Spring Temprature, pH & Osmotic Pressure
Mineral springs can be classified into many categories according to Japanese law. Just like wine in France, onsen in Japan have a detailed classification system. Almost every onsen will have a sign that, along with basic information like the address of the well, when it was drilled, the well temperature and so on there are four important factors in describing each spring.
Classifications are done using 4 criteria.
Type of Spring
Cold spring - 冷鉱泉 - wells that have a temperature up to 20 C.
Warm spring - 低温泉 - wells that have a temperature from 25 C to 34 C.
Hot spring - 温泉 - wells that have a temperature from 34 C to 42 C.
Extra hot spring - 高温泉 - wells that have a temperature above 42 C.
The common temperature for a bath is 41 C. Many places with a spring colder than 41 C will heat up their spring water. This is called kaon 加温 in Japanese. Conversely, springs that are too hot will cool the spring by adding cool water to the baths. This will dilute the amount of minerals in the bath. It is called kasui 加水 in Japanese. If the water is used straight out of the well without any heating or cooling it is called kakenagashi 掛け流し or gensen 源泉.
Acidic - 酸性 - Wells that have a pH of 3 or lower.
Weak acidic - 弱酸性 - Wells that have a pH of 3 to 6.
Neutral - 中性 - Wells that have a pH of 6 to 7.5.
Weak alkaline - 弱アリカリ性 - Wells that have a pH of 7.5 to 8.5.Alkaline - アリカリ性 - Wells that have a pH of 8.5 or greater.
Acidic wells are rare in Japan. Most common you’ll find neutral to weak alkaline wells. The pH of the water has a large effect on how the water feels in the bath. The more alkaline the baths are the softer and soapier the water feels. Alkaline baths are thought to be better for the skin and also called "beauty springs" or bijin-no-yu 美人の湯 in Japanese. Acidic baths will, conversely, feel much harder or firmer on the body. Both sensations are enjoyable and are a matter of opinion. Some bathers dislike the strongly alkaline baths as they dislike the slippery feeling while others dislike acidic baths because of the dry skin feel.
Low - 底張性 - 8g/kg of dissolved solids or lower and a freezing point of -0.55 C or higher.
Isotonic - 等張性 - 8g/kg to 10/kg of dissolved solids with a freezing point of -0.55 C to -0.58 C
High - 高張性 - 10g/kg or higher and a freezing point of at least -0.58 C
This is the total weight of the dissolved solids in a given amount of the spring water and the water’s ability to hold those minerals in suspension. In a very basic way of talking, It tells us how much pressure is needed to squeeze the water out of the solids suspended in that water. Sometimes you can tell the difference between the onsen in this category by just looking at the water. Those that contain a high percentage of dissolved solids will typically be opaque. Baths that are cloudy or entirely opaque are called nigori-yu. The word nigori is also used to describe cloudy sake. For more information about this please watch this wonderful video from The University of Texas at Austin that filled it in for the non-chemist that writes this page. https://www.youtube.com/watch?v=l33rQYic5ro