The World’s Largest Piece of Glass

The 200-inch telescope blank moving eastward on Market Street near Pine Street intersection. Photo courtesy Corning Inc. archives

While the Great Depression enveloped the country in the 1930s, a great scientific challenge faced what is now Corning Inc.: how to cast the largest piece of glass to date for the world’s largest telescope.

The 20-ton, 200-inch diameter disc made of industrial Pyrex took months and months to cool and years and years to grind into a fine optical device, a reflector. It would be fitted into a telescope – the Hale Telescope on Mount Palomar, 85 miles south of Pasadena, California. The telescope would see four times farther and peer into a volume of space 300 times larger than any existing telescope.

When Corning announced the project, NBC radio newsman Lowell Thomas called it “the greatest item of interest to the civilized world in 25 years, not excluding the World War.”

It did pique interest and curiosity. The first attempt at casting it on March 25, 1934, at Corning Glass Works drew over 5,000 scientists, astronomers, newswriters, science magazine authors and representatives of the motion picture industry.

The top floor of the local Baron Steuben Hotel (now home of the Corning Tourist Information Center), back then known as the Corning Club, became the press center. The journalists ranged from seasoned veterans from The New York Times, The Wall Street Journal and Popular Science magazine, to a 14-year-old cub reporter (me) with my boss, Joe Hayes, from the Elmira Advertiser and Sunday Telegram. These were the predecessors to the current Elmira Star-Gazette.

For the company, then known as Corning Glass Works, the demanding task provided an opportunity to learn from and interact with the research and development scientists and the glassworkers on the Main Plant blowing-room floor in the factory.

It took two attempts in 1934 to successfully cast the specialty pieces of glass that measured nearly 17 feet across. It took six months to cool it down, and it wasn’t until April 10, 1936, that the huge piece of Pyrex glass, transported by a special NYCRR railway car, arrived at its destination in California, the Mount Palomar Observatory Optical Laboratory.

There, the flat front face would be ground into a concave mirror surface, a feat that was interrupted by the attack on Pearl Harbor (and World War II) and wasn’t completed until December 1945. It wouldn’t arrive at Mount Palomar Observatory until 1947, and it was dedicated there as a telescope mirror for the Hale Telescope in 1948.

The mold for the large disc was set up on the floor of the “A” Factory blowing room and housed in a specially designed fire brick structure shaped like a giant beehive. Around the sides of this structure were three large openings through which the molten borosilicate compound, code-named Pyrex, was poured in to build the final mirror up to a thickness of about 25 inches.

The “doghouse,” as it was called by the floor workers, was a short distance from the 75-ton glass melting tank from which the molten Pyrex was drawn. An overhead set of rails carried three large ladles of glass to the mold.

For the project, the late Dr. George McCauley, a company physicist and the director of operations, had bought three huge iron ladles originally made for pouring steel in the Pittsburgh area. A veteran of the company melting department, Charley Wilson, was chosen to handle the actual pouring of the disc, using the large ladles as his carrying device.

To fill the mold, more than 65 tons of glass were melted in the nearby tank and heated to about 2,700 degrees Fahrenheit, a process that took more than two weeks before the pouring.

Wilson, protected from the intense heat by an asbestos apron, gloves and a makeshift wooden mask, controlled the path of the giant ladles, each of which held 750 pounds of liquid Pyrex. He positioned the filled ladles over the three pouring holes in the “doghouse” every six minutes. To maintain constant temperatures in the mold, only the first 400 pounds of glass in each ladle were used, with the remainder returned to the melting tank for reheating.

Pot-like ceramic cores were attached to the bottom of the mold, which formed ribs on the back of the disc to which metal fasteners would be attached. The fasteners mounted the disc-turned-mirror on the telescope.

Early in the operation, it was noted that a few of the cores had broken loose from stress on the overheated metal anchors and the weight of the glass, and they had floated to the surface.

At first, the problem with the cores was considered minor, but more broken cores surfaced after lunch. On March 27, 1934 (two days later), newspapers reported that 15 cores had broken away, and while the company called the first casting “95 percent successful,” a decision was made to pour a second disc nine months later.

The company considered what happened in the first pouring to be a necessary, though unhappy, learning experience, and two seasoned employees were called upon to help remedy the cause. One was Walter Smith. At 99 years old, he is the only company official who worked on the project that is still alive today. Smith came forth from his job as an electrical engineer in manufacturing to design a better method to anchor the core. The second casting on December 2, 1934, proved successful.

Only a small group of onlookers was on hand for the second attempt. Charley Wilson again handled the pouring. He was greeted with much applause and cheers from his fellow plant workers.

At the end of the day, Charley was the celebrity interviewee for local newsmen. Some noted that this time Wilson was wearing a different face mask. A plant official explained that Charley had had his teeth pulled a few weeks earlier and was unable to hold the mask in place with his mouth holding, a spool attached to the inside of the mask. The new mask was just a board with a glass-covered opening for his eyes, stapled to a baseball catcher’s mask to hold it in place. Not fancy but efficient, “it served the purpose,” Charley told the interested newsmen.

After the first pouring, the still-cooling disc was moved to a nearby large oven for final annealing to prevent cracking throughout the mirror. The annealing oven had 304 electrical heating elements to control the cool-down, according to newspapers and science magazines.

Several hundred glassworkers, company officials and area residents were on hand when the Pyrex disc (the “good one”), insured by Lloyd’s of London for $100,000, left the Main Plant entrance March 26, 1936. The train hauling the disk traveled only during daylight, was restricted to 25 mph and moved along advertised routes, drawing thousands of onlookers along the way. It arrived in California on April 10.

The first failed disc was not forgotten. It was initially used as a tourist attraction to the city’s business district. It was sponsored by the local Chamber of Commerce and eventually housed in an observatory-shaped building in what is now known as Centerway Square adjoining the Baron Steuben Hotel building. Then, a few years later, the big piece of glass was packed up and carefully transported across the river to become the centerpiece in the newly opened Corning Glass Center/Museum of Glass in 1951 (then opened to mark the 100th anniversary of the company).

Since 1951, thousands of tourists and dignitaries (including President Eisenhower, Nelson Rochefeller, Tom Dewey, and the best of Broadway and Hollywood) have stood in awe at the giant piece of glass as they began their tours of the Glass Center and the Glass Museum and Library tracing the 5,000-year history of glassmaking.

I am glad that I, as probably the dean of upstate New York reporters, editors and corporate public relations writers, am still around to “pen” this headline-grabbing news event in the world of science and industrial development.


by Donald Bonnell
Donald Bonnell, a lifetime resident of Corning, is a veteran in the media world. From 1933 to 1951, he worked as a cub reporter, sports writer and photographer in the Cornings News Bureau office of Gannett’s then Elmira Advertiser and Sunday Telegram, now both published as the Elmira Star-Gazette. In 1951, he joined the new public relations department of Corning Glass Works as a writer, editor, photographer and spokesman to newspapers and magazines across the country.


Of all the hundreds of news events that he covered between 1933 and 1985, nothing stands out more to him than the casting by Corning Glass of the world’s largest piece of glass to date for the world’s largest telescope.