ASTM C336 Fiber Elongation to check Annealing and Strain Point of Glass

ASTM C336 is used to measure the viscosity of the glass and provides valuable data about the strain points, annealing points, and setting thresholds for seals. It was proposed that the viscosities at the strain point and annealing point should be 1014.50 and 1013.00 poise respectively at 4°C temperature. Viscosity is expressed in poise.

Talk to an Expert

ASTM C336 Fiber Elongation to check Annealing and Strain Point of Glass

TRUSTED BY

Precision-driven testing for dimensional accuracy and compliance

Overview
Scope, Applications, and Benefits
Test Process
Specifications
Instrumentation
Results and Deliverables

Overview

ASTM C336 is a standard test method for determining the annealing point and strain point of glass by measuring the elongation rate of a glass fiber under a defined load at elevated temperatures. These two thermal parameters define the temperature range within which residual stress in glass can be relieved or becomes permanently locked into the material.

This method is fundamental for glass manufacturers and processors to establish correct annealing schedules, prevent thermal stress-induced fracture, and ensure dimensional stability of finished glass products. It applies to a wide range of glass compositions used in optical, technical, architectural, and specialty glass manufacturing where precise thermal processing control is critical.

Scope, Applications, and Benefits

Scope

ASTM C336 covers the determination of annealing point and strain point of glass using the fiber elongation technique. A drawn glass fiber is suspended under a standard load inside a controlled furnace, and elongation rate is measured as temperature is varied.

The scope includes:

Determination of annealing point (viscosity ~10¹³ poