Understanding wood, moisture and guitar making

You may have gauged from our knowledge section that I am quite keen on guitar players understanding humidity and how it can affect precious guitars.  It’s important that guitar owners understand the basics about humidity and what they need to do, but if you’re after a bit more detail about why wood as a material is especially affected by humidity changes – and how it affects the guitar making process - you’ve come to the right place!

Wood is ‘hygroscopic’ which means it takes in moisture and loses moisture to balance with the environment that surrounds it. It literally absorbs the moisture from damper air and releases it when the atmosphere is drier than its current moisture level – or vice versa. It wants to be in moisture equilibrium with its surrounding environment. It does this when it’s alive as a tree, but the process still continues when it’s cut and sized into lumber - it never stops!

Similar to plants, when trees are alive they continually suck up water through their trunks. When they are initially cut down their cells are usually full to the brim with water – this is normally referred to in timber circles as ‘green lumber’.

Green lumber can have a total moisture content of up to 100%. This means that the tree’s free water (sloshing around outside the cells) and cell water (held within the tree cells) are full to capacity. When wood starts the drying process, it firstly begins releasing its free water to reach a moisture content state called fibre saturation point – this is usually around 22-28% total moisture content - depending on the wood. So the only water now remaining is that which is bound within the cell walls. Coming down from 100% total moisture content to fibre saturation point, the wood does not change in size at all – no shrinkage. That’s because it’s the water in the cells that determine the size of the wood. So what does this mean? Changes and movement only occur in wood below the level of fibre saturation point.

When wood continues to dry past the level of fibre saturation point – it loses moisture from its cells and shrinks in size. If the wood is already dried below fibre saturation point and the environment around the wood gets damper again, for example in very humid conditions, the wood takes on moisture into its cells and it expands again.

You’ll probably have encountered this type of movement with wood products in your house.  A common one is if you’ve experienced fairly damp conditions and noticed a wooden door suddenly struggles to close. One of the likely reasons for this is that the timber in the either the door and/or the frame has taken water into its cells and expanded, so they no longer fit together.

Our guitars are made to such a precise specification, that if we were to meticulously thickness a guitar neck blank and then leave it for a week it could lose some of its water and shrink – this is really no good for us, makes us pretty annoyed and should be avoided. One of the areas of the guitar most susceptible to moisture changes is the soundboard, because it is so thin and the exposed surface area so large.

So this mischievous aspect of wood begs for luthiers to be aware of the moisture content of the wood we work with and the moisture content (relative humidity) in the atmosphere of where it is stored.

Humidity is defined as the amount of water the atmosphere can hold at any given time. It is called ‘relative humidity’ as it’s directly related to the current temperature. That’s because when it’s hot the atmosphere can hold more water than when it’s cold. So 40% humidity in 5 degrees is not the same moisture level as 40% in 25 degrees.

So when we’re building guitars we need to control our building environment so that our wood is in moisture equilibriumwith the surrounding air, which in turn means that it doesn’t decide to expand or shrink as the guitar is being put together. Sudden shrinkage in a too dry an environment can also cause wood to crack – and that results in a very unhappy guitar and luthier.

So in the Turnstone workshop and in a lot of guitar making workshops, we try to maintain a relative humidity of around 45% - 50% at a temperature around 20 degrees. Moisture content in timber stored in these conditions usually averages out at about 7-8%. So that means it’s well below fibre saturation point. How long it takes to get to this point obviously depends on what state we receive it in and how it was originally dried, but also the dimensions of the wood come into play (open surface areas and thickness) and the species of wood.

As a general rule, the longer wood can be kept in a stable environment before it is worked, the more stable the wood will be overall and can deal much better with moisture changes without a detrimental effect on a guitar. This is one of the main reasons why luthiers buy wood long before they intend to use it.

There is a lot of information here. It’s not necessary for guitar players to understand (or even care!) about all of this but for those interested I hope it gives a little insight into what we need to take into consideration when building your guitars.

We make the most stable instruments possible so players just need to take some simple precautions to keep them healthy. To see what you need to do to do to keep your guitars in tip top condition, please check out our knowledge section.