Casting the Giant Mirror - Part I

I have been on vacation and have missed a few important Palomar dates that I would like to commemorate. I'll have to put them off for a while because today's anniversary is a big one.

Seventy four years ago today (March 25, 1934) the dedicated crew at Corning Glass Works made their first attempt at casting the 200-inch mirror.

Creating the 200-inch mirror was a monumental task.

Initially General Electric was hired to fabricate a 200-inch disc made out of quartz. Nearly a million dollars later, that idea was abandoned. George Ellery Hale approached the Corning Glass Works of New York with a proposal to instead cast the 200-inch mirror out of a special blend of glass called Pyrex. Changes in temperature make Pyrex expand and contract much less than ordinary glass, so a Pyrex mirror would be much less prone to the focus and distortion problems that plagued other glass mirrors.

The top side of a mirror has to be perfectly shaped, while the back is used for support. The back side of the 200-inch disc is honeycombed. The triangular pieces are hollow areas to make the mirror weigh less, while the round area are places where the mirror is supported from below to help the topside to maintain its critical shape. Making the honeycomb undersurface provided some challenges.

A mold was built inside of an oven (think bottom of a waffle iron) to give the mirror’s underside its special shape. The hollow spots were created by bricks that were held down by steel bolts.

After taking nearly a month to melt enough glass, on March 25, 1934 the molten glass was ladled into the mold. Alas, the glass was hot enough to melt the steel bolts and the bricks floated to the surface and ruined the mirror. This first disc is on display at the Corning Museum of Glass in New York. The successful second casting was performed in December 1934.

The Corning Museum of Glass website has a great article on the making of the mirror called The Glass Giant.

I have seen many wonderful photographs of March & December, 1934 castings. Thankfully it was also captured in two paintings by artist Christian Jacob Walter. Shown above is "Pouring Glass". The other painting is called "Ladling Glass." The paintings are on display at the Earth and Mineral Sciences Museum at Penn State University.

Later this year I hope to make the trek to visit Corning and to see the first disc. It is a part of the Palomar heritage that I need to visit.