How To Build A Sauna #1: Choosing the right sauna stones
You can’t have a good sauna without good sauna stones.
For thousands of years, the most fundamental design element of any sauna or sweat lodge has been the hot rocks onto which water is periodically thrown. When preparing a sauna, we heat the stones and not the sauna.
Not all stones are equal in the sauna though.
Some rocks absorb and release heat better than others. In fact, certain types of rocks explode when subjected to high heat, which makes them really not ideal for the sauna. Unfortunately, some people have learnt that the hard way.
So, in this article, we’re going to cover everything you need to know about choosing the right stones for your sauna. The same principles apply whether you’re building a luxury spa, a traditional Nordic sauna cabin, a humble home sauna, or even just a makeshift camping sauna.
Introducing the ‘How To Build A Sauna’ series by Estonian Saunas
This is the first in a new series of articles we’re writing at EstonianSaunas.com to answer the most common questions we get from sauna builders around the world.
It’s not a step-by-step guide to building a sauna from scratch (but we’re already working on that with our US partners at Cedar & Stone Nordic Sauna). Instead, we’ll take a different element of what makes a good sauna for each article then break it down in as much detail as is useful and interesting to us (and hopefully you too), while exploring the history and culture around it too.
In this article:
Intro: The rocky history of heating saunas
Frequently asked questions about sauna stones:
- Which types of rock are most suitable as sauna stones?
- Can I use decorative rocks in the sauna?
- What size and shape of stones are best for sauna stoves?
- Do rounded or split-faced stones produce better heat and leil?
- How can I check my sauna stones are safe?
- What’s the best way to arrange stones in my sauna stove?
- When should I replace my sauna stones?
The rocky history of heating saunas
The oldest evidence of sauna style bathing dates to 3,500 BC — and it was the stones that provided the essential clues for archaeologists.
The location of this ancient sauna might surprise you. It was found in England. Marden Henge, to be precise, close to the much more famous Stonehenge. Archaeologists from the University of Reading led by Jim Leary found the remains of a small building which once had a large fit pit at its centre.
There was something particularly interesting about this fire pit though. The ground had been scorched beneath it and there were fragments of stones heated to very high temperatures.
However, the only burning material was found in a second fire pit outside the building. This indicated that the stones had been heated outside then carried inside. This would have provided plenty of warmth inside the small building, the purpose of which is difficult to explain other than for use as a sauna.
This doesn’t mean the Brits invented saunas. It just means that this where the oldest surviving evidence for sauna-style bathing has been found so far.
One theory is that the sauna is likely to be almost as old as the discovery of ways to control fire. That was a crucial turning point in the history of humanity and enabled humans to migrate and settle in new lands.
Fire has its downsides too though. For one thing, you can only enjoy its warmth outside until chimneys have been invented — unless you can figure out a way to transfer and store that heat into something that you can bring inside without the smoke.
The solution, as early humans soon would have figured out, is stones.
Stones are basically natural batteries, able to absorb heat energy from fire and then slowly release it. They are readily available everywhere too, which makes them very convenient for early humans who wanted to keep their huts or caves warm without choking on smoke.
After that, it may not have taken them long to figure out that pouring water onto those hot stones creates an even more intensive environment that is ideal for bathing.
And that’s why, if you go back far enough then you can find evidence of sauna-style bathing in many different cultures all over the world.
We’ll never know who built the first sauna or where. Ancient saunas are most commonly built in very simple, low impact, tent-like structures, which are unlikely to leave any archaeological evidence for future generations to uncover. But what’s more important is that we can embrace this part of our common human heritage and keep the tradition going today, wherever we are in the world.
In fact, this ancient British technique of heating stones outside then carrying them inside is still the same method used to heat native American sweat lodges today.
Here in Northern Europe, people have also been enjoying saunas with hot stones at their centre since ancient times — although using a slightly different heating method to ancient Britons and native Americans. Our technique is known as the smoke sauna.
