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The following is a transcript of a document that was reprinted from THE TELESCOPE, Vol 1, No. 4, June 1934. It was provided to Major deCourcey Brown of Hollywood, California after a request was made to Corning Glass Works. The publicity department graciously sent this article and a summery on Feb 5, 1936. All original documentation is in the files of the Land ~ Sea Discovery Group. It pertains in particular to the making of the 200-inch disc for the Palomar Observatory.

Preparing to Look Farther into the
Universe of Stars

A Pyrex Disk is in the Process of Manufacture for the Mirror of the
World’s Largest Telescope

By Dr. George V. McCauley Corning Glass Works, Corning, N.Y. The curiosity of Alice to see what lives behind the looking-glass may be linked to the desire of the astronomer to see beyond the range of his vision. No matter how remote the star that is added to the “Catalogue of Stars” by each addition to the light gathering power of his instrument he soon yearns for a glimpse of things farther away and pines for a larger telescope. Such has been the story of the development of astronomical telescopes from the days of Galileo up through the centuries.
The demands of the observer have always been met by the skill of the builder until today the world boasts of the 100 inch Hooker Telescope of the Mount Wilson Observatory and is feverishly awaiting the completion of one twice this size, which, from its mountain site at the California Institute of Technology Observatory, will increase thirty fold the present volume of the stellar universe.

The first step in the assembly of this huge instrument is now in progress at Corning Glass Works of corning, N.Y. Here for the past two years in the seclusion of the sharp hills and narrow winding valleys that constitute the beauty of Southern New York State the forces of the industrial firm that supplied the glass envelope for Edison’s incandescent filament to complete the electric lamp and later gave to the world the Pyrex brand of heat-resisting glasses were quickly building and testing each step in the process by the actual production of a new type of telescope disk of ever increasing size. First to be produced in this series was the 30-inch disk, then a 60, then followed the 120-inch, and finally the 200. The latter is at present incomplete but, when finished, twenty tons of the lowest expansion borosilicate glass ever used in telescope mirrors will have been fashioned into a ribbed disk two hundred and two inches in diameter and twenty-seven inches thick; and the whole will have been annealed to a degree of stress no greater than is to be found in the optical parts of field glass.

The various steps in the process of casting a large telescope mirror are in all essentials to the manufacturer of glass articles the same as he would employ in the fabrication of a bean pot or of a locomotive head-light cover glass. He recognizes in either case the need of melting raw materials of sand, soda, and borax to obtain his glass, of transferring the molten glass to a mold of proper size and design, of regulating the cooling of the mass to room temperature to prevent breaking and limit permanent internal stress, and finally, of removing the mold in preparation for inspection and shipment. These processes he ordinarily describes as:”melting,” “working,” “annealing” and “finishing.” The only departure in the procedure from that of producing a bean pot lies in the methods employed for carrying out the various processes and for conveying the heavy disk through the several steps in its manufacture.

The exterior of the 120-inch casting furnace formed by the inverted dome above and the mold rim below. Eight vertical steel members suspend the dome from an overhead support. .
As in the case of most scientific research and engineering achievement and preparations preliminary to the actual casting of the first large disk constituted the major task. First of all there had to be made ready an annealing kiln of sufficient size equipped with accurate and automatic temperature control in which the hot glass blank could be annealed after casting. Then there was the mold to receive the hot glass from the melting tank and give to the disk its shape and size. And finally means had to be provided for moving the heavy load of glass and mold into the annealing kiln and for supporting it there for nearly a year while it cooled at a rate of less than one degree Centigrade each day.

The annealing kiln consists of two huge cans with insulated walls eighteen inches think that telescope one within the other to form an enclosure twenty feet in diameter and approximately five feet deep. One of these cans hangs in an inverted position by a multitude of small thermally insulated rods from an overhead steel structure. The other in an upright position forms the table top of a sixty-ton screw hoist. Exactly centered in this one, on an iron frame too rigid to warp and yet constructed to expand freely in all directions from the center, rests the mold to receive the glass. Four screws four inches in diameter and forth-eight inches long, situated at the corners of a rectangle sixteen by fourteen feet, turn in unison to raise and lower the hoist in and out of the suspended portion of the annealing kiln at the rate of only two inches each minute. A carriage and track beneath the hoist, by means of which the whole can be moved between the annealing and casting positions, completes the equipment for handling the disk.

For the usual type of large telescope mirror, the construction of the mold is relatively simple. A ring of refractory insulating brick is set with the proper height on a base of the same material and bound with flat iron straps. The mold is then ready to receive the fluid glass. For disks of the type requested for the large mirror of the California Institute of Technology Observatory, however, the mold is complicated by the rib structure, which is needed to reduce weight and retain rigidity. By this structure the weight of the two hundred-inch mirror is reduced from the forth-two tons of a solid disk to approximately eighteen tons. The mold for such a disk besides having the outside rim of the simple mold described above had to be fitted with numerous cores. One hundred and fourteen such cores are required to produce the system of straight and circular ribs in the two hundred-inch reflector.


The lower section of the 200-inch annealing kiln testing on the table of the screw hoist. The heating units and mold frame are shown in place.

