I am trying to figure out how plausible it would be for people without electricity or steam power technology to use the sun to smelt and forge iron and steel. The people developing this solar furnace have limited fuel options for combustion.

The mid 20th century Odeillo solar furnace is 54 meters high and can obtain a temperature of 2,500 °C (4,530 °F). This is perhaps overkill, because iron ore smelts at 1800°C (3270°F), though I do not know how large an area is heated by the Odeillo furnace so perhaps something even bigger (and more diffusely directed) would be needed to be useful. Here is a video of Odeillo melting a hole in steel in just a few seconds.

The ability to make mirrors that reflect and concentrate sunlight is ancient. Even if Archimedes' "burning glass" never existed, the fact that this idea is so old suggests that the technology to focus sunlight has been around for awhile.

This solar furnace need not produce industrial quantities of steel. Instead, consider this a very high fixed cost, low operating cost alternative to a clay bloomery.

Question Could the ancient Greeks, or else 15th century Europeans or Chinese, smelt iron with concentrated sunlight? If yes, how big would the mirror array have to be to smelt a football sized bloom?

Assume favorable weather conditions.

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    $\begingroup$ "Iron smelts at 1800°C (3270°F)": this sentence has no meaning. If you already have iron, then it has already been smelted. Smelting and melting are two very different things. In fact, throughout most of history iron was smelted from ore at temperatures much lower than the melting point of iron or of any of its the oxides. (And anyway, there is no relationship between smelting iron from iron oxides and melting the oxides. Smelting is a chemical reaction; you need to have the oxides react with something to be reduced to iron. In the real world, that something is most usually carbon.) $\endgroup$
    – AlexP
    May 4, 2022 at 23:59
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    $\begingroup$ @AlexP - the interesting question here is what they might have that could serve as a reducing agent like coke / charcoal but not be suitable as a fuel. That is a good question for a Mars iron operation too. $\endgroup$
    – Willk
    May 5, 2022 at 0:08
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    $\begingroup$ Ancient mirrors were polished bronze, with glass involved sometimes later. However, to get your bronze and/or glass you need high-temperature processes, albeit not as hot as required for working iron. So are you assuming enough combustible fuel to create lots of remarkably high quality mirrors, just not enough for smelting and forging? Otherwise it's a bootstrap problem - if you can't create high temperatures then you can't create the mirrors to create more high temperatures. $\endgroup$ May 5, 2022 at 0:15
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    $\begingroup$ @AlexP 1800C is the temperature needed to smelt iron ore. 1,538C is the melting point. I think you jumped to pedantry too quickly. I've added the word "ore" to the sentence, though no one else seemed to be confused by it. $\endgroup$ May 5, 2022 at 4:47
  • $\begingroup$ @KerrAvon2055 Very good point about the bootstrap problem. There is some fuel, but not much, and less than there used to be. Enough to get started, but not enough for consistent production of metals. $\endgroup$ May 5, 2022 at 4:50

5 Answers 5


As you point out the concept that solar could be concentrated was well known for a long time, so there is some potential to explore an alternative time line where it could have been developed.

The Nimrud lens was a 8th century BC polished piece of rock crystal archeologists found in Assyria that some think may have been used to start fires. Even more intriguing is in The Epic of Ishtar and Izdubar, Column IV, Coronation of Izdubar, written about 2,000 BC. 10th stanza. That reads:

The King then rises, takes the sacred glass, And holds it in the sun before the mass Of waiting fuel on the altar piled. The centring rays—the fuel glowing gild With a round spot of fire and quickly spring Above the altar curling, while they sing!

So perhaps there are several motivations for concentrating sunlight, besides the Archimedes example.

Visby lenses made of polished rock crystal have been found in 11th and 12th century Viking graves and are of very high quality.

In Europe (1515), Leonardo da Vinci proposed the idea of designing a Compact Linear Fresnel Reflector (CLFR), but according to what has been reported in the literature, his study of this system remained only an idea on paper and was not supported by experimental work [31], as he said that this system is valid for many industrial uses [32].

More specifically for smelting metals this was investigated by Lavoisier in 1774 with temperatures achieved of 1750 C. The larger concentrating lens was 52 inches in diameter, made of two convex sheets of class filled with white wine. It was capable of melting steel and even platinum. Ben Franklin made high temperature experiments with smaller lenses.

