Updated 09-XII-2018

Prof. Robert L. Coble

This article was written by fellow lamp engineer and collector Edward J. Covington, and originally appeared on his own website of biographical sketches of persons involved in the lamp industry. Following his passing in February 2017, and with kind permission of his family, Ed's words have been preserved here in the hope of maintaining access to his writings for the benefit of subsequent generations.

Dr. Coble pictured with a piece of Lucalox Ceramic

Robert Louis Coble was born on January 22, 1928, in Uniontown, PA, being the son of Gomer Lawrence and Dorothy Marguerite (Phillippi) Coble. On April 14, 1952 he married Joan Walker and they had five children: David W., Eric R., Stefan G., Catherine, and Janet4,8.

Robert Coble earned his bachelor's degree from Bethany College in West Virginia in 1950 and his doctor's degree from the Massachusetts Institute of Technology in 1955. He started to work at the General Electric Research Laboratory in Schenectady, NY in that same year.

Robert Coble's life was closely attached to MIT, where he went to school and later researched in the area of ceramics for a large part of his career. However, between 1955 and 1960 he sojourned to Schenectady where he started his career with a major development development and success. He worked with a group that was assembled by Joseph Eldrid Burke (1914-2000). Burke's earlier work with aluminum oxide helped to point Coble's research in the right direction. From a Smithsonian site10 it is pointed out that this material (aluminum oxide) turned out to make a good container for highly reactive sodium and it could withstand high pressure. The end result of Coble's research was an aluminum oxide tube (containing small amounts of magnesia) that had adequate translucency to result in a marketable high pressure sodium lamp.

In 1960 Coble returned to MIT as an assistant professor of ceramics in the Department of Metallurgy. In 1962 he became an associate professor and then in 1969 a full professor. Upon retirement in 1988 he became professor of ceramics, emeritus.

Quoting from a news release from the MIT News Office6:
"His honors included election to the National Academy of Engineering in 1978. He was a Fellow of the American Ceramic Siciety and a member of the Ceramic Education Council and the National Institute of Ceramic Engineers. He was awarded the Institute's Professional Achievement Award in 1960 and the Raytheon Award for 'Outstanding Ceramist of the Year' in 1976.

"In 1974, he went to Japan as a visiting scientist and lecturer under an appointment by the Japan Society for the Promotion of Science. He took a year's leave from MIT in 1975-76 to program planning for materials sciences with the United States Energy Research and Development Administration."
Details of the working career of Robert Coble at GE and MIT were presented in 1994 by Handwerker, Cannon and French7. The article reveals personal characteristics of Coble that publications alone cannot convey. It is recommended reading for those interested in the makeup of this scientist. Of great value in their retrospective is a compilation of 85 of Coble's 114 (or so) published articles.

The life of Robert Coble was cut short in 1992 when he drowned off the coast of Maui in Hawaii, where he had a residence. The body of Coble was cremated in Hawaii5.

Contribution to the High Pressure Sodium Lamp
Useful inventions sometimes result from scientific explorations in the materials sciences. A good example concerns the work performed by Dr. Robert L. Coble when he was employed at the General Electric Research Laboratory in Schenectrady, New York between the years 1955 and 1960.

One of the most efficient light sources developed in the past was used extensively in lighting highways in Europe. It was a low pressure sodium vapor design that emitted a yellowish light, which, while not suitable for interior use, could be used outdoors. In part the lamp consisted of glasses and glazes that had to be developed in order to resist the corrosive nature of sodium. The sodium pressure in the lamp was of the order of several microns.

The origin of the high pressure (about 200 torr of sodium) lamp of today can be found on a Smithsonian website9 as well as in the book by Waymouth2 and one by Elenbaas3. Quoting from Elenbaas:
"...an interesting lamp may be made with a high-pressure sodium-vapour discharge. It combines an efficacy of 100 lm/W or more with color-rendering properties which are good enough for many applications. The difficulty of finding a tube material that can stand sodium vapour at a temperature of about 1000°C, was solved by G. E. Inman of the American General Electric Company; he used 'Lucalox', a highly sintered aluminium oxide (with a small addition of magnesium oxide). This material is sodium-proof at the required temperature, and is about 90% translucent at a thickness of 1 mm. Tubes with a 1 mm wall are thus suitable for making high-pressure sodium-vapour lamps.."
It was in this development of a translucent aluminum oxide envelope that Coble was able to succeed. The word "Lucalox" is a registered trademark of the General Electric Company. The following comments were made by Louden and Schmidt1, in a 1965 article in the Illuminating Engineering, of a 400-watt high-pressure sodium discharge arc tube.
"The arc tube is sintered, high-density polycrystalline alumina manufactured by a process that promotes controlled grain growth. The alumina tubes produced by this process are translucent and have a total transmission of light in the visible region greater than 90 per cent. The translucent alumina is highly resistant to alkali vapor at high temperatures. As a comparison, a high-pressure sodium arc operating in a conventional quartz (silicon dioxide) arc tube would cause chemical darkening of the quartz by forming sodium silicate in less than an hour of operation. The translucent alumina shows no attack even after 10,000 hours of operation."

Lamp-Related Patents
  1. US 3,026,210 - Transparent Alumina and Method of Preparation - 1962-03-20
  2. CA 681,744 - Transparent Alumina - 1964-03-10
  3. SE 300,379 - No title available - 1968-04-22
  4. DE 1,278,318 - Verfahren zur Herstellung von Polykristallinen Formkörpern aus Sinterkorund - 1968-09-19

  1. "High-Pressure Sodium Discharge Arc Lamps", W.C.Louden & K.Schmidt, Illuminating Engineering, Vol.60, 1965, pp.696-702.
  2. "Electric Discharge Lamps", J.F.Waymouth, The M.I.T. Press, Cambridge, Massachusetts, 1971, pp.196-205.
  3. "Light Sources", W.Elenbaas. Crane, Russak & Company, Inc., New York, 1972, p.194.
  4. "Robert Louis Coble", Supplement to Who's Who in America, 45th Edition, 1989-1990, Marquis Who's Who, Macmillan Directory Division, p.217.
  5. "Obituaries - Robert L. Coble, MIT Professor, Researcher in Ceramics; at 64", The Boston Globe, Sep 2 1992, p27.
  6. "Professor R. L. Coble Dies" http://web.mit.edu/newsoffice/1992/coble-0916.html
  7. "Robert L. Coble: A Retrospective", C.A.Handwerker, R.M.Cannon & R.H.French, Topical Issue of Alumina of the Journal of the American Ceramic Society February 1994 Vol.77, pp.293-298. See also -
  8. "Robert Louis Coble", Who Was Who in America with World Notable, Vol.11, 1993-1996, Marquis Who's Who, p.55.
  9. http://americanhistory.si.edu/lighting/20thcent/invent20.htm#in3
  10. http://americanhistory.si.edu/lighting/scripts/s20b.htm