Just a short reply and explanation to the instructions I sent to Richard.
There is a big difference between a newly built instrument and a vintage one. Our instruments end up all over the world in all kinds of environments.
We need to make sure they are able to adapt to these variations.
We import wood from different parts of the world ourselves and need to dry/cure/stabilize them to a uniform r.h.value. Wood is much more stable at low values. You do not want to combine different types of wood with different r.h. And expansion values. another factor is the hide glue, which cures and functions better at low hum.
it is much easier and safer for an instrument consisting of different types of wood with different expansion values to over time adjust to higher rather than lower hum. values. Wood flexibility increases with increased hum. Which allows adjusting without damage. On the other hand, shrinkage (high to low humidity) can cause cracks etc. We recently delivered an instrument to Europe that ended up in a 80% r h. And showed extreme wood tension (warping). After a controlled drying process, the instrument was brought back to ca. 40% with no damage.
imagine going the other way, shrinkage would have cracked every flat surface.
Our instructions are for new instruments, especial ones with engelmann or Sitka sound boards. Over time the instrument will adjust to any value without damage.
regarding vintage instruments, especially ones imported from the UK, we have repaired countless cracked sound boards, failed glue lines, warping, etc due to moving the instrument to a lower hum area. Keep in mind that mainly coastal regions have high hum. Inland usually is a lot drier.