Humidity, arch shape and bridge fitting

[tom.gibson]

After putting the finish on a new mandolin I've found that the bridge, which fitted perfectly when it was strung up in the white, now fits quite badly. I'm sure this is due to a big increase in humidity, combined with the fact that one side of the soundboard is now sealed and the other is not, thus changing the arch contours.

My question is whether it's a good idea to fit the bridge to the instrument at the same sort of middle-range humidity (40-50%) used for gluing braces etc. If I refit the bridge now that its steamy summer, will that just mean it's poorly fitted in dry winter?

[sunburst]

My question is whether it's a good idea to fit the bridge to the instrument at the same sort of middle-range humidity (40-50%) used for gluing braces etc. If I refit the bridge now that its steamy summer, will that just mean it's poorly fitted in dry winter?

That's actually 2 questions, but the answers are the same: yes, and yes.

On thing, though. Once the top is glued to the rim and thus restrained from "cupping" (warping across grain), the fact that one surface is finished and the other is not makes very little difference in the shape of the top arch. What happens is: if the top takes on moisture from the air (high relative humidity) it swells/expands in width. Since it is contained because it is glued to the rim and cannot get wider as it would if unglued, the arch increases in height. Conversely, when the top releases moisture and shrinks (low RH) the top arch will sink (shortest distance between two points being a straight line, the top "tries" to get closer to that if it continues to shrink). So, since the top's response to changing moisture content is to change the shape of the arch, the bridge can only fit well if the relative humidity (and thus the moisture content of the wood) is controlled.

[tom.gibson]

Thanks, John. I've never come across advice to fit the bridge at the right RH, but it makes sense. When the humidity shoots up, and there are no strings on to counteract the movement, the change in arch shape is quite dramatic. I'll turn my dehumidified box back on and see if the perfect fit returns.

(I can see you're right about the finish being on one side only making no difference: in fact if it did do anything it would do the opposite of what I was thinking, since the cupping would be concave, not convex.)