1774 solar furnace Lavoisier

Making mirrors or lenses for that matter, since the emphasis is on concentrating sunlight, the image quality doesn't need to be that good. However it is something of a problem to keep the light focused on one spot due to the apparent motion of the sun.

So you have a choice of moving the item being heated or adjusting the lens or mirror.

Polishing metals is one way to make mirrors, there are alternative routes that one could take cleaving crystals or fracturing obsidian or using flat sheets of mica to have very flat or smooth surfaces. That is all in reach of 15th century technology.

But it the mirrors are small, then the issue is positioning lots of small so the reflect the sun. You could have lots of small pieces that could be positioned in the side of a hill focusing onto a furnace area.

Silvering the mirrors can be another issue, but there were some silvering and gilding techniques in 10th century Persian alchemy texts, and different glass mirrors made with tin and mercury in the 15th century, and other small glass mirrors made in Ptolemaic Egypt ~ 300 BC backed with lead, tin or antimony. Since you want to reflect the IR as well as the visible gold will be reasonably good and can be beaten very flat. Gold also doesn't corrode and would withstand the elements.

As for how big to make the concentrator, that depends. But back of the envelope you get about 1 KW per square meter of sunlight energy deposited. The latent heat of iron slag is about 209 kJ/Kg (Cast iron about 123 kJ/Kg) so depending on what want to melt, it would depend. Probably more importantly you need to have very good insulation and not conduct the heat away. 1 kw is 3600 kJ/hour. A football is about 5575 cm3. The density of iron is about 7.8 g/cm3. So the mass is 43.5 kg.

So best case is with perfect 1 meter square mirror, no heat loss, 200 kJ/kG and 50kg of material to heat up, you would need about 2.8 hrs melt the iron, assuming all the energy is absorbed... or say 3 meters of mirrors to melt the iron in 1 hr, or 12 square meters to melt in 15 minutes.

If you lost 50% of the heat due to conduction... then 24 square meters. If the mirrors for some reason only 50 % efficient... then 48 square meters. Then if only 50% of the energy is pointed to the furnace... then 100 square meters of mirrors.

So potentially something 10m x 10m could probably get your football sized material really hot.

By the way the nice thing about solar furnace is that the heating rate can be really, really fast, especially it the object being heated is small and thermally isolated.

So in the 15th century, you could build an amphitheater like structure into the south side of a mountain or a hillside, postion your mirrors with a common focus and you could probably get a few thousand square meters of mirrors. Or if you had 100 mirror have maybe 100 people help keep the mirrors adjusted. Paying attention to eye safety etc.

Or you could have a number smaller setups to melt smaller amounts.

In the late 19th century there were attempts to use solar for steam engines etc., but really they couldn't economically beat coal.

Other solar heating trivia... include the recent melting of a Jaguar car in London by a shiny skyscraper.

The science fiction author Arthur C. Clark also wrote the story " A Slight Case of Sunstroke" where the fans in a football stadium in South America vaporize a corrupt referee by using their shiny programs to focus the sunlight on the referee.

  • 1
    $\begingroup$ Lots of fun info here. I had read about the French Academy of Sciences' lentilles ardentes, but had no idea it contained white wine. Amazing. Thank you for all of the different ancient mirror making techniques, and in particular for the numbers. Very surprised by how plausible this sounds. Hugely helpful! $\endgroup$ May 5, 2022 at 4:57
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    $\begingroup$ @SeanOConnor Definitely! $\endgroup$
    – UVphoton
    May 5, 2022 at 11:30
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    $\begingroup$ Great answer on the mirror side of the issue, but you completely neglect that smelting isn't just about the heat - the chemical reaction that liberates free iron from the ore is very unfavourable on its own, even at high temperatures. You need to add a reducing agent - which even today, is carbon (more specifically, carbon monoxide formed from imperfect burning of the coke). There's a reason arc furnaces need a feedstock that's already reduced (i.e. metal scrap and/or pig iron or directly reduced iron). $\endgroup$
    – Luaan
    May 5, 2022 at 13:17
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    $\begingroup$ @Luaan I was trying to dodge the whole carbon and chemistry issues, so thats why I assumed that the mirrors were heating iron for the calculation. There are tons of other issues in how ore would be crushed or pre-processed and I don't know much about those aspects. I guess hematite iron oxide melts at about 1530 C and that you need about a 1 :1 ratio of charcoal or carbon, so if you crushed and mixed it up ahead of time it might be easier energetically. I liked MolbOrg 's answer below that got into some of that. $\endgroup$
    – UVphoton
    May 5, 2022 at 14:36
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    $\begingroup$ @SeanOConnor The problem with that is that oxygen dissolves readily in molten iron, though. You need a way to get it out - normally, the carbon monoxide also fills that role, aggressively "attacking" the oxygen and carrying it out in bubbles. If carbon is that scarce, maybe there would be something else that could fulfil the same role, but I can't think of anything that wouldn't also be a practical fuel. Maybe we could do something like photodissociation of water to produce hydrogen (or just say they have wells of carbon monoxide), and just say they don't have the tech to use gas fuels well. $\endgroup$
    – Luaan
    May 6, 2022 at 6:36

