leaving water in the tub?

I am an architect in NYC.
I have a project on Long Island for which your tub could be a great feature.
I have one concern that I wonder if you could please address so I can decide if we should move ahead and get into the details.
I have heard that traditionally these types of tubs are meant to be left with water in them. This would make sense for a wood vessel made of separate pieces of wood (such as a wooden boat). I did not see any reference to leaving water in your tubs. I would like to know from you: what is the tradition and recommended maintenance routine for your tubs?

Thank you for your interest in our japanese bathtubs.
There are some precautions to be taken when installing a wooden bathub.
The main point is to maintain a balanced relative humidity between 40 and 60%, which is by the way also an optimal environment for pianos, antique furniture, and of course human beings!

For this reason you should avoid elements that can dehydrate the wood:
– floor heating under the tub
– heating elements in the bathroom in general
– central air conditioning
(most types include a humidity control battery)
– expose the tub to direct sun radiation
– leaving the tub unused for an extended period of time

Good practice is:
– use the tub at least 3 times a week. This routine restores the optimal wood humidity.
(Also, while the tub is full of water, you can use heating and AC or expose the tub to direct sun radiation

Acceptable practice is:
– leave some water inside the tub and cover it.
This should be done in case the tub is left unused for one week or more.
Anyway you should still not use heating or AC as the humidity of the wood of the external side of the tub will differ from the humidity inside and may cause the wood to warp.
Since leaving water inside the tub may cause stains and originate a water-line, we recommend to leave a bassinet with water inside and place it inside the tub. If the tub is covered, the humidity will be trapped inside and will be enough to keep the wood hydrated.

I hope this answered your question.

We are looking forward to working with you and remain at you disposal for any question/problem.