All About Glass

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The Glass Giant

All About Glass

For the American astronomer George Ellery Hale, bigger was always better. In 1897, at the age of 29, he had become director of Chicago’s new Yerkes Observatory, whose 40-inch refracting telescope remains the largest instrument of its kind in the world.  The lenses of refractors collected and focused light, but Hale found that, at sizes bigger than 40 inches, those lenses would distort under their own weight and absorb too much starlight.  Hale dreamed of probing the mysteries of the universe by applying the principles of physics and chemistry to the stars.  In order to realize this ambition, it would be necessary to build much larger telescopes.

Hale redirected his interest to reflecting telescopes, which permitted the use of mirrors to collect more light and yield brighter images than were possible with refractors.  He started with a 60-inch mirror that was donated by his father.  Later, with the backing of the steel magnate Andrew Carnegie, the 100-inch Hooker Telescope was erected at Mount Wilson, California.  But Hale wanted an even bigger telescope.  In 1928, he wrote an article titled “The Possibilities of Large Telescopes” for Harper’s magazine.  While recounting the successes of the Yerkes and Mount Wilson telescopes, he worried about the restrictions they imposed.  “Starlight is falling on every square mile of the earth’s surface,” he noted, “and the best we can do at present is to gather up and concentrate the rays that strike an area 100 inches in diameter.”

200-inch disk

Having addressed concerns that large mirrors would result in poorer image quality, Hale concluded that “a 200-inch or even a 300-inch telescope could now be built and used to the great advantage of astronomy.”  He even managed to secure a pledge of $6 million from the Rockefeller Foundation to construct a 200-inch telescope for the California Institute of Technology.  The principal challenge was casting a satisfactory blank of glass.  If temperature fluctuations caused the glass to expand and contract unequally, images of the stars would be badly distorted.  What was required, Hale determined, was a type of glass with a low coefficient of expansion.  The mirror of the Mount Wilson telescope was made of plate glass, but that material would not work for a 200-inch mirror.

Experimentation and prototypes

Detail of disk

Hale’s first choice was fused silica glass. From 1927 to 1931, engineers at General Electric in Lynn, Massachusetts, used this material in an attempt to produce mirror blanks.  Their effort was plagued with problems, and it eventually yielded a disk of only 60 inches at an expense of more than $600,000.  Looking for faster results at lower cost, Hale turned to Corning Glass Works and Pyrex®, a kind of glass that was designed to resist heat expansion.

The task of producing the giant mirror blank—then the largest single piece of glass ever made—was entrusted to Dr. George V. McCauley, a physicist in Corning’s research laboratory.  When McCauley, a quiet man, was asked how he intended to cast the glass, he replied, “It will be no different than making a bean pot, except in the methods employed.”  One of those methods called for an elaborate pattern of ribbing on the back of the disk to reduce its immense weight.  Convinced that practice would make perfect, McCauley started his work by building small disks (26 and 30 inches) and moving up to bigger models (60 and 120 inches).

The ribbing was formed by placing silica firebrick cores in the mold.  During the casting process, sufficient glass was poured to fill the spaces between the cores and then to cover them with a solid layer of the desired thickness.  These cores were removed after the disk had cooled, producing holes that gave the back of the glass a wafflelike appearance.  While the cores themselves successfully resisted the tremendous heat of the molten glass, the materials used to anchor the cores to the mold proved to be far more problematic.  The first casting of the 30-inch disk was ruined when the cores broke loose and rose to the surface as the glass was poured.  Firebrick dowels provided enough support to produce a 30-inch disk, but they failed with the 60-inch.  McCauley’s attempt to attach the cores with steel bolts appeared to pay off.  He successfully fashioned 60- and 120-inch disks without incident.

The glass posed problems of its own.  It would not flow through the complex mold unless it was maintained at a much higher temperature than that normally employed with high-expansion glasses.  To address this concern, McCauley devised a series of domed ovens that resembled large igloos.

The First Attempt

Ladling molten glass

At last, it was time to attempt a 200-inch disk. One hundred fourteen cores were anchored to the mold with steel bolts.  A tank 50 feet long and 15 feet wide contained 65 tons of molten glass.  Distrusting mechanical pouring systems for a job this big, McCauley intended to fill the mold from ladles suspended from the ceiling on trolleys. Each ladle contained 750 pounds of molten Pyrex heated to more than 2,700 degrees. 

The pouring commenced early in the morning on Sunday, March 25, 1934.  The observation platform above the pouring room was jammed with onlookers.  One of them was the radio commentator Lowell Thomas, who termed the event the “greatest item of interest to the civilized world in 25 years, not excluding the World War.”  

Every six minutes, another ladle full of glass was dumped into the mold.  But while the pouring teams were taking a break for lunch, someone rushed into McCauley’s office to inform him that one of the cores had broken loose.  Soon, more cores were floating on the surface of the glass.  “Break them up!” McCauley ordered. His workers attacked the cores with long steel rods. Then, fearing that the rods would melt and contaminate the glass, McCauley called a halt to that effort.  The pouring continued through the afternoon.  Finally, after 10 hours of intense work, in which 105 ladles containing a total of 21 tons of glass had been poured, the task was completed.

