Sauna Rocks
If the heart of the sauna is the kiuas, or sauna heater, then the soul is the rocks.
Every brand of sauna heater I’ve installed has a different approach to the rocks. Some, like a wall hanging six or eight kilowatt unit use less, maybe forty pounds. And some, like the Harvia Cilindro or Club heaters use more, up to two hundred pounds. Wood burners vary too. Kuuma’s heater takes one hundred fifty pounds or so, but you have to provide your own. My custom-built stoves (Lämpimämpi brand) take a similar amount of stones and like the Kuuma stove, the rocks are piled on top, mounded as high as you can. The Harvia Legend, an elegant wood burner from Finland, has a basic cage surrounding the stove. By the time you are finished loading the stones, you won’t see the stove. There is also an optional cage that surrounds the stove pipe that hold even more rocks.
The Finnish and Swedish heaters use olivine diabase or peridotite, igneous rocks found in Scandinavia. The rocks I have used that come with the electric heaters look the same: grayish chunks, some flatter, some chunky, all in the size range you’d find in a bag of potatoes. These are amazingly cheap, considering that they come all the way from some quarry in Finland or Sweden. The UPS driver stopped asking what was in the boxes (“yup, rocks”). Once, I found a note in one from a young Swede, hoping that I was happy with my rocks.
My favorite rocks are hand-selected glacial erratics: various igneous granitic and metamorphic stones that I find in places where glacial melt waters sluiced out potato-sized rocks.
These are heavier and denser than our native shale, which is worthless for the sauna since it tends to explode when heated. The erratics have a crystalline structure that you can usually see and a solidity and heft evident when you pick them and bang them together (they are not easy to chip).
A significant consideration when selecting a heater is the rock capacity. The more rocks, the longer you should let your sauna heat up. Clients always ask: “How long should we let it heat up?” While I usually say, “About forty-five minutes,” I really want to answer with the same sort of retort I give when someone asks how long it will take for paint to dry (“as long as it takes”). The correct answer should always be: the sauna is ready when the rocks are hot enough to produce good löyly or steam. Löyly is the soul of the sauna’s mysterious expression. And how hot is that? Four hundred fifty degrees Fahrenheit, more or less. If you want more steam, then you want more rocks.
Löyly is the soul of the sauna’s mysterious expression.
More rocks are good If you expect several rounds or people coming and going. No one wants to pour water on the rocks expecting a stimulating burst of steam (but not too sharp), only to hear the fizzle of water barely boiling. That’s like trying to make an omelet in a pan that is barely hot. Disappointing, to say the least, but also bad for the heater. There’s a Finnish saying that goes something like this: when you leave the sauna, put another log on for the sauna elf, or he will pee on your stove. Which means don’t keep throwing water on the stove, especially at the end, if the rocks are not hot. The warm water will just sit on the steel stove and rust it. When you throw water on the rocks, it should all turn to steam, and when you are done, the stove should dry quickly. When I finish eating my omelet, I rinse my cast iron skillet and let it dry over the flame; I don’t let it languish all wet in the dish rack. The same principle applies to the sauna stove.
Jagged rocks will catch and momentarily hold more water, allowing less to get past the rocks and into the heater. The dense, roundish rocks I use produce a nice soft steam, but they have to be hot. I’m not so worried about water getting down to my stove, which is always very hot. (Lämpimämpi stoves have 3/8” steel plate on top so they will never rust out.) By the way, always use clean water and be wary of city water with chlorine or even country water with minerals or methane. We use only use filtered water or rain water for löyly at home. Spring water or distilled water might be prudent in some cases. Never use pool water or, worse, let some disrespecting novice wring out their bathing suit over the rocks, as I have seen in a gym—pleasant smells should emanate from the rocks. Special sauna oils or even Sauna Brew can be added to the löyly water for aromatic effect. Heck, we used to add beer to the water (cheap lager was always best).
If your electric heater comes with the jagged, grey olivine rocks, I suggest using them. They are selected to fit between and around the elements. When setting the stones, first I rinse them, then I take care to arrange the rocks one by one. It can take me forty-five minutes to fill a large capacity heater. For the Cilindro, I place large rocks at the bottom to hold up the others and then work a flattish layer against the cage wall. I then backfill against the elements with smaller and irregular rocks. On the smaller, wall-mounted units, I try to fit the flat stones between the elements without forcing or bending them. The goal is to loosely fill the cage so there is good airflow and to completely conceal the elements. Lastly, I top off the heater with larger stones and try to arrange them so they will catch water and force it inward, not splashing it out. In Finland there might a “spirit stone” placed here: a special stone or stones from a favorite place (make sure it is igneous or metamorphic—test it in a campfire first if not sure). The top might even have a layer of more decorative stones, which in Finland you can buy from sauna dealers, but here in the US, I have only seen the same dull grey stones. The Harvia web page has a good section on stones and how to place them into the pillar.
Once you have the sauna going and have been using it for a year or so, expect the stones to settle and flake off. You should remove them, vacuum out the heater and replace the rocks loosely, discarding the broken ones and adding more if needed. Failure to do so will cause the heater to eventually overheat and lead to the elements failing.
Care in selecting and placing the rocks will ensure that your sauna…Rocks!