Instead of heating rocks outside then carrying them inside, a smoke sauna has stones permanently inside above a fire that we light for around half a day. The room does fill up with smoke, but there’s no one inside during that time and this smoke is useful in other ways — such as for smoking meat and sterilising the surfaces.
After about half a day of heating, the fire is left to die out and the smoke is ventilated through the door and windows. People can then go inside to bathe in the radiant heat coming from the stones, which will remain hot through the night.
Although many people still cherish these two ancient heating techniques around the world, there were two sauna innovations that made it considerably easier to heat the rocks in a sauna. The first — around the time of the industrial revolution — is chimneys. This meant for the first time that saunas could be fired continuously while people bathed inside, enabling greater ease and efficiency. Here in Estonia, chimney saunas overtook smoke saunas in popularity around the 1920s.
The second big innovation — during the later 20th century — is the electric stove. This meant that saunas could be used inside buildings with even greater ease and therefore helped popularise saunas globally.
There was an unfortunate side effect to these innovations, however. The importance of stones became increasingly forgotten. Stoves became clunkier, boxier, and more industrial. The number of stones on top of them kept getting fewer and fewer. Sauna builders stopped thinking about stoves as the central design focus and instead thought about how to cover them up.
Even worse, some ‘saunas’ started being built with no stones at all — such as infrared ‘saunas’.
Claims from infrared manufacturers about the superiority of their technology and how it enables ‘deep sweating’ doesn’t have any scientific basis. ‘Deep sweating’ just doesn’t exist.
Removing the stones doesn’t just affect the authenticity and aesthetics though. It also results in an inferior heat and steam. There is considerable difference in the heat distribution radiating from a dense stone compared to other heating sources, including an open fire.
Most interestingly, there’s increasing evidence that leil (löyly / sauna steam) contains more negative ions when it rises directly off heated stones as opposed to metal or other elements. The first studies were by Niilo Teeri at the University of California.
The more negative ions in the air, more refreshed we feel — like when walking along the sea coast or standing next to a waterfall. The more positive ions in the air, the more tired and sluggish we feel — like when being in an air conditioned environment, especially an airplane. It’s not just the lack of legroom and the quality of the inflight meal that gives you that feeling.
Smoke saunas — which require a huge quantity of stones — were also found to have the highest amount of negative ions in the air. All saunas had this positive (well, negative ion) affect, but the more stones the better. This in part explains why people often report feeling more refreshed after a wood-burning sauna, which typically contains more stones and more adequate heat transfer to them.
While the popularity of infrared ‘saunas’ is still increasing around the world, there is a much more interesting design trend emerging within the (authentic) sauna stove industry. More people want more authentic sauna experiences and also to invest in saunas that reflect high quality “Instagrammable” design.
As a result, stoves are becoming more minimalist while holding a larger number of stones to provide a superior heat and steam. And sauna builders are no longer trying to hide stoves away. They are treating the stoves as works of art. The most notable are HUUM stoves, although other manufacturers are following their trend.
More people are rediscovering what it means to have a good sauna — and nothing is more fundamental than good stones.
Frequently Asked Questions about sauna stones
1. Which types of rock are suitable as sauna stones?
There are three main types of rock: sedimentary, metamorphic, and igneous. Igneous rocks are also commonly called volcanic rocks, although these are actually an additional subcategory.
The first two categories — sedimentary and metamorphic — should not be used in a sauna. They’re not dense enough to retain enough heat and they are also porous so may contain moisture in them, which leads to them cracking or exploding in high heat. This has resulted in injuries at New Age retreats where sweat lodges have been built by those seeking to imitate ancient traditions without sufficient knowledge of how to do it correctly.
Sauna rocks should instead be igneous rocks, which generally tend to be dense and non-porous. There are some exceptions for igneous rocks that don’t make good sauna rocks, but that’s because they don’t have these typical characteristics. For example, obsidian is too smooth while pumice is too light.
