Yufu-no-Oyado HOTARU

Yufuin Onsen boasts the second largest volume of hot springs in Japan, following Beppu Onsen in Oita Prefecture.
Most of its springs are simple hot springs, but each of the source wells in the Yufuin basin has slightly different characteristics.
Yufu-no-Oyado HOTARU's spring water is an Alkaline Simple Hot Springs with a Hydrogen ion concentration (pH value) of 9.7, one of the highest among Yufuin's hot springs.
Since human skin is generally slightly acidic, soaking in alkaline hot spring water causes the surface of the skin to react and dissolve dead skin cells. This results in a silky smooth skin texture. However, because the pH value of the hot springs here is particularly high, the texture of the hot spring water is more than smooth; it would be more accurate to describe the water as thick and creamy. The feeling of the water clinging to the skin can be felt as soon as one takes a bath. The clear, colorless water is also referred to as “Hot spring water like lotion” by regular visitors.

After bathing, the skin feels smooth and renewed because the dead skin cells have been removed. The alkaline nature of the water is one of the conditions for its being called “Hot springs for beautiful skin”.
Since dead skin cells are removed, skin metabolism is also activated. The skin tissue newly created by bathing comes to the surface in about a one-month cycle, depending on the individual, so it is known that the skin beautifying effect can be obtained not only immediately after bathing, but also about one month later.
However, it is important to note that there may be hidden pitfalls even in the beautiful skin baths. The removal of dead skin cells means that a barrier on the surface of the skin is removed. In other words, the skin is more prone to dryness, so in the winter when the air is dry, or if you have dry skin to begin with, you should put on some cream after taking a bath.

The hot spring is rich in Metasilicic acid. It contains 123.8 mg/kg of metasilicic acid, more than twice the hot spring standard value (50 mg/kg). Metasilicic acid is a natural moisturizing component and is sometimes used as component of some lotions. It has a moisturizing effect and leads to moist skin after bathing. The alkaline component removes excess dead skin cells from the skin, and the moisturizing component metasilicic acid penetrates into the skin.
Metasilicic acid meets one of the requirements for qualification as a hot spring when present at 50 mg/kg or more. At 100 mg/kg or more, it provides a noticeable moisturizing effect. Although it plays a supporting role, this can also be considered one of the conditions for “Hot springs for beautiful skin”.

Carbonate ions (CO₃²⁻), which have the most abundant content among anions, are also noteworthy. When alkaline simple hot springs contain high levels of carbonic acid ions, they more effectively wash away sebum and dirt on the surface of the skin, resulting in a strong feeling of wetness and smoothness after bathing. And it also exhibits a higher cleaning effect.
Sodium ions (Na⁺), which make up the majority of the cations, combine with Hydrogen carbonate ions (HCO₃⁻), producing the so-called “baking soda” effect and further enhancing the smooth feel of the water. The hydrogen carbonate ion is said to enhance the skin's ability to retain moisture in the stratum corneum, which means that the skin tends to feel less tight and more moist after bathing.

A simple hot spring is a hot spring with less than 1,000 mg of dissolved substances in 1 kg of the source. Because of the low dissolved substances, this type of hot spring is said to be “Gentle hot spring” because it is hypoallergenic. Therefore, it is said to have a particularly relaxing effect, and indications unique to this spring include Autonomic instability, Insomnia, Depression, and other mental health issues.
These conditions are thought to have been selected as indications for Simple Hot Springs because of the gentle nature of the water.
With a relatively small amount of dissolved substances and its low-irritation nature, the water places less strain on the body, making it easier to enjoy a long, relaxing soak.
The thermal effects, buoyancy, and water pressure of hot spring bathing help relieve physical and mental tension and promote relaxation. In addition, the gradual decrease in core body temperature after bathing helps promote natural sleepiness.
For these reasons, Simple Hot Springs are thought to help relieve autonomic instability, insomnia, and depression.

