I’ve seen this in movies. There’s a nasty blizzard and the wise Indian digs a shallow pit, starts a fire, and scatters the coals. Cover with earth and you have the worlds first electric blanket.
However, would this really work? Fire needs oxygen to burn. :dubious: How long would those coals last covered with dirt? If it works then why doesn’t every camper sleep in a warm bed over coals in the winter?
I have heard of cooking a whole pig in the ground. But, its the same issue, wheres the oxygen?
I don’t know, but embers can keep going for a really long time under ash. However I guess oxygen can permeate relatively easily ash, and I’m not convinced it would happen with dirt compacted by your lying body’s weight.
Here’s a Field & Stream article. They mention the famous scene in Jeremiah Johnson.
I still think its something someone made up for a movie. I haven’t read anywhere that North American Indians or other indigenous peoples actually did this.
When you pit cook a pig (or goat or other animal) you line the pit with rocks and start the fire on those. The rocks absorb the heat from the fire, and by the time you put the pig on the fire has burned down to coals and ash that fall down the cracks between the rocks. It’s really the hot rocks that cook the meat. I know from experience that the rocks will remain hot enough to sizzle for a long time (8-24 hours). I don’t see why you couldn’t use this principle to make a warm bed. It would take a couple hours to heat the rocks up, but once they’re hot and buried they’d release the heat slowly.
I guess you could pit cook your pig or buffalo and lie on the warm ground above for a great nights sleep. 
But, your body weight might compress the pig too much and mess up the cooking.
I wonder which indigenous people pit cooked animals? I know the native Hawaiians did it. I’m not sure if the plains Indians did or not.
Which is why the choice is very important. You really want rounded ones if you’re to be satisfied. To help them absorb heat, you’ll want to paint them a dark colour.
The technique would work if done exactly right but I doubt people really did it much in real life. The human body’s comfortable temperature range is really small. You can go from cold to slow cooking in just a few degrees with an external heat source directly on you. You can heat the ground with a long burning fire in a pit that you bury and it will release heat for a long time. However, the fuel requirements to do that are really high and not practical for common use. It would be better to sleep close to the coals but not on top of them so that you can move if spots get too hot.
Part of the OP’s question isn’t exactly right however. Normal fire needs oxygen to burn but coals can smolder and release heat underground just fine. You have to get them really hot to do that but there is oxygen underground as well. They have fissures and cavities that allow continued burning.
By sheerest coincidence, I am just now reading a book about Pancho Villa; the book says his men did exactly that, on very cold nights in the desert. So, the practice wasn’t extinct as late as 1916, anyway!
Some people are still teaching (and using) this technique today. Because of “minimal impact” philosophies these days it’s mostly thought of as a survival technique rather than something you would just use camping.
Whether or not North American Indians used it or not, I have no real clue.
FYI Google “fire bed”
Bear Grills in Man vs Wild made a “smoke blanket”(i think I remember him calling it that) where he was nearly on top of two fires on either side of him, but not directly on top.
As noted above, you probably don’t want the coals to keep burning while you’re lying on them. You’re using the residual heat, not the actual heat of ongoing combustion.
In Peru with have a traditional cooking technique called pachamanca.
You heat rocks and when they reach the appropriate temperature you out them in hole about 1 or 1.5 m deep. you cover the rocks with banana leaves and on top you put different meat (beef, pork, chicken, lamb, guinea pig), another layer of banana leaves and you ut your veggies (potatoes, favas, seet potatoes, cassava) then you cover that with a blanket, and finally cover with the hole with the dirt you took out. An hour or so later you’re ready for you-don’t-need-knives meat like you’ve never tasted.
Because carrying a -20 sleeping bag around is a whole lot easier than the work and effort that goes into building up a big fire, digging a pit, etc. Ever tried to dig into the ground in the winter? Additionally, it’s usually a giant breach of park rules to create a big fire, especially using brush and wood from the area. Also, tents are very useful things, but tend to melt when in contact with fire.
Materials scientists have developed a range of phase change materials with really high heat capacity/latent heat of phase change. These can be used to store available heat and release it slowly. One project has developed bed rolls for Mongolian herders - the roll is heated by the fire during the day, and keeps the herders warm all night. A related project has heat storing threads woven into the herders clothes. As they walk their herds to the grazing area, they get warm and store the heat in their jacket. This then keeps them warm for hours afterwards, while they sit around watching the flock.
There are phase change materials for buildings, too. They are hugely more efficient density wise than concrete, brick or stone. The trick is getting the stability, safety and transition temperature right for the target use.
You’ve never heard of coal or peat burning underground, and how difficult it’s to extinguish?
As for why not every camper does it: rules at the national park may forbid it; digging into the earth in winter in the first place is more effort than erecting a tent and getting out your sleeping bag; you need more fuel for the fire; it’s difficult to get the temperature right compared to a tent.
I suspect that’s why Indians, if they used it at all, thought of it more as emergency measure than regular camping usage. Under normal circumstances, if they were about in winter at all, they would have a tent, fur blankets and a fire inside as a proven method.
I remember reading a book that took place in Sibiria, and the boy walking through the wilderness used a similar but easier technique: after cooking his food over the fire, he spread the embers over a piece of ground his body size, and when going to sleep, brushed the embers away. Thus, the ground had been warmed but he wasn’t sleeping directly on hot coals.
Outdoor Survival Skills, by Larry Dean Olsen, has a diagram for a heated bed.
You use a pair of Y-shaped stakes to support a wooden rod a few inches above the ground. Set up a row of these rods, and you have a platform on which to put your bed. The space under the platform acts like a duct, drawing hot air from your campfire. At the other end of the bed, you build a chimney-like structure to draw air through the duct.
Olsen says that it is a lot of work, but he claims to have used it on many winter nights.
Okay. I’m someone who as actually done this. Let me address some of the comments in people’s speculations from the discussion.
You do use hot burning coals to make your ‘Fire Bed’ and these coals continue to burn all night.
You cover the burning coals with dirt. If you get too warm you add more dirt. If you get too cold you remove some dirt.
Is there enough oxygen for the fire to keep burning? YES! Definitely. In fact you want the coals to only slowly smolder and covering the coals does this.
Can you get burnt? I think it’s possible but that didn’t happen to me. Six inches of dirt seems to be decent insulation from the heat source.
We did this technique of staying warm in wilderness survival training where you go into the mountains and stay up there with only a knife and flint and light clothing and no food. For me I could only get my Fire Bed to give me heat for six hours at night then the coals burned out. Also it only heated the half my body laying on the dirt, all night long I had to turn over again and again as half my body was terribly cold. I didn’t sleep great but got adequate sleep and I survived the nights, it was too shivering cold to sleep without heat. Sleeping bags are far quicker and much more comfortable. Reasons indians would do this is they would be on long hunting trips and traveling light and away from camp.
My friend, who is an elderly native Alaskan Tlinget Indian, tells me that he heard of stories- where Athabaskan Indians did sleep in beds prepared this way, when they traveled and came into severe weather.
Sounds almost identical to ahangi(sans banana leaves as they don’t grow in New Zealand). Again, the heat doesn’t come from combusiton, it’s just been ‘stored’ by the rocks.
Do you know what these are called? I’d love to get some for my own use.