DC

[Ron Nossaman]

>"Humidity goes into wood faster than it out gasses out."
>
>Where does this information come from and why would it be so?
>
>Terry Farrell


Terry,

I don't recall ever being able to find this in any authoritative source 
literature, nor the one about drying wood to 0%MC forever changing it's 
characteristics. Here's where I think this premise comes from, based on 
information from reasonably official sources -

Wood does lose or absorb moisture at a rate proportional to the MC of the 
surrounding air compared to it's own current MC. The greater the 
difference, the faster the moisture transfer rate. That's the official 
stuff - the rest is my conjecture. When drying a soundboard panel, for 
instance, the closer the panel MC gets to the MC of the air in the hot box, 
the slower the drying. No one notices that the panel drops from 11% in a 
70°F shop at 60%RH, to 7%MC in a 120° hot box at 20%RH pretty quickly, but 
they do notice that it takes a long time, at an ever slowing rate of 
change, to get that panel down to 4%MC in that same hot box. So the 
important part of the drying (the part that actually gets the wood close to 
where you want it) takes what seems like forever. Taking the panel out of 
the box and exposing it to the 70° shop at 60%RH will take the panel from 
4%MC to 7%MC MUCH faster than it took to dry it through that range because 
of the wide MC difference between the wood and the surrounding air in the 
shop. It will take a considerably longer time for the panel to reach 10%MC 
from 7%MC, and even longer to reach 11%MC from 10%MC, than it did to reach 
7%MC from 4%MC, but it's too late to rib it by then anyway, so no one cares 
to notice.

So the wood doesn't necessarily absorb humidity faster than it releases it 
(at least not in this example), but in the humidity difference ranges in 
which we work with it, it gives that impression.

So does wood really absorb humidity faster than it releases it? I don't 
know, nor know where to find out.

Ron N