How to build a DIY timber drying kiln

[agg221]

i,m confused when it comes to kiln drying, my question is always "why?". If you can get timber down to say 10% moisture,but store the logs in your log store wont the logs absorb the moisture in the air and you end up with logs no dryer than if they had been air dried, i realise there must be a reason why you kiln dry them, but i cant figure out why.

 

Big J has already commented that wood takes the water back up again, but this is only to a certain extent.

 

The structure of wood is like a big bundle of tiny drinking straws all bundled together. The straws have little membranes across them every few mm, which partially block them. They also have small holes running between adjacent straws. These holes (lumens) allow liquids to pass through. In the living cell structure, if there is damage within a straw, the membranes above and below can be closed off, allowing fluids to pass to adjacent straws which helps the tree to keep all its cells functioning and stops it from bleeding to death under the high osmotic pressures involved in transporting fluids 100ft up in the air.

 

These membranes, both within the straw and in the lumens, are soft and act like flexible diaphragms. When they dry out they change irreversably and become permanently shut. This typically happens at about 12% moisture content. Hence, once you have dropped the wood below this figure, water uptake is much slower as it can't get in so quickly.

 

This is a slight generalization, as obviously wood can still take up water so they aren't a complete seal, and in some species (notably radiata pine from New Zealand) they are absent, but the principle is generally correct.

 

Hope this explanation helps!

 

Alec

[j warwick]

Really useful - thanks for taking the time

That gives me a bit to think about. If wanting a smaller setup would it be possible to insulate a shed (obviously with the floor beefed up to allow for the weight) was thinking something like kingspan with a ply lining.

 

Have you tried looking at the smaller insulated units like those on the back of the tesco, asda, etc vans. They do come up for sale the biggest problem would be moving them.

[Dog Tag]

Hi, have you looked up solar kilns? saw one recently basically a wooden shed with large sloping south facing window, has some vents, fans and screens to protect timber from direct sun. apparently produces very good results.

 

Hi L W, I thought wind, air, and sun were the best way to dry timber. Why the sun screen?

[spence]

Sounds great, any chance of a few photos?

Have you got a batch of firewood going through at the moment?

[Big J]

Sounds great, any chance of a few photos?

Have you got a batch of firewood going through at the moment?

 

I will do a quick video tour of the kiln instead of photos as it seems simpler and quicker. I'm putting some firewood in today, so I'll report back as soon as I can.

[Big J]

Here is a short video explaining the kiln:

 

 

I wasn't able to get the bloody firewood in today, despite my best efforts. The crates I have (constructed from pallets) and for the most part unsuitable for use with a pallet truck so I need to have a little think about how to do it.

 

Jonathan

[Copford sawmill sussex]

Be aware kiln drying timber too soon from cutting can produce honey combing! We air dried a stack of 2" qtr sawn planks for about 9 months got the moisture down and stuck in our kiln, a month or so later we pulled it out and the hole lot has exploded!! A few swear words and a lesson. Air dry an inch a year for oak anyways and then kiln.

Honey combing is where the moisture is sucked out too quickly resulting in cracks opposite and through the grain.

[Big J]

Be aware kiln drying timber too soon from cutting can produce honey combing! We air dried a stack of 2" qtr sawn planks for about 9 months got the moisture down and stuck in our kiln, a month or so later we pulled it out and the hole lot has exploded!! A few swear words and a lesson. Air dry an inch a year for oak anyways and then kiln.

Honey combing is where the moisture is sucked out too quickly resulting in cracks opposite and through the grain.

 

I do think that this is dependent on both quality of log in the first place and also the kiln schedule.

 

I had moderate success with kilning Oak from green in the early days of my kilning. Basically, if it was a good log to start with, it would be OK in the kiln. Anything less than very good wasn't though.

 

These days I always air dry Oak for a good while, but even then the length of time required is down to storage conditions. All of my timber is stored in barns in very meticulously stacks packets with no direct sunlight. As a consequence, I put through about 70-80 cubic foot of 1, 1.25 and 1.5 inch Oak milled in April that just came out of the kiln. It was in the 25-30% range going in and is in the 9-14% range now. There is no surface checking, end checking or honey combing and the boards are perfectly flat. They were very good logs though.

 

I have had timber I've milled honey comb, but not in my kiln. A friends vacuum kiln had a heater malfunction and overcooked some cherry. That honeycombed pretty spectacularly, but other than that, it's an issue I've never experienced.

 

Jonathan

[slackbladder]

Many thanks for a great thread and video

This will help lots of people

[broonie]

Sure has been very helpful, got me thinking ... I am looking at IBC tank metal crates , fold down metal crates of the large trolleys they use for transporting dry goods to supermarkets in to dry the firewood. Noticed you had 3 heaters in the kiln is that to guarantee 38 degrees for the firewood drying ?