I've noticed something unexpected while testing PVA glues for strength.

Glues tested: Titebond Original, Titebond III Ultimate.

Wood species: Beech and Pine (and other unknown hardwood species).

Transverse to transverse grain joinery. All mating surfaces sandpapered.

Control: Glue applied evenly to both surfaces with brush then clamped straight away for 24h.

Test 1: Glue applied evenly to one surface with brush, then left to dry 1 hour. Then glue applied evenly to both surfaces before being clamped 24h.

Test 2: Glue applied to one surface with brush, excess glue scraped off with sandpaper (leaving surface moist), left to dry 1 hour. Glue applied evenly to both surfaces then clamped 24h.

I had expected the control joint to be stronger than test 2, and test 2 to be stronger than test 1.

In reality, the joints would seem to be equally resistant to sheer stress and longitudinal traction. All joints had barely visible glue lines, the difference between test 1 and the others is in the range of microns suggesting most of the dried glue had been absorbed into the wood during clamping.

The results seem to be reproducible, meaning this could have implications for correcting errors in assembly, or repairs and maintenance of cured glue joints.

Do you think this is unusual, given what we know about how PVA glues work?

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    "In reality, the joints would seem to be equally resistant to sheer stress and longitudinal traction." You have scientific equipment that measures that ? How do you plan to effect the entire woodworking industry with your findings ? – Alaska Man May 10 '20 at 18:06
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    Your testing methodology has exposed a flaw, as you suggest in your own answer. PVA glues, once cured will tend to not form a good bond with fresh PVA glue. A full cure is dependent on a number of factors you might be able to control for, but 1 hour is probably not long enough under most conditions. – jdv May 11 '20 at 0:14
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    There very well may be some conditions where the received wisdom does not hold true, but these wisdoms are often practical simplifications in a complicated world. I'd look to what folks who do a lot of regluing have to say about practical experience. Furniture restorers and luthiers have a lot of experience with a variety of glues, and how to glue up previous glued surfaces. I know most guitar makers would remove cured PVA from surfaces if they wanted a guitar not to fold up in half under stress. – jdv May 11 '20 at 0:17
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    I suspect that 1) conventional wisdom is correct, and also 2) what you found is true insofar as the joint is "strong enough", and 3) often we don't need the strongest possible glue joint. This is true a lot in woodworking; we don't need the "best" joint, we only need one that is good enough. Or said another way, what is "best" is dependent on context. All other things being equal, you want the strongest glue joint possible. You definitely want one that won't fail in use! Whether your method achieves that, I don't know. It might! I don't think that invalidates conventional wisdom, though. – Katie Kilian May 11 '20 at 5:44
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    @anthony (This comment was provided by Graphus and coppied here): Titebond, and all other yellow carpenter's glues, are versions of PVA, with yellow dye added! "Aliphatic resin emulsion" is just another way of describing the chemistry (there are others). Over here and in other markets overseas the water-resistant and waterproof version of PVA are frequently left white, and while some are described in yet another way a few are simply called PVA on the labelling. – Ashlar Feb 23 at 17:41

In the absence of helpful comments, I've done some research on PVA glue chemistry to see what this could be. The general rule of thumb is that wood glue does not stick to cured wood glue.

Wood glue binding undergoes two phases.

  1. A short-term drying phase (< 2 hr), during which the material is simply dehydrated.
  2. A curing phase (24 h+), in which electrostatic and covalent cross-links are formed.

Source: Ülker O (2016). Wood Adhesives and Bonding Theory. DOI: 10.5772/65759

I'm no expert on PVA glues, but what I'm observing could be as simple as partially dried surface glue being mobilized again through re-wetting before cross-linking occurs.

This would then allow it to be absorbed into wood or cross-link with glue on the adjacent surface.

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    Also, make sure you provide a reference for that image. – jdv May 11 '20 at 0:08

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