Double basses and winter humidity issues

We are experiencing an unusually long and cold winter this year, and in my workshop there is a queue of cracks and buzzes to be attented to. Humidity levels in winter and how they relate to instrument care is something I spend a lot of time discussing with customers. My own opinion is not necessarily gospel truth and I'm still collecting data - when it comes to basses it's an inexact science - but this is what I have to say.

Relative humidity: Cold air cannot hold nearly as much moisture in it as warm air. What happens in winter is that cold air from outside, which contains low but relatively speaking quite acceptable moisture levels, comes into our living spaces and is warmed up by our heating systems. While the actual amount of moisture in the air remains constant the warm air is now relatively speaking very dry and this can cause problems with musical instruments.

What dry air does to your instrument:

Wood: When wood dries out it shrinks, mostly between the grain lines. This means that when you look at the table of your instrument, with the grain lines running down the length of the instrument, the width will decrease slightly over time. If it shrinks too fast a crack could pop open running along the length of a grain line. What compounds the problem is that the instrument is constructed in such a way that the grain lines in the ribs, table and back run in different directions, so that when shrinkage occurs it creates tension in the instrument. What can then happen is that the ribs, which shrink along their width much more than their length, become too long for the shape of the table and pop out of line. Added to this is the fact that a many basses are made from wood that is at least partially slab cut (you can see this in the pretty circular grain patterns in the upper and lower bouts). This can create a tension which pushes the two edges of a crack out of alignment, making a straight forward gluing job impossible. Wood that has been well seasoned will present with fewer problems.

Glue: Dry air also affects the hide glue used in instrument making. This is less serious. The glue shrinks as it dries, bringing the two sides of a joint tightly together. Moist glue is quite elastic, but it become more brittle as it dries. This means that in dry conditions seams of glue can become so brittle that they open up, usually between the ribs and table or ribs and back. This is easily rectified.

String height: On some instruments the string height changes with the season, so that the strings end up lying too far or too close to the fingerboard, affecting playability. An adjustable bridge or summer/winter bridges is usually the answer to this. Some instruments need a higher bridge in the winter, some need a lower bridge, and many are quite stable and don't need adjustment at all. I have not yet found rhyme or reason why this is so. There are just too many varying factors involved. Wood is a very complex material and behaves in a unique manner in each instrument. On instruments that do change a lot seasonally it is a good idea to keep an eye on the tightness of the sound post. Some instruments need different length sound posts for different seasons both sound wise and as insurance against possible sound post cracks.

Humidity control for bass owners

We have several hygrometers (humidity gauges) in the workshop and they all show different measurements. There are ways to callibrate them, but it's important to realise that they can be unreliable. Luckily we humans, like our basses are also sensitive to humidity levels. If you feel the dryness in your nose and your lips, you know that you need to need to do something about the humidity for your own sake as well as your bass's.

Room humidifiers: This was one of the first thíngs I invested in, but it made so little difference to the humidty in the workshop that I just returned it. There are simpler methods which I use e.g keeping water containers by every radiator. When the temperature drops below minus ten outside I start boiling water in the kettle with the lid off 10 mintues at a time and I'll have a steaming pot of water on the stove all day (which I use anyway for glue). The cheaper humidifiers available may be sufficient for a small room, and I'm sure that there are sophisticated expensive ones available too which would be effective. I'd be interested to hear of other people's experiences.

"Dampits": I'm not a "dampit" fan. The danger with them is that they drip water inside the bass which leaves stains in the wood and mixes with the dust inside to form a kind of cement which is a nuisance to clean. The drops of water can also dissolve glue and seams can open. As general rule when it comes to drying or moistening wood, the slower the better. The structure of the wood can be damaged both when the cells collapse suddenly due to harsh drying and when the cells expand suddenly when water is dropped on them. So if you use "dampits" make sure that you wipe off the drips. Otherwise just keep your bass inside its cloth cover when you're not playing.

What to do about cracks and buzzes:

Open seams and some cracks are easily fixed with a bit of warm hide glue and a clamp. For others it may be necessary to open the bass so that they can be properly glued and reinforced. Sometimes I'll choose to leave a crack open until a later time when the instrument can be opened. Some cracks can be glued provisionally and then reinforced from the inside at a later date. Some cracks will create a buzz, some won't affect the sound at all. Some are really obviously, others are quite secretive. Buzzes are a whole mystery in themselves and can have many different causes. If you find a crack or open seam on your instrument that does not affect its playability, it can be left until a convenient time to fix, say within a few months or so, depending. But if it is left too long there is a danger that it can get worse if it has been caused by tension and it can also collect dirt and be more difficult to fix invisibly. 

So my advice to players is check your instrument reguarly and bring it in if you have questions, to me or another repair person whom you trust. What I have written here is as much as I can say without actually looking at your instrument together with you.