Using the right sauna rocks is an important aspect of your overall experience, as well as your safety and extending the life of your heater. You’ll need a special class of rocks, known in Finnish as konnos, and they aren’t just any old stone you can go pick up down by the river.
The best sauna rocks are actually igneous rocks which come from cooled lava. Yes, that does cover a variety of rocks from granite (which are too heavy) to pumice (too light), but there exists a special type of dark-colored volcanic rocks which are just right.
The ideal type of rocks for a Finnish sauna must have a good resistance towards chemical and thermal weathering, and should not contain harmful mineral deposits.
That means no fibrous rocks that might have minerals, asbestos, or sulphur in them and which could give off harmful fumes inside the sauna. Additionally, the rocks should not be able to hold water as they can explode when intense heat turns the water in them to steam.
It doesn’t take a degree in applied geology to know that the sauna heater rocks must be able to hold in heat and vaporize water instantly. The dark-colored volcanic rocks contain magnesium and iron which allows them to have a relatively high heat capacity (900-1500° F ) and hold the heat enough to produce a steady supply of steam. That makes them perfect rocks for a sauna stove.
Once you have the right stones, you must remember not to operate the heater without the rocks in the tray. Size is important too. The sauna rocks should be about the size of your fist or a large potato. Loosely place the rocks around, between, and on top of the sauna heater’s elements.
Take care, however, not to force the rocks between the elements. They should pack close enough together so that water doesn’t pool on the heater elements. You may not need all of the rocks at first, but save them for later to replace broken rocks.
After about a year of continuous sauna use, the sauna rocks will be on their last gasp. They may be cracked, or crumbly, turning to dust, or just plain smaller and have lost their dark color. It’s time to replace the whole lot.
When you pour water onto hot rocks, it’s a good idea to use warm water as cold water vaporizes too slowly and will work to cool the rocks. The water should also be clean and not be from the pool or Jacuzzi, as any water with chemicals in it will cause rapid oxidation of the internal parts of the heater.
The hiss of sauna rocks lets you know your heater and rocks are hot enough. Löyly is invisible; it’s the steam beyond steam. So if you pour water on the rocks and get “just steam” and some bubbling, you aren’t there yet. Löyly is dramatic, a crackle, a loud and instant vaporization, followed by the quick thought of “Gee, nothing hap... - wait, it sure got hot in here!” That’s invisible löyly steam and that’s an authentic Finnish sauna.