Ceramic Sauna Stones Outlast Natural, and a Researcher Calls the Bans Unjustified

Sauna researcher Lassi Liikkanen has published the results of what he calls the world’s first realistic durability test for sauna stones, and the findings rewrite assumptions that operators have relied on for years. Using a protocol he developed at Saunologia, Liikkanen ran natural and ceramic stones through hundreds of heating cycles and thousands of water throws in a laboratory heater, then modeled their long-term survival rates. Ceramic stones outlasted every natural variety he tested. The gap between the worst-performing ceramic and the best-performing natural stone was close to fourfold.

That number matters because it lands in a market where several major heater manufacturers explicitly prohibit ceramic stones in their installation manuals, warning that ceramics can damage heating elements. Liikkanen writes that those bans are not justified by his data, and that the heater itself survived ceramic use without incident.

What the Protocol Actually Measures

Liikkanen developed the Saunologia Sauna Stone Protocol 1.0 to fill a gap he identified in existing research: no credible, repeatable method for measuring how long sauna stones last under realistic conditions. The only prior study he cites is a 2005 fragmentation test by TM Rakennusmaailma, which used thermal shock methods borrowed from industrial ceramics rather than simulating actual sauna use.

His protocol subjects stones to repeated heating cycles with directed water throws in a 3 kW heater, simulating the thermal stress and thermal shock a stone experiences over months of regular sauna sessions. The primary measurement is survival rate at the end of the test, supplemented by visual inspection of cracking, crumbling, surface discoloration, and stacking collapse. For natural stones, the standard protocol ran 150 cycles. For ceramics, he doubled the duration to avoid ceiling effects, since early results suggested ceramics might survive the standard test with minimal degradation.

The exponential model he uses to compare varieties projects the daily rate of decay from a single known data point (survival at day X) rather than assuming a constant linear decline. That approach produces more realistic estimates of when a batch of stones would cross a replacement threshold, but it relies on extrapolation rather than observed multi-year field data.

Natural Stones: Moderate Differences, Clear Winner

Among natural stones, the spread between the weakest and strongest performers was about 30 percentage points in survival rate. Peridotite came out on top, followed by olivine, vulcanite, and olivine diabase. Larger olivine diabase stones were more durable than smaller ones, but still trailed the other stone types.

Using linear extrapolation (which Liikkanen flags as likely optimistic), the most durable natural stone could remain usable roughly three times longer than the weakest. For an operator following recommended maintenance schedules, that translates to the difference between replacing stones once a year and replacing them every two to three years, depending on session frequency.

The qualitative differences mattered too. Stones did not just shrink uniformly. Surface stones showed heavy discoloration from water evaporation, stacking collapsed as stones crumbled into debris, and the resulting fragments fell into the heating element channel. That debris pattern is what makes stone aging an operational cost, not just a materials cost: a clogged element channel restricts airflow, slows heating, increases energy consumption, and, in commercial settings, can trigger downtime.

Ceramic Stones: Longer Life, and a Heater That Survived

Ceramic stones were not part of the original natural-stone study. Liikkanen added them as a follow-up in winter 2019 to 2020 after finding that earlier studies by others had failed to produce clear durability data on ceramic products. Using the extended protocol (twice the standard duration, nearly one-third of a year of continuous cycling), he tested two ceramic varieties against the natural-stone baseline.

Both ceramics outlasted all natural stones. Between the two, Tiileri proved more durable, partly because of its larger piece count in the test batch. The largest performance difference was in surface integrity: ceramic stones maintained their shape and stacking structure far longer than natural alternatives, which meant less debris, less stacking collapse, and less risk of exposed heating elements.

Kerkes is the Finnish ceramic-stone brand most visible in this debate. Prisma’s product listing for Kerkes Tetra identifies the stones as made in Finland and describes them as ceramic heater stones fired above 1300 degrees Celsius, while IKI singles out Kerkes as the ceramic exception in its professional-heater instructions.

The exponential survival model showed close to a fourfold gap in estimated lifetime between the worst ceramic and the best natural stone. That figure comes with caveats: it is an extrapolation from a limited run, not a direct observation over years of field use, and the fourfold ratio specifically compares the weakest ceramic performer against the strongest natural performer, not averages across categories.

Perhaps the most operationally significant finding was that the heater itself survived the ceramic test without damage. Liikkanen notes this explicitly because it contradicts warnings in several manufacturer manuals.

What the Manuals Actually Say

Harvia, the world’s largest sauna manufacturer, prints the following warning in multiple heater manuals, including the TRC70XE/TRC90XE, the Spirit, and models sold through its North American channel: “Neither light, porous ceramic ‘stones’ nor soft soapstones should be used in the heater. They do not absorb enough heat when warmed up. This can result in damage in heating elements.” The warranty explicitly excludes faults resulting from using stones “not recommended by the heater manufacturer.”

IKI-Kiuas takes a different approach. Its Pillar and Monolith installation manuals state that “the use of ceramic heater stones (other than those from kerkes.fi) is prohibited,” while simultaneously recommending Kerkes ceramic stones around the heating elements in communal and professional saunas because they “support and help preserve the heating elements and they do not disintegrate prematurely.”

The IKI carve-out is telling. It means the company’s engineering team does not consider ceramic stones inherently unsafe or damaging to elements. The prohibition applies to other ceramic brands, which suggests the restriction is at least partly a supply-chain decision rather than a pure safety concern. Finnish heater makers generally recommend peridotite, olivine-dolerite, and olivine as approved stone types, with annual re-piling and replacement of weathered stones as standard maintenance.

What This Means for Operators

For commercial operators running heaters on heavy duty cycles, the maintenance math changes substantially if ceramic stones hold up the way Liikkanen’s data suggests. Stone mass is a core design parameter in modern heaters, and the cost of stones themselves is modest compared to the labor, downtime, and energy-efficiency losses that come with a degraded stone bed.

If an operator using olivine diabase (the weakest natural performer in the Saunologia test) replaces stones annually, and ceramic stones last four times as long, the replacement interval extends to roughly every four years. The stone cost itself might be comparable or slightly higher per purchase, but the labor and downtime savings compound. In a commercial facility running daily sessions, each stone replacement means taking a room offline, unpacking the heater, restacking, and running a break-in cycle. Cutting that from once a year to once every three or four years is a meaningful operational improvement.

The caveat is that Liikkanen’s protocol uses a 3 kW residential heater, not a commercial-grade unit running at 10 kW or higher on 208-volt three-phase power with daily duty cycles. Commercial heaters subject stones to more aggressive thermal stress, higher peak temperatures, and more frequent water contact. Whether the fourfold ratio holds under those conditions is an open question that his data cannot answer.

There is also the warranty question. An operator who loads ceramic stones into a Harvia heater is voiding the stone-related portion of the warranty, regardless of what Liikkanen’s data shows. Until manufacturers revise their manuals, operators choosing ceramics are accepting that trade-off.