Thread: solar kiln for drying timber question.

Pointless? I don't think so.

But I suspect you may be starting a WIP for years to come... even if only on a small scale.

"Perhaps if I altered the positions of the fans..."

"I'm not happy with the access, I might try..."

You know, the perpetual "slight improvement" path. Even in a worst case scenario, total failure, you could remove a side and use it as a year-round covered stack.

Originally Posted by Skew ChiDAMN!!

"Perhaps if I altered the positions of the fans..."

"I'm not happy with the access, I might try..."

So true Skew, i'm a chronic 'tweeker'

The thing to watch is if you get the wood too dry you will have even MORE problems with wood movement. If you dry the wood down to 6%, build something and move it into your house where equlibrium is 12 % it's going to gain moisture and expand / move.

The trick is to get your wood at equlibrium with YOUR environment. If it's like your (Western side of NZ) then the average humidity is about 80% and wood should be sitting at 12-14% moisture. It will be prefectly stable there, it's just if it changes you will have problems.

But as for your kiln idea, yes it will probably work, as long as the wood you put in is air dried first. Working from green you need a bit more control and a poorly designed kiln can really mess things up. But once you get the wood down to under 20% it's pretty hard to damage.

Ian

Thanks Ian. I'm guessing that u have similar problems with humidity on the other side of the ditch?
Sounds like a moisture meter could be a good investment. The prices of the machines vary so greatly from bearable to downright painful. anybody got some suggestions on good moisture meters?

You get what you pay for, although there have been a few exceptional value for money ones offered on these forums at times.

The cheapies from Crabatec are alright if all you want is something to help you decide when to move the timber into the solar kiln. ie. when the actual percentage isn't important and you're more interested in "too green" or "close enough."

However if you want good, accurate readings, then $$$.

a solar kiln that heats up during the day and cools off at night, accompanied by nightly humidity swings may do more damage to your wood than good. there's more to it than just getting the moisture out of the wood as fast as you can- the hotter it gets inside the higher the humidity in there needs to be to keep from getting end check.

with pitchy softwoods like conifers (may not apply at all down under) getting the wood up to pitch set temperature is important- otherwise it will continue to bleed sap indefinitely.

Would this still be the case tho with 'air dried' timber? As i'm sure alot of people have already found, sometimes air dried timber isn't as dry as we like especally if the actual drying period has been 'fudged' by the timber seller.

Originally Posted by bridger

a solar kiln that heats up during the day and cools off at night, accompanied by nightly humidity swings may do more damage to your wood than good. there's more to it than just getting the moisture out of the wood as fast as you can- the hotter it gets inside the higher the humidity in there needs to be to keep from getting end check.

Aussie wood boffins have done experiments on this for Aussie conditions on Aussie timbers and provided the daytime temperatures generated are not too high (which is possible using adequate ventilation) they have found this reduces checking (splitting). It is surprising how little ventilation is needed. Two whirly bird extractor fans keeps my seatainer at no more than about 10ºC above outside temperatures. (I only did one experiment but when I blocked the fans the temperature rose to 30ºC above ambient.)

Commercial solar kilns are designed to dry the wood as quickly as possible in as small a space as possible so they recommend
- Adequate Ventilation and internal circulation. It's not just a matter of sufficient airflow into and out of the kiln it's also important to move air around inside the kiln itself to even out the humidity. It's surprising how much internal circulation is recommended - something like 100 W of ventilation fan power per cubic metre. This can add significantly to the cost of drying. If this is not done the wood can go mouldy.
- A thermal buffer (usually done with rocks or water) to store heat so that the timber stays warm overnight.

What I have found is,
If the kiln itself is big enough and contains enough wood then wood itself (or the water in the wood) acts as a heat store. Non-thermally buffered kilns still work they will just take longer to dry the wood.
To avoid supplying internal circulation I have used bigger stickers (at least 1" or 1.5" thick for wider slabs) so the humdity can escape easily - I have only had minor mould growth when using 3/4" stickers in winter. In Tassie you may need bigger stickers. This of course means you fit less timber in your kiln but for an amateur this is not an issue.

I also agree with Ianab, "The trick is to get your wood at equlibrium with YOUR environment". if your wood is already air dry, setting up a kiln to dry the wood below this is somewhat pointless, you just have to learn to live and work within your environmental constrictions.

Originally Posted by BobL

I agree with Ianab, "The trick is to get your wood at equlibrium with YOUR environment". if your wood is already air dry, setting up a kiln to dry the wood below this is somewhat pointless, you just have to learn to live and work within your environmental constrictions.

The reason this idea came to mind was that some people arn't as dilligent with their air drying as they should be, pushing out timber before its really ready.
I recently bought some Blackheart sasafrass that had been air dried & after it had sat in my shed for a good month or more to aclimitise i worked with it only to get some chronic cupping & twisting out of it. I spoke to a local woodworking guru & he told me that sasafrass is bad for it in the first place but also if it was air dried then it could well have not been air dried long enough. According to him its quite common for air dried timber not to have been dried long enough before it was sold.

Originally Posted by Rattrap

The reason this idea came to mind was that some people arn't as dilligent with their air drying as they should be, pushing out timber before its really ready.
I recently bought some Blackheart sasafrass that had been air dried & after it had sat in my shed for a good month or more to aclimitise i worked with it only to get some chronic cupping & twisting out of it.

Does you mean the twisting happened during the month it was acclimatizing or after you worked it?

How thick was the wood?

I spoke to a local woodworking guru & he told me that sasafrass is bad for it in the first place but also if it was air dried then it could well have not been air dried long enough. According to him its quite common for air dried timber not to have been dried long enough before it was sold.

Yep - this is common.

It does sounds like you need a moisture meter, but a good one (electromagnetic sensor type) will set you back quite a few hundred $.

The pin type ones only work properly on freshly milled surfaces and even then they need calibrating for wood type (all of them really need that to some extent). If the timber is not too thick you can use the pin type ones for relative measurements and if make a piece of wood that you know the moisture content of then you can always compare it to that. To get the moisture content of a piece of wood dry the wood to near zero moisture content (constant weight eg 500g) in an oven and then let it re-equilibrate with ambient humidity (it now weighs 550g so that is equivalent to 100x(550 - 500)/550 or 9.1% moisture. However to measure the moisture content of big pieces of wood they will still need to be cut open to the middle of the piece to reveal a fresh surface.

Hi Bob,
The boards 'were' round 30mm thick. They twisted like buggery during the month they sat in the shed then when i milled them up they twisted some more then when i re-milled them to size about 2 weeks later & made them into a document box it then twisted & warped all over the place once it was made. The solid lid of the box warped a good 20mm. It was so bad that i had to remove it, re-mill it again & its currently sitting in a clamping set keeping it flat. Its now only 8mm thick & only good for an insert panel.

Originally Posted by Rattrap

Hi Bob,
The boards 'were' round 30mm thick. They twisted like buggery during the month they sat in the shed then when i milled them up they twisted some more then when i re-milled them to size about 2 weeks later & made them into a document box it then twisted & warped all over the place once it was made. The solid lid of the box warped a good 20mm. It was so bad that i had to remove it, re-mill it again & its currently sitting in a clamping set keeping it flat. Its now only 8mm thick & only good for an insert panel.

I'm guessing they were in a stack and not completely dry. On being sold to you they dried some more and also twisted - bugga!

Moisture meter sounds like the way to go there. Then you can test before buying and don't buy them if they are too moist - well certainly don't pay top $ for them anyway

yup i recon that'll be the go Bob, a moisture meter is now at the top of my list of 'needed' items. For now i'm going to clear an area in the shed where i can stack my timber stickered.