Fire and exploding rocks.

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Janne

Sent off - Not allowed to play
Feb 10, 2016
12,330
2,293
Grand Cayman, Norway, Sweden
Melt Swedish granite or gneiss?
Not possible with a wood fire.
What happens is the surface develops cracks, that the ice fractures open. If you extinguish the fire and embers properly, by pouring water on it, the stone will crack up.
That is the technique they used in mining in the past.

It is ok and legal to use smaller rocks and stones, of the size you can carry, but making a fire on any larger, and specially the ice age smoothed bedrock, is a total no-no.

Yes, the ice pushed some stones down to you. Your state also own quite a few beautiful, very large stones of the unique Black Granite. It is quite unique because it is homogenous pure, deep black.
You guys paid for them in the 30’s, but never collected.
That stone is very popular as a wall cladding stone for magnificent buildings.
Also called Diabase.
 

Erbswurst

Bushcrafter (boy, I've got a lot to say!)
Mar 5, 2018
4,079
1,766
Berlin
Ah, I understand!

We usually find Scandinavic stones in the size between an egg or a football, and they do not crack in fires.

I guess, they are to small to produce any inner tensions, or every stone who had inner tensions was cracked in the ice age on the way to us.

Larger ones are really seldom here on the surface around Berlin, but perhaps they are deeper in the earth. When they build in Berlin the subway, they found some really large ones, as high as a man, and when we digged a 1,50 metres deep trench for our new water tube in the garden colony we found several as large as dogs.
 
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John Fenna

Lifetime Member & Maker
Oct 7, 2006
23,106
2,833
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Pembrokeshire
How did you find out which rocks not to use around a fire? Is there a specific type and if so, what causes it to explode when heated in fire?
l
Slate and shale tend to absorb a lot of water - our garden is full of it and after a while any bonfire tends to start spitting out high speed rock fragments!
If I wanted to use rocks as a fire surround I would go for something like quartz - of which there is also a plentiful local supply :)
Metamorphic rock tends not to absorb too much water and is therefor not likely to explode - it has already been through heats that a camp fire could never replicate :)
 

Erbswurst

Bushcrafter (boy, I've got a lot to say!)
Mar 5, 2018
4,079
1,766
Berlin
Just by a concreat pipe ring!

Ugly, heavy in the rucksack, should it fit in, but it works.

Saw it yesterday in a youth hostel.
Romantic building side holidays!
 

Janne

Sent off - Not allowed to play
Feb 10, 2016
12,330
2,293
Grand Cayman, Norway, Sweden
Ah, I understand!

We usually find Scandinavic stones in the size between an egg or a football, and they do not crack in fires.

I guess, they are to small to produce any inner tensions, or every stone who had inner tensions was cracked in the ice age on the way to us.

Larger ones are really seldom here on the surface around Berlin, but perhaps they are deeper in the earth. When they build in Berlin the subway, they found some really large ones, as high as a man, and when we digged a 1,50 metres deep trench for our new water tube in the garden colony we found several as large as dogs.
You are getting Granite and Gneiss. The calcium based minerals from around Gotland, and the sandstone from around Scandinavia got turned to sand. That is why there is so many nice sandy beaches around the Baltic sea.

Of course, the harder stones also got pulverized.

You can take both Granit and Gneiss , of whayever size ( watch out for Hernia!) and put them around the fire, only very superficial damage will happen. Unless you pour liquid ( beer, water, bodily fluids) on them.

Some, used in Saunas, do not even change the surface, but I do not know the name of them in English. The Sauna stones are fine if you pour water or even Alcohol like vodka on.
 

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