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Wood can be dried in a kiln or air-dried. The former is usually faster than the latter, but not always. For pieces that are too large to fit into an oven type kiln, for example in Canada in order to dry big timbers for construction purposes, lumber mills will build special wood sheds where the wood is hung sideways in tiers off of beams and allowed to dry slowly over time under very carefully controlled conditions.
Kiln drying works by applying heat which kills fungi and bacteria that cause decay while promoting shrinkage so that water flows out of the cells allowing them to collapse. Wood dries best when there’s good airflow around it all through the process. Wood continues to lose moisture (water content) until it reaches around 12% MC (moisture content – the weight of water as a percentage of dry wood).
Wood needs to stay in the kiln until it is at this level. If you take it out too soon, it will warp badly as it loses moisture and outside conditions such as temperature and humidity affect how quickly this can happen. When removed from the kiln, we re-stack into new packaging that looks like big boxes made of chipboard – with 10 – 20 pieces per box and lots of insulation all around them. This lets them settle down and minimizes the risk of warping during transportation to our warehouse and then onto your home.
Step 1:
the first step is to cut the tree into boards that are large enough to fit into your kiln. This can be any size that you want as long as it will fit through the door. Our machine has a carriage that goes back and forth across the width of the rack, so we set our saw head to take full-sized slabs about 24 ” wide. These are then sawn down into standard board sizes like 4/4 (1″), 8/4 (2″), etc.
Step 2:
The second step is to dry these newly cut boards in a separate drying room for one or more weeks depending on their moisture content levels. We usually let them sit 2-3 weeks at least before bringing them into our kilns. This gives us a good idea of their moisture content and lets us make sure that they are ready to go into the kiln. The boards have been sitting around losing water from their cells, so when they first come out, they are very heavy.
Step 3:
We then load them into one of our two large drying kilns. When you first walk into a drying kiln it feels REALLY hot because this is where all the heat is being generated to dry the wood. It would be too much for most people to stay in there more than a few minutes before feeling uncomfortable, but we spend hours on end in here every day! To start, we spread each slab as level as possible across a moving rack with lots of insulation between each piece. We then use a blower to force air through the kiln at a rate of 25,000 cubic feet per hour. This helps dry the wood even though it’s surrounded by insulation on all sides, pushing warm air through the boards causes water vapor to flow out of the wood cells.
Step 4:
After about 5-6 days in here, we can feel which boards are ready by touching them. They may not be completely dry inside because there is still moisture moving up into the outer part of each board, but they are no longer cold or damp feeling to the touch. I wouldn’t stand on them at this point however unless you really want to go for a swim! When you enter the kiln again after it has been turned off, the humidity is really high.
Step 5:
Finally, we move them into our second large drying kiln for 3-5 days to drive as much water as possible out of the boards and then they go through a stacker which flattens them down. The board now dries from the inside out by pulling in warm dry air across its surface, so it can take up to 12 weeks or more for full moisture content to reach the 12% mark and be ready to use in your project…
Summary: That’s how we sterilize wood at -20C and store it until you need it later! We hope this was helpful and not too confusing…thanks for reading!
