Standard Test Method for Annealing Point and Strain Point of Glass by Fiber Elongation

The annealing point of a glass is defined as the temperature at which a round fiber, nominally 0.65 mm in diameter, elongates under a load of 1.0 kg at a rate of 0.14 mm/min when it is cooled at a rate of 4°C/min. The strain point is determined by extrapolation of the annealing point data as the temperature at which the elongation rate is 0.0316 times that at the annealing temperature. A representative sample of 50 gm or more of flame workable glass in pieces a minimum of 5 mm in diameter is required. Fritted or ground samples must be remelted to obtain a piece large enough from which fibers can be drawn.

$300 per glass to 800°C
$300 per glass for fiber preparation


Significance and Use

This test method provides data useful for (1) estimating stress release, (2) the development of proper annealing schedules, and (3) estimating setting points for seals. Accordingly, its usage is widespread throughout manufacturing, research, and development. It can be utilized for specification acceptance.


1.1 This test method covers the determination of the annealing point and the strain point of a glass by measuring the viscous elongation rate of a fiber of the glass under prescribed condition.

1.2 The annealing and strain points shall be obtained by following the specified procedure after calibration of the apparatus using fibers of standard glasses having known annealing and strain points, such as those specified and certified by the National Institute of Standards and Technology (NIST) (see Appendix X1).

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.


Copyright Notice

Extracted, with permission, from ASTM C336 Standard Test Method for Annealing Point and Strain Point of Glass by Fiber Elongation, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the standard may be purchased from ASTM International, phone 610-832-9555, e-mail:, website:


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