The sauna stones that we deliver with our HUUM stoves are collected in Finland and known as olivine dolerite in British English or olivine diabase in American English. This is a really heavy stone with a density of approximately 3.5 kg/dm3, which is why it is highly valued as a sauna stone for its durability and high heat capacity.
The next best igneous rocks after olivine are peridotite and vulcanite rocks. However, there are many other different types you can choose from, including basalt, granite, and gabro.
Before searching for sauna stones, it’s worth starting your search online to better understand what types of rock are most common in your local area. You can then type those locally common rocks into Google Images to help you spot them.
Our US partners at Cedar and Stone Nordic Sauna are based in Duluth, Minnesota along the shore of Lake Superior, which is a particularly great place to collect sauna stones. Check out this Instagram video made by their founder Justin Juntunen as he collected his own rocks and gave a few tips.
Don’t be too daunted by identifying them, however. You don’t need to be a geologist. You can also focus on finding rocks with the right characteristics. They need to be dense so find the heaviest rocks and avoid any with tiny holes or ones that are comprised of layers. If you are still not sure, there are some safety tests you can do on the rocks that you collect, which I’ll cover later.
2. Can I use decorative rocks in the sauna?
‘Decorative rocks’ are rocks that have had effects applied to them to change their appearance, such as rounding and polishing. This is, therefore, a very vague category of rocks.
In fact, even the olavine dolerite / diabase rocks that we deliver with HUUM stoves have been rounded and polished, but we wouldn’t consider them to be decorative as they retain all their properties as highly functional sauna rocks.
Sauna builders usually inquire about the use of decorative stones when they want to achieve a very specific, modern look to their sauna.
Most decorative rocks on sale for non-sauna purposes are way too light and delicate to be of any use in a sauna.
However, you can now find white sauna rocks for sale online, which are sometimes used in our HUUM stoves. Although people do tend to fill their entire stoves with them, it’s generally recommended to use higher quality sauna rocks on the inside then use these kind of white rocks as an outer layer.
3. What size and shape of stones are best for sauna stoves?
Most sauna stones today are typically about the size of a hand palm.
Old style smoke saunas traditionally used much larger stones as these had to retain their heat for half a day after the fire died out. But saunas that are continuously heated by either a wood fired or electric stove don’t require such large stones.
Depending on the size and style of the stove, they may need to vary between about 3 to 15 cm in diameter. You want to make sure the stones fill the whole space provided without pushing on the heating elements and with enough air flow between the stones. Slightly different sizes and shapes will make it easy to fit them all in.
If you’re reading this, there’s a good chance you are interested in buying a HUUM stove so here’s a quick overview of the stone sizes needed to fill them all:
- Drop electric stoves (all): 60 kg of 5 to 10 cm diameter stones.
- Hive electric stoves (6 & 9 kW) without air tunnel: 120 kg of 10 to 15 cm diameter stones and 30 kg of 5 to 10 cm diameter stones.
- Hive electric stoves (6 & 9 kW) with air tunnel: 80 kg of 10 to 15 cm diameter stones and 30 kg of 5 to 10 cm diameter stones.
- Hive electric stoves (12, 15 & 18 kW) without air tunnel: 220 kg of 10 to 15 cm diameter stones and 30 kg of 5 to 10 cm diameter stones.
- Hive electric stoves (12, 15 & 18 kW) with air tunnel: 180 kg of 10 to 15 cm diameter stones and 30 kg of 5 to 10 cm diameter stones.
- Hive wood-burning stoves (13 kW): 90 kg of 5 to 10 cm stones.
- Hive wood-burning stoves (17 kW): 100 kg of 10 to 15 cm stones and 30 kg of 5 to 10 cm stones.
3. Do rounded or split-faced stones produce better heat and steam in the sauna?
Opinion on this matter is…split. Neither is necessarily better though. It just comes down to personal preference.
