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Real smelters typically use fire. Unfortunately, fire doesn't work so well underwater, and so an alternative method of heating must be sought by aquatic animals

One potential method would be to use some optical system to focus light from the sun onto ores to heat them, thus smelting metals. One problem is with materials, namely, what materials can the optical system be made from?

The materials would need to be able to make an image-forming lens or mirror, which is capable of properly focusing sunlight for heating. It would also need to be creatable and workable with stone-age technology. The world is alien, but is earthlike and follows normal physical rules; hence the animals, vegetables, and minerals can be assumed to be as on Earth

Is there any material that could fulfil these requirements?

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    $\begingroup$ What does underwater have to do with smelting? Besides, things cool down quick in water increasing energy requirements and the steam gets in the way of the light beam. The mirror/lens is going to need to be above water anyhow, why can't the minerals/metals just be taken up to them? - Breathing gear for the aquatics would be an issue perhaps for another question. $\endgroup$ Commented Mar 5, 2022 at 23:05
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    $\begingroup$ Even the purest H2O isn't perfectly transparent. A substantial part of the light energy will be lost to the water between the lens and the smelting material. To say nothing of the unfocused light between the light source and the lens. $\endgroup$
    – Tom
    Commented Mar 5, 2022 at 23:17
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    $\begingroup$ "[Do something to] ores to heat them, thus smelting metals": That is not how smelting works. You may heat, for example, iron oxide as much as you want, and you still won't get any iron. You need to make it react with carbon, so that the carbon combines with the oxygen in the iron oxide and liberates the iron. In short, heating stuff up may melt it, provided you heat it up to a high enough temperature; smelting metals is something else entirely. $\endgroup$
    – AlexP
    Commented Mar 5, 2022 at 23:59
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    $\begingroup$ Oh yes, applying heat is necessary. The reaction environment is also necessary. Heating up iron oxide and carbon in water will only get you a lot of steam, because water boils at much lowe temperature than what is necessary for the carbon to reduce the iron oxide to metallic iron. $\endgroup$
    – AlexP
    Commented Mar 6, 2022 at 0:22
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    $\begingroup$ The presence of heat absorbing water is a bigger problem than the lack of heat producing fire. You could literally have fire that could burn underwater and it still wouldn't be enough because the water is there. $\endgroup$
    – DKNguyen
    Commented Mar 6, 2022 at 5:57

4 Answers 4

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If you limit this question only to a material for focusing sunlight, then you have no problem at all using plain silica glass. I am saying this on the condition that your alien sun is similar to our own yellow dwarf sun.

I would feel wrong about leaving out some important engineering considerations your aliens will be running into. So, maybe think about creating a couple other questions to make this system come together.

  1. Water will take most of the power from your sunlight. Creatures deep on the ocean get no sunlight. In fact, after only 30 meters you can barely see the sun. There will be very little energy reaching your ore by the time it gets focussed. The only answer is to build some sort of tube that has a very clear fluid or air in it. Fluid would be best because you will need to go very deep.
  2. Water is going to boil before your ore is smelted. By the time your sunlight is focussed on the ore you want to smelt, it will need to be very VERY deep in the ocean, or the heat it makes will be boiling the water around your ore. In fact, if you want to smelt even lead (which needs to be heated to 327°C before it melts), you need to be in water at 1800 PSI. You didn't talk about your planet much, but on Earth, this means you would be 1.2 kilometres deep in the ocean. If you let the water boil, then the light will not really reach your ore very well because the boiling bubbles will refract the light you just went through all that trouble to focus.
  3. If you smelt in water, the water is going to be cooling the ore extremely fast. When we smelt in air, the metal will slowly cool off as the air carries heat away. But water carries that heat away much faster. There are complex calculations to show what the exact difference will be, and we don't know anything about your planet's water (not even if it's salt water); but just using our sun as a reference, there is no way at all to make a metal hot enough to melt under water.

Sorry for the discouraging news, but again, glass is a very plausible material for an optic smelter, if you are not under water. Your sea-cave-men will not be entering the iron age with a solar smelter.

Undersea volcanoes are how your people will get the heat to make their first swords. It will not be easy at all.

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    $\begingroup$ Not to mention how you get the glass in the first place. $\endgroup$
    – DKNguyen
    Commented Mar 6, 2022 at 5:52
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    $\begingroup$ Fortunately the question has a very narrow scope. $\endgroup$
    – Vogon Poet
    Commented Mar 6, 2022 at 6:01
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I'd worry about energy and environment before materials. Because you're in a Catch-22 if you require a complex methods to smelt underwater even if you do find materials to support that method since you have the ingredients but have no progression to get there. How do you process that material without the methods you want to enable with that same material?

Consider this: Some modern materials could not be processed hundreds of years ago because they require a vacuum or inert gas atmospheres. Your situation is similar but easier in that to achieve a liquid-less environment all you have to do is go to the surface.

There's no reason that just because humans can smelt in the same air we breath (well, not really. See later. ) that aquatic animals need to smelt in the same water they live in. It's not that crazy because it isn't like humans can breath the the air in a furnace or reach into the furnace with our bare hands.

I think the biggest difficulty in this is actually gravity. Water flows down while air goes up. So in air you can work from above or the side whereas in water you can only work from below.

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Glass works great to redirect light. It's also fairly heat resistant.

Another option is the concave "Umbrella cooker," where you line the inside of an umbrella with tin foil or something else reflective, and then put your pot on a shortened handle.

However, as mentioned in the comments, both of these options would be made nearly pointless underwater.

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Considering the difficulty of making glass before you already have a high heat source, it could be possible to make a solar furnace out of carefully selected, polished shiny stones.

  • Select shiny stones.
  • Polish the stones with grit.
  • Dig a large parabolic bowl-shaped structure underwater in the shallows, and line it with the shiny stones.
  • Place the raw ore and other materials at the focal point atop a pillar of rocks.
  • Set up a wall around the bowl-shaped structure and pump out all the water using a pump made of driftwood or perhaps bone, to obtain a cofferdam. Use the tide to assist in removing the water.
  • Wait until the sun is in the right place, and you could obtain high temperatures at the focal point.
  • Let some water back in to the cofferdam so that your aquatic animals can retrieve the finished product.

There are many ways of getting rid of the water, besides a cofferdam:

  • A boat
  • A diving bell
  • An airlock, kept dry with pumps
  • Just work on the shore

The aquatic animals can sit in a pool of water while they reach out into the air, perhaps using long tongs or pokers, to do their work.

The easiest way for aquatic animals to get smelting temperatures would be to burn biomass. Animal fat could work for this, if there aren't readily available large plants.

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