A traditional Finnish sauna that utilizes sauna rocks in its operation is often referred to as a rock sauna, and the name aptly implies just how important a feature of the conventional sauna the rocks are. The sauna heater holds and heats the stones, and the sauna bather tosses water onto the hot stones to create the loyly or vapor that increases the humidity in the sauna room. Even in the modern era of far infrared sauna therapy, many sauna enthusiasts refuse to be swayed in their belief that sauna stones are essential to achieving the definitive sauna experience.
While sauna fans are free to forever debate what defines an authentic sauna bath, it is an absolute must that rock sauna owners always use proper sauna rocks in their saunas. Many manufacturers and dealers often provide ks with the Finnish-style saunas and kits they sell, but sauna owners who enjoy frequent dips in their rock saunas will likely someday have to replace old stones with new ones.
Under the intense pressure of the sauna's fluctuating temperature and humidity levels, even the best sauna rocks eventually crack and crumble. Once a sauna rock starts to disintegrate like this, it can't hold heat as well as it used to. It will also produce less steam, and it may even clog your sauna heater. Before the day comes when you have to replace the rocks in your sauna, it's imperative that you learn to differentiate good sauna rocks from bad sauna rocks.
For obvious reasons, sauna owners should avoid using rocks that could explode in the sauna as well as those that give off poisonous or offensive odors. Neither should rocks that contain sulphur minerals (minerals with a yellowish color and/or metallic luster) or asbestos minerals be used. Rocks with sulphur minerals do not pose a health risk; they just wear down very quickly. However, rocks with asbestos minerals do present a legitimate risk, as asbestos has been proven to cause cancer.
To avoid placing potentially explosive rocks in your heater, follow the advice of expert Mikkel Aaland, who recommends a simple test: “Thoroughly heat a sample for two hours or more. Drop it into a pail of cold water then look for cracks. When the rock is cool, test it further by hitting it with a hammer or against another rock. If the rocks cracks or makes a soft grinding sound when rubbed against another rock, discard it. If it survives, you have a safe sauna rock.”
Because the best rocks are those least exposed to weather, certain quarried rocks are held in high esteem by traditional Finnish sauna lovers. One of the most popular is peridotite, a quarried Finnish rock and one of the most common rocks in the earth's upper mantle. Peridotite is a type of igneous rock, as are olivine and vulcanite which likewise perform well in saunas. Many purists prefer using dark-colored igneous rocks because these rocks contain magnesium and iron and therefore have a relatively high heat capacity.
Be sure to refer to the manuals and other literature that came with your unit and heater before you replace your sauna rocks. You might inadvertently void your stove's warranty if you use a type of rock that is not specifically approved or recommended by the heater's manufacturer.
The information packaged with your heater should also indicate the amount or weight of rocks recommended for use with the heater. As a general rule, though, you should use enough rocks to cover the heating elements of an electric heater. If you heat your sauna with a wood burning stove, the rocks should form a small mound that rises above the stove's rim. In both cases, remember to pack the rocks loosely to allow for sufficient air circulation.
Substantial information on sauna rocks can be found online, but not all of it is accurate or reliable. For example, one website recommends using smooth stones from rivers and seashores as sauna stones, while another advises owners to steer clear of them. Disagreement also exists on how suitable rocks with high iron content are for use in saunas. Questions about the ideal size, shape, texture and mineral content of your sauna rocks are perhaps best answered by the manufacturer or dealer from whom you purchased your sauna heater.