Is is well known that a hole cut into a metal piece would expand when the metal is heated, just as if the piece would expand without the hole.

Is it the same for a hole cut into wood, when the wood is exposed to water or moisture? I have nothing to test this now, but it seems that, contrary to the hole cut into metal, the hole cut into wood actually shrinks as the wood swells. Just a hole in raising dough would close off.

If it is actually shrinking, why? What physically differs between heat expansion and wood swelling?

  • $\begingroup$ The reason that the hole in the dough closes off is due to the fact that other parts of the dough are constrained. If the dough actually could undergo a uniform expansion then the hole would expand as well. With a hole in wood the question would be, is the moisture content increase uniform throughout the wood and can the wood expand freely? If yes, then the hole will expand too. If there has been more moisture absorbed around the hole boundary then the hole could close up due to the constraints from the surrounding material that has not absorbed moisture. $\endgroup$ Feb 16, 2020 at 13:56

1 Answer 1


Let me first get something out of the way:
It seems to me that wood is an unsuitable material for this thought experiment in the following sense: wood is very resistant to change of dimension parallel to the grain, but not perpendicular to the grain. Wood changes very unevenly.

But of course your question can readily be reformulated to feature a material that also changes dimension dependent on how much water is absorbed, but evenly. Such even swelling is a better fit for comparison to thermal expansion.

When a material is heated, with corresponding thermal expansion, this expansion does not mean any internal tension will arise.

For absorption of water into a material I imagine the following: the material has structural integrity (otherwise it wouldn't be a solid), so I expect that when this absorbant material becomes saturated an internal tension will arise.

Presumably the material absorbs water because of forces of cohesion. I imagine that these forces of cohesion will cause water molecules to be wedged into openings that are barely large enough to acommodate a water molecule, hence development of internal tension. The point of saturation will be the point where the internal tension has risen to a point where there is an equilibrium of force. An equilibrium between forces of cohesion that tend to pull in more water, and force of internal tension that tends to push water out.

I expect that this internal tension will affect the size of a hole in the material.


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