Maybe, if they had the idea of Fresnel lenses

When Bottger and Tschirnhaus made porcelain, they used solar furnaces because European furnaces were not up to the task.

100 years earlier, lenses simply wouldn't have been big enough; they're hard to make and horrendously expensive. Lens making advanced enormously during the 17th and early 18th centuries.

However, in the 19th century, they figured out a way to make giant lenses for a fraction of the cost. By breaking the lens into separate arcs, you save a tonne of glass and hard work. Those are Fresnel lenses. They could have been made centuries earlier, had anyone had the idea.

Your people would probably be better off copying the enormous Chinese pottery furnaces, which were dug into excavated hillsides and reached temperatures easily hot enough to make steel; in the 1500s, they made porcelain items in batch sizes only matched by the Europeans on the 19th century....but solar furnaces would be waaay cooler.

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    $\begingroup$ The porcelain stuff is really neat. I found muse.jhu.edu/article/33270 Apparently Tschirnhaus was melting bricks with lenses and mirrors in the 1680's partly becasue he figure out ways to make cheaper large mirrors. Also it seems like there was a lot intrigue since the Chinese had porcelain and Europeans didn't. $\endgroup$
    – UVphoton
    May 5, 2022 at 11:50
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    $\begingroup$ Hmm, ceramics might be an interesting choice, in fact. You don't necessarily need a chemical reaction to produce those - heating alone is often enough. Even our civilization used to be primarily a ceramic civilization, long before embracing metal (and even then, compare how much ceramics we had compared to metal - now imagine how that would change in a world where metals are even more expensive and rare). $\endgroup$
    – Luaan
    May 5, 2022 at 13:34
  • $\begingroup$ @Luaan I've assumed they're mixing some carbon in with their ore. Bear in mind though, iron oxide is NOT stable much above 1500 deg C. Any oxide gets less stable as temperature gets higher due to the huge entropic value of hot gas. The reason no one ever smelts iron that way is that energy costs in practice are exponential with temperature. With a solar furnace, that might not be an issue. $\endgroup$
    – user86462
    May 6, 2022 at 4:07
  • $\begingroup$ There's probably no reason alchemists couldn't make Tollens reagent, and then make mirrors using the silver mirror reaction. $\endgroup$
    – user86462
    May 6, 2022 at 4:27
  • $\begingroup$ Ooh ooh I ran across a viable, super-IR reflecting gold mirror reaction! Gold, dissolved in aqua regia (forming tetrachloroauric acid), can be reacted with citrates on very clean, polished glass to form gold mirrors with great IR reflectivity! $\endgroup$
    – user86462
    May 6, 2022 at 9:58

I think you are missing a key point in the whole process of smelting iron ores to get iron, that is that the carbon burned in the process does two things at the same time:

  • provides the heat to increase the temperature
  • provides the atomic species which, at the temperatures reached in the furnace, shift the oxygen from being bound to the iron atom, leaving it alone and capable of forming metallic iron

Now, while as you state a solar furnace can reach way beyond the melting point of iron, even by eating up iron oxide to that point you have nothing to remove the oxygen from the melted mass.

Therefore by only having solar mirrors to concentrate solar energy one won't be able to go from iron ores to iron.

