I had a few tins open for too long, and the tobacco contained therein is now too dry. What is the best way to reintroduce the right amount of moisture? Mason jars with small humidifiers perhaps?
I usually spray some distilled water in the tin, shake it up and wait a day or two. Always use distilled water so you don't impart the chemicals from tap water.
Finally, I’ll offer my preferred method of moistening tobacco in a controlled, if not scientifically precise manner. Put the tobacco in a large bowl, and cover the bowl with a moist towel (plain water is fine). Do not let the towel touch the tobacco. Over time, and it can take anywhere from hours to days, depending on how dry the tobacco is to start with, and the ambient temperature and RH, the tobacco will equilibrate with the moisture provided by the moist towel. Check it often, and re-moisten the towel if necessary. This works like a charm, and minimizes the risk of over hydration, and mold. The target is strands of tobacco that are pliable, but don’t stick together when pressed into a ball.
I've used this method many times. Works to perfection.
If you can drink your tap water there is absolutely no need whatsoever to use distilled water.
Check the tobacco occasionally, give it a turn and a mix when you do.