Wood loves water. This guide gives details for finding the ideal wood moisture content when wood turning, as well as the benefits and drawbacks of turning either dry or green wood.
Determining ideal wood moisture content before turning
A much better approach when leaving wood to dry or when choosing the ideal moisture content for woodworking in general is to consider the relative humidity that the workpiece would be exposed to, once finished, and to dry or choose the wood with a similar equilibrium moisture content (EMC).
EMC is the moisture content at a particular level of relative humidity at which the amount of moisture content neither increases nor decreases (that is, stays in equilibrium). EMC and relative humidity do not have an exact linear relationship, but a good approximation of EMC can be figured by dividing the relative humidity percentage by 5.
So, for example, if the relative humidity is 60%, then the approximate EMC to which the wood blank should be dried before turning is 60% ÷ 5 = 12% EMC.
For those living in the States, the EMC generally ranges from 6%-9%. Usually it is okay to keep the wood between a ± 2 % margin of the EMC for fairly accurate dry wood turning. But if you want a better approximation for more precisely turned pieces, just follow the above method.
The old way of determining how dry wood should be for turning
Store a completely dry wood blank in a rather humid room, within a few days the blank will have absorbed a reasonable amount of water. It may even begin to look a bit bloated or deformed as a result.
The reverse is also true, in that, if one were to place green wood in a dry enough room, then day by day, the green wood would continue to lose its moisture and slowly turn into dry wood.
An old rule of thumb is that green wood needs to be air-dried for one year, per inch of its thickness.
While this rule will generally be a sound basis for considering the air-drying period, most would agree with the fact that this is quite a long time to wait! And quite excessive especially for those who live in a humid climate, since the required dryness of the wood would be higher than this rule would dictate.
Measuring moisture content of wood with wood moisture meter
A lot of woodworkers like to go by feel and age when it comes to judging the suitability of dry wood blanks for turning. If you are working on a piece which needs high precision and/or low tolerances; or even if you are just new to wood turning and do not feel ready to gauge the dryness by feel and age alone then a handy way to measure the moisture content of wood is through a wood moisture meter.
Firstly though, it is important to clarify that nearly all moisture meters support a wood calibration (usually from 5% to 40% or slightly lower), but the recommended type of moisture meter for measuring moisture content in wood turning blanks is the Pin-type moisture meter.
A pin-type moisture meter is an accurate way of measuring the moisture of an object. The two pins at the head of this type of moisture meter are designed to be pushed within the wood. The moisture percentage reading obtained by this meter is at the depth of the inserted pins.
Most wood blanks are thick enough that the puncture marks left by a pin-type meter do not pose any hindrance when it comes to turning. More importantly, the pin-type moisture meter allows the wood worker to measure moisture content at varying depths, this is important because thicker wood blanks would generally be wetter near the core than near the surface.
Importance of turning dry wood
The reason for drying wood before turning is important for a number of reasons. The first and foremost reason is, dry wood is simply better and more consistent to work with. You will find that dry wood turns a lot more predictably. The turned object also tend to hold its shape well.
The second reason is cracking. Since all turned wood articles would eventually dry out to the EMC, by letting the wood dry before turning, one ensures that any would-be cracks within the wood structure either become apparent or are avoided. This is unusual done by using Anchorseal or something similar on the ends that are prone to cracking. All this is done before the work of turning the blank into a masterpiece is put in.
The third important reason for turning dry wood has to deal with contraction. Green wood will eventually reduce its volume and contract its three-dimensional size regardless of whether it is dried before turning or afterwards.
This contraction is also what causes cracking. But more than that, if you are working on a project that involves turned wood pieces fitting together with low tolerances, then you must use dry wood blanks for this so that it doesn’t contract over time and the fittings do not come loose or deform under the pressure of the joint.
Advantages of turning dry wood
In addition to the above 3 important benefits, turning dry wood also yields the following advantages:
- Turning a dried blank saves time on finishing after the article has been completely turned since you do not have to wait for the piece to dry out before applying polish, varnish or a weatherproof seal.
- Turning dry wood is a lot easier for beginners since it provides immediate results that do not change or deform over time.
- One can turn a lot finer detail into dry wood than would be possible with a green wood blank since the dry wood would retain all the turned features.
- Dry wood blanks provide many more options for wood turning techniques as we do not have to worry about any unsightly cracks rearing their face on the turned wooden object.
Disadvantages of turning dry wood
Using dry wood is definitely the recommended way to go for beginners and casual woodworkers. However, it still does carry certain drawbacks:
- First and foremost, is the cost of dry wood. A wood worker pays this cost outright in dollars and cents, or if they dry their own wood, then they pay this cost in time. No matter which way one looks at it, dry wood is costly in its use.
- Dry wood is physically harder due to the lower moisture content. This consequently means that for a similar turning, dry wood blanks require a more powerful lathe to work with.
- Turning dry wood, especially air-dried wood blanks tends to produce a lot of saw dust and so more safety precautions must be taken when working with dry wood blanks. Including a breathing mask and better ventilation.
Why you would want to turn green wood
Green wood is not recommended for beginners. However, once you’ve got your feet wet with turning dry wood, you may quite enjoy the benefits of turning green wood every now and then:
- Green wood is physically softer than dry wood. This makes roughing green wood blanks (turning the blanks within 10% or so of the desired shape and size) a breeze.
- Unlike dry wood, green wood can be turned even when it is chopped straight from a freshly felled tree trunk or log, which makes it great for when inspiration suddenly strikes.
- Dry wood tends to lose a lot of the natural wood features. Such as the bark and tree rings become diminished. Not so with green wood, which allows for very fresh-looking designs as if carved straight from the tree trunk.
- Green wood is cheap, and if there are frequently felled trees in your area, then sometimes green wood can be acquired virtually free of cost.
Disadvantages of turning green wood
- Green wood turned objects tend to crack more often when not dried evenly or properly.
- These objects may also morph and deform into a different shape. A common example is that a wood bowl turned entirely with green wood tends to acquire an oval shape instead of the round shape once finished.
- Green wood contracts a reasonable amount. This contraction differs between wood species. This makes green wood wholly unsuited for projects that require predictable turning behavior as well as those that need low tolerances between connecting joints and pieces.
- If wood is not allowed to dry in a reasonable amount of time, in some conditions of prolonged moisture it may began to break down.
Turning green wood is certainly not for beginners. But some wonderful results can be achieved when one mixes green wood turning with dry wood turning by allowing the turned green wood piece time to dry before finishing it off dry.
A very common approach for this is to rough up the green wood blank, then allow it to dry, before finishing it with the final fine grained turning on the dry wood. No matter which method you choose to adopt for your next wood turning project though, we hope that this article helps you achieve the best results possible.