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I had a thought last night about how to build a more efficient furnace in a fairly low tech society. (I build Games, so this is part of my world mechanics)

My question is: How feasible is this idea - could it actually be done?

So I had a thought, "Lodestone" or more precisely "Magnetic Lodestone" has been around for a long time (people have known about it for a long time, and magnetism was understood~ish).

If you could get hold of enough Magnetic Lodestone, and you attached all the pieces to the inside of a barrel. If you were to spin that barrel around a metal object would you induce enough heat to forge that metal object?

Lets say I want to make axe-heads, so 2kg? (that's a big axe) And I have iron in the form of bog-iron (pretty pure I believe, but not exactly sure)

Could I do this? How much lode stone would I need? Would the lode stone wear out (lose magnetism?) Would I have to spin the barrel at an accomplish-able speed? Or would I have to spin the ore instead?

I've done some research, but my understanding of inductance and the finery of the field means I'm struggling to understand or search the right things.

Thanks for any help :)

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    $\begingroup$ Not only does magnetite not have sufficiently strong magnetic fields, but the speed at which you would need to spin you barrel would be an engineering challenge even today. $\endgroup$ – pluckedkiwi Mar 20 '18 at 13:07
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    $\begingroup$ It sounds like you're essentially talking about creating a generator. Except it's being used strictly for it's ability to generate heat, instead of electrical current, correct? Would it also be fair to assume that it would fit accurately within the world to have it water-wheel driven? sites.google.com/site/denergysystems/labs/… $\endgroup$ – Dustin Halstead Mar 20 '18 at 13:08
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    $\begingroup$ @pluckedkiwi makes a great point. Lodestones were great for compasses, but they're relatively weak. It also has too low a magnetic coercivity (resistance to demagnetization) to stay magnetized for long. Generators use electromagnetism (magnetism generated by electrical current). Even the strongest, manufactured magnets would be insufficient (and would quickly lose their magnetic property). $\endgroup$ – Dustin Halstead Mar 20 '18 at 13:19
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    $\begingroup$ For clarification are you trying to smelt the ore or are you trying to forge the metal into an axe...or both? $\endgroup$ – James Mar 20 '18 at 14:02
  • $\begingroup$ The idea was to 'smelt' ore of high~ purity, but knowing that would take a lot of power, I wondered if you could at least get enough induced heat to forge iron. Does not seem like it's feasible from the comments here (and below). I knew Lodestones were weak, but have no idea 'how weak' (lack of concept) hearing that even very powerful modern magnets would struggle with this, puts it in pretty good perspective :) (PS: The water wheel link is great thanks :) ) $\endgroup$ – TolMera Mar 20 '18 at 14:48
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It is possible to use induction heating to generate temperatures capable of forging metal (with relatively low power consumption).
https://www.youtube.com/watch?v=HMYtTO8LIAw

Induction Heating

Unfortunately, although lodestones were great for compasses, they're relatively weak and have too low of a magnetic coercivity (resistance to demagnetization) to stay magnetized for long. Even the strongest, manufactured magnets would be insufficient (and would quickly lose their magnetic property).

Generators use electromagnetism (magnetism generated by electrical current) to operate.
This was true even with the first generator, the Faraday disc generator, in 1831 (by Michael Faraday).

Faraday disc generator

Even if you were to introduce a fictional mineral, it would still be the process of generating electricity -- and once technology reaches that point, your world would be on the verge of entering a Victorian equivalent era.

