air dried v kiln dried

[spiral]

There can be, depending on the species. You can think of the structure of wood as being like a bundle of drinking straws, with membranes blocking each straw at periodic intervals along the length. There are little holes connecting the straws and through each membrane, allowing water to flow (this is how sap is transported). This is the same route by which water flows out during seasoning and back in when left in the shed.

 

The little holes are covered with flap valves. This is why trees don't keep on bleeding, despite the enormous pressure from the head of sap at the top to the bottom. These flap valves can normally open and close. In some species, once you get below a certain moisture content, the flap valves close permanently and cannot re-open even if the moisture content is increased. This makes water transport much harder, so the timber is far less prone to swelling and contracting. Spruce is a particularly good example.

 

Alec

 

Good post Alec!

 

I agree kiln dried is with some timbers more stable, Thanks for explaining why!

 

I think that, that stability goes with the reduced toughness on occasions though, so better for some things not some others. Horses for courses...

 

Interesting you mention spruce, 18 years ago I was given 10 tons of unsorted Swedish & finish spruce shorts. {most 1 metre planned 6x1 , some up to 1.7 m.} I kept the best clear straight grained, slow grown 3 tons.. Rest I gave away. What I kept was the Best spruce Id ever seen or worked{The firm it came from were getting through 5 artic a day of it! }

 

Wish the company was still going! They hadn't room to store there offcuts!

[se7enthdevil]

toughness is definitely something that is reduced in the kiln in my opinion as some of the sycamore and beech i used is just so much tougher.

 

i have a beam of oak that was air dried in a porch for 30 years that i got from ebay and turning it is like turning stone in some places like knots. you don't get that from kd euro oak.

 

i've also nearly got through the last of my slabs for the sycamore skittles and the timber in that is so much superior to the stuff i get from lumber yards.

 

the only exception to that is the colour. i do not need nice white sycamore for skittles it just needs to be sound wood.

[spiral]

I agree, Air dried is what you need for your skittles.

 

On another track, Would pressure impregnating of super glue or similar, even if just in otter layers make a difference? {A friend of mine does it himself to stabilise & harden knife hilt timbers.} Although I guess costs would rise.

[ucoulddoit]

I suspect there is a differerence between timber dried in a steam kiln and timber dried in a dehumidification kiln which works at a much lower temperature? So maybe the smaller scale operators using dehumidifiers might be producing better quality timber?

 

I read years ago but can't remember where, that although the moisture content of timber rises when stored outside after being 'kiln' dired, many species don't behave elastically. So the width of a board dried down to say 8% mc, then left outside until the moisture content rises to say 16%, will still be narrower than the board width when it was originally dried down to 16%. So I suppose you could say that some of the drying shrinkage is 'locked in' which reduces the risk of movement in service which corresponds to a note for some species in timber data tables 'low movement in service'.

 

Andrew

[spiral]

I suspect there is a differerence between timber dried in a steam kiln and timber dried in a dehumidification kiln which works at a much lower temperature? So maybe the smaller scale operators using dehumidifiers might be producing better quality timber.

 

Possibly so, I am not sure.

 

one would need evidence to be certain though.