In dry conditions (below 40% relative humidity) the arching of the front and back will shrink down onto both ends of the sound post, which can make the response of the cello dry, harsh and excessively resistant. If the wood contracts further, stresses from the changes of shape will tend to build up near the top and bottom of the instrument because the arching is fairly flat in these areas and therefore cannot change shape. If these stresses increase gradually, the animal glue attaching the front and back to the ribs will release at the points of tension. The tone of the cello will suffer due to the newly-opened seams, but no damage will have been done to the plates. However, if a sudden and extreme reduction in humidity occurs and the seams do not come unglued, cracks may form in the front or back. Another common symptom of low humidity is a reduction in string heights; differential rates of shrinkage in the neck root cause the neck angle to change, encouraging the bottom of the fingerboard to rise up closer to the strings. (see table below showing common symptoms of low and high humidity in cellos)
In conditions of high humidity (over 60% relative humidity) the cello may lose resistance and projection as the plates slacken and the sound post becomes too loose. The neck root will expand on the varnished side, thus making string clearances higher than usual.
Air temperature has a big impact on relative humidity. For example, if the relative humidity of the air outside in winter is 40% and the outside temperature is 8°C, as soon as the air moves into a heated house and is warmed to 20°C, the relative humidity of the air will drop to a dangerously low 18%. For this reason, the most hazardous time of year for cellos is during the winter months when central heating drives down the relative humidity of air which is already lessened by the cold conditions outside. Relative humidity in the USA plummets as low as 10-15% in heated homes and even in the UK, a centrally heated home on a freezing January day set to 20°C could drive the indoor environment down to a dangerously low relative humidity.
The best way to monitor humidity levels is to use a battery powered digital hygrometer in the room or case where you keep your cello. Hygrometers are not expensive and they are small and portable. If the humidity in your room is too low you can use a room humidifier but a far cheaper, simpler solution is to keep your cello in its case whenever you are not playing it and to use a case humidifier to keep the humidity at a suitable level. It is also sensible to use a digital hygrometer in your case so that you can check the environment around your cello and be reminded when you need to moisten your case humidifier. Built-in circular (non-battery powered) case hygrometers are notoriously inaccurate but some modern digital case hygrometers are very reliable.
Hygrometer test. We tested three digital hygrometers for this article: Planet Waves, Oasis and Stretto. Planet Waves consistently registered a lower humidity reading than either Oasis or Stretto and was also less sensitive to humidity changes. Since Stretto agreed most consistently with our control hygrometer, we chose Stretto as our favoured hygrometer, but Oasis was also very reliable and a little less bulky than Stretto. Further information about these models is listed below:
Humidifier test. We then tested three case humidifiers (Planet Waves, Stretto and Oasis) using Stretto hygrometers in three identical fibreglass cello cases, placing a different humidifier in each case. We kept the cases in a humidity controlled room, left each case open for about one hour each day, and took daily readings from the three cases for seven days. We used large-sized Stretto and Planet Waves humidifiers in our cases; the Oasis humidifier is only available in a small size.
In this first test the Stretto humidifier was consistently the best performer. The Planet Waves came a credible second but the Oasis did not make much impact on case humidity at all.
In a second test we used Stretto as our control humidifier and compared its performance with a ‘Dampit’ style green snake humidifier and a homemade device made from a 35mm plastic film canister with holes drilled in the ends and a piece of dampened sponge inside. In this test the ‘Dampit’ style humidifier raised humidity even more than the Stretto during the first 24 hours but its performance soon dropped away. The home made canister device was inadequate for a cello case (its performance was similar to the Oasis).
This second test showed that the traditional ‘Dampit’ style humidifier has a powerful initial effect but it must be re-moistened daily to maintain its performance and it must also be used very carefully to avoid damage to the cello and varnish. The Stretto maintained its performance throughout the test period and we would suggest re-soaking its crystal bag every week. The Stretto is less likely to damage an instrument than a ‘Dampit’ style humidifier, as moisture is absorbed by a bag of hydroscopic crystals and cannot leak onto the instrument. (See further details below).
Humidifier manufacturers recommend re-charging humidifiers with water every 1-2 weeks but the safest approach is to check the case hygrometer every morning when you open your case, and re-charge the humidifier if necessary. Another factor to consider is the air-tightness of your case; if the case is not well sealed or if you tend to leave it open for extended periods, you will need to recharge your humidifier more regularly. It is most effective to use distilled or de-ionised water in humidifiers so that the absorbent sponge or crystal does not get clogged up by mineral deposits as the water evaporates. For more detail about distilled or de-ionised water, see below.
Case study. Colin Carr and his Gofriller cello travel between the UK and the USA many times a year in a busy solo touring schedule. Colin explains how he keeps his cello happy:
‘Over the years it has become clear that the cello does not do well in extreme dryness or humidity, but I have gradually learned to control the environment around the cello so that I never have to worry about changing bridges for summer and winter. I use a room hygrometer as I find they are more accurate, and try to maintain my cello at 40-50% relative humidity. Whenever possible I use a room humidifier, but if the dryness is extreme I will use a Dampit as well. I always put a Dampit in when flying. In hotels, running the shower for ten minutes is an effective humidifier (keep the plug in so the humidity lasts longer). When I can’t use a room humidifier I drape a large damp towel over the whole cello case with the cello inside. It’s possible to raise the humidity in an instrument locker from 25% to 60% using this damp towel method and as long as the towel isn’t dripping wet, there’s no danger of over doing it. In hot concert halls I don’t worry too much about the cello as I create a lot of humidity when I play!’
© Robin Aitchison and Sarah Mnatzaganian 2010.