A short course — sauna stones 101

April — the (unofficial) Month of Sauna Stones

Rocks matter. Here’s what I learned from Lowell, my geologist friend, and two Internet sites, Rock Collector and Visit Sauna.

The qualities important in sauna rocks are durability (how quickly stones degrade) and thermal conductivity (how long they hold, conduct and radiate heat).

All stones eventually crumble (“friable” is the geological term) and fluctuating temperatures and water speed that process, so rocks in a sauna stove artificially weather even faster than they do in nature. That decreases their durability.

Since metamorphic and sedimentary rocks degrade more quickly than igneous rocks (those created by vulcanism), enthusiasts have preferred igneous rocks, like granite. Granite, however, can crumble at as low as 300 degrees Fahrenheit, a temperature not uncommon in the sauna stove (not the air surrounding).

Stones that best withstand the repeated temperature extremes of the sauna are those least exposed to weather — quarried from deep down where lava never reached the earth’s surface. (Geologists call them “plutons.”) Stones with iron and magnesium will conduct heat more efficiently and last longer. Quarried stones have more surface area than rounded rocks, thus more places to emit heat.

Peridotite, quarried in Finland, and olivine are dense, granular, igneous rocks, dark and heavy. They are among those with the highest thermal capacity. Others include plutonite, vulcanite, diabase, black basalt, chrome ore and diorite — minerals I’m quickly becoming familiar with. 

Conecting technology to geology is the subject of my next post.

Nikki