DRYING TIMBER
There are several protocols to be adhered to when drying timber cut from small diameter logs, as it has a tendency to return to its original shape, if left to its own devices.
The end grain should be sealed (preferably whilst still in the log form) with paraffin wax or similar, and where possible the ends covered in polythene. Preventing moisture loss through the end grain, greatly enhances the stability of the drying timber, and helps prevent "end check", (which is where the ends of the planks split).
"Sticking" should be started, as close to the ends as possible, this will combat "cupping", (where the planks curl), common in timber from small diameter logs, sticks should be inserted at every 12", (the sticks must dry to prevent staining the timber)!
"Banding" should also be started as close to the ends as possible, at a maximum of 24" between bands, (closer is advisable).
Protect the stacks from extremes of weather where possible (sun & rain), whilst allowing the wind to circulate through the stack, (the illustrated "curtain-sider" is ideal for this purpose).
These precautions will significantly increase the percentage of timber that will be suitable to go onto the next stage, kiln drying.
Kiln Drying
Kilns can come in many shapes and forms, from purpose-built brick buildings, to insulated "pie-van" backs. However there are several features that are vital in making an effective kiln, especially one that works on the "heat & vent" principle, that Coed Cymru has found to be effective, especially with the timber from small diameter logs.
Don’t put the timber in whilst the moisture content is too high! Timber with a moisture content higher than 30% is "too green".
You can’t have too much insulation, under, over, or around your timber, this helps stabilise and maintain the internal temperature of the kiln, which is vital with this type of timber, and it saves energy & money!
Keep the timber tightly banded or weighted to prevent movement whilst it is drying in the kiln.
Ensure that the circulating air can only pass between the planks! Not under pallets, around the ends of the stack, over the top of the stack! Blank off any gap that will allow the circulating to take the least line of resistance! Drive the circulating air across the exposed surface that you want to dry, nowhere else!
Temperature should be maintained as close to 30°C as possible, (which is controlled by the thermostat). Humidity in the kiln should be set at 55% relative humidity, (which is controlled by the humidistat).
A "box" made from 8 x 4 sheets of plywood or OSB will produce a kiln with a capacity of approx 60cu ft, taking into account heating elements, circulating fans etc. Deducting the space taken up by the separation sticks, the amount of kiln-dried timber per cycle is approx 1.3cu mts (1.5 tons). Enough to keep a joiner going for at least a week.
As illustrated the top of the kiln can be used as a bench top for machining or gluing up, with the added bonus of warm air venting into the workshop!
Operation
Charge the kiln with as much timber as possible, and block off all air spaces except the gaps between the stacked planks, set thermostat at 30°C and humidistat at 55%, start the circulating fans, close the door.
The heating elements (or heat source) will quickly raise the temperature inside the kiln above the ambient air temperature, this will cause the timber to "sweat" its excess moisture into the circulating air, causing a rise in the humidity.
The elevated humidity level will be detected by the humidistat, and when it rises above the preset figure (in this case 55%), it triggers an extractor fan, which vents the moisture-laden air to atmosphere, (warming the workshop)!
Syphonic action draws unheated air into the kiln via the inlet, to replace that which has been vented, this joins the circulating air, and travels across the surface of the heating element which raises its temperature, completing the cyle!
Tables are available which calculate the % M.C. (moisture content) of the timber, when there are given figures of temperature/humidity within the kiln.
At 30°C & 55% R.H. (relative humidity), the timber should be dried to a moisture content of ~ 10% (+/- 2%), and ready for working!
For further information please contact David Jenkins at Coed Cymru, 01686 650777
(OR see Staff Page for email addresses and officers' telephone numbers).
Coed Cymru, The Old Sawmill,
Tregynon, Newtown,
Powys SY16 3PL
Tel 01686 650 777
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