Hand Tools Archive

Subject:
Shrinkage tables and the hysteresis effect

Wiley Horne--So. Calif.
Thanks to all contributing! This is just a note to pick up on Bill's observation,

" In my experience, and for some valid reasons, they [shrinkage tables] over predict wood movement."

and John in NM who noted the same thing.

Back in 2007 on this forum, we had a discussion of this very subject, and Larry Williams said there was a phenomenon called 'hysteresis' which partly explained why wood, once dried, tended not to pick up the amount of moisture expected during seasonal humidity increases.

Sorption hysteresis. Doing some googling, I found a bunch of papers on 'sorption hysteresis'. It's a well-known phenomenon, though the theory and mechanism of how it happens is still being argued. Most of the papers on the subject become quickly impenetrable without a lot of background or further study. But here is a study from 1957 which is clearly written, whose terms are defined, and whose summary results comport well with other experimental results:

http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/6022/Some_Aspects_ocr.pdf?sequence=1

The nut of the matter is that the amount of water a piece of wood will hold at a given temperature and relative humidity (it's equilibrium moisture content, or EMC) depends on that wood's history. Suppose we're talking about the EMC of a piece of wood being held at 60% humidity and 75 degrees: The eventual moisture content of the wood--it's EM--will depend upon whether the wood is coming from a wetter condition (i.e., it's desorbing), or whether it's coming from a drier condition (i.e., it's adsorbing). Desorbing wood will reach steady state at a higher moisture content than adsorbing wood.

This study's experiment with aspen indicated that the desorbing or drying wood would arrive at an EMC~ 12%, whereas a similar block of wood which had been drier, would arrive at EMC ~ 8.5 to 9%. Both blocks being held at 60% humidity and 75 degrees, come to significantly different EMC's. The previously dry piece stayed drier. In the study, see Figure 1 towards the bottom of the article for summary results.

This hysteresis effect, together with the moisture-retarding effect of film finishes, begins to explain why actual furniture shrinks or expands less than the quoted shrinkage figures, which I believe are based on the initial drying from wet wood.

Again, thanks to all!

Wiley

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