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    $\begingroup$ How are you defining low energy? They're using 25kW for at least a minute in that video, that's 1.5MJ. $\endgroup$ – Samuel Mar 20 '18 at 18:09
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    $\begingroup$ I barely glanced at the video, and only posted it as an example of induction heating of metal. Definitely agree with you @Samuel -- certainly not on the low end. I was thinking of one like this: amazon.com/dp/B01C70G7Y8 -- which only uses around 1-2kW (the same range as a clothing iron, but less than a hair dryer). I'll see if I can find another video. $\endgroup$ – Dustin Halstead Mar 20 '18 at 19:29
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    $\begingroup$ Ah, you're interchanging power and energy. They're quite different. If energy was how much money you had, power would be how fast you spent it. That is, energy is a quantity, power is a rate. In this case if you have enough storage you can store a lot of energy over a long time (low power) and then use it very quickly to heat metal quickly (high power). Quickly is key because it's cooling down all the time, so you have to get the energy in quickly (high power). Think of it like filling a bucket that's leaking. $\endgroup$ – Samuel Mar 20 '18 at 19:37
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    $\begingroup$ Excellent point @Samuel. I've edited the answer to use the correct terminology. The analogy you posed, by the way, was a wonderfully insightful explanation. Thank you. $\endgroup$ – Dustin Halstead Mar 20 '18 at 20:29
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    $\begingroup$ I have an M.S. in electrical engineering. I would hope I understand the basics well enough to explain them simply :) $\endgroup$ – Samuel Mar 20 '18 at 20:30
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There are a few issues with your idea:

  1. magnets lose their magnetism when they are heated above their Curie temperature, which is lower than the melting temperature of iron. Your lodestone would heat up by radiation.
  2. To have a decent effect you need to be able to align all the magnetic fields in a somehow coherent way. Just stacking various cobbles will be less effective.
  3. To melt iron you need to produce a good amount of power, which means a pretty high induced current in this case (order of the hundreds of Amp at least). Not so sure you can achieve it with such a simple design.
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    $\begingroup$ Actually you can generate a lot of energy with very low current and voltage. To melt iron you need high power, which can be produced from the energy that was stored up over a long time then discharged in a short amount of time. $\endgroup$ – Samuel Mar 20 '18 at 18:07
  • $\begingroup$ @Samuel, power is voltage times current. If both are low the power is low. $\endgroup$ – L.Dutch - Reinstate Monica Mar 20 '18 at 19:06
  • $\begingroup$ That correct. I was commenting on your statement "you need to produce a good amount of energy, which means a pretty high induced current", which is false. It can be a high current, but it isn't required. A good amount of energy can be generated with low power. Perhaps it's worth clarifying that energy and power are not the same thing. Energy is a quantity while power is a rate. High energy is easy if you have the time, high power is easy if you have the storage. $\endgroup$ – Samuel Mar 20 '18 at 19:10
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    $\begingroup$ @Samuel, fixed it $\endgroup$ – L.Dutch - Reinstate Monica Mar 20 '18 at 19:52
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If you're looking for a low-technology, low-cost means of working iron, have a look at history. As other posters have noted above, any naturally-occurring magnetism will not be strong enough or persistent enough to be able to heat metal the way you're thinking of. But a simple wood-charcoal fire can be sufficient to not just forge metal, but even smelt it.

https://www.google.com/search?q=viking+iron+smelting

And if you can smelt iron, you could take the next step and cast it -- which might work a good deal better for getting the initial form of a 2kg axe head than trying to simply soften a lump of bog iron and then bash it into shape.

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    $\begingroup$ Exactly my thought: KISS and use the fire you already know about... $\endgroup$ – RonJohn Mar 20 '18 at 20:10
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    $\begingroup$ There are lots of reasons why people may want to avoid using fire. Firstly, "FIRE!!!" :) - so yes, this is an option and an answer, but it's not in the spirit of the question, nor the spirit of 'worldbuilding' where thing's are not exact replicas of reality. Suspension of disbelief is easier when it 'feels' right. $\endgroup$ – TolMera Mar 21 '18 at 18:48
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    $\begingroup$ @TolMera: Can't get much more low-tech than fire, and from your initial question, it sounded like perhaps you just weren't familiar with some of the real-world furnace possibilities. If you're just looking for low-tech alternatives, would mirrors still be low-tech enough? If so, consider a solar furnace. Even easier to set up if your planet's rotation keeps the sun at a more even angle (less pronounced north-south wobble, a.k.a. seasons). $\endgroup$ – Eiríkr Útlendi Mar 21 '18 at 20:00

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