Yunohana is the spring's minerals made solid
When spring water reaches the surface it cools, loses pressure and meets air. Some of what was dissolved can no longer stay dissolved and becomes solid. This is precipitation, and the result is yunohana.
Its form depends on the minerals: fine white-to-yellow grains or thread-like clumps in sulfur springs; stony layers (travertine) from calcium carbonate; white salt crystals in very salty water; reddish-brown sediment from iron. Flakes floating in the tub are not dirt but evidence that the water is rich.
The yunohana huts of Myoban
At Myoban in Beppu, yunohana is "grown" by a method dating from the Edo period. Blue clay is spread on the floor of a thatched hut; volcanic steam rising from below passes through it, and over about two months crystals rich in sulfates form on the clay's surface. Scraped off, they are sold as bath salts.
The method was designated a National Important Intangible Folk Cultural Property in 2006. You can visit the huts and see for yourself how yunohana forms.
Belle Equipe · CC BY-SA 3.0
Myōban Onsen
明礬温泉
Oita · 100 Onsen to Know Japan — No. 87
On the floor of a thatched hut, crystals grow a millimetre a day from volcanic steam and blue clay: 'yunohana' bath salts, made this way since 1725.
Kusatsu, Tamagawa, Yunotsu
At Kusatsu's Yubatake, the sulfur that settles in the wooden channels is collected as a local speciality. Tamagawa's acidic water grows a radioactive mineral called hokutolite over long years (collecting it is prohibited). At Yunotsu's communal bath Motoyu, decades of deposits have thickened the rim and floor of the tub like stone.
Yunohana is the spring's character made visible. When you see it, ask yourself what is in the water.
Aspere · CC0
Kusatsu Onsen
草津温泉
Gunma · 100 Onsen to Know Japan — No. 25
Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.
あおもりくま · CC BY-SA 3.0
Tamagawa Hot Spring
玉川温泉
Akita · 100 Onsen to Know Japan — No. 13
At pH 1.2 and roughly 9,000 litres a minute from a single vent, Japan's most acidic hot spring pours out of a steaming valley.
663highland · CC BY-SA 4.0
Yunotsu Onsen
温泉津温泉
Shimane · 100 Onsen to Know Japan — No. 77
Two public bathhouses in the port that once shipped Iwami's silver. The water seeps naturally from the rock, and decades of brown mineral crust have built up around the tubs.
Glossary
- Travertine
- A stony layer built up from calcium carbonate deposits.
- Hokutolite
- A lead-barium sulfate mineral containing radium, known only from Beitou in Taiwan and Tamagawa in Akita. A Special Natural Monument.
Related onsen
Belle Equipe · CC BY-SA 3.0
Myōban Onsen
明礬温泉
Oita · 100 Onsen to Know Japan — No. 87
On the floor of a thatched hut, crystals grow a millimetre a day from volcanic steam and blue clay: 'yunohana' bath salts, made this way since 1725.
Aspere · CC0
Kusatsu Onsen
草津温泉
Gunma · 100 Onsen to Know Japan — No. 25
Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.
663highland · CC BY-SA 4.0
Yunotsu Onsen
温泉津温泉
Shimane · 100 Onsen to Know Japan — No. 77
Two public bathhouses in the port that once shipped Iwami's silver. The water seeps naturally from the rock, and decades of brown mineral crust have built up around the tubs.
あおもりくま · CC BY-SA 3.0
Tamagawa Hot Spring
玉川温泉
Akita · 100 Onsen to Know Japan — No. 13
At pH 1.2 and roughly 9,000 litres a minute from a single vent, Japan's most acidic hot spring pours out of a steaming valley.