I get questions all the time about sauna rocks. Whether a wood burning sauna stove or an electric sauna stove, sauna rocks play an important role.
Some swear by igneous rocks, or volcanic rocks. Though lava rocks have a high heat capacity, we find that they don’t hold thermal mass as well. Why is thermal mass so important? We find that an integral part of the sauna experience is the thermal heat, or dense heat, that is only created by heating mass (think heavy water heated radiators vs. tinny electric baseboards). There is a theory that sauna manufacturers sell and promote volcanic rocks for sauna stoves because they are lighter weight so more reasonably priced to ship.
Specifically, rounded aged granite rocks from the icy shores of Lake Superior. Rocks without cracks or crevices. There is a stigma and fear that sauna rocks can explode. Sure, if you’re using rocks that allow for water to get in cracks, they will explode! If you’re worked up about that, you can read about Hydrofraction. Bottom line: use non porous granite rocks on top of your sauna stove.
Take two proposed sauna rocks. Smash them together. If they don’t crack or break, chances are, you are holding two good sauna rocks. This is one way to ensure that the rocks you are using for your sauna are strong and stable.
Softball to golf ball. This size range works well. Why? Different size sauna rocks hold heat (thermal mass) differently and release steam (löyly) differently from water being tossed on hot sauna rocks. A nice hot wood burning sauna stove or electric sauna stove will heat your sauna rocks, then as water is tossed on the sauna rocks, the water turns to steam, and that process will temporarily cool your sauna rocks. Softball size sauna rocks hold more heat and will maintain thermal mass better. The golf ball size sauna rocks react quicker to the water, turning it to vapor but at the expense of thermal mass.
You want different size sauna rocks.
Consider that savusaunas, the earliest form of saunas, are “simply a room containing a pile of rocks, but without a chimney.” The rocks are heated by fire, with lots of wood and for a long time. Then, the fire goes out and all that heat is contained within the rocks. This same principle, heated rocks, is what differentiates a sauna from a lake. (here’s where we throw infrared light bulb closets under the bus).
Consider building your collection of sauna rocks as a journey, not a destination. Hiking, walking along the beach, snooping around your neighbors backyard are all times to have your granite sauna rock radar detector going. Oh, and how about how much water to toss on your sauna rocks?
Just as Minnesota Fats, the famous pool player, would advise that there are two ways to hit a pool shot – soft and softer, there are two ways to toss water on sauna rocks: start with a little water, then add a little more. You can always toss more water as you go.
As Clint points out, all sauna stoves are made to take water. Rocks on sauna stoves are a thermal mass of heat energy. That heat is transferred to steam, as water gets tossed on rocks. That steam then gets transferred to your body as it comes in contact with your skin. You can toss a liter of water on the rocks and try to “ride it out” but this macho ploy is best reserved for the art of reverse cycling.
Steam from water being tossed on sauna rocks, Löyly, is a spiritual thing, involving negative ions. Fire (sauna stove), Earth (sauna rocks), and Water (via löyly) create an aura akin to waterfalls and rainbows, something beyond this writer’s ability to put into words without mind altering assistance.
Starting with a little water, then adding more water won’t shock your sauna stove, so in theory, your stove will suffer less fatigue. Rocks will be less depleted of thermal mass and respond quicker to more water with a smile and a sizzle.
The temperature in a sauna does not go up when you toss water on sauna rocks. It just feels hotter because heat is transferred via water vapor onto your skin.
Welcome Dick from Scandia, Minnesota. Dick works in Research and Development at the Andersen Corporation (Windows). He has worked in design, project management and materials research for 33 years.
If you are still with me I know you probably have a wood burning sauna. Someone who would wonder about collecting their own sauna rocks would probably burn wood. There is a certain person who cuts wood and dries it, carefully splitting and stacking it in preparation for taking sauna- it’s the “Gestalt Principle”, in action! The preparation is just as important as the Sauna and when combined together give us an exhilarating experience. That same person is likely also to search out and find their own “Sauna Stones” enriching your experience further.
If you are wondering why I switched back and forth from rocks to stones I think of them as rocks when they are in the wild and they become “Stones” once they are placed in my sauna heater.
Gathering stones for your sauna is not the same as Rock collecting. Everyone grabs a pretty agate from the restaurant landscaping or even at a city park. Taking an entire 5 gallon pail of rocks from public beaches, parks and private places is not alright. Take care and only collect rocks with permission or from land you own. You might want to try a local landscape company or nursery supply business. Mine lets me even sort through the pile and pick just the “Perfect” ones.
