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Continuous Perfection: Optical-Quality Glass

All About Glass

They make glass. By day and night, the fires burn on … and bid the sand let in the light.
     — Carl Sandburg, In Reckless Ecstasy, 1904.

To see the unseeable: the quest is unending. But lenses and prisms are only as good as their glass. Optical-quality glass must be flawless. Even tiny flecks, streaks, or bubbles can cause distortion.

With World War II approaching, optical glass was in great demand, and making it was a slow, crude process. The ingredients were put in ceramic pots, melted, stirred, then cooled. Only about 10 percent of the glass was usable. Corning was among the glassmakers struggling to mass-produce optical glass in a continuous process.

Physicist Charles DeVoe knew that the problem holding back the mass production of defect-free glass was in the stirring. It was impossible to see what was going on inside a hot glass tank, so he built the world’s first oil model of one. He used what he learned to design a delivery system with a platinum paddle-stirrer that thoroughly mixed the glass as it left the tank. By 1945, he had fitted his stirrer to an electric melting tank. Exceptionally high-quality optical glass flowed from the tank in a continuous stream.

In a constant flow

optically pure glass

  1. Purified raw ingredients and recycled optical glass—called batch—are fed into the tank, which is lined with refractory ceramic. An electric furnace, sometimes aided by additional gas heat, turns them into molten glass.
  2. The molten glass flows into a platinum fining chamber. There, it is heated even more, causing the bubbles in it to grow larger, rise to the surface, and break.
  3. After passing into a delivery tube, the molten glass is homogenized—thoroughly blended—by a DeVoe-type platinum stirrer with curved paddles.
  4. A continuous stream of optically pure glass emerges from the platinum delivery tube at a steady rate, night and day.

Not good enough

flawed glass

This piece of glass doesn’t meet the criteria for optical quality because it contains defects:

Stones: tiny chunks or pieces of unmelted raw materials, small pieces of the melting tank that break off and contaminate the glass, or crystals that grow in the molten glass.

Seeds: bubbles that form during the melting process and become trapped inside the glass.

Striae or cord: streaks that aren’t completely removed if the hot glass isn’t thoroughly mixed.

The Corning Museum of Glass
This article was originally published in Innovations in Glass, 1999, pp. 34–35.

Published on October 25, 2011