Duncan (d-baron) and Keith (Corptaxman) and any other interested parties): Well, I think I waited long enough, and with no input (yet) from others.... I think I will pipe in again in this subject.
Duncan.
To try to answer your question in a long and roundabout way... For human comfort (as well as for wood chess sets, ivory, and bone....) I would think that an indoor humidity level of about 50% is appropriate (if it can be maintained) throughout the year. Some folks and sources talk about a range of between 40 - 60% to be acceptable.
However, in winter time - in northern dry climates, for example, here in the N.E. and Central USA, having humidity levels of above 60% (but especially above 70% can lead to mold growth. That level can be harmful to house structures, since it promotes mold growth. And while, generally speaking, the mere presence of mold, mildew, and mold spores and not necessarily harmful to humans, it can be problematic if a.) The exposed person is allergic to mold, b.) the person has asthma, c,) The person has a compromised immune system, and/or if the amount of mild spores inhaled is too high
While there is not an actual regulatory or set-in stone "level" (here in the USA) of mold spores or parts that is considered "safe" or “unsafe” for a person in general, too much of anything is not good. For example, drinking too much water is not good. Too much salt intake is not good. Breathing too much of any one particular gas is not good. And inhaling too many particulates, regardless of the material or source, is also not good. Etc. etc. etc. (I say all of this, having worked within and headed the non-industrial) Indoor Air Program for the Vermont Department of Health a number of years ago.) And of course, one does not want mold or mildew growing on paper (books) or ivory, etc.
Keith:
The information you provided about your own dwelling is interesting. And your question to the group is good one, I think. And even though I think I understand why you asked that question, there might be a point that got missed. I am not sure.
In the northeastern and north central climates in the USA, outdoor winter air tends to be fairly dry. Indoor humidity levels can also be dry, since there is usually an exchange between the outdoor levels and the indoor levels.
Unless one's house is extremely "tight" and one supplements the house’s humidity level by way of many (mechanically) unvented showers or cooking activities, and/or one uses room humidifiers or house humidification systems, then the indoor air can humidity levels can drop to the 20's and 30's. Occasionally, it could be as low as in the teen's! I know this is not good for human comfort, and I suspect could possibly lead to increased susceptibility to respiratory infections, if exposed to viruses, etc. I also don't believe a humidity level in the teens and low 20's is good for the "life and health" of things such as ivory (and pianos!)
Over the last couple of days and weeks, I have been struggling to get and keep the indoor house level at about 45 (or so) % humidity. The other day, the outdoor temperature was down to zero degrees F or so. Many (but not all) of the time here in Vermont, the outdoor air gets much drier when the outdoor air is so cold.
I think it is time for me to use my new, relatively small, humidifier. (As it was, I have only been using the small glasses of water in the display cases. That seems to work for ivory. But for the pianos and human comfort, additional indoor moisture is needed. That is, the only sources of indoor air moisture I have used this far have been a.) the unvented shower moisture in an individually heated bathroom) b.) the unvented (electric stove) cooking activities, c.) a pan of water atop the wood stove in the basement and d.) human respiration and skin evaporation.
In summary, as far as my ivory and bone and wood chess sets which I have collected for nearly 19 years..... so far, so good, as far as not (seemingly) having any new cracks the pieces.
I do not, however, know how long it would take such materials to crack (due to heat and/or dryness) over a long period of time.... say, decades and centuries. My guess is that would depend on a number of factors. But I will leave it to all of you to add whatever knowledge or guesses you'd like to take...
"Johnny V."
