The aim of this page is to provide you with the bare essentials of how you can dry wood. From my own experience, I find you gradually get a feel for what you’re doing when drying wood and the more you do it, the better you get. For additional information it is well worth searching the internet.
There are 3 factors that determine how fast a piece of timber will dry
• relative humidity (RH) – how much water is in the air around the wood
• air circulation
As soon as a tree is cut it will lose moisture. To begin with this is water that is held in the cells of the wood (free water). As the wood reaches between 25% – 30% moisture content, water from the cell walls begins to evaporate (bound water). When this happens the wood starts to change shape and shrink. When drying your own timber you need to know where the timber is going to be used afterwards. If it is outside you will be aiming for a moisture content (MC) of 18%. If inside then it will be 10% to 12%. This is an important point to consider. As you dry your timber, it will need to be in equilibrium with the environment it’s going into. For instance, if you made a table out of wood that had an MC of 18% and put it inside a modern house the wood will continue to shrink and crack. This is sometimes not a bad thing for rustic furniture, as it adds character.
So what do you do with the wood after it has been sawn?
01 First of all you need to treat the ends.
This can be done with waxoil, PVA glue or something similar. If I have time I often end seal the whole log end before milling. You do this to slow the evaporation of water from the ends of the wood. But surely you want the wood to dry quickly you might ask? It’s a hard point to get your head around to start with, but you need to dry the wood in a controlled way. If you do not treat the ends, water rushes out causing the wood to split and deform. Water needs to be released out of the timber in a smooth, controlled way.
02 Stacking timber
You stack the timber in a pile with small wooden spacers called stickers. These stickers are soft wood usually ¾” by ¾”, which need to be dry already. Don’t be tempted to use fresh cut stickers as these can stain your timber which will lower the quality of your wood.
03 Air drying timber outside
If you are going to air dry the timber naturally outside, you need to construct your stack somewhere under cover where there will be some air flow. How long will the timber take to dry? This can vary depending on the 3 factors and also the type of wood and its thickness. As a very general rule of thumb you should allow 1 year per inch thickness. I.e. a 1″ thick plank will take a year to dry (18% moisture content in this country), and a 2″ plank will take 2 years.
04 Kiln drying timber
These days you can dry wood much faster in a kiln, which gives a more constant and controllable environment. The word kiln conjures up something mystical and expensive, something unobtainable by the common man, but it’s really not that hard! I would recommend buying an old refrigerator back off a lorry, because after the lorry is redundant there is little demand for the refrigerator unit. I picked one up in excellent condition for £350 including delivery. This gives you a sealed and insulated place to dry your timber and is faster, cheaper and more effective than building a kiln yourself. You also need to buy yourself a medium to large dehumidifier. These can be bought from places like ebay for £100 to £200. Then get a bathroom extractor fan with a humidistat build in. This does not need to be big, a 4 fan is fine. Also equip yourself with a desk style fan and a hygrometer (a device which measures temperature and relative humidity). Once you’ve done this you’re ready to go!
05 Stacking the kiln and starting drying
Cut a hole in the freezer body (I have done this at the far end and at the top) and position your small extractor fan. Load your wood and sticker as you go. If possible, try and use planks that are all uniform thickness or stack the thinner bits at the top. These will dry faster and can be taken out sooner. Start the dehumidifier and fans and shut the door.
06 Inspecting the wood
To begin with it’s best to keep a close eye on things, as you don’t want the wood to dry too fast as this can cause defects. If the wood is drying too slowly you might find mould begins to form on your boards. A little mould isn’t too serious as it shouldn’t penetrate deep into the wood. If the kiln is full of timber I find it best to have everything switched on full straight away. Humidity in the kiln tends to be around the 83% mark to start with. If it drops below this I tend to turn the power down on the dehumidifier. After a couple of weeks the humidity will drop down. However, this doesn’t necessarily mean you have to turn everything off. The wood has lost moisture and so you want the environment to become drier to keep the water coming out of the wood. Over the weeks the relative humidity will drop in the kiln as there is less water in the timber. At a RH of 50% I start to examine a couple of planks to look at their moisture content. You can do this with a moisture meter. I take plenty of readings and I’m looking to get consistent measures of 10% to 12%. If the wood’s MC is above this, then it needs a little more time.
07 & 08 Final stages of drying wood
When the wood’s MC reaches 10% I do a couple of things. For 3 days I stop the bathroom fan extracting and I turn off the other fan. I now have the dehumidifier running flat out because it produces heat. The temperature in the kiln rises to 55 degrees or just over. The reason for raising the temperature is that it kills any pests that might be present in the wood. It will kill woodworm and eggs and is a very effective way of doing it. I’ve spent a lot of time researching woodworm and although I’m no expert this is the best way I’ve found to kill the pests. Also, because the timber is now dry it will not suffer any defects due to the temperature increase. Often milled timber has woodworm and in the first few weeks it will still be alive eating into your wood. You’ll often see tell tale dust as they feed (image 07). Sometimes it seems woodworm has burrowed in just after I have cut the timber as a series of holes are visible. The holes are actually caused by the adult beetle chewing its way out of the wood to mate and not the woodworm burrowing in. When the beetle lays its eggs on the wood they hatch and the larvae burrow in, but the holes are too small to see (image 08).
09 Dried timber
Once the timber is dry it is ready for use as fencing, furniture or selling on. For furniture we find oils nourish the wood and bring out the natural grain and texture more effectively than varnish. Please have a look at treet, a rustic furniture business I started from wood milled from an Alaskan Mill (image 09).