Fire from Ice: How an Ice Lens Focuses Sunlight to Start a Fire

Starting a fire with a lump of ice sounds like a contradiction, but it is real physics. Shape a piece of clear ice into a lens, hold it up to the Sun, and it can focus sunlight onto a small spot of tinder until it smoulders and catches. The ice stays cold the whole time; it simply redirects the Sun’s energy.

The fire-from-ice video

In his video “How to make Fire from ICE”, Tim Jones demonstrates the trick outdoors. Fire from an ice lens is a favourite demonstration among bushcraft fans because it turns a winter landscape into a source of fire without matches.

How an ice lens focuses sunlight

Light changes direction when it passes from air into a transparent material and out again, an effect called refraction. A convex lens, thicker in the middle than at the edges, bends incoming parallel rays towards a single point. Gather the sunlight falling on a lens the size of your palm into a spot a few millimetres wide, and the energy at that spot becomes high enough to heat dry tinder to ignition.

This is exactly how a burning glass works. Ice does the same job as glass, as long as it is transparent and properly curved. The energy comes from the Sun; the ice only changes where it arrives. That resolves the apparent paradox of “fire from ice”: cold material is not a fuel, but it can still be a very effective optical tool.

Making a clear ice lens

• Start with clear ice. Bubbles and cracks scatter light, so look for clear ice from a frozen lake or stream, or freeze boiled water slowly in a shallow bowl.
• Shape both sides. Use the warmth of your hands, or a knife, to round both faces into a smooth lens that is thicker in the middle.
• Polish the surface. A thin film of meltwater from your gloved hands smooths small scratches and makes the lens clearer.
• Find the focal point. Hold the lens between the Sun and the tinder, and move it closer or farther until the bright spot is as small as possible.
• Use fine, dry tinder. Dark, fluffy material catches far more easily than pale or damp tinder. Work quickly before the lens melts.

The method needs strong, direct sunshine, so it works best around midday on a clear day. Carry a proper lighter or firesteel in the backcountry and treat the ice lens as a skill to practise.

Burning glasses from ancient Greece to today

The idea is ancient. In Aristophanes’ comedy The Clouds, staged in Athens in 424 BC, the character Strepsiades asks whether anyone has seen “a beautiful, transparent stone” that can kindle fire, one of the earliest references to a burning lens. A later legend claims that Archimedes used burning glasses to set the Roman fleet on fire at Syracuse in 212 BC. Historians think a bronze mirror was more likely, and modern tests, including an MIT experiment in 2005, suggest the weapon would have been impractical in battle.

For more about light and ice, see why thick glacier ice looks blue and ice crystals producing a different optical effect.

Related reading

• The Drinks-Can Burner: A Compact Design Demonstration
• A Roof-Snow Release Beside a Ladder
• A Wet T-Shirt Freezes in Dubuque: Temperature and Wind Chill
• A Wood-Splitting Clip That Keeps the Pieces Together

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