No two hot springs in Japan are the same, and Japan keeps count. Official guidelines sort each spring by what its water carries: salts such as chloride and sulphate, or an element such as carbon dioxide.
A bath's analysis hangs on its wall. Regulars read it the way you might read a wine label.
Chloride塩化物泉
The salt water. It feels heavier on the skin, and Japan knows it as the water whose warmth stays with you after you leave the bath.
Alkalineアルカリ性
Not a type of its own but a quality. Where a spring's water reaches pH 8.5 or above, Japan labels it alkaline. It feels silky, almost slippery, on the hand. Many of the springs Japan calls bijin-no-yu, waters of beauty, are like this.
Carbonated二酸化炭素泉
Water that rises with carbon dioxide dissolved in it, at least a gram in every litre. Fine bubbles gather on the skin. At Arima, near Kōbe, it runs clear as ginsen, the silver water.
Beyond the three
Japan has other springs too. Sulphur springs are milky and smell of struck matches, and you'll find them at places like Kusatsu and Noboribetsu. Iron springs turn rust-red in the air, like kinsen, the golden water at Arima. Neither is well suited to bathing at home. They are best enjoyed at the springs themselves.
Yunohana: the hot spring's flower湯の花
Yunohana (湯の花) are the mineral deposits a hot spring leaves behind. Many kinds of spring leave them, white, grey or rust-brown, depending on what the water carries. At Myōban in Beppu, they are still grown in thatched huts of blue clay and steam, by a method that dates to the Edo period. Japan made it an Important Intangible Folk Cultural Property in 2006.
Yunohana is how the onsen first travelled: gathered at the spring, carried home, stirred into the bath.
At home
A bath at home is not a spring. We carry three of these waters: chloride, alkaline and carbonated. Some are made with water or minerals drawn from the springs themselves. Others are made in the character of a spring, not drawn from it. Each one tells you which.
Most of what you can buy off the shelf in Britain is bath salts. Ours will be mineral powders, tablets and yunohana, made the way Japan makes them.