Ask a Finnish bather what makes a good sauna and the answer will not be the temperature. It will be the löyly — the character of the steam that rises when water meets the stones.
What is actually happening
Water poured onto stones at 300 to 400 °C flashes almost instantly to vapour. Relative humidity in the cabin jumps from perhaps 10 % to 30 % within seconds, and the apparent temperature on the skin rises sharply even though the air temperature barely moves — briefly, it drops.
Why stone mass decides everything
A thin stone bed gives one sharp burst and then nothing. The stones have surrendered their heat and the next ladle produces a wet, disappointing hiss. A deep bed of split stone holds enough energy for repeated löyly across a full session.
Split stone matters as much as quantity. Tumbled river stone has a smooth surface and a fraction of the effective area; split olivine diabase presents fractured faces that flash water far more efficiently.
Ventilation is half the story
Humidity that cannot leave the room turns a sauna into a damp box. The outlet grille should sit diagonally opposite the inlet and high on the wall, and it should be open during bathing, not closed to “keep the heat in”. A cabin that clears its humidity between rounds feels dramatically better than one that does not, at the same set point.
Practical guidance for operators
- Two to three decilitres per ladle. More than that floods the bed and cools it.
- Wait sixty to ninety seconds between ladles so the stones recover.
- Use soft water where possible; hard water leaves a scale crust that insulates the stones.
- Re-stack the stone bed twice a year and replace fractured stones annually in commercial use.
