By Yasunori Kuroha / Yomiuri Shimbun Staff Writer Inky black hot water the color of cola is a feature of some sento public baths in Tokyo, mainly in Ota Ward and the waterfront area, but they are also dotted around the Ueno, Asakusa, Azabu and Edogawa districts.
I visited Kaiseiyu in Ota Ward, which takes pride in having particularly black water for its area.
Water from the hot spring is mixed with hot tap water, making it raven black. A sign near the bathtub warns, “Watch your step,” because the water is too dark to see the bottom. You have to soak in this bath very carefully.
Before taking a bath, I was told by bath attendant Naoya Maikawa, 31, that he rarely catches a cold since he started bathing in the black water almost every day. He may be right; my body felt like it became warmer than from other baths.
Drying off, I noticed the white towel was subtly tinted with brown.
Waterfront rich in black onsen
There is no data available at the Tokyo Sento Association regarding the number of sento with black hot springs in Tokyo.
Yasuo Kanroji, 88, an adviser at the Hot Spring Research Center based in Kita Ward, Tokyo, said: “Black hot water is commonly found in flat land made up of a sedimentary stratum. This layer can be seen from Hokkaido to Kyushu.”
The black hot springs in the Kanto Plain originate in a relatively shallow stratum about 500 meters deep, and that’s why sento with black water are seen so often in Tokyo.
Ota Ward in particular is known for having a number of sento with black water, and the ward office introduces this fact on its website. “Hot springs known as kuroyu [black hot springs] are found widely in the waterfront area including Ota Ward and have long been used in public baths.”
In Ota Ward, 15 of the 39 member facilities of the Tokyo Sento Association have black hot springs, while in neighboring Shinagawa Ward six out of 24 members have such hot springs.
When asked why these sento are concentrated around the waterfront, Kanroji said: “As the area has been an industrial district since before the end of World War II, many wells have been drilled to pump water to cool machinery. The area has been known for hot, black spring water gushing out. On top of that, the hot springs were deposited in relatively shallow locations and were easy to drill.”
In fact, the water used at Kaiseiyu comes from a well about 90 meters deep.
Decomposing ancient plants
Kanroji also explained the reason for the dark color of the hot springs.
Plants that sank underwater between tens of thousands and hundreds of thousands of years ago were gradually decomposed by microorganisms in an oxygen-poor environment. Eventually they changed into a black, carbon-rich substance that mixed with groundwater. The process is similar to wood slowly smoldering to become charcoal rather than being burned to ash in a single stroke.
As for the efficacy of the black water, Kanroji said, “[The kind of effect it has] hasn’t been discovered yet although research has been done.” But he added that the water is rich in sodium hydrogen carbonate, which has the effect of removing dead skin cells so that the skin becomes beautiful.
His explanation made sense to me and reminded me of my experience at Kaiseiyu. Despite the shocking color, the water is almost scent-free and has a smooth feeling. Maybe it is because the blackness is derived from plants.