Wood and moisture content

[cessna]

But of a theoretical question/s.

What holds water in a log/tree?

Does a length of wood say 2.4mtr want to loose moisture rather than gain it , when in a stack, of course depending on outsides air temperature and humidity?

What I am wanting to know in practical terms is, if a stack of wood split lengthways by cone splitter, has a tarp on the top of it will it keep loosing a small amount of moisture through the winter providing the relative humidity of the outside air is less than 99.0% humidity.

 

[BillQ]

I would have thought so as the wood is split so has a cut face larger than unsplit wood so can lose moisture as air moves across it and I would presume that the less moist area draws moisture from the rest of the log???

However I did notice that the poplar I cut and split a year ago had dropped to 7% in September but by November had risen to 12%, my wood is stacked in an enclosed shed with lots of vents so slightly different to your scenario. 

[openspaceman]

Most of the water in a green log is simply filling the wood cells and is lost relatively easily if  the surrounding air is low humidity and it can migrate out of the wood to the surface.  As air becomes warmer it can hold more moisture as vapor, so warm dry air can remove a lot of moisture wheras cold 99% RH air can carry no more.

 

Once the moisture content falls below about 25% all the free cell water is gone and the remaining water is lightly bound to the woody molecules. This water is a bit more difficult to remove but as it is lost the wood shrinks, a bit like the way copper sulphate holds waters of crystallisation, if you heat it this is driven off and the colour changes from blue to white.

The other thing, as @BillQ notes, wood is hygroscopic, once it is below 25% and only contains the bound water its moisture content rises and falls a it reaches an equilibrium with the ambient air, so in a wet winter it will re adsorb water to up to 17% if it is cold and humid and in a hot summer it will dry down below 10%.

[Muddy42]

The cell structure of wood holds the water.  Water is mainly held just under the bark in the sapwood.  splitting the wood helps by increasing the surface area but also by breaking through the bark which can hold in the water.

 

The way I think about drying is surface water evaporates first, then water diffuses and travels from the inside of the wood to the outside 'evening itself out' and then evaporates from the surface again, this time more slowly.

 

High surface area and airflow aid this evaporation process. Hot low humidity air can absorb more water.

 

Ultimately with time and patience any wood will be dry enough to burn in 1-2 years.  Personally I like a bit of weathering and keeping wood outside before brining it undercover for the final stages.  But I have lots of space and a loader to shift IBC crates around.

Edited 6 hours ago by Muddy42

[openspaceman]

1 hour ago, Muddy42 said:

Hot low humidity air can absorb more air.

edit required

[Muddy42]

2 hours ago, openspaceman said:

edit required

thanks, done.  too much hot air !!