Thursday, February 15, 2007

Some low-tech approaches to drying wood

I acquire a lot of my wood while it is still green; kiln dried boards are expensive and often not available in thickness' greater than one inch. In the maritime climate in which I live, air drying is slow and only gets wood down to about 15% EMC (equilibrium moisture content). This is not low enough for work that is to be glued, or if it is going to find a home in more arid climates.

Anyone setting out to dry their own lumber will benefit from a basic understanding of the physics involved. The rate of drying is determined by the temperature, relative humidity and air flow. The final EMC is determined by the relative humidity:
  %     %
 80   15
 70   13
 60   11
 50    9
 40    7

Raising the temperature not only makes the moisture more mobile within the wood, but also reduces the relative humidity. If you want to know more about drying wood I highly recommend reading Wood and How to Dry It (Fine Woodworking)

I usually err on the side of caution when seasoning wood to avoid defects such as splits or honeycombing so I try to be patient and let the wood dry slowly. Every piece of green lumber has its end-grain sealed with either paraffin wax or proprietary end-seal.

It is then stickered somewhere cool to air dry to below 20%, preferably 16%. This may take some months, but unless I am in a particular hurry, this is better than risking it splitting. The back room in my workshop is cool, and makes a good starting point. After a month or two I may move it into more normal room conditions until I think it is safe to put it into a 'kiln'.

A moisture meter is an invaluable tool to help you assess how drying is proceeding.

One simple way of drying wood that I have used in the past is to place a table (plywood and folding legs, not a good one) over a hot-air floor register. A cloth is draped over the table top, large enough to reach the floor all the way around. It is weighted down at the back and sides but left free at the front to encourage hot air to pass all the way through the chamber.

Wood is stickered inside, leaving a space above the register to promote even airflow. The advantages of this system are that it is cheap and easy to set up, and costs virtually nothing to run as the heat eventually warms the house. The main disadvantage is that the operating temperature and humidity can only be controlled by adjusting the register or lifting the table cloth. As a rough guide I find that it takes one month for every inch of thickness. The whole contents are removed and checked once a month. Anything dry enough to use is removed. The remainder is returned to the back of the chamber nearest the register and any free space filled with new wood. The temperature can get up to 40°C and the humidity down to 40%, but generally it runs at about 30°C and 55%. This will depend a lot on the humidity in the house and how much the furnace is running. I find that most woods get down to about 8% EMC. It doesn't work anywhere near as fast as a commercial kiln, but is far better than air drying alone.

I have dried 3.5" oak from green to 10% in less than 9 months using the two techniques above..

The 'kiln' described above obviously isn't going to work during the summer months, so I have a simple solar kiln in the back yard. It is essentially a greenhouse, a wooden frame covered in UV stabilized polythene. Black polythene is stapled to the inside of the frame in order to keep direct sunlight off the wood, and to absorb heat.

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