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Thanks Fillipo.
The live back braces are Gore-size - a beefy 10mm deep and 18mm high, tapering down to 3mm.
RobRus, don't have issues with the tops, just the backs.
Per your comments, as the plates are bookmatched, if the issue was with the woods, one would bow one way, and one the other (as i have seen in some spruce plates)
In this case, they bowed evenly, telling me it is surely an RH problem, and most probably the result of over-zealous scrubbing of the excess glue with dampness.
Lesson learned.

I've had exactly the same problem with a yew back and plan to make some laminated spruce carbon fiber braces and brace with them

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Formerly JJH

I learn more from my mistakes than my successes

Now that you point it out that makes sense they would bow in opposite directions. Mine are bowed evenly as well so I wonder what the deal is with mine. My RH is between 40% and 45%.

At least you found the problem and can avoid it in th future.

Bob

I use fish glue for the back braces. I also wipe down the braces after I have finished removing most of the glue -- a little bit of moisture helps with removing the glue residue which I scrape away with a razor blade. I use a damp paper towel but certainly not one that is dripping wet. Once again, I've never had any issues at all. In other words, the problems some of you are experiencing are likely RH or a combination of factors -- but fish glue and wiping with a damp cloth will not cause these issues. If any of you are continually struggling with this issue -- please feel free to give me a call and run down your entire process with me -- maybe I can pinpoint a problem.

Best Regards,
Simon

If you join a plate and lay it flat it'll likely curl up away from the surface it's sitting on. If you flip it over it'll flatten out then curl away again.

If you catch it when it's flat then lean it so air can circulate around it then likely it will stay flat.

I leave my plates unjoined until I'm ready to brace them and then immediately glue them to the ribs.

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Old growth, shmold growth!