Milky white: not only sulfur
The commonest cause of milky water is hydrogen sulfide meeting air and forming tiny particles of sulfur. A sulfur spring may emerge clear and only turn white in the tub (Naruko, Zao, Manza). The particles scatter light, so shallow water looks bluish-white and deep water milky.
Another cause is fine calcium carbonate. Shirahone's whiteness is of this kind; there is a sulfur smell, but calcium carbonate does the clouding. Metasilicic acid can cloud water too.
Milkiness changes daily with air temperature, water temperature and ventilation; the same tub can differ between morning and evening. "Not white today" is nothing unusual.
Soica2001 (talk) · Public domain
Shirahone Onsen
白骨温泉
Nagano · 100 Onsen to Know Japan — No. 50
Clear as it emerges, white once it meets the air. Shirahone's milkiness comes not just from sulfur but from calcium carbonate that solidifies, even building the 'funto-kyu' spring mounds.
SpringField1967 · CC BY-SA 3.0
Manza Onsen
万座温泉
Gunma · 100 Onsen to Know Japan — No. 29
At 1,800 metres, 5.4 million litres of sulfur water a day. Thin air, deep snow, and a milky bath in the middle of it.
Zhaitengli · CC BY 4.0
Naruko Onsen
鳴子温泉
Miyagi · 100 Onsen to Know Japan — No. 18
Seven of Japan's ten official spring types in one valley. The communal Takinoyu by the station costs 300 yen, and the town lends you wooden geta to clatter from bath to bath.
Blue: particles scatter blue light
Beppu's Myoban, Marukoma in Hokkaido, the "blue open-air baths" of Niseko and Hakone: blue is not the colour of a dissolved mineral but of very fine particles (sulfur or silica) scattering short-wavelength light, the same physics as a blue sky. As particles grow the water turns white, and with more of them, milky.
Water advertised as "cobalt blue" or "emerald" varies with light, so knowing this narrows the gap between expectation and reality.
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.
Soica2001 (talk) · Public domain
Marukoma Onsen
丸駒温泉
Hokkaido · 100 Onsen to Know Japan — No. 4
The bath is exactly as deep as Lake Shikotsu is high that day: an open-air tub joined to the lake.
Green: sulfur and pH in balance
The green water of Tsukioka (Niigata) and Kunimi (Iwate) comes from the combination of sulfur compounds and pH. It ranges from yellow-green to deep green and, again, changes from day to day. Iron-bearing water can also take on a yellow-green tint.
663highland · CC BY 2.5
Tsukioka Onsen
月岡温泉
Niigata · 100 Onsen to Know Japan — No. 38
A green so vivid people think it is bath salts: the sulfur in the water shows itself as colour in the tub.
Brown and red: iron rusting
Iron springs emerge clear. On contact with air the iron rusts (oxidises) and the water clouds brown to reddish-brown: Arima's gold spring, Ikaho's golden water, Furofushi. Every iron-bearing water does this, which is why tubs and floors stain red — and towels too.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.
ShuichiNakayama · CC BY 4.0
Ikaho Onsen
伊香保温泉
Gunma · 100 Onsen to Know Japan — No. 28
The spring runs beneath 365 stone steps and is tapped off to the inns tier by tier. A town built around the iron-browned 'golden water'.
おにく · CC0
Furofushi Onsen
黄金崎不老不死温泉
Aomori · 100 Onsen to Know Japan — No. 12
A rust-brown pool cut into the rocks at the very edge of the Sea of Japan, with nothing between you and the sunset.
Black: ancient plants
Tokyo's "kuroyu", Tokachigawa in Hokkaido and several Niigata springs are the colour of strong tea or cola. Not volcanic, they are groundwater that has passed through peat or lignite and dissolved plant-derived organic matter (humic substances). Called "moor springs", they feel silky. Tokachigawa is known as one of the world's few plant-derived moor springs.
663highland · CC BY 2.5
Tokachigawa Onsen
十勝川温泉
Hokkaido · 100 Onsen to Know Japan — No. 6
Not volcanic heat but ancient plants: the brown, silky 'moor' spring that defines the type in Japan.
Kamata Onsen
蒲田温泉
Tokyo · 100 Onsen to Know Japan — No. 35
Water so black your hand disappears in it, in a neighbourhood bathhouse in Tokyo, for 600 yen.
Springs that change colour
Yunomine's Tsuboyu is said to change colour seven times a day, moving between clear, white and blue. Some baths at Sukayu and Naruko also vary in cloudiness with the weather. The reasons are as above: particle load and light. That is why this atlas does not boost the colours in photographs — the real water is whatever colour it is that day.
Nekosuki · CC BY-SA 4.0
Yunomine Onsen
湯の峰温泉
Wakayama · 100 Onsen to Know Japan — No. 66
You can bathe in a World Heritage site, thirty minutes at a time. The Tsuboyu, a pool hollowed from rock, is where Kumano pilgrims purified themselves.
ゆ＾ω＾び · CC BY-SA 2.1 jp
Sukayu Onsen
酸ヶ湯
Aomori · 100 Onsen to Know Japan — No. 9
A 160-tatami bathhouse of hiba cypress without a single pillar, where men and women share acidic water under Japan's deepest snow.
Glossary
- Moor spring
- Dark brown water containing plant-derived organic matter; from the German "Moor" (peat bog).
- Precipitation
- Dissolved substances turning into solid particles — the cause of cloudiness and yunohana flakes.
Related onsen
Soica2001 (talk) · Public domain
Shirahone Onsen
白骨温泉
Nagano · 100 Onsen to Know Japan — No. 50
Clear as it emerges, white once it meets the air. Shirahone's milkiness comes not just from sulfur but from calcium carbonate that solidifies, even building the 'funto-kyu' spring mounds.
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.
663highland · CC BY 2.5
Tsukioka Onsen
月岡温泉
Niigata · 100 Onsen to Know Japan — No. 38
A green so vivid people think it is bath salts: the sulfur in the water shows itself as colour in the tub.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.
663highland · CC BY 2.5
Tokachigawa Onsen
十勝川温泉
Hokkaido · 100 Onsen to Know Japan — No. 6
Not volcanic heat but ancient plants: the brown, silky 'moor' spring that defines the type in Japan.
Nekosuki · CC BY-SA 4.0
Yunomine Onsen
湯の峰温泉
Wakayama · 100 Onsen to Know Japan — No. 66
You can bathe in a World Heritage site, thirty minutes at a time. The Tsuboyu, a pool hollowed from rock, is where Kumano pilgrims purified themselves.