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Old 02-11-2013, 08:09 PM   #31
Trail_Mix
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Keep us updated please! This is such a great find!
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Old 02-11-2013, 11:21 PM   #32
lochaber
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There are some rocks that are dangerous. Rocks like lava rock can have air trapped inside that can explode when boiled. I agree that the warning has been exaggerated, as most rocks are safe to boil, but there are a few that aren't.
Have you ever witnessed this, or heard a first hand account of this happening? -I'm rather skeptical that any would actually explode. I could possibly see cracking, but I don't think it would be dangerous at all.

The reason not to put rocks on a camp fire is because of the water present, and there is a tremendous amount of expansion involved when water changes from liquid to vapor form. Rough estimate, I think it's something like a 1000x increase in volume.

As to a gas pocket that's already in the rock, it's probably not even going to expand 2x, since you are only heating it to ~100 celsius
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Old 02-12-2013, 01:30 AM   #33
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Keep us updated please! This is such a great find!
Rocks are back! The new dumpster is a bit higher, but they filled up half already. What sizes are you specifically looking for? I can pull one or two out (maybe - wife dependent) and hold them for you. They range from 5 inches to large 20 pound pieces.
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Old 02-12-2013, 02:12 AM   #34
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Have you ever witnessed this, or heard a first hand account of this happening? -I'm rather skeptical that any would actually explode. I could possibly see cracking, but I don't think it would be dangerous at all.

The reason not to put rocks on a camp fire is because of the water present, and there is a tremendous amount of expansion involved when water changes from liquid to vapor form. Rough estimate, I think it's something like a 1000x increase in volume.

As to a gas pocket that's already in the rock, it's probably not even going to expand 2x, since you are only heating it to ~100 celsius
I have had no personal experience, but hear are a few supposed accounts:
http://www.greenscenelandscape.com/W...sMarch2011.htm
http://www.contractortalk.com/f11/la...re-pits-48144/

You are right that it is the moisture, not air, trapped in the rocks. Water expands a lot when converted to steam.
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Old 02-12-2013, 05:11 AM   #35
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Those links seemed to be about sticking rocks into/around fires.

I think boiling is different: the temp doesn't go as high as it would sitting in a fire. The outside of the rock is going to heat/boil before the inside does (probably a miniscule difference unless you are dealing with really big rocks). Also, water trapped in a cavity would be under pressure as it's heated, and this pressure would effectively drive up the boiling temp for the bit of liquid in that cavity. I'm wondering if it's something similar to why boiled eggs don't explode, but microwaved ones will?

Anyways, yeah, don't stick rocks in the oven, or on fires unless you know what you are doing (granite, basalt probably fine, sandstone, shale, slate, limestone - not so much).
and I don't see much point to boiling them anyways, but I don't think there is really any risk to it.

Back to topic, cool find, it does sound like maybe they are excavating or something. Hope some of you Manhattan folk can make some use of it.
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