/indent into the timber before and never had issues with "glue starvation". It's always been my impression that when gluing up, the stronger clamping you can use the better (up to the point where you damage the work, obviously). I'd say it's basically impossible to clamp the wood with enough pressure that too much of the glue squeezes out. The wood would disintegrate first. If anything you might require more pressure than you've been applying, but certainly not less.

5 - Reinforcement. With face to face glue joints this shouldn't be necessary, and not easy to do in a neat/hidden fashion as you say. We generally use dowels or biscuits but these are for location rather than strength. If you really need to then there are hidden joint systems out there, such as this magnetically driven fixing: Axminster Lamello Invis - Youtube - this is quite expensive though, I think individual fixings are about £6 ($10) and the tool to tighten them is about £100 ($140ish).

There are also other factors which you have not mentioned such as application: Make sure you fully cover both surfaces with glue, do not let them skin over before pushing together, and then clamp such that excess glue is squeezed out. Clean any excess off with a scraper.

On joints generally. Butt joints (end-grain to side face) are the weakest kind of joint, except for end-grain to end-grain which are weaker. If you're gluing the end-grain of the joint to the face of another piece of timber the joint won't be inherently strong. Depending on the exact nature of the joint it may be possible to do a sort of hidden spline as seen here:

Hopefully this all gives you something to think about and try.

Answer 2

Since the joint is pulling apart where it should be tightest it seems unlikely to me that the concavity is causing the problem. Rather, it seems that I might be making the concavity too subtle. Thoughts?

You're doing what are called sprung joints, or a spring joint in other places. These should do exactly what you want them to do, help the two ends not come apart.

It is possible the concavity is a little too subtle as you put it, however, the important point is the joints shouldn't come apart even if the jointed surfaces were completely straight.

So I think it's extremely unlikely this is the cause, although it could be a contributing factor if there are multiple causes.

It is often said that any of the major woodworking glues are stronger than the wood itself...

This is an example of sloppy wording, one I particularly hate because it's so commonplace. As I've covered in previous Answers it is better expressed as glued wood joints are stronger than the wood, not the glue itself.

In reality many wood glues are not that strong themselves! Standard white PVAs are soft enough that once set they can easily be dented with a fingernail, soft enough to glue paper together and the joint can bend. But still, used properly they can create a joint in wood stronger than the wood around them... even in particularly strong species the joint will always be stronger than the grain of the wood.

And in fact ALL glues used for structural joints in woodworking are capable of creating joints stronger than the wood, so the whole question of which glues are stronger or weaker, which glue is the strongest of all, is really just a distraction.

I've done some testing where I glue two bits of wood together, leave it overnight, then break it apart with a hammer. The break would generally be fairly clean with only a bit of splintered wood. This would suggest a glue issue to me.

Glue is not the issue, I think you ruled that out yourself using multiple bottles.

Since this is test pieces and not full jointed edges I'm betting your problem is either your wood is a little too wet or your clamp pressure is too low, or possibly both.

On longer joints the clamp pressure being insufficiently spread along the entire length of the joint can be a definite problem (even VERY strong clamps are not enough if only placed centrally) but this kind of thing seems unlikely in tests like this.

I've read about glue starvation from excessive clamping pressure but I've also read that it is a myth. I would describe my clamping as "pretty damn tight". Maybe I need to either increase or decrease my clamping pressure.

As I've covered in a few previous Answers here it is as far as we need be concerned a complete myth. No clamps we use are capable of squeezing out too much glue from a well-glued joint and having the joint end up starved.

Many tests have proven this conclusively and I've done my own to see the effect with my own eyes, using multiple large C-clamps on small test blocks (so many thousands of psi at the joint face, beyond the crush limit of the wood) and the resulting glue joint was exactly as it's reputed it can be, stronger than the wood.

And this points to a key recommendation — clamps should, ideally, be tightened to the point where they will mark the wood. So you should always need to use waste blocks to take the denting and to help spread the clamp pressure.

5 - Reinforcement.

No need to bother. Unreinforced butt joints are used extensively (to say the least) so if you find the issue or issues that are causing the problem for you you won't need to waste any time thinking about this.

One other possible cause
Glueing old surfaces. Again this is touched on in previous Answers, only freshly-worked surfaces bond reliably. Yes people do glue pieces that they prepared days or even weeks before together all the time and get decent results, but where you want to ensure the joint is as strong as it can be you must only glue surfaces that have been freshly worked (within the hour, ideally shorter than this).