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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.

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    $\begingroup$ This question is slightly worrying with regards to your username. $\endgroup$
    – Virusbomb
    Commented Oct 4, 2017 at 17:13
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    $\begingroup$ Would you believe that's not the first time I've heard that? $\endgroup$
    – Friendlysociopath
    Commented Oct 4, 2017 at 17:14
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    $\begingroup$ What does this have to do with worldbuilding? $\endgroup$
    – sphennings
    Commented Oct 4, 2017 at 17:17
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    $\begingroup$ Who is grinding the bones to dust and leaving it in a closed room? $\endgroup$
    – A. C. A. C.
    Commented Oct 4, 2017 at 17:27
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    $\begingroup$ 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. $\endgroup$
    – Ash
    Commented Oct 4, 2017 at 17:53

2 Answers 2

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Let's see, so you need roughly 235974cm3 of dust to achieve your desired depth of fill, bone has a mineral density of 3.88g/cm3 in men and 2.9g/cm3 in women, assuming an even gender split among your "donors" that's 3.39g/cm3 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 adults (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 117987cm3 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.

As a note if the dust is fine enough you can get quite legible footprints in considerably less than a millimetre (5/127th of an inch) of it, in fact to some degree the less dust the better in terms of tracing the tread pattern. If you want tread depth as well then your half inch is more like it but the dust needs to be fairly "fluffy" and uncompressed before it is walked on to capture and hold good impressions.

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  • $\begingroup$ What about in pregnant women? $\endgroup$
    – anon
    Commented Oct 4, 2017 at 18:02
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    $\begingroup$ How dense is bone dust in comparison to actual bone? I would assume the dust to be less dense by a significant factor. binmaster.com/_resources/dyn/files/75343622z9caf67af/_fn/… 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. $\endgroup$
    – A. C. A. C.
    Commented Oct 4, 2017 at 18:02
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    $\begingroup$ @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. $\endgroup$
    – Ash
    Commented Oct 4, 2017 at 18:08
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    $\begingroup$ @anon And I thought the question was in questionable taste. $\endgroup$
    – Ash
    Commented Oct 4, 2017 at 18:15
  • $\begingroup$ Careful, bones aren't typically dry. Especially the juicy marrow. Mmmmm. Marrow. $\endgroup$
    – CaM
    Commented Oct 4, 2017 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.

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