Naked Science Forum
General Science => General Science => Topic started by: Batroost on 27/04/2007 21:11:48
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Here's something I remember from when I was at Primary School and have never found a sensible explanation for:
I remember that if we used a magnet on the surface of the playground we could find lots and lots of very small metallic spheres. They looked a bit like fine lead shot, 1-2 mm across, though rusty and magnetic so I'm guessing they were iron.
When we discussed it at the time (and please remember that I was only about ten years old!) we thought they might be the remains of small meteorites. The playground was just a standard tarmac affair - nothing special - and there were no obvious metal buildings around.
So what were we finding with the magnets?
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Were they completely spherical? Or quite rigid and bumpy?
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I don't remember them as being perfect speheres, nor perfectly smooth; but 'round' certainly - a good approxmiation.
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Hmm.. I think they were just substances found with the rocks. Magnetic rocks can be found within rocks, just like metals, gold, silver, tin, can be found in rocks.
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I recall they looked like rusty iron. But what you're saying makes sense as an explanation - the aggregate in the tarmac could have been an iron-rich mineral?
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Yup. I have a rock, which is my star sign rock (scorpion), and it is magnetic. I guess as the tarmac became more weak and old, the rocks which were compressed to make it tight, gradually loosened and revealed rocks, falling apart, some of them being magnetic.
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I think you've got me convinced. I was trying to imagine why anyone would put iron in tarmac, but magnetic rock is a lot easier to believe.
Thanks Seany.
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Heehee. Your welcome! I hope someone else comes along, like George to give a better explanation [;)] Mine was just a guess [;D]
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Did the construction of your school involve any steel? As I would have thought that welding it may produce lots of little steel balls if lumps of molten steel fly out. I know that if you put wire wool in the microwave you get lots of little steel balls as you melt the tiny steel wires and surface tension pulls them into spheres. They then tend to weld themselves onto the inside of the microwave so don't do this with your mum's/wife's microwave if you ...
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Did the construction of your school involve any steel?
Good idea but, I'm not sure in this case. As far as I remember it was a traditionally built brick building with a tiled roof.
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when you say playground, what exactly do you mean? grassed field/area, tarmaced or what?
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when I was young in the 1940 - 50s there used to be a dark grey surfacing material we called "cinders" and nasty stuff it was too, because if you fell down on to it it caused grazes that did not heal well. I think it was crushed slag from ironworks or similar places and did contain little metal spheres in it as well as a lot of nasty sharp bits like broken glass. It was often used to surface open areas like playgrounds Now your playground may have been covered with this originaly and later covered with a layer of tarmac when this sort of surface was no longer acceptable because of a growing interest in health and safety. What you were seeing was a bit of the original surface material or spillover from another area that still had that sort of surface. You just don't see that sort of material on surfaces nowadays.
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when I was young in the 1940 - 50s there used to be a dark grey surfacing material we called "cinders" and nasty stuff it was too, because if you fell down on to it it caused grazes that did not heal well. I think it was crushed slag from ironworks or similar places and did contain little metal spheres in it as well as a lot of nasty sharp bits like broken glass.
this is why i asked the previous question, Ian.
my thinking was some sort of slag, fromeither iron or coal works. slag and pit top works are or were also used in roadmaking.
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Natural, iron-bearing, strongly magnetic minerals (magnetite, maghemite, pyrrhotite) found in rock are usually fairly small in size. Due to their small size and ready oxidation, these magnetic minerals weather out more rapidly than does the surrounding rock (they turn to "rust" before the rock breaks down). Therefore, it's more likely that the tarmac used some sort of iron slag, as suggested by Soul Surfer and paul- easily picked up by a small magnet. Some slags also contain higher concentrations of toxic metals- which is why they are no longer acceptable building materials in many parts of the world.
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when you say playground, what exactly do you mean? grassed field/area, tarmaced or what?
A simple light grey tarmac surface as far as I can recall.
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A simple light grey tarmac surface as far as I can recall.
that being the case, i think we may have identified the answer previously. That it was something to do with the materials contained in the slag that was used to make the tarmc.