Rounded stones allow water to fall deeper into the stove as it evaporates ever so slightly slower. This produces a ‘softer’ leil (sauna steam). Split-faced stones offer more surface area and so tend to vaporise water almost instantly, producing a slightly stronger blast of leil. The difference between these is very marginal though. There is far more variation in steam depending just on how you pour the water onto either of those types of rocks.
Proponents of split-faced stones also argue that there is better air circulation between split-faced stones, although you can achieve the same benefit from rounded stones if you stack them well.
After that, you’ll want to consider which ones simply look nicer. This might seem trivial to some, but we think the sauna stove should be a work of art and so you want to get the design aesthetic right for you.
The world’s largest sauna stove manufacturer, Harvia, sells split-faced stones and argues they are superior, while HUUM prefers rounded stones. It is, of course, also possible to choose your own stones for any heater type.
4. How can I check my sauna stones are safe?
Your sauna stones should be strong and sturdy, especially when subjected to high heat, so there are quite simple ways to test this.
Firstly, try scratching them with a nail. If material comes off then they are not safe. Next, drop them to a hard ground and inspect them to ensure they remain unchipped. While doing this, just be very careful of any flying fragments that could get into your eyes.
Finally, you should give them a baptism of fire. Place the stones into the centre of an outdoor firepit and let them reach a high temperature. You can then carefully remove them and plunge them into a cool water. This extreme temperature change is fine for a sturdy sauna stone, but others will crack and crumble under the pressure.
5. What’s the best way to arrange stones in the stove?
The way you stack your stones inside your heater is going to affect the quality of the heat and steam that is produced, and also the life of your stove.
No matter what type of sauna stove you have, the most important advice for filling it with stones is to enjoy it like a puzzle and don’t rush it!
We always start by sorting the stones into three piles for stones that are slightly smaller, slightly larger, and slightly flatter. This makes it easier to complete the puzzle.
You’ll want the stones to fit into every part of the heater with smaller stones covering up the gaps between bigger stones.
In a wood-fired stove, especially a smoke sauna, you place larger stones at the bottom where they will be first in the line of fire. In an electric stove, you instead need to think about fitting the stones around and between the heating elements in a way that doesn’t push on them. Place the big stones where you can and use the smaller and flatter stones to fill the tighter spaces.
In both cases though, you also need plenty of air circulation between the stones. So they need to be slightly loose, although without the ability to see through them to the heating elements. When you reach the top, you should layer stones more tightly together because that’s where most of the steam will be rising from. You don’t want the water dripping down directly onto the heating elements or the metal base because that reduces the quality of the steam.
Once you’ve finished, you’ll hopefully have some stones leftover. That’s fine. You’ll need to add some of them in future anyway as the stones sink a little bit over time. You’ll also find that they expand and contract with the heat so some of the stones may occasionally fall out. That’s normal too.
6. When should I replace my sauna stones?
Every type of stone will eventually break down in the sauna and start to crumble, although not for several years if you’ve followed this guide when collecting your own or bought the highest quality sauna stones.
Stones in public saunas, which are heated every day, need to be replaced more frequently of course.
Every six months or so, while the sauna isn’t heated, simply look out for stone fragments from where the stones have disintegrated and inspect them for any cracks.
Thanks for reading
This Estonian Saunas blog is run by Anni and Adam. We export Estonian sauna design, technology, & culture at EstonianSaunas.com.
As it happens, we sometimes get asked by sauna builders if they can order our HUUM stoves without the stones. We would be more than happy to do this. However, in 9 times out of 10, they then return to us to order the stoves after realising the complexities of gathering your own. Plus, our stones do look really nice! Hopefully though, this article may inspire more people to collect their own locally — or at least appreciate the complexity and beauty of the sauna stoves we can deliver.
You can follow our own adventures on Instagram, Facebook, YouTube and Twitter. We also have a Facebook group for Sauna Builders & Explorers.
You can also contact us at tere@estoniansaunas.com.