Even though the amount of dissolved substances in simple hot spring water is low, compared to commercially available highly concentrated bath additive (with approximately 100 mg of dissolved substances per kilogram), the dissolved substances in the Yufu-no-Oyado HOTARU source are approximately 300 mg/kg, which is considered about 3 times higher.
The greatest advantage of simple hot spring is that it is hypoallergenic, so people of all ages and both sexes can bathe in it at their leisure. Therefore, it is sometimes used for stroke rehabilitation, and is therefore sometimes referred to as “Hot springs for strokes” or “Hot springs for neuralgia”.
When a hot spring contains less than 1,000 mg/kg of dissolved substances, it is generally classified as Simple Hot Springs. However, lumping all such springs under a single label makes it difficult to convey their true individual qualities. For example, according to the Hot Spring Analysis Report, if the total amount of dissolved substances in the hot spring at Yufu-no-Oyado HOTARU were to exceed 1,000 mg/kg, it would be classified as Sodium‒Sulfate / Hydrogen Carbonate Springs.
In other words, it would possess the properties of both a Sodium‒Sulfate Spring (formerly known as Glauber’s Salt Spring) and a Sodium‒Hydrogen Carbonate Spring (formerly known as Sodium Bicarbonate Spring).
The Sodium‒Sulfate Spring promotes skin regeneration by stimulating collagen production, earning it the nickname “Hot springs for rejuvenation”.
Meanwhile, the Sodium‒Hydrogen Carbonate Spring has a cleansing effect similar to soap, gently removing excess sebum and leaving the skin smooth—making it a quintessential “Hot springs for beautiful skin”. This demonstrates that even “simple hot springs” can possess powerful and diverse benefits—ones that can only be discovered by taking a closer look at the spring’s detailed analysis.

The temperature of the spring water is 43.6℃, which is the ideal temperature for a free-flowing hot spring water.
Free-flowing hot spring water is a luxurious use of water, in which the hot water is poured into the bathtub and any overflow is disposed of as it is. The possibility of always being able to soak in fresh hot spring water is a great attraction for hot spring enthusiasts.
If the temperature of the source is not at the perfect level, it is necessary to go extra steps to make the hot spring flow, such as adding tap water to lower the temperature or using a boiler to raise the temperature, which may compromise some of the original power of the hot spring.
This process is also costly. The temperature of the spring is also important, especially for those who are particular about hot springs. In the case of free-flowing hot spring water (when no chlorine or other disinfectants are used), it is said that a standard amount of 1 liter/minute of spring water per person in the inn is necessary to keep the baths clean and enjoyable. The hot spring here is about twice the standard value, which means that the source can be used in a luxurious and hygienic way.

Furthermore, the hot spring here is a private source.
The benefits of this are that the distance between the spring and the baths is close. This reduces the risk of impurities mixing with the hot spring water before it is poured into the baths, and most importantly, the hot spring water is fresh and ready to soak.
The further away the hot tub is from the spring, the more likely it is that the hot spring components will deteriorate. Whether or not a hot spring has its own hot spring source is a point that hot spring enthusiasts should not overlook.
The hot spring here is 100% natural free-flowing hot-spring water, which is poured into the large baths, private baths, and open-air baths in guest rooms.

Once you soak in the excellent hot spring with its particular use of hot water as mentioned above, you will be so surprised by the tender feeling of the hot spring that you may say that this hot spring is so sinful that your impression of other hot springs in Yufuin may fade away.

“Yufu-no-Oyado HOTARU” is a hot spring inn located a little far from the central area of Yufuin Onsen. Its location overlooking Mt. Yufu is also an attractive feature of this ryokan.
You can experience the lotion-like hot spring water from its own hot spring source in the men's and women's open-air baths, as well as in the private baths available free of charge.
The guest rooms are in a detached style with a roofed corridor connecting them. The guest rooms include a variety of types, such as a guest room with an open-air bath made of rocks, a Japanese-style room with a half-open-air hinoki bath, a Japanese-style room with two adjoining rooms with a great view, and a Western-style room for the barrier-free.
The inn's atmosphere, which enhances a sense of the extraordinary, and its sumptuous dinner prepared by a veteran chef with a special attention to detail, yet at a reasonable price, make it a popular choice for many repeat guests.
Once you stay here, you will see that the owner's wish is strongly reflected: “I want anyone to feel free to come and enjoy this hot spring and know what Yufuin has to offer”.
Travel Journal