Experimentation and prototypes
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.
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