I was thinking about Helen's comment about how she uses glass jars and sometimes leaves them as long as a year. I realized that perhaps climate has something to do with the methods we choose when using similar techniques such as solar dyeing. Our winters are on the harsh side. They are cold and can be long. Our first frosts come in September. We can have snow, which stays as early as November. I have a high/low thermometer in the greenhouse and during the winter, there is little advantage even there. If it is -15C outside, it is -15C in the greenhouse. I've only vaguely thought about using glass jars. Large ones are hard to find these days and I would have to bring them inside during the winter. If I were to leave a full glass jar outside, it would likely crack from the frozen liquid expanding, unless I was certain to leave enough space in the jar. Even then I'd worry about it just from the freezing and thawing that might take place at various times, from temperature fluctuations.
Even our summer weather can be erratic. We're having extreme and unusually hot weather, but often we get very warm days and cool nights. I was doing some Indigo fermentation one year, starting early in June to take advantage of a few really early, warm days. I had 2 white buckets and one which was blue and white. After the 2 warm days, I noticed that the water in the buckets had cooled down noticeably. The outside of the buckets were almost cool. In thinking about this, I sat down on some plastic bags of top soil and compost and found that it was hot, very hot in places, which were the black writing. I immediately spray painted my buckets black and a few hours later, the water was steamy warm. I'll have to do some checking on water temps.
Bettina - my regular old water is well water. Our water is fairly heavy in iron, so we filter it somewhat. Not sure about the hardness factor. I don't think it's super hard, but at least is free from chlorine.