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Polished Sheet Metal

3 min read

Where mirror glass is unavailable, aluminium sheet polished with progressively finer abrasives reflects 80-90% of sunlight, against about 95% for glass, and can be formed over a wooden or sand mold. It bends into a curved trough but cannot bend into a bowl without wrinkling, and every scratch scatters light, so use it for troughs and cookers and accept a lower power per square metre.

Part of Building Reflectors from Salvage

Polished sheet metal is a reflector made by smoothing aluminium until it is bright enough to reflect sunlight, then bending it to shape.

What You Lose Compared To Glass

At 1,000 W/m² a square metre of glass mirror at 95% reflects 950 W; polished aluminium at 80-90% reflects 800-900 W. A 2 m² trough of aluminium therefore reflects 1,600-1,800 W against 1,900 W for glass: 100-300 W less. This is a modest loss for a large saving in effort, especially when the sheet is also the structure.

The finish matters beyond reflectance. A surface scratched in different directions scatters light in all directions, so a bright but scratched sheet sends part of its light off the target. Polishing in one direction along the length of a trough keeps the scattered light close to the focal line.

Bending: One Direction Only

A sheet can be rolled into a cylinder or a parabolic trough, because every point can be unrolled flat. It cannot be pressed into a bowl: a bowl needs the metal to stretch or to fold into wrinkles. For a trough, shape the sheet over a wooden or sand mold of the parabola, or fix it to cut ribs (Parabolic Trough). For a dish, use segments or tiles (Mirror Tile Parabola).

What You Need

  • Aluminium sheet — thin enough to bend by hand, thick enough to hold shape between ribs
  • Abrasives, finest last — sandpaper, then polishing compound, then metal polish (parent page)
  • A wooden or sand mold, or ribs cut to the parabola
  • Fasteners or rivets that do not pierce the reflective face
  • Gloves and eye protection

Method: Polish And Form

Step 1: Clean the sheet of grease and oxide.

Step 2: Sand with progressively finer paper, always in one direction along the length. Move through the grades without skipping.

Step 3: Apply polishing compound, then metal polish, until the surface reflects a sharp image of a window.

Step 4: Form it over the mold or ribs, supported along both edges, without kinking.

Step 5: Test the shape with a straightedge and the focus with a card, as for any trough. Keep the sheet protected from fingerprints and sand.

Common Mistakes

  • Polishing in circles. The scatter goes in every direction.
  • Kinks. A crease sends light to the wrong place. Re-roll gently.
  • Using aluminium sheet that is bright but dented. Dents scatter light as widely as scratches.

Safety

Bright metal sends the sun's full beam where you point it

A polished sheet is a mirror: it ignites dry material at its focus, reflects glare into eyes at a distance and burns skin at the focal line. Cut edges are razor sharp, so wear gloves, and aluminium dust from abrasion is a breathing irritant. Cover the reflector whenever you leave it (Cutting & Shaping Metal).

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