The tops and backs of most dulcimers are made with 1/8" hardwood. The finish is also as thin as possible, laying on top of the wood without sinking in too much so as not to dampen the tone of the instrument.
Cold weather is a direct cause of finishs cracking. It is usually just a cosmetic thing and does not really effect the sound. Once we start heating our homes, cars and work areas the air gets very dry which causes the majority of damage.
Most dulcimer wood is dried to around a 8 percent moisture content. However, the wood will forever allow moisture to pass in and out. Each variety of wood has what is called its equilibrium moisture content (EMC) with most instrument wood being at about 12 percent. This is the moisture level at which the wood is at rest naturally at a given atmospheric temperature and humidity. It is not constant. When the air is wetter, the wood will absorb moisture to a new wetter level, which is not the same amount as the moisture level in the air.
The EMC is different for each type of wood and so you have a conflict going on within the instrument when severe moisture changes occur, particularly when you have an instrument which is made out of several different types of woods. You have different amounts of stress going in different directions at the same time. This is where the pressures come from that result in the glue joints pulling themselves apart or the actual wood members cracking.
Most homes have a humidity level of about 10 percent and usually most instrument builders try to keep their workshops at about 40 percent. This allows instruments to be shipped worldwide to slightly higher or lower humidity ranges with little or no effect.
Instruments are relatively undamaged by exposure to higher humidity levels. However, when humidity drops extremely low or extremely fast, the resulting loss of moisture and shrinkage of the unfinished wood surfaces (like the inside) while the fibers of that piece of wood are still moist and unshrunken causes severe pressure and distortion of the wood. The results can be glue joints coming apart, cracks, warping, loose braces and loose and uneven frets. Wooden instruments can shrink so much that the strings will buzz. Often these strange buzzes will disapear when the humidity is brought up again.
With the unfinished fretboard wood expanding and contracting with the weather changes, it is not uncommon for the frets to lift away from the fretboard causing any number of buzzes, rattles, muted notes and more. A problem can occur when the fretboard shrinks and the fret ends are exposed. This can cause the edges of your fretboard to be so sharp it can draw blood!
All acoustic instruments must be kept in a good hard case. A quality hard case is the only way to guard against dings, scratches & bruises. This should be followed with a thermal case cover. Standard high quality wood cases offer little to no thermal protection.
When you are not playing your instrument, it should always be returned to the case. And the case should always be latched closed and be kept somewhere away from direct heat like heaters and stoves. The reason for latching is to keep the instrument in a stable temperature situation and many instruments have been damaged severely when the unlatched case was picked up and the instrument falls out.
If it is necessary to play outside where it is cold, and your instrument has been in either a warm car or house, take the instrument outside (in the case) about 30 minutes before you want to play. Leave it closed up in the case for about 30 minutes. Then 10 minutes before you want to play, open the case. This gives your instrment time to gradually acclimate itself with little shock. Do the same thing in the reverse also, if you are going from cold to warm conditions, say from the trunk of the car to the gig. Either of these senarios might mean planning on getting there a little earlier than the last minute if you want to minimize the possibility of hurting your instrument.
All musicians should understand that owning a high quality dulcimer requires that you know how to care for it. If you follow the advice outlined here, your favorite instrument will play better, last a long time, and be a valuable asset in years to come.