This is a problem in climates with cold winters and warm summers caused by heated dry winter air alternating with humid summer air. When cold air is heated in the winter, the indoor relative humidity drops way below the outside relative humidity. This dries out the wood slowly over the winter, causing it to shrink. In summer, the humidity slowly restores the moisture in the wood and it re-expands. Some woods expand / contract more than others. Jay Bereck told me that he chose cherry because it had the lowest coefficient of expansion with moisture of any of the woods he was considering. Coefficient of expansion, or stability, measures the amount of shrinkage/expansion of wood given a certain change in relative humidity. This is different from coefficient of expansion for temperature changes, a negligible factor for congas. The main factors that affect wood expansion are extremes in relative humidity beyond 35%-65% range, species (see chart
http://www.planethardwood.com/documents/WoodStabilityChart.pdf), grain orientation (flat sawn more expansion along the length and width of the stave than quarter sawn), and how well the finish seals the wood.
In my personal experience in New Jersey (humid summers, cold dry winters), I had two quarter sawn oak Skin on Skin drums finished only with Watco oil both crack in multiple places during their first several winter/summer cycles. One ash SOS with polyurethane finish cracked in the winter as the wood shrank around the alma. Two quarter sawn oak SOS drums finished with polyurethane on the outside have never cracked. My cherry SOS with polyurethane has never cracked. Most of the cracks form in the glue line between the staves. Once fixed and refinished with polyurethane, the oak drums never cracked again. This is the first winter for the ash drum after I fixed the cracks and moved the alma to make a less tight fit in winter. Humidity in my studio ranges between 70-80% in summer and 35-45% in winter.
In mid winter, I have taken the screws off the loose bands, moved the bands down or up the drum to a larger circumference area until they are moderately tight, drilled new screw pilot holes (partial thickness) and replaced the screws. I have not yet had a SOS band pop in summer, but this can happen, especially with the thin aluminum bands like some Gon Bops drums. I did this with a Gon Bops drum many years ago and several of the rivets popped in the summer.
Oak The bands don't shrink and expand, so the relative circumference of the shell and the bands changes with the seasons, leaving loose bands in the winter and tight bands in the summer. Some of the drums still have bands a bit loose in the winter that can rattle, others stay tight. On most makes of drums, the bands are mostly for aesthetics and loose bands will not affect the integrity of the shell.
Ways to reduce the expand / shrink process in the cold climates include:
1) Use a dehumidifier or AC 24/7 in the summer and a humidifier in the winter to reduce the seasonal variation in humidity. Expensive and fossil fuel dependent, but comfortable. Fresh air ventilation of damp basements may help.
2) If your drums have oil finish only on the outside, seal the outside with polyurethane. Do not seal the inside. Polyurethane inside the drum would reduce expansion/contraction but would make most drums ring and sound much worse. Partially effective.
3) Fix existing cracks before sealing the wood.
4) Cherry probably is better than oak (even quarter sawn) and ash. Not sure about mahogany. Anyone with mahogany drums who can report on your experience?
5) Never heat your home in the winter. This can be bad for your hands, not to mention marriages, kids, dinner guests, etc.
6) Move to Cuba or other Caribbean island or take your drums there every winter and never miss a year. You may have difficulty leaving every spring. Risk of jail, large fines or losing some of your Cuban friends if you live in the US.
Posted: Sat Jan 29, 2011 2:09 pm
I have just not dared so far because I am not too familiar with bands, so seriously, thank you for this tip.