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    $\begingroup$ Could a solar furnace at least save some carbon/coal? A bloom uses a huge amount of fuel - hopefully most of that is for heat and the chemical process part only requires a smaller portion. Thank you for the info, I'll have to do some research on how to incorporate carbon into the process. $\endgroup$ May 5, 2022 at 5:01
  • $\begingroup$ @PinkSweetener Probably not all that much. There's a reason why the iron production has always been limited by the availability of wood (-> charcoal), that is until we figured out how to make coke out of coal. I'm not even sure you could use solar to efficiently preheat the blasting air in a blast furnace, for example - you need something that can heat a lot of air very fast. Maybe you could operate it in bunches, preheat some heating fluid that would serve while the furnace actually runs, and then wait until it recharges again. People never did that, though - even though fuel was scarce. $\endgroup$
    – Luaan
    May 5, 2022 at 13:22
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    $\begingroup$ @PinkSweetener However, you have control over what your world is made of. If you find a reducing agent that is plentiful (while fuel itself isn't, especially wood), but doesn't introduce unwanted impurities to the iron, that would work. But then again, if you have a reducing agent, you also have a fuel source :D Like, you can use syngas, for example. But then why not also use that to provide the heat? This isn't avoidable - a reduction agent literally is a fuel. And most of the energy goes toward liberating the oxygen from iron ore, not the heating itself. $\endgroup$
    – Luaan
    May 5, 2022 at 13:26
  • $\begingroup$ @PinkSweetener I think these comments are wrong. For sufficiently small quantities of iron, a solar furnace will save a tonne of carbon. Normally you'd never run a furnace at 1700 deg C using purely thermal decomposition when you can make steel at 1300 deg C using carbothermal reduction. But for a small quantity of steel in a world where there is very little fuel, why not? $\endgroup$
    – user86462
    May 6, 2022 at 4:20
  • $\begingroup$ Note, 'wrong' <> 'silly' or 'unreasonable'. This is good discussion and I'm happy to be corrected. $\endgroup$
    – user86462
    May 6, 2022 at 10:00

Good answers already, and every adds some points to consider, so will I add it too.

@UVphotons' answer is good, but one thing to consider is - forget about 50% efficiency in the case, as there should be no obstructions for light between mirrors and material heated, so thermal insulation in the case can't be done for the side where light enters, and hot material will actively emit energy as old light bulbs did, so as air will cary a lot of it, so as heat transwer in the material itself does not make things better.

At the same time as there is a problem to insulate stuff, there is a problem to protect it from interaction with oxygen, and while the problem can be tackled with some shield gas flows, as it is done in welding torches it requires understanding and practice in few fields - get that shielding gas requires undertanding it is a solution(so chemistry understanding is required, some basics), cryogenics (physics of gases, rectification columns, mechanics to do all the cryogenic stuff, pipes, insulators, etc) which is quite heavy area for not prepared folks even when there are good simple to make solutions for the problem it requires afterthougth knowledge.

  • it possibly to adress two of those problems, insulating and oxidising by introducing some heat carrier - some gas and idk saphire(transparent heating area with high melting point, trough which the heat carrier is blown trough to be heated and then to the ore)

So blumeries aren't that bad for what they do - combining few physical chemical processes in one simple and easy to make setup which a quite straigth forward continuation of processes people used at the time.

And compare it to all kinds of problems which required to be solved, where complexity jumps rigth to 1900's technologies in one setup.

@L. Dutch correctly pointed out that one of the functions of coal there is to reduce oxides to metall, and do the same with solar furnance is not impossible but it requires those top temperatures which you mentiont about modern setups, and just right now without further research I even can't tell may it work in practice, is it enough(seems so, and physics of processes hints it may he possible but specifics are important here - do evaporation points and such are right for it)

And considering that coal is available when wood or plant matter is available by coal pits burning it and it is a renewable resource in many places, not all but most places.

It may make more sense to use solar to purify Iron - main problem of blumery is that Irons does not liquify there in this process(no crusible form it all), and slag goes not float up, does not separate well from the reduced metall, but if you liquify this blumery product with another method (like solar furnance) then such separation may happen.

