Urban myth or what?

[durulz]

I was always told you put the stones around the fire to stop it spreading.
Anyway.
I've never used stones from a river, but I can see how they would explode - the water inside heats up, expands and then...POP!

Like a bunch of complete bell-ends, we once put flints around the fire. And yes, they DID explode!

 

[John Fenna]

Our garden soil is full of shale (only about 12" of topsoil then we are onto bedrock shale) and the topsoil is full of small chunks of the shale...
If we have a bonfire things are fine for a while - then we have to step back as the shale starts popping and whizzing about.
I have had a couple of "near-misses" with red hot chunks of shale pinging past my head.....

 

[Wayland]

Many years ago, before I knew any better, I tried to construct a bank oven with some dry paving slabs. ( I presumed that as long as they were dry they would be OK, big mistake.)

The resulting explosion, which is the only way I could describe it, threw chunks of concrete up to 15 yards from the site of the construction when I left a fire burning in it to heat it up.

Very fortunately we were some distance away training a schiltron when we heard the loud "CRUMP" and a great deal of consternation coming from the camp area.

 

[Toddy]

Over the years I have seen quite a few "pop." My experience has been that any smooth stone that has a surface that you can tell by looking would not absorb water, may heat and cool too rapidly and simply break. There will be some noise, perhaps even fairly loud, but the stone will simply break and not explode.

Any stone that looks like it would absorb water if you poured some on it, probably will and as stated in one of the posts above, it will heat the water, the water will turn to steam and it will probably explode. Violently enough to possibly cause injury.

Stay away from sandstone, slate, and anything porous and you will be completely fine. I examine each rock, build the fire ring and never give it another thought. There is really nothing to worry about if you can tell porous from nonporous. If you don't trust your judgement, you best not use rocks.

Archaeologically these types of stones are what we call *pot boilers*. They are just as you say, smooth, non absorbant, generally well rounded, anywhere from an egg to a melon sized, though the ones I've found have been like large grapefruit sized.
They do split, but it's a cracking apart not an explosion with flying pieces. Inside you can see rings of different colours where the heating and cooling has created colours from black to brick red.
I would have no qualms using stones such as these as fire stones.

Stones around the fire give a place to put things down; keep a pot simmering instead of boiling; angled just right to the heat they are good for cooking bannocks and oatcakes; they act as a windbreak or a reflector too if needed.

However, it really is know your stones. Get it wrong and it's not usually funny.

cheers,
Toddy

 

[John Fenna]

I have used the "pierre chaud" method of cooking a couple of times after having been introdued to it in a retaurant in the Alps.
Basically a flat topped rock is made very hot (in an oven and kept hot by a spirit stove in civilised surroundings or in the fire and then kept hot at the side of the embers in the wilds) and then greased/oiled before your gobbets of meat are placed on it to cook to your own prefered level of "raw - welldone". The meat is then dipped in a choice of several sauces and consumed. Each bit of meat contributes a little more grease to the rock so that it is "non-stick".
My Venture Scouts used to love this very sociable form of communal cooking and we never had a rock explode on us though we hada well proven "cooking rock" - an off-cut from the local Monumental Mason's work....a bit of Gravestone Granite!

 

[Zingmo]

As a 17 yr old, a mate and I built a fire using stones from a stream. The evening started pleasantly enough, until the rocks started exploding. We had to take cover from the flying shards behind a boulder and the detonations stopped us getting back to the tent. We had to stay up til the small hours until everthing had quietened down. Fortunately we had enough cider to keep us warm!

Z

 

[mr dazzler]

Stay away from sandstone, slate, and anything porous and you will be completely fine.

We used sandstonne as our preferred stone of choice for a good explosion (tons of it everywhere you look in S W Durham, even the names like STAINdrop, STAINton, STAINmore etc-are all derived from old words for stone). The best was if you could get a big flat piece to delaminate after it was "quenched" in the river, sometimes they did....... The other cool thing was hoying big flat pieces off the top of cat castle (when it was still derelict in the early 70's, before someone reopened it a few years later) and hearing them whistle like bombs as they went down. We always tried to get them to hit flat on, then they would shater into smaller pieces.

 

[trewornan]

It's not just porous rocks like some people are saying. I once had several big bangs from flints in a fire - embers and splinters of hot rock all over the place. I would say the main danger is getting minor burns rather than actually getting hit by shrapnel (as you might say). Although after the first bang we were all pretty far away. Until you experience it you can't appreciate the shock of your nice relaxing camp fire suddenly exploding.

 

[John Fenna]

Even the "experts" can get it wrong - the trouble is they can pass bad advice on to Newbies....
In his book "The Commando Survival Manual" H. McManners shows a loverly picture of a fire with a rock wall surround - page 61.
Although he clearly states "Select dry, non porous rocks and avoid slate, which can explode when heated." the photo clearly shows the rock surround - which the fire is surounded by on 3 sides and up to quite a hight, with the fuel leaning on the rocks - made out of flint!
Not quite the safest of rocks to get realy hot......
On page 60 HM also states "Safety is an essential consideration. Fires are potentially dangerous.....they can also scar the landscape...."
No _ _ _ _ Sherlock? How about faces and minds?

Did HM train BG?

 

[Graham_S]

I stick to logs for my fire surrounds, far safer.
I hate seeing blackened rings of stones where people have made fires.

 

[Boreas]

I can attest to it being true. I've had quite a few rocks explode on me in the fire, not necessarily violently enough to do any harm but it really does pucker up your butt hole when it goes.

Don't think because a rock appears non-porous that it won't explode. Stone may absorb water slower the less porous it is, but they all have some. Furthermore the outside stone may not look like swiss cheese but until you crack it open you never know the entire make up of stone.

Concrete scares the crap out of me. It naturally contains some water in it's make up, even if it appears dry. That's one of the reasons why it's so fire proof. However, when it get's enough sustained heat all of the water will expand and it really does explode, or if you're lucky and it heats slowly it just crumbles. Firefighters are trained to be aware of that possibility. Dry wall also contains quite a bit of water, that's why it's a great material to build with, makes houses less flammable.

Anyway, I don't use rock rings on my fire. They can explode, and they are really difficult to put out properly, Improperly put out fire rings are one of the major causes of forest fires.

 

[dorkingbushcraft]

we decided for a laugh to put a concrete slab on the top of the chiminea a while later blew off the top and went about 7 ft in the air