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How to Heat a Metal Smelting Crucible With a Fresnel Lens

Fresnel lenses don't look like traditional lenses. Where a traditional lens has at least one surface that curves in a smooth spherical arc, Fresnel lenses are flat sheets of glass or plastic that are ridged on one side. A Fresnel lens is what you'd get if you took the central section of a traditional lens and shaved down the thickness of the flat side, then took a circular ring around the central section and shaved it to the same thickness, then added more rings until the lens is complete. The end result is a lightweight Fresnel lens that collects light very efficiently, which is why it's a good choice for concentrating sunlight.

Things You'll Need

Large Fresnel lens
Tiltable support frame
Cast iron crucible
Welding goggles
Cardboard
Scissors
Temperature resistant bricks

Instructions

- 1
  Assemble the Fresnel lens into the support frame with the ridged surface facing the sun. Because Fresnel lenses are thin, they can curve if they are not supported and the curve will decrease the efficiency of the concentration.
- 2
  Cut a series of small holes in the cardboard. The details are not that important, something like 12 to 15 1/2- to 1-inch diameter holes should work. You just want the holes to let light through different parts of the lens.
- 3
  Set up the lens so it faces the sun and put the cardboard in front of the lens to find the focal length of the lens. Take a piece of paper and put it on the opposite side of the lens. Move the paper along the beams to find the spot where all the beams come most closely together. Measure or mark that location.
- 4
  Tilt the lens so it faces the sun and put the crucible on the bricks so it is one focal length away from the lens. The beams through the holes in the cardboard will help you put the crucible in the right place.
- 5
  Remove the cardboard from the lens. Your lens will now concentrate up to 1,000 watts of power on the crucible, enough to melt metals such as tin, zinc and aluminum.

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