Designation: C336 − 26Standard Test Method forAnnealing Point and Strain Point of Glass by FiberElongation 1This standard is issued under the f i xed designation C336; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 This test method covers the determination of the anneal-ing point and the strain point of a glass by measuring theviscous elongation rate of a f i ber of the glass under prescribedcondition.1.2 The annealing and strain points shall be obtained byfollowing the specif i ed procedure after calibration of theapparatus using f i bers of standard glasses having knownannealing and strain points, such as those specif i ed andcertif i ed by the National Institute of Standards and Technology(NIST) 2 (see Appendix X1).1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards: 3C162 Terminology of Glass and Glass ProductsC338 Test Method for Softening Point of GlassC598 Test Method for Annealing Point and Strain Point ofGlass by Beam Bending3. Terminology3.1 Def i nitions:3.1.1 annealing point, n—that temperature at which internalstresses in a glass are substantially relieved in a matter ofminutes. 4-6 During a test in accordance with the requirementsof this method, the viscous elongation rate is measured by asuitable extensometer while the specimen f i ber is cooling at arate of (4 6 1) °C ⁄min. The elongation rate at the annealingpoint is approximately 0.14 mm/min for a f i ber of 0.65 mmdiameter. 63.1.2 annealing range, n—the range of glass temperature inwhich stresses in glass articles can be relieved at a commer-cially practical rate. For purposes of comparing glasses, theannealing range is assumed to correspond with the tempera-tures between the annealing point and the strain point.3.1.3 strain point, n—that temperature at which the internalstresses in a glass are substantially relieved in a matter ofhours. The strain point is determined by extrapolation of theannealing point data and is the temperature at which theviscous elongation rate is 0.0316 times that observed at theannealing point.4. Signif i cance and Use4.1 This test method provides data useful for (1) estimatingstress release, (2) the development of proper annealingschedules, and (3) estimating setting points for seals.Accordingly, its usage is widespread throughoutmanufacturing, research, and development. It can be utilizedfor specif i cation acceptance.1 This test method is under the jurisdiction of ASTM Committee C14 on Glassand Glass Products and is the direct responsibility of Subcommittee C14.04 onPhysical and Mechanical Properties.Current edition approved July 1, 2026. Published July 2026. Originally approvedin 1954. Last previous edition approved in 2020 as C336 – 71 (2020). DOI:10.1520/C0336-26.2 Calibrating glasses known as standard reference materials (SRMs) are availablefrom the National Institute of Standards and Technology (NIST). See NIST SpecialPublication 260, SRM Program, NIST, Gaithersburg, MD 20899.3 For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at www.astm.org/contact. For Annual Book ofASTM Standards volume information, refer to the standard’s Document Summarypage on the ASTM website.4 Littleton, J. T., and Roberts, E. H., “A Method for Determining the AnnealingTemperature of Glass,” Journal of the Optical Society of America, Vol 4, 1920, p.224.5 Lillie, H. R., “Viscosity of Glass Between the Strain Point and MeltingTemperature,” Journal of American Ceramic Society, Vol 14, 1931, p. 502;“Re-Evaluation of Glass Viscosities at Annealing and Strain Points,” Journal ofAmerican Ceramic Society, Vol 37, 1954, p. 111.6 McGraw, D. A. and Babcock, C. L., “Effect of Viscosity and Stress Level onRate of Stress Release in Soda-Lime, Potash-Barium and Borosilicate Glasses,”Journal of the American Ceramic Society, Vol 42, 1959, p. 330.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1
