Drying Wood in Small Volumes
Tony Nicol introduces us to a home-made drier for preparing his turning timber
Like many woodturners, I started out by buying pre-prepared blanks of wood, but it wasn’t long before the disadvantages of this approach became apparent to me. It is convenient, but it can entail a number of shortcomings:
• Cost – The supplier has to cover his costs, as well as make a profit, and the retailer adds his margin. The cost of the blank includes the waste as well as the usable timber
• Species – Although many suppliers can offer a wide range of species, exotic timbers are expensive and many carry a hidden environmental cost, whereas many fine local timbers aren’t commercially available
• Quality – Can be extremely variable. Establishing a personal relationship with your supplier may help, but may not be practicable for people with no local suppliers or who only use wood in small volumes
• Orientation – I find it frustrating having to tailor the end product to suit the orientation of the grain and figure of a piece of wood that the supplier has determined
• Moisture content – Correctly stored kiln dried timber should be of satisfactory moisture content for turning without further treatment, but so-called ‘part dried’ timber is highly unlikely to be suitable for turning to a finish and taking into a heated environment
Many of these problems can be overcome to at least some degree by sourcing and preparing your own timber. This will also give you an appreciation for the amount of waste that is involved and why purchased blanks may seem expensive. You can buy kiln dried timber by the board and with careful board selection, this gives you some control over grain orientation.
Another way is to look for local timber that may be freely available from neighbours or friends, such as garden trees that have been storm damaged or have outgrown their surroundings. In this case, the recipient will be faced with the tricky task of drying it. Many comprehensive articles have been written about drying timber and this commentary is not intended to be yet another. Instead, it describes aspects of timber drying that are relevant to my own individual approach to timber processing.
Small pieces of timber can often be dried ‘in the round’ with little preparation other than sealing the cut ends. Air drying larger timbers entails cutting the log into boards, a process known as slabbing. The rough cut boards are then stacked with small pieces of wood between them known as stickers, to separate the boards and allow air to circulate around and between them. Typically, the stack is built in a shaded area and covered by waterproof sheeting on the top only, leaving the sides and ends exposed to the air but not direct sunlight. If space is available, the stack can even be placed inside a well-ventilated building.
The process is slow and as a rule of thumb you should allow a year per inch of thickness for the board to reach equilibrium moisture content. Even so, the moisture content is unlikely to get much below 18% in the UK without further effort being required.
The slow nature of the process should mean that the wood is reasonably unstressed. This is still too wet for turning to a finish, however, unless you wish to turn your piece thin and accept the distortion that will occur during final drying.
For speed and convenience, an alternative technique called rough turning can be employed. This involves cutting and turning pieces of work from green timber to a thickness which is about 10% greater than the intended finished product and air drying it until it is at a moisture content low enough to be worked to a finish. This is a popular and effective method, although it is quite likely that there will still be the odd failure due to cracking or excessive warping.
Commercial producers dry their timber in environmentally controlled structures known as kilns. Correctly used, a kiln can bring timber down to a satisfactory final moisture content value very quickly. Experience is required to stack the kiln efficiently to produce optimum results. This method involves significant financial outlay both in capital and operating costs. In recent years, however, small kiln heaters have become available which means that kiln drying is becoming viable for small professional businesses and enthusiastic amateurs or groups. This can be made even more affordable if basic DIY skills using recycled materials are applied to the building of the drying chamber, so that the kiln heater is the only major outlay.
The other primary method of artificial drying, and the one this article will focus on, involves the use of commercially available domestic dehumidifiers. These are readily available and there is also a lot of information freely available from suppliers which will be invaluable for the selection of a suitable unit.
The two principal types of dehumidifier currently on the market are either refrigerant or desiccant. Space precludes a detailed discussion of these two technologies but typing ‘dehumidifier guide’ into your favourite search engine will reveal many sources of information.
The primary drawback of the refrigerant dehumidifier is that it becomes increasingly ineffective as air temperatures fall below 10°C and such units can be prone to freezing. Moreover, although modern units are filled with more environmentally friendly gases than used to be the case, they are by no means benign and many still have the potential to pose an environmental hazard. The desiccant type of dehumidifier, on the other hand, can operate to 0°C and (with suitable precautions to protect the water tank or
drain line) even sub-zero temperatures. Importantly, these units do not involve the use of harmful gases for the heat exchange process.
Whichever dehumidifier type is chosen, make sure that it has the following essential features:
• Easy to clean air filter to keep out dust
• Safety cut out if starved of air
• Safety cut out if tipped over
• Over temperature protection
Drying chamber design
I needed a drier capable of handling:
• Smaller timber in the round
• Boards that I would either buy in as air dried or slab by myself from green logs
• Rough turned pieces, all to be dried down to suitable moisture content ready for finish turning.
While wood bought by the board is generally available in standard lengths, there are no standard sizes for green wood that comes from windfall. However, there are important factors that need to be considered. Water is quickly lost from the ends of the log or board and this promotes cracking and splitting, so if I am not going to slab a log, I usually split it lengthways through the pith to considerably reduce the radial cracking that occurs in the two half-round sections that result. However even when the ends are sealed, shrinkage is rarely negligible. For example, if a log is 1m long and 300mm has to be discarded from each end, there isn’t very much left to work with!
So longer is better, but longer also means heavier, imposing limitations unless you are fortunate enough to have means of mechanical handling. Consider that in the year 2013/14, manual handling was responsible for 24% of workplace injuries (Source: UK Health and Safety Executive). We may not consider our garages or workshops to be workplaces, but in practice they most certainly are.
Eventually I decided that for me, anything longer than 2.5m was rarely going to be practicable. From a construction point of view, it is also worth considering that plywood and other sheet materials are typically supplied in 2.4m lengths.
I also wanted the flexibility to add or remove wood in an ad-hoc manner, rather than to have to dry it in batches. For example, a few roughed out bowls would clearly require much less time in the drier than several 50mm thick slabs of green wood. It would be useful to have the ability to arrange the wood according to its relative moisture content: a single chamber wouldn’t have suited this. After some sketching, I decided on a three tier arrangement. With hindsight, two tiers would probably have been quite sufficient
and much simpler to build. Since I cleared my initial backlog, I have rarely found the need to use all three.
As the air is warmed by the dehumidifier, its density is slightly reduced. If left to its own devices, warm air will rise over damper, cooler air. To reduce this tendency, plywood baffles are used to direct the airstream down to the bottom of each tier as it enters to encourage it to flow around and between the wood waiting to be dried.