This topic is close to my heart and is low-key a huge part of my life. I obsess about humidity, in a good way. I’m not sure when this started, perhaps when making my first guitars, but at some point in my guitar making journey, the importance of humility control was drilled into me. Humidity regarding woodwork is an interesting and nuanced topic. Being well informed about humidity means you understand and respect your guitar better, and this knowledge really makes guitars and even other wooden things, seem more alive. I will write about humidity in general first, then my advice to guitarists, and finally about my own humidity journey.
Wood and humidity:
Wood adjusts/expands/contracts to the relative humidity (RH). There is another measurement of humidity called ‘absolute humidity’, however this isn’t relevant to woodwork.The RH is the amount of water in the air, relative to the air temperature’s ability to hold water. Guitar makers usually build around 45-50% RH, so there’s leeway for the wood to expand and shrink in wetter or dryer environments. If a guitar was made a 90% RH and travelled to a drier environment, the wood would shrink so much that it would crack, and the opposite situation would cause problems too.
Different species of wood move in different ways. For example, Indian Rosewood is particularly stable. I’ve noticed maple tends to expand and contract quite a lot, though without cracking. Brazilian rosewood has a reputation for cracking a lot (ie, see an earlier post with pictures of a 100 year old Arias guitar of brazilian rosewood). There is a wood called Cocobolo that I was advised by London Guitar Studio against using, as it’s gonna crack. Cedar tends to move less than spruce. The beautiful wood called snakewood moves unpredictably and cracks, so is usually reserved for small decorative functions such as the binding. Ebony has a reputation for shrinking forever, which can cause cracks along the side of the fretboard, so must be extensively dried before use.
Showing how different cuts of wood move in different ways.
Within these species, individual pieces of wood can have different levels of hygroscopicity. In a book for intermediate luthiers, Daniel Friedrich recommended testing soundboards for their levels of hygroscopicity, and using those examples less prone to absorbing water for the guitars destined for tropical places.
It is also important for guitar makers, and also players when buying a guitar, to consider the cut of the wood. If the wood is quarter-sawn, it expands and contracts in a more predictable way, and only half as much across its width, than plain-sawn/ flat-sawn wood (recognisable by more elaborate grain patterns).
Advice to guitarists regarding humidity:
It is important to take all these things into consideration as a guitar is made up of many individual small pieces of wood of many different species. The guitar is expanding and contracting, each species of wood at different rates, in the worst cases each piece perhaps pulling in different directions or twisting. This is why it is important to keep your guitar in a stable environment, keep an eye on the RH, and take the type of wood into consideration when you buy a guitar.
The first step is to buy a hydrometer so you at least know what the RH is. Then basic measures such as not storing your guitar near heaters, drafts can be taken. Keeping your guitar in its case is a good option as it’s easier to control the humidity in that small space. I do this, and I use D’addario’s Two Way Dehumidification kit or something similar, to keep the humidity in the case at 45%. If you have a room where you keep all your instruments, I’d recommend getting either a humidifier or dehumidifier depending on your situation.
Many of the factors regarding humidity which make life easier or harder for you as a player, are actually in the control of the luthier. But by keeping the guitar in a stable environment and not exposing it to extremes, you will be limiting any problems and keeping your guitar happy.
Some cracks visible on this Brazilian Rosewood Vicente Arias guitar. If using a wood that may crack one day, it’s also worth bearing the finish in mind. For example, it may be better to French polish it, rather than using a conversion varnish which would result in an uglier repair.
My dehumidification journey:
I work in England. For most of the year the RH rises to about 90% at night and falls throughout the day to about 60% in the evening. I’ve been tracking the humidity religiously for about 7 years so don’t need to look those figures up. Therefore, in England, regarding humidity, unless you have your central heating on all the time, it’s usually a matter of dehumidification.
When I was studying guitar making, I was sixteen, and I rented a workshop, as my course was only two days a week (!) and I wanted to be working all the time. However, this workshop wasn’t the most suitable; a concrete floor, the thinnest brick wall and single glaze windows, and freezing cold in the winter. I bought an industrial dehumidifier; the thing was big and loud and due to the large size of the room and nature of the walls and windows, the effort was quite futile.
The next workshop when I was 22, was also quite unsuitable as far as humidity went. However, this time I built a cupboard, which was airtight. I put a small dehumidifier inside and stored everything in there when I wasn’t working on it. Not a perfect solution, but an improvement.
For the next workshop, keen to have a handle on the humidity, I bought an expensive dehumidifier from the brand Maeco. I had it running all the time to get an RH of 45%. And I do the same thing in my current workshop. The dehumidifier is running literally all the time. It’s not loud but I must say there are times when I would prefer silence and the hum of the dehumidifier sounds like a roar. However, for the sake of my guitars, it’s a small sacrifice that I’m more than willing to make.