Thread: air drying

Originally Posted by pjt

I was reading another thread on bar tops and drying and decided to start a new thread,

Hi all, just thinking about this 1"/year air drying thing,

Am I thinking this right?

This is number thrown around in the northern hemisphere where they have cold winters and mild summers. In Australia that number probably still applies to Tassie but overall it depends where you are in the year. For example if a 2" slab is cut in say November of year X it could be dry by the end of Jan in the year X+2 so than means it has only taken 15 months to dry. If you cut it in April of year X is will probably still take until Jan in year X+2 to dry so that means it has taken 21 months.

the rate at which moisture is lost would be greatest at the start of drying (greatest difference between timber MC and outside air) (hence we seal the ends to slow down drying) and will be continually slowing down as the moisture is lost to the air, in other words the rate of drying is not linear but a negative exponential moisture loss rate or said another way the last bit of moisture takes the longest to be removed.

Correct. BUT inverse exponential assumes a constant external MC but this changes even over the cycle of a day and then over a year. So the MC sort of decreases by a tiny squiggle added to a couple of longer period squiggles on top of the inverse exponential.

And once the MC has come down to a point where it doesn't lose any more water (free water + cell wall water) it's as dry as it's gonna get without some sort of forced drying.

Once the wood is close to dry, one average Australian Summer will be more than enough to pull it past a constant MC.

And I would wonder whether timber force dried to below what it would normally dry to when just left to air dry is worse cos it's just gonna suck moisture back in untill it reaches equillibrium with the surrounding air.

Correct, how quickly this happens depends on the thickness and type of wood the range of the MC forcing function.

Has anyone measured the MC in a bit of green wood over time say monthly and noted the drying rate or weighed a piece of green wood and noted the weight loss over time, I have wanted to do these tests myself but havent as yet

Yeah I started doing this with a MC probe meter but it is not the best device. If you do this by just weighing a small cross section of slab it dries quicker than the who slab. The wood being measured should be from the inside of a slab hence an ElectroMagnetic meter is needed.

And the drying time taken would be different for the same green timber dependant on where the timber was, say in Tas or Qld..

Yep

And this is why timber is cut into thin planks before it is kiln dried - it dries quicker closer to the surface and you get less differential pressure causing cell collapse.

And why 100x100 pine is commonly 2 pieces of 100x50 laminated together.

Cheers

Graeme

Thanks for that, seems I is somewhere in the paddock
Is the cell wall collapse more prone to happen in the softwoods than hardwoods?
Or is it just a function of the larger section sizes?

Originally Posted by pjt

Thanks for that, seems I is somewhere in the paddock
Is the cell wall collapse more prone to happen in the softwoods than hardwoods?
Or is it just a function of the larger section sizes?

Don't know about pines - we only kilned hardwoods.

Its a function of trying to dry the timber too fast. Trying to increase the kiln throughput.

Cheers

Graeme

I have a program call drywood it works from air dried in shed open air to kilm.
Density seems the most important thing.
When I dry i cut a piece and place it in the middle near the bottom of the stack on sliding stickers. I cut a bit off this plank and weigh both then insert the plank in the stack and take the small piece and put it in the oven and dry it untill it does not get any lighter then calculate moisture content then how heavy the plank will be when it gets to whatever% them pull it from the stack every now and again and weigh it.
The program drywood is good because after a couple of weighings you can work out how long it will take to dry under your conditions.
I would take this 1"/year with a pinch of salt. May give some idea but not enough to be usefull. You can use the weight method to check moisture meters as well as they all vary for different timbers even different parts of the same tree as density is not consistent.

A couple of points:
Firstly- Drying timber is necessary to prevent dimensional change or shrinkage.
Once free moisture and combined (cell moisture) are removed the timber becomes dimesionally stable.

Secondly - Air drying will remove all the free moisture and some of the combined moisture however some of the combined moisture remains bound (ionically) to the cells. Longer air drying can remove this bound moisture but it is best done in a kiln. Hence the cabinet makers preference for kiln dried timber. Air drying timber down to say 20% is pointless as further dimensional change (shrinkage) will occur.