The stones deliver the steam. Stone size density, porosity, and surface all play a role in transferring heat from the fire to you. If we use too small of stone it doesn’t hold enough heat to last very long. Too large and it takes longer to heat up. A stone with a nice rough fracture is nice because it has a little more surface area to hold on to the water and gives off more heat. Some stones might stack too close together and prevent the free flow of heat and water through the pile. Too loose and too much water passes through too quickly.
I use stones about the size of my fist with a few larger and smaller to fit just right when I lay them on my stove. Note that folks with those new fangled electric stoves need to be careful about their stones because stone size and placement can shorten the life of the elements.
I had to shamelessly say it. Darker rocks give off heat quicker. It’s a physics thing. The “Finnish” stones for sale are all dark. Now that I’ve got that out of the way let’s move on.
I noticed some companies tend to say things like “the only correct stones”, “the only real sauna rock” or “the correct type”…These are written by “Ad men” and are simply trying to convince you that they have the very best rock for your money. Now be assured, proper sauna rocks are a safety concern. But any igneous, structurally sound rock will work in a sauna and you will be hard pressed to appreciate the differences unless you are 100% Finnish.
Without getting into a geology class let’s just say if you can scratch material off the rock with a nail, DO NOT USE IT!.
Sedimentary rocks (sandstone, shale, limestone, etc) and Metamorphic rocks (slate, marble, Quartzite, etc…) all are too soft, have faults in them or have moisture in them and should not be used. These rocks with moisture in them can “POP” when subject to hea
We want an igneous rock (Vulcanite, gabro, peridotite, Basalt, Olivine, Granite, etc…) Note I did not include Obsidian and Pumice, both bonafide igneous rocks but they have issues. Obsidian is so smooth that the water will run off so quickly it will not have time to evaporate much. Pumice is so light and porous that it will cool too quickly and not have enough mass to give you the sustained heat a sauna needs.
If you are having trouble identifying your rocks. Do an internet search for igneous rocks and that should help you. You can also search for a map of your area showing what the predominant rock type is in the area. Some folks might have to wait until vacation to find suitable rocks and depending on who’s with you. You might have to only bring a bucket full at a time home.
Before loading your stones in the Rock Chamber, wash and rinse them to get off anything that might soil your floor or impart an off odor when heated. Next depending on your heater, start with the largest stones first and then use smaller and smaller, stack them in and around the Rock Chamber until it’s full and heaping. You do not want them so tight that air has a hard time getting through but you want them close enough that when you pour water on, it doesn’t run quickly off and doesn’t have a chance to turn to steam.
Some rocks will last longer than others. I’ve heard that some only last less than a year. Whoa! Those are not rocks we want to use. But after a while rocks will crack and get smaller and smaller. Usually the hottest rocks next to the fire pot degrade first. They are subject to more thermal expansion due to the heat / cold cycle. You might want to use room temperature water rather than cold water to limit this stress on the rocks and also use only a cup of water at a time to limit the stress also. It’s better to dump a cup or so each a few minutes apart than it is to dump 2 cups all at once.
When your rocks are cracked and getting small, some are ending up on the floor. It’s time to replace them. Simply remove them all and combine the old usable good ones with your new rocks. See #3 above.
“A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral sauna.”
So, we are approaching the “Finnish line” of our authentic sauna build. We’ve followed along with every step of the way of the DIY ebook. We’ve cured our sauna stove with a good long burn, burning off the factory paint. We’ve put some finishing touches in place, like attaching our towel hooks and our hot room door hardware. We’ve checked the calendar and have now invited our friends over for the inaugural Friday sauna.
We’re all set to go, but wait! “I need sauna rocks!”
Well, a quick trip to the shores of Lake Superior is out of the question. And despite the metaphor of a million grains of sand, there’s a guilt that comes with dragging a 5 gallon pickle bucket out to the beach shoreline to collect free range organic sauna rocks.
What about railroad tracks? There’s a fringe line of thinking that “there’s asbestos from train brakes in dem der rocks.” And then we can get caught up with the idea that if everybody got their sauna rocks from train tracks, would anybody hear a train whistle blow if the train ran off the tracks?
What’s a morally conscious sauna enthusiast to do?
For a few bucks, at a place like Landscape Love in South Minneapolis, a sauna enthusiast can bring a milk crate or 5 gallon sauna bucket, and hand pick through the pile of landscape rocks to get a great assortment (golf ball to softball size) of sauna rocks.