The softening point of a glass is the most widely used production control parameter.
Changes in the softening point temperature are indications of chemistry changes. According to ASTM C-338 the softening point temperature is the temperature at which a uniform fiber of glass (0.65 mm in diameter x 23.5 cm long) elongates under its own weight at a rate of 1.0 millimeters per minute when the upper 10 cm of its length is heated in a special furnace at the rate of 5°C per minute. This test is performed in a laser optic extensometer system that is manufactured by Orton. The Orton Model LSP Series has been designed to automatically monitor the elongation of the sample fiber and calculate the Littleton softening point according to the ASTM C-338 method.
(Note: If forming a fiber is not desired, is impractical, or impossible, the softening point temperature RANGE can still be determined by the Parallel Plate Viscometer method, ASTM C-1351M.)
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