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(Sorry in advance)

From the maker of How many people would have to be killed to make the streets 'run red with blood'? we have yet another question about dead bodies and how to use them.

In effect, say we have a 10 x 10 foot room (3.05 x 3.05 in meters). How many skeletons would have to be demolished to produce a covering 0.5 inches (1.27 centimeters) thick of powder over the floor that you could leave a footprint in?

Assume an airtight room and no bone pieces are ever lost or removed from the room. Assume the bones of a person were crushed or burned- whichever produces more 'debris'. Assume no blood, flesh, or anything other than pure bones is destroyed to make this inch of powder.

Friendlysociopath
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    This question is slightly worrying with regards to your username. – Virusbomb Oct 04 '17 at 17:13
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    Would you believe that's not the first time I've heard that? – Friendlysociopath Oct 04 '17 at 17:14
  • in order to make this question not broad and singularly answerable you need to address: How thick/dense the coating needs to be to count as a coating (1 body could easily coat a whole floor in molecules though I don't think that's the density you want) and how many surfaces (I could argue the walls don't count). – anon Oct 04 '17 at 17:15
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    What does this have to do with worldbuilding? – sphennings Oct 04 '17 at 17:17
  • Because I eventually need a larger expanse covered in bone dust and was hoping to get a number to ballpark it here.

    Presumably at least the density of snow, the important bit is the footprint, or more exactly being able to trace a pattern in it.

     – Friendlysociopath Oct 04 '17 at 17:17
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    Who is grinding the bones to dust and leaving it in a closed room? – A. C. A. C. Oct 04 '17 at 17:27
  • Technically, you could leave a footprint in a lot less than 1" thick bone dust... – Ghotir Oct 04 '17 at 17:42
  • @Ghotir True, in theory I should probably change it to half an inch as that's closer. – Friendlysociopath Oct 04 '17 at 17:47
  • Fe fi foe fum, I smell the blood of an SE-man. Be he alive, or be he dead, I'll grind his bones to make my bread. – CaM Oct 04 '17 at 17:53
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    I'm down voting you not because I think the question is in bad taste but because it took less than ten minutes and four Google searches to answer, lazy dude. – Ash Oct 04 '17 at 17:53
  • What has this question got to do with world building? – Slarty Oct 04 '17 at 17:59
  • So the whole grinding bones to make bread? Yeah, that was thing. Briefly. It failed badly. http://www.troynovant.com/Franson-JM/Essays/Bones-to-Bread.html – CaM Oct 04 '17 at 17:59
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    @Slarty You are, unsurprisingly, not the first person to ask that question. – Ash Oct 04 '17 at 17:59
  • @Slarty It's something I use in two different worlds. The first one it's just a byproduct of a dude getting rid of bodies. He eats everything but bone and smashes those to dust. The second involves an entire battleground basically being covered in the stuff and used for a ritual. The former seemed easier to get a result for than the latter. – Friendlysociopath Oct 04 '17 at 18:03
  • VTCed because this seems like a pretty straightforward math question. Length, width, depth, density should do it. – Green Oct 04 '17 at 18:39

2 Answers2

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Let's see, so you need roughly 235974cm^3 of dust to achieve your desired depth of fill, bone has a mineral density of 3.88g/cm^3 in men and 2.9g/cm^3 in women, assuming an even gender split among your "donors" that's 3.39g/cm^3 so you'll need 799952g or nearly 800kg of bone dust to coat the room. How many people's skeletons that is depends on how big the people you're knocking off are. Skeletal dry-weight is 15-20% of total body weight so on average you need about 4571kg of person to achieve your goal, if they're "average" Americans (latest estimate I could find was 196lbs in 2014, that's 89kg), and erring on the side of caution, you obviously don't want to go too thin, that's 52 people's bones you have to powder.

Post the "half inch" edit you only need 117987cm^3 for a total of 26 skeletons.

Based on your comments about specific scenarios this might be useful; you're talking about 2.8 bodies per square metre to cover any space that half-inch deep.

Ash
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  • What about in pregnant women? – anon Oct 04 '17 at 18:02
  • How dense is bone dust in comparison to actual bone? I would assume the dust to be less dense by a significant factor. https://www.binmaster.com/_resources/dyn/files/75343622z9caf67af/_fn/Bulk+Density.pdf this links ground bone as 0.8 g/cm^3 and dry ground bone as 1.2g/cm^3. I have no idea what bones these are and if the ground bones are actually in dust form. – A. C. A. C. Oct 04 '17 at 18:02
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    @A.C.A.C. I did actually think about that and since he said "powder" I assumed single mineral grains that would pack down without significant air gaps thus I would expect that the density of this powder would very closely resemble the mineral density of the material. If in fact the bone was less thoroughly crushed you would need less of it but the footprint would be less legible in direct proportion too. – Ash Oct 04 '17 at 18:08
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    @anon And I thought the question was in questionable taste. – Ash Oct 04 '17 at 18:15
  • Careful, bones aren't typically dry. Especially the juicy marrow. Mmmmm. Marrow. – CaM Oct 04 '17 at 18:40
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Average male weight is 72 kg, average woman weight is 67 kg (that's for Europe though), with 15% bone for males, 10% for females, giving around 10.8 kg and 6.7 kg of bones respectively. I've found wildly varying estimates for skeletal weight, going all the way down to 3 kg. I'm assuming that the figures above are for dry bone, otherwise the results are likely off by a factor of 2.

Dry bone density of 1.25 g/cm3 means 800 cm3 per kg, but uncompacted "ash" is around double that, and translates to 17280 cm3 for a male and 10720 cm3 for a female skeleton; on average, that's 14000 cm3 - a nice round number.

One square meter at 1.27 cm height needs 12700 cm3, so you get approximately 1.1 square meters out of the average person, which is in the same order of magnitude of Ash's results (the difference mainly due to different bone density estimate).

Depending on the method used to pulverize the bones, you can get small roundish grains (resulting in the footprint legibility issue Ash raised) or ash-like flakes, that would be compressed rather than displaced by the incoming foot, which would actually improve legibility - think Armstrong's footprint on the Moon.

LSerni
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