So in your potencial scenario they are not limited by coal or blumery iron but it then goes to second stage of producing sun iron. In a simple setup one probably will burn back plenty of that blumery iron, but one may get slag free beads of iron, and that iron oxide can be reduced again, and even produce better blumery iron(not necessarly) as it easier to separate iron rust from other stuff as a dust thing. Maybe in a more convoluted way, in reality, ability to melt sticks of blumery iron which just need to drip down in some water bucket(or oil or whatever) may be a process to refine blumery iron.

Historically plenty of eforts were put in this process of refining blumery iron, and only a smal portion of it (best portions) were used for sword making in Japan, when batches were few ton in size, and other time consuming methods like letting it oxydise in earth, moist, water for a decade or more were used.

So people did try all kinds of methods, and solar concentration does not necessarly look that complex and effort hungry and time consuming compared to what people actually did in history to achieve more pure iron.

So I propose instead of melting football balls of ore at once - make candle sticks out of blumery material, and heat a limited spot(its tip), few cm area and let it drip as wax and just supply (horizontaly) shift that blumery candle in the spot. Let it drip in some coolant - fat oil water, then crush the stuff, collect beeds of iron, and then regular heating and smashing the stuff together.

Limiting the area of heating it also possible to create some oxygen free environment a small chamber where it will just burn out, so as slag may protect stuff to some extend(secondary schielding not so important), have some transparent window to the chamber(which may be secondary lens as well (or window and a lens is probably better, or flat window+water(coolant)+lens sandwiche - so you can replace flat window easier clean it easier etc)), no need to supply reducing agent etc.

  • maybe it can be used this way as well to substitute blumery process, candles are pressed sticks of ore and reducing agent(coal dust) and some binding agent(clay)

In this setting it can be a reasonable way(compared to actual efforts which had place in history) and maybe a viable way, which improoves things as quality and even quantity of better iron.

There may be other ways, but it my 2 cent for the case.

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    $\begingroup$ I think from an economics stand point there would be a huge capital investment for any medieval type town to build a large solar furnace, but if it could be built there would be a lot of work to figure out how to prepare the materials, just crushing ore and prepping it before heating would be a big deal. The moving spot would be a big deal if the mirrors were stationary and your candle like approach would solve that. My guess is that in addition to iron there would also be competition from people who want to make better glass which could be done at lower temperatures and likely easier. $\endgroup$
    – UVphoton
    May 5, 2022 at 11:39

The problem with the question is that while as a single problem the solar furnace is something that can be implemented in a pre-industrial society. There are however, several other related problems to solve simultaneously for a society to start producing iron from a solar furnace.

Problems to solve:

  • The overall lens and or mirror system.
  • Keeping the system focused/aimed at a target.
  • Determining crucible tech to contain the ore and other reactants.
  • Determining reasonable chemistry to use depending on source ore.
  • Reasonable temperature control.

Solar furnace

Mirrors have been made in antiquity. Focusing onto a point was a known idea. Other answers address this.

Maintaining focus

This will either require a bunch of people to assist or a mechanical system. Very solvable, but increases overall complexity.

Needs a crucible:

To go from iron ore to iron you need a reducing atmosphere.

Simply putting a lump of iron oxide at the center of a solar furnace will have about the same odds of getting iron as extracting hydrogen from water placed at the focal point.

Might be obvious but it is not mentioned in the question. A crucible would essentially a pottery container, but its additional mass to heat up. Thus increasing time and thus expense over "Just need to heat ore.".

Chemistry to smelt.

Ie what else needs to go into to the iron ore batter before putting in the solar furnace for baking. Probably already known from other traditional processes. But might have to be adjusted for a solar furnace that has less carbon then a charcoal fire.

Temperature Control.

This is a tricky problem. Other traditional processes relied on experience. For a solar furnace with less thermal mass involved it will be easier to over or under heat the crucible. Which will affect properties of the final result.


There are all problems that can and could have been solved in a pre-undustrial society. It has the big negative in terms of it is a novel approach and several problems are needed to be solved together. Not a good combination for adoption of new tech. Never mind pre-industrial tradesmen and guilds tended to be secretive.

  • $\begingroup$ True porcelain crucibles could be used. If you have a solar furnace that can thermally decompose iron oxide (or even just help carbothermal reduction), then you can make porcelain. I'm guessing slag and the movement of the sun will be the biggest problems. $\endgroup$
    – user86462
    May 6, 2022 at 10:06

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