That evening, the disk was transferred to a special annealing oven.  McCauley had planned to cool the glass very slowly, but since the disk was already damaged, he decided to increase the annealing rate by a factor of 10 to see if the glass could withstand it.  In June, when the cooling was completed, he found that the disk had remained intact.  Today, that disk is on display in the Glass Innovation Center of The Corning Museum of Glass.

The Second Attempt, Grinding and Transportation

Transporting the disk

In an unpublished manuscript titled “Corning Glass Works and Astronomical Telescopes,” written in 1965, McCauley reflected on the options available to him. “It must not be assumed for a moment that all thought of casting another 200-inch disk was abandoned,” he wrote. “While we would have wished to salvage the present one, the work of grinding away the great quantity of glass required to reestablish its intended rib structure seemed very questionable as a cost-saving measure. Besides, no one could predict with certainty that the grinding would be completed without that one single fracture to a rib that would be sufficient to doom the disk to the scrap, or cullet, pile.  Consequently, the necessary preparations were going forward . . . to make another attempt at casting. . .”

On December 2, 1934, McCauley and his team were ready to try again. This time, the cores were anchored to the mold with chrome nickel steel rods, and the cores were hollow so that cool air could be pulled through them to prevent the rods from overheating. Two crews completed the pouring in just six hours.  The completed disk was placed in the electrically heated annealer, where it was to remain for the next 10 months. There was a brief scare during the summer of 1935 when the nearby Chemung River overflowed its banks, forcing a 72-hour power shutdown. A small earthquake caused additional worries. But when the disk was removed from the annealer and tested, no flaws were detected.

The disk was to be ground in Caltech’s optical shop in Pasadena. A special railroad car was built to contain the glass giant, which was padded with rubber sheets and encased in steel. The disk was hoisted onto the flatcar, where it stood upright for its trip west.  That trip took more than two weeks, since the telescope train traveled only by daylight, and only at speeds that never exceeded 25 miles per hour. At each stop along the way, curious spectators lined the rails to catch a glimpse of the disk. About 10,000 turned out in Indianapolis. At night, the train was parked on a siding, illuminated with floodlights, and protected by guards with loaded rifles. When the train arrived safely in Pasadena on April 10, 1936, one local newspaper reported, “There has not been such excitement since Ambler’s Feed Mill burned.”


Dr. Hale died in 1938, while the grinding of the huge mirror was still under way. He would have expected the telescope to be completed early in the following decade, but war intervened, and the nearly completed mirror was not transported to its final home on Palomar until November 1947.

By that time, grinders had used 62,000 pounds of abrasives to remove more than five tons of glass from the disk.  On January 26, 1948, the astronomer Edwin Hubble took the first photographs from the Hale Telescope’s prime- focus cage.  Those photographs, he later reported in Scientific American, “confirmed the most optimistic predictions of [the telescope’s] designers.  They recorded nebulae at least four times as faint, and hence twice as far away, as had ever been photographed before.  This early result was better than we had any right to expect, because the photographs were made at a time when further work still had to be done to bring out the full power of the mirror.  When the mirror is adjusted . . . its range should surpass all advance expectations.”

200-inch disk at The Corning Museum of Glass

And it has. The 200-inch Hale Telescope has allowed astronomers to look farther into the universe than ever before.  On almost every clear night for more than half a century, researchers have used this powerful instrument to extend their studies of objects in our solar system, the Milky Way galaxy, and beyond.

Shortly before his death in 1976, at the age of 93, Dr. McCauley was asked about the future of telescopes.  “If you’re attempting to build a land-based telescope,” he suggested, “you could assemble a group of relatively small mirrors into a single mirror that would be larger than any mirror cast to date.  But for far better results, you could take even a relatively small mirror and send it into space.  Eliminating the distortion caused by Earth’s atmosphere would allow you to see farther and produce sharper images.”  Fourteen years later, McCauley’s belief was confirmed with the launching of the Hubble Space Telescope, which has produced some of the most spectacular space pictures ever taken.



Hale Telescope and The 200" Disk: Prototypes, Failure, and Success: A Selective Bibliography
The Rakow Research Library, The Corning Museum of Glass

  • diCicco, Dennis. "The Journey of the 200-Inch Mirror." Sky & Telescope, v. 71, no. 4, April 1986, pp. 347-348
  • Florence, Ronald.  The Perfect Machine: Building the Palomar Telescope.  New York : Harper Collins, c1994. 451 p (CMGL: QB82.P18 F63)
  • Learner, Richard. "The Legacy of the 200-Inch."  Sky & Telescope, v. 71, no. 4, April 1986, pp. 349-353
  • Rhodes, Richard. "Reflected Glory: How They Built Palomar." American Heritage Invention and Technology, v. 1, no.1, Summer 1985, pp. 12-21.
  • Woodbury, David Oakes. The Glass Giant of Palomar.  New York, Dodd, Mead & Company, 1953. 385 p. (CMGL: QB88 .W88 1953)
  • Yerkes Observatory, University of Chicago:
  • Space Telescope Science Institute:
  • Caltech Astronomy:

Published on October 3, 2011