Mark Kinsler wrote in the 070328 MMDigest about a book, written
by J.E. Gordon:

  "One of his (J.E.G.) investigations revealed that the plywood
  being used to construct aircraft had been rendered un-glue-able
  because it had been rolled with hot, smooth metal rollers during
  its manufacture.  This closed the ends of the cellulose tubes,
  thus preventing glue from entering and being soaked into the wood.
  The joints seemed strong at first, but failed under stress.  Only
  careful sanding fixed the problem, but Prof. Gordon's general
  opinion of wood as an aircraft material under conditions of war
  production was not a high one."

This is an intriguing text.  Plywood, in those days, was produced
using natural glue like hide glue, so under tropical circumstances it
would not last very well since hide glue cannot stand high temperatures
and/or high humidity over time.  That's the reason why piano makers
built special instruments to be shipped into tropical areas: these
instruments were remarkably heavier, since the glued joints, and even
the soundboard, were reinforced with screws.

During WW2, using plywood as a cheap construction material for gliders
was clever.  These gliders were meant to be used only once, and if
the battle was lost (like Arnhem) the enemy would not score a lot of
reusable metal.  And, in those days, it was not very long ago that wood
was a common material for airplanes, cars and boats.

Plywood was produced using heated rollers, thus applying pressure and
heat, so the glue would melt and bond.  I do not understand why the
plywood became un-glue-able.  At least, I do not understand what
cellulose tubes are.  I never heard of them.

Plywood normally consists of several layers of thin wood.  These layers
are cut from a log lengthwise, so the surface will show a lengthwise
oriented structure.  Even if speaking of tubes of any kind, plywood
will show no, or hardly any, open "tube" ends.  So when gluing layers
together, the sides of these "tubes" will be glued together.

Sanding will make a wooden surface smoother, which is not a good thing
to do if you want to glue it.  Making it rougher does actually expend
the contacting surface, making the glued connection even stronger.

I looked for the book by Prof. Gordon but was unable to find it.  I
would appreciate a copy of the chapter in which those "cellulose tubes"
are mentioned, because it's an intriguing subject.

Jan Kijlstra
The Netherlands

 [ Long, long ago, Jan worked at the Rippen piano manufacturing firm.
 [ Read his article about the cheap but innovative Lindner piano, at
 [ http://mmd.foxtail.com/Archives/Digests/200002/2000.02.11.09.html
 [ -- Robbie