A wall of mirrors in the French Pyrenees focuses sunlight to 3,500C, hot enough to melt almost anything

High in the mountains of southern France stands a building that looks like a giant curved mirror. The Odeillo solar furnace gathers nothing but sunlight, and yet it can reach a temperature that would vaporise steel.

The huge concave mirror at Odeillo concentrates sunlight to a single blazing point. Illustration: Watts & Wild.

We usually think of the sun as warm rather than fierce, something to sunbathe under, not to fear.

Gather enough of it onto one tiny spot, though, and sunlight becomes one of the most powerful tools on Earth.

How hot does the Odeillo solar furnace get? The Odeillo solar furnace in the French Pyrenees can reach a temperature of around 3,500 degrees Celsius at its focus, using only concentrated sunlight. That is hot enough to melt or vaporise almost any material in seconds, with no fuel and no flame.

A furnace that burns with sunlight

Completed in 1970, the Odeillo solar furnace is the largest of its kind in the world.

Its most striking feature is an enormous concave wall of mirrors, around 54 metres wide and built from thousands of smaller mirror panels.

That curved wall works like a colossal magnifying glass, bending the sunlight that hits it down to a single point.

The focus is no bigger than a cooking pot, yet it holds a temperature close to a third of that on the surface of the sun.

All of that heat comes from sunlight alone, with not a drop of fuel burned.

How to bottle a piece of the sun

The giant mirror cannot track the moving sun on its own, so it gets help from the hillside below.

There, a field of 63 flat mirrors called heliostats turns slowly through the day to follow the sun and bounce its light onto the big curved wall.

The wall then concentrates all that gathered light by a factor of around ten thousand.

It is a two-step relay, from the tracking mirrors on the slope to the focusing wall and finally to that searing point of heat.

In effect, the whole hillside is arranged to pour the power of the sun into a spot you could cover with your hand.

Hot enough to melt anything

At the focus the temperature climbs to roughly 3,500 degrees Celsius in a matter of seconds.

That is hot enough to melt steel, ceramics and almost any other material we know, and to boil many of them away as vapour.

Because the heat comes from light rather than burning fuel, it is wonderfully clean and pure.

There is no soot, no smoke and no chemical contamination to spoil the sample being tested.

For scientists who need extreme, spotless heat, that combination is almost impossible to get any other way.

What it is actually for

The solar furnace was the idea of the French physicist Felix Trombe, who had built a smaller one at nearby Mont-Louis years earlier.

Odeillo was chosen because it sits high in the mountains at around 1,500 metres, with clean, thin air and roughly 300 sunny days a year.

Researchers use the heat to study how materials behave at extreme temperature, from spacecraft heat shields to advanced ceramics.

It has even been used to test the punishing conditions a capsule faces when it slams back into the atmosphere.

For half a century this mirror has quietly served as a laboratory powered by the sky.

The honest catch

For all its drama, the Odeillo furnace is not a power station and never was.

It produces no electricity, it is a research instrument for making heat, not for lighting homes.

It also only works when the sky is clear, falling idle under cloud or at night.

More broadly, the wider family of concentrated solar technology has struggled to compete with cheap, simple solar panels for generating power.

But as a tool for reaching clean, ferocious heat on demand, this wall of mirrors still has no real equal.

The Odeillo solar furnace is a reminder that the oldest energy source we have, plain sunlight, still hides astonishing power.

It belongs with the other bold attempts to put the sun to work, from the solar tower that stores the sun's heat to run at night to the dream of gathering solar power in orbit.

If a wall of mirrors can melt metal with nothing but sunshine, how much of the sun's power do you think we are still leaving on the table, and what would you build with a furnace like this? Tell us in the comments.