Glass turned violet in color due to long exposure to sunlight is thermoluminescent when heated to temperatures above 100°C. Thermoluminescence was also observed in some samples free from color while others showed no luminescence. Effects similar to those for sunlight were also observed from glasses exposed to the carbon or mercury arc and in some cases for exposures of only a few minutes. Special glasses were made from a chemically pure zinc borate glass to which was added a trace of one of the following metals: barium, calcium, strontium, magnesium, manganese, aluminum, nickel, cobalt, chromium, silver, cerium, thallium, and thorium. Each of the above was exposed to sunlight and tested for thermoluminescence, the most intensely luminescent being thorium, cerium, silver, chromium, cobalt, and manganese. The following curves were determined: growth of luminescence with time of exposure, both for sunlight and the carbon arc; effect of concentration of solute; decay of thermoluminescence and periodic changes of luminescence as dependent upon time after excitation. All light measurements were made with a polarization photometer.
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