For this reason I stand by my comment that a slab 50mm thick will require 2 years air drying to approach dimensional stability.

Most of the shrinkage occurs in the latter part of drying.
ie under 30% moisture content. Very little shrinkage occurs in the early part of drying but in the latter part of drying the shrinkage is almost directly proportional to moisture loss. ie a straight line.

As a rough guide:
30% Moisture content - Fibre saturation point or FSP where all free moisture has gone.((FSP for cypress pine is 22%) Below this the wood starts to lose the combined moisture.
From 25% down shrinkage vs moisture content becomes linear. eg for cypress pine every 1% loss in moisture content results in 0.3% change in the timbers dimension (shrinkage). The same occurs in reverse. If the timber is overdried it will expand proportionally as it takes up moisture.
12 to 10% All combined moisture removed.
0% - Oven dried.
Interestingly timber continues to shrink down to 0%.

So for dimensionally stable timber (which is what this is all about) you need to get the moisture content down to a level where the combined moisture is removed (under 10% MC). The timber can then re absorb moisture to reach local EMC.(equilibrium)

With regard to collapse, this usually occurs when the timber is heated too quickly while moisture content is above FSP. This boiling effect causes surface tension effects in the cells changing their shape.

Anyway that's my 2 Bobs worth.

cheers
Steve

Great information, worthy of a sticky in my opinion. Where I am air dying won't get you below 18% or so. So when making furniture, serious consideration and allowance must be made for further shrinkage.

Thanks guys, that is great info, the 1'/yr thing always sounded a bit non scientific to me, a bit how ya going, so many factors come into it specially when drying.

From the info then we could say that a 2" thick slab of say mango will be down to about 20% MC after 2 yrs but then need further drying in a kiln to further remove the bound up moisture to 10% ....or will longer air drying still bring it down?

I always thought the the old time joiners/cabinet makers much preferred air dried timber or is it cos of all the defects that KD causes? and Is the best timber air dried then KD to remove that last bit of moisture?

Has anyone got one of those small solar kiln set ups, they look like they might do a reasonable job specially in Central Qld.

Peter

it is worth a sticky ,very informative thread given all the info, my problem is this, air dry timber, be it blackwood,cypress or redgum. In my area the timber gets to 12%-14% emc. my question is ;once the slab has been machined, sanded and coated in whatever finish required ,will it still move with the moisture still left in it?

Originally Posted by pjt

Has anyone got one of those small solar kiln set ups, they look like they might do a reasonable job specially in Central Qld.

Peter

There are CSIRO plans on the web for solar kilns that dry timber to 12% in about 60 days in QLD.

I have a sort of a solar drying shed. It's a seatainer that has vents along the sides and 2 whirly birds on the roof.

Surprisingly, provided the shed is kept close to full, the air temperature inside is only around 7º hotter than the outside temperature in summer, and about 5º hotter in winter. The bulk timber temperature varies a lot less than this depending where it is in the stack, but is always above outside air temperature. This temp difference does not sound like much but the shed still dries timber in less than half the time it normally takes to air dry. If the shed is not kept more than half full the temperatures varies to a greater extent which may damage the wood.

What this drying shed does is push the MC content of 2" thick timber down to about 12% during one winter and one summer. It doesn't go any lower than this probably because the shed is less the 5 km from the ocean. This is enough to remove most of the combined moisture to a point where it can be close to dimensionally stable. As stopper says, be aware that there is still a chance that even 12% MC timber can shrink even further in some environments - for example our builder put some floor boards with a 14% MC content down in an upstairs bedroom in our house and the MC content went down to 8% and the boards shrank noticeably over one summer! One way around that would have been to put the timber into that room and let them equilibrate with the ambient MC over one summer - but of course if you can't want for a whole summer to do this there is nothing else for it but for them to be fully kiln dried.

Sorry to hijack the post guys, get reading by the way..
But I have an air drying related question
Just finished a simple chainsaw jig and plan to slab some stringy bark and peppermints, my question is what is the best way to stack and dry the timber?
This is a website I have found most helpful
http://sres-associated.anu.edu.au/fp...ir.drying.html
Cheers
Justin