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Building a Kiln

As a woodworker, you may or may not know the benefits of drying your own wood. Years ago, as a furniture maker, I found myself paying substantial bills for wood to timber merchants. I was also finding that the timber that I was getting was not dry enough to go into my customers’ houses without ‘moving’. I needed to reduce the moisture content from 14% - 15% down to 10% - 12%. I found that the timber would go through the workshop without picking up much moisture, and if I could have wood at 10% moisture content, and it reached the clients home at 12%, I would have very few problems from shrinkage, or indeed expansion of the oak and ash that I was using.

However, I was finding it very difficult to buy wood that dry in the U.K. although I am sure that someone would have supplied dry enough timber at a price, or a quantity. My only option was to dry my own.

Air-drying is a matter of nature taking its course, so it is cheap, but the problem remains of reducing the moisture content sufficiently. Then you have several methods of kiln drying wood within a chamber so that you can control the drying process, including a dehumidifying kiln, a conditioning kiln, and a vacuum kiln. I chose the only option available to me at the time, which was the cheapest. Drying wood takes time, and that means capital being tied up during that time, which does not help the cash flow of a young business. It was going to be hard enough to generate spare income to build up a supply of my own wood to air and kiln dry, and still pay timber merchants for theirs while mine was drying.

Modern kilns use ventilation, temperature, or humidity to control the drying process. A dehumidifying kiln works by heating the chamber in which the wood is stored until the wood releases moisture, and circulating the air within it. We can pass the moist air through a refridgeration unit to cool the humid air back into water, where it can be collected to give the operator an indication of how much is being removed. A timer on the fridge unit can be set to the operators requirements, remove water from the chamber, as quickly or slowly as necessary. The timber has to be given time to release the moisture, with a maximum differential of 6% between the inside and outside of the board. A greater imbalance will result in putting stresses within the wood, showing as cracks. The other problem to be aware of is drying the wood really quickly on the outside, while the centre cannot release the moisture fast enough, and the wood becomes almost baked, or case hardened on the outside. Rather like a cooking a cake too quickly, and ending up with something over done and hard on the outside and raw in the middle.

I have made several chambers for use as kilns, and we seem to have improved them along the way. The latest has even impressed the manufacturer of the dehumidifying unit that is used within the kiln.

The latest chamber consists of 4” x 3” stud walls screwed together, with a plastic sheet on the inside, acting as a moisture membrane.

The floor of the box was made up of 6” x 3” external nailed or screwed together, with several cross members placed length at 16” intervals between. 2” x 1” battens were nailed around the bottom of the inside of the stud work, to support new, but reject 4” insulation, saved from a landfill site. We then tacked a sheet heavy grade builders’ plastic, used under concrete onto the framework before sheeting the whole surface with ½” marine quality ply.

The ends were made in a similar way only using 4” x 3” softwood instead. If you make the rest of the panels on the base, you can use dimensions of the base to go by. The ends are constructed the same width as the base, but remember that the back and front (the door) need to be the twice the thickness of the walls less, so that it all fits. We used shuttering ply as the outside surface, insulation between the stud work, with a polythene membrane, on top before putting another sheet of marine ply on the inside of the chamber. The front and back are constructed in the same way, although w did make the front about ¼” smaller in length and height to ensure an easy fit. The top is made in the same way on the base and exactly the same size, then lifted out of the way (even suspended in the air if your feeling brave. We then dropped an end in place, screwing it to the base with some large angle plates. Alternatively, if you leave the shuttering ply off until you have put the walls up, you can screw straight through the studding. When you have the back, and both ends in place, the drop the lid on. Bearing in mind that these panels are heavy, so I say drop loosely, a forklift was used.

When we put the front in place, we screwed a 6” x 1” board around the outside overhanging by about 3”, forming a rebate so that the door would not fall in too far. It also gives us somewhere to screw the door tightly shut.

We tacked felt over the roof of the chamber before putting a couple of strips of 2” from on end of the box to the other. The one at the back was 2” x 2” while the other was 6” x 2”, to create a slope for sheeting with some corrugated tin. Then we nailed 2” x 1” strips of batten in the edges and cut featheredge boards to fit in between to form a waterproof wall. It is worth putting a couple of coats of preservative over the whole box, when it is finished. On the inside of the floor we have fixed 2” x 2” from front to back every 12”, for the packs of wood to sit on. These are 5” or 6” short of the inside of the door, and this gives us a point to place pack, to allow about 6” gap between the wood being dried, and the door. It is worth mentioning that the door has to be removed with the help of a forklift. One man can manage it with ease once you have done it a couple of times. You could choose to put two hinged doors on that meet in the middle, but we felt that these would blow in the wind when open, and be quite dangerous considering the weight on the hinges.

The dehumidifier unit is hooked onto a rebated strip bolted to the back wall. We fix a strip of ½”ply either side of the unit out to the end walls, with a sheet of polythene attached at the bottom. This covers the timber being dried, and forces the warm air to circulate through the stacked timber evenly.

Put the cables out through a small hole in the back, with the controls in a little lift up box to prevent the rain getting on the electrics. You will also need to drill a hole out for the pipe for the water drainage, which I direct into a five gallons container. We dry about 50 cubic feet at a time using the smallest dehumidifier unit available from our suppliers. I will catch two or three gallons of water per day after the timber has been air-dried for a while. You would collect more if you started with green (freshly sawn) timber. The chamber is 10’ long internally, and although I do not have to, it is worth putting an extra fan in either side of the drying unit, to help even out the drying and the airflow within the kiln.

We kept the costs of making the kilns by using timber that we had milled to make it, including the featheredge boarding. The most expensive materials were the sheets of ply. We used a compressor and nail gun, so it does not take long to put them together, especially when you look at the returns.

When you have kiln dried a few loads and gain your confidence, it becomes less stressful. (The first few loads were a constant worry because I did not want to spoil the timber, and I found myself constantly checking to look for splits, temperatures and on). You will also start saving a great deal of money as you have cut out another middleman.

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