John Rumm wrote:

On 23/11/2017 22:14, Roger Hayter wrote:
John Rumm wrote:

The direction of the bend in the OPs case would suggest it was the ply
swelling that was causing most of the movement.

Perhaps I was inaccurately calling it engineered oak. In fact it is
made of small blocks compressed and glued together and is the same both
sides. Is that better or worse.

"Engineered" normally implies a man made board either bonded to one side
or inserted as a core (depends a bit on what you are describing - e.g.
engineered floor will usually be ply on the underside. An engineered
door would be real wood on all the facing sides and MDF or ply in the core.

Are these all oak blocks or is the core made from something different?

(lots of "solid oak" furniture these days is made from reconstituted
boards assembled from lots of small offcuts finger jointed end to end
and then bonded up into laminations)

Either way, blocks will tend to behave differently from solid wood
because the glue between layers will stop the moisture travelling
through the wood in the same way it would with solid. That can make
bending more pronounced when unevenly exposed to moisture since it
concentrates the expansion in the blocks exposed directly to the moisture.

Actually I have re-examined the problem and it looks as though there may
be something in my original theory that the plaster may have had a
chemical effect, rather than purely acting as a reservoir of water.
The wooden blocks themselves have hardly warped but the three glued
joints between the blocks have widened on the plaster side from an
imperceptible width before to about 1/2 a millimeter now. This seems a
more destructive effect on the glue than I would expect from plain water
seeing that this wood is sold for kitchen use with only light varnish.

--

Roger Hayter