Tencho no Yu is the ryokan's own source at Hirayu in the Okuhida hot-spring district — a sodium-calcium bicarbonate water with a salt-tinged chloride finish. Neutral in pH and soft on the skin, it sits in Japan's beauty-water family, juso-sen in style and soda-soft against the skin. At the wellhead it runs colorless and tasteless but carries a faint note of hydrogen sulfide, and a generous 136.8 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards. It emerges among Japan's hottest natural sources.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺203mgP52 national61%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺70.9mgP64 national24%
- Magnesiummild; a laxative if you drink itMg²⁺13.9mgP66 national8%
- Potassiumminor; matters for drinking limitsK⁺32.4mgP77 national6%
- LithiumLi⁺0.70mgP63 national0.7%
- ManganeseMn²⁺0.50mgP66 national0.1%
- StrontiumSr²⁺0.70mgP74 national0.1%
- AmmoniumNH₄⁺0.30mgP39 national0.1%
- Iron (II)Fe²⁺0.30mgP43 national0.1%
- BariumBa²⁺0.20mgP71 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻462mgP75 national51%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻220mgP58 national41%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻54.1mgP43 national8%
- FluorideF⁻0.90mgP43 national0.3%
- Hydrogen sulfideHS⁻0.40mgP50 national0.1%
- BromideBr⁻0.80mgP55 national0.1%
- ThiosulfateS₂O₃²⁻0.30mgP57 national0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitriteNO₂⁻NDmg
- NitrateNO₃⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃137mgP66 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.6mgP72 national
- Metaarsenious acidHAsO₂NDmg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂222mgP89 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.0mgP75 national
- LeadPb0.01mgP58 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.