The making of these cores is an exercise in jig-saw puzzles of three dimensions. With carborundum wheels shapes are cut from standard size insulating brick. These are then cemented together and the exterior of the resulting block is rubbed to the proper slopes and curves to give the desired taper and shape to the cast ribs of the disk. When completed the cores as well as the entire inner surface of the mold are painted with a flour made from sand and mixed with water to give a smooth finish to the casting and to prevent the sticking of the casting to the mold. The cores are now ready to set into place in positions determined by a large wooden template containing the pattern of one-sixth the mold plan. To keep them from floating when the mold is filled with liquid glass, metal rods anchor the cores to the iron plates of the cores to the iron plates of the mold frame below. These anchor rods are then protected with a stream of air drawn around them with an exhaust fan. In this way a negative pressure is maintained in the interior of the cores and the formation of blowholes in the casting is avoided. Since the anchor rods are assembled when cold, their lower ends outside the heated zone are equipped with springs under compression, which enable them to expand without relaxing their downward pull of several hundred pounds on the largest cores.

Because the disk to be made is large and because it is to be made of a very viscous glass it must be kept hot while casting and for some time thereafter. Hence an oven is required in which the mold can be held while it is being filled. A large round dome built of the same material, as the mold suffices for this purpose. To its under rim as it hangs suspended from overhead steel beams, the mold is fitted by elevating with the screw hoist. Thus the mold and dome form a casting furnace into which gas flames are directed from numerous burners around its periphery. Three doors in the side of the suspended dome permit glass to be poured into the mold.

The supply of glass needed for making a large disk is best obtained from a type of furnace commonly known to the glass industry as a tank. It consists of a rectangular pool enclosed on the bottom and sides by heavy clay walls and is covered by a high arched roof of refactory brick under which gases are burned to produce the heat required for the melting process. The raw materials for the glass are introduced into the pool through a door at one end of the structure and the finished glass is worked from the opposite end.
Because of the very viscous nature of the glass being used for these disks the rate of melting is much slower than ordinarily obtained with other glasses. Only four and one half tons can be satisfactory melted each day of twenty-four hours in the particular furnace available. Thus approximately fifteen days are required to fill the tank with its sixty-five tons of molten glass. Still another six days are needed for fining, which is the name given by glassmakers to the process of freeing the liquid mass from bubbles of gas.
Adding ten more days as the necessary time to safely heat the large furnace to a temperature of 1575 degrees Celsius where melting can proceed, a total time of one month elapses in preparing the glass for the mold.

With all preparations made and the melting done all is in readiness for the casting. This is accomplished by means of long handled iron ladles suspended from trolleys on overhead monorails and guided by a crew of workmen between the tank and the casting furnace. The ladles are filled after being chilled in a vat of cold water, by insertion through doors in the tank in an inverted position and immersion in the fluid glass with a combined dipping and rotating motion about the ladle handle as an axis. In this manner seven hundred and fifty pounds of glass are removed from the tank at a single operation. As the ladle is withdrawn from the pool of glass its point of suspension automatically shifts forward along the handle and equilibrium of load and counter balance is maintained for the workmen. The sheet of glass clinging to the rim of the ladle as it comes from the tank is broken away by a workman using for this purpose an iron rod resembling the stove poker of fifty years ago. Then along the monorail with another workman spraying its rim with cold water from a pressure tank carried on his back the ladle is guided to a door in the casting furnace and its cargo poured into the mold. As each ladle is emptied approximately three hundred pounds of glass adhere to its walls. This “ladle skin” as it is generally known in the glass industry is then dumped into a wheelbarrow and conveyed to the filling door of the tank or to a chilling vat of water and then to a storage for subsequent remelting as desired.

After the mold is filled by successive ladles of glass the whole mass is heated in the casting furnace to a temperature of approximately 1350 degrees Celsius for several hours to rid the glass of any large bubbles that may have been introduced during the pouring operation. The disk in its mold is then allowed to cool to approximately 800 degrees Celsius when it is lowered with the screw hoist and transferred to the annealing kiln.

The 120-inch disc crated and anchored to its car ready for shipment. This disc arrived safely by freight at Los Angeles, California,. Six days and sixteen hours after leaving Corning, NY. .
The slow regulated cooling necessary for annealing is provided for by electric heaters of the ribbon type that completely cover the inside walls of the two sections of the annealing kiln. Hanging from the roof of the suspended section are one hundred and four of these units. A like number occupy the space in the floor of the section on the hoist, and the equivalent of ninety-six more cover the sidewall of the upper can. Thus when the two sections of the kiln are closed with the mold and disk within, the latter is completely surrounded by an electric heating pad whose temperature is regulated by ten automatic temperature controllers governed by ten symmetrically placed thermocouples over the top, bottom, and side wall of the kiln. The use of direct current controlled reactors makes it possible for the controllers, without the use of switches, to produce variations of five hundred amperes in an alternating current power supply by making and breaking a direct current of less than one ampere. Under the guidance of this equipment operators lower the temperature a fraction of a degree Centigrade each day for a disk of the thickness of the two hundred inch mirror.

At the expiration of ten or eleven months the cooling period is completed. The disk is then removed from the kiln and its mold cleared away. Now for the first time is to be seen the rough blank ready after inspection to be shipped to the optician who must spend several years of painstaking grinding and figuring before the astronomer may direct its broad polished surface into the universe of stars and satisfy for a few years more his desire to look farther into space.


For a 4000 plus word summary covering the final phases of making the 200” Telescope disc for the Palomar Observatory 50 miles north east of San Diego see the marketplace, articles and research section. The downloadable version from original material written by the Corning Glass Works, Corning, New York, Dated February 5, 1936 can be obtained for a small fee, to cover costs.