Massive Rock Heater

OK here is the deal. In order to heat up rocks the traditional way for a sweat lodge takes way to much wood.
A wile back I got an old compressor tank with the idea of heating the stones with far less wood.
My question is how to distribute air ,to the burn, evenly, the full length of the tank?

I was thinking of a pc. of 4''pipe entering from one end running along the bottom with holes drilled along the sides then capping the end. To create a draw insert a 6'' stack in the center on top.

Maybe set the tank on end, bury it about 1/3 of the way, plumb an air inlet that runs down the outside, and have a chimney from the top?  Lighting it from the top would help a lot, too.

If that seems unsafe, maybe a 45 degree slope would work.

woodman wrote:
OK here is the deal. In order to heat up rocks the traditional way for a sweat lodge takes way to much wood.
A wile back I got an old compressor tank with the idea of heating the stones with far less wood.
My question is how to distribute air ,to the burn, evenly, the full length of the tank?

I was thinking of a pc. of 4''pipe entering from one end running along the bottom with holes drilled along the sides then capping the end. To create a draw insert a 6'' stack in the center on top.

What's the traditional way?

Your pipe-down-the center sounds like an open-pan forge; those work fine.  You can have the exhaust at the opposite end from the air intake, or almost anywhere uphill.

The vertical or horizontal versions sound effective too.  I'd want the fire underneath, and flames tickling up past all the rocks like a charcoal briquette starter in a coffee can.  The trick is making it safe to unload.

To really save firewood, I'd want a way to heat the rocks up, and then let them 'mellow' at temperature until they're needed.  (I also don't want red-hot rocks exploding in my face when I open the door and cool air hits them, so letting them sit awhile gives me a better idea what temperature I've achieved). 

Holding at temperature says 'insulation' to me.  Can you bury the sides of the burn container in loose ashes for insulation?  Maybe in a sloping trench in a bermed hill or something.  You could even pile more ashes, earth, etc. over the top.
Then the rocks could stay warm longer, and any differences between the colder and warmer corners would even out.


The reason I ask about the traditional method is that primitive technologies often contain hidden efficiencies: they are the local culture's 'first choice' for getting the job done with the locally available materials and energy.  Back before firewood came by the truckload for a day's wages, there were probably some pretty efficient fire-keepers out there.

Many of the traditional technologies have been lost, and others depend on a lost availability of time and resources.  But it's still worth taking a look for old methods.

If your local fire-keepers don't have any special secrets, maybe look at traditional methods for firing bricks, pottery, lime-kilns, charcoal-pits, or pit-roasts.  Ancient furnaces.
  These are all ways to achieve a uniformly hot temperature, by using time and insulation as well as fuel.  You can scale up/down to the temperature you want your stones to reach.

Hope that helps.  Good luck with the project.

The rocks are usually stacked on a bed of sticks, more the size of 5'' or bigger splits. Then more wood is stacked over the rocks. The rocks are heated for 3 or more hours, all the time adding more wood to keep the rocks covered. Red glowing rocks are preferred and volcanic rock is ideal ,they do last for several firings.

You can't imagine how much wood one goes through to heat rocks up this way.


Native Americans are excellent in adapting things cast off for something usable.