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Solar furnaces using mirrors to concentrate sunlight on a small area do not produce much power - perhaps 1kW per square metre of mirror - but can reach very high temperatures. From the earliest metalworking through to the beginnings of the industrial revolution, metal refining and working was limited by the difficulty of supplying enough heat to the furnace. For a small scale operation - the sort of activity carried out before the industrial revolution - could a solar furnace have been used instead of one using a fuel (usually charcoal) and air mixture?

I'm imagining a sunny hillside with perhaps 10 to 100 polished metal mirrors, perhaps a square metre apiece, aligned by humans running about, all pointing at a small crucible or forge inside as much insulation as could be wrapped around it.

Would it be possible to use that for refining or forging?

I want to ask, over on history stack exchange, whether such an arrangement was ever used, but I thought I'd make a fool of myself asking a question over here before I make a fool of myself asking one over there. Thanks!

Edit: I did find these:

I'm guessing it would have been hard to make a large Fresnel lens until relatively recently. Mirrors are much easier, they have lovely shiny steel armour in many museums, and child labour to polish the metal and align the mirrors would have been cheap in the past. The Woodward forge looks good but the blog says that it hasn't been used for metalwork. Has anyone made this work on a small scale with mirrors?

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I'm not aware of any small scale historical versions, but it seems more-or-less possible. There is a good myth about the ancient Greek's using shields to focus sunlight and light Roman ships on fire.

I see a few engineering challenges. You either have to have a nice roughly parabolic shaped mirror, which was really tough to manufacture until the last century, or approximate one with many small flat mirrors. The small flat mirrors (check out linear fresnel concentrated solar collectors) quickly become a controls nightmare without automation. Turns out, when you're trying to focus the sun with a mirror, the sun actually moves annoyingly fast (well, in your frame of reference). Modern CSP heliostats adjust position on a one minute type timescale.

Temperature control and solar flux are also challenging problems. If you work through the geometry to collect enough sun to produce the requisite heat, you'll find the focal distance between not hot and burned-a-hole-in-your-work is not very big.

On an only sort of related, but really interesting, note, someone in maybe the 70s decided it would be fun to make a concentrated solar power plant using a liquid sodium/potassium alloy as the heat transfer fluid. As you likely guessed based on this answer and a little bit of knowledge about liquid sodium, it burned down, killing an operator. The moral is to only forge non-flammable metals, I guess?

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  • $\begingroup$ Thanks very much, I think that's the best answer I'm going to get here. "Maybe"! $\endgroup$ – emrys57 Jul 2 '18 at 6:29
  • $\begingroup$ Proving a historical negative is pretty hard. Probably Jens the Viking tried it but didn't get anywhere? I have enough experience in that field to think that if it was well known, I would know about it, and I don't. So if its happened, I'm guessing its obscure. $\endgroup$ – ericksonla Jul 2 '18 at 21:22
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Well, I think the French built a large one... See https://anglophone-direct.com/mont-louisfont-romeu-odeillo-via/

As for historical, the issue may have been the quality of the mirrors...

And, 1000 years ago - understanding that the power was available as well as reliability ie not weather dependant.

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  • $\begingroup$ Yes, I know we can make very big ones now. Would a small scale one have worked, say, 1000 years ago? $\endgroup$ – emrys57 Jun 29 '18 at 11:00
  • $\begingroup$ As for historical, the issue may have been the quality of the mirrors... $\endgroup$ – Solar Mike Jun 29 '18 at 11:08
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I think the first step is to decide if you are refining or forging. If refining you are pretty much limited to making steel from pig iron : certainly not practical on a small scale; it is more about blowing hot air + oxygen into the bottom of a ladle than a heat source. Forging sounds possible ; build a insulated chamber that the beam can enter and a hatch for access. Then why or what are going to forge ? An almost practical objective would be to melt copper alloys to pour castings . Start by melting zinc for castings , low melt temperature and fairly e.asy to handle

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You asked if it is practical. The answers will be subjective because it depends on the pal solar performances in relation to the latitude in which the work is being done and under what weather conditions allow it. Whilst mirrors seem obvious, focal distances are imperative and relative to surface curvature of the reflective surface. In my engineering view, sustaining a temperature in excess of 1746°C will require a flat mirror surface area of about 400 {m^2} or an equal parabolic surface area of that divided by the depth of the parabola. A Fresnel lense of 1 square meter can transmit enough energy to heat a local object up to 800°C but that takes time. You only have 120° of daylight and of that 45° is optimum power meaning 3 hours of sunlight a day IF weather conditions are clear . It may take a couple hours to obtain smelting temperatures but sustaining them is going to be the challenge. You may be better off using solar to generate power for arc furnace use through parabolic solar heated steam turbines instead, in my opinion.

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It sure is possible but be prepared to face a lot of engineering challenges especially if you are planning to install all the equipment, mirrors etc by yourself. A lot of metalworking tasks used in the 16th & 17th centuries are no longer in use by firms but you can find hobbyist videos on YouTube doing that kind of thing. Common household mirrors have less than the required reflectivity index and all concentrated solar projects make use of heliostats that are designed to withstand harsh environments.

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