I Learned Three Things About Saunas from a Fin

I stole a quick heat at the Finnish Baths the other day and met a Finnish gentlemen there. It's kind of like meeting a porn star in Van Nuys -- you know they're there somewhere, but you never expect to see one.

Anyway, I picked his brain about saunas and schvitsmenship and he gave to me these lessons three:

1. A sauna oven needs its rocks changed regularly or else they loose their ability to hold heat effectively. 

I asked a follow up: How often? He said, "A place like this? Every three months."

Now, I've been in a lot of heat houses and I don't see them changing their rocks but hardly ever, and by hardly ever, I mean never. Especially in the Russian furnaces. Granted, those tend to be bigger rocks, but if this guy is right and rocks loose their ability to hold heat after a while, this would apply to these rocks as well.

But I wonder what exactly changes in the composition of a rock? It's not like something's burning off...is it? 

Here's an internet opinion:

Each heating of a sauna rock equals thousands of years of natural erosion. Only the strongest rocks can survive constant heating and cooling, and even they will eventually crack and crumble. When this occurs, they lose their capacity to hold heat and in time they disintegrate and clog the [oven].

But every three months? Just on principal? Sans crumblage? Finnish or no, I doubt this one.

2. All modern apartment complexes in Finland are being outfitted with saunas.

This, indeed, is something to boast about. But is it true? Seems to be, at least, according to the internet I get at my house.

3. If you build your own sauna, you need a sealant on the back of the wood to protect the surrounding concrete or whatever from moisture.

That I believe without need of an internet search.

Thanks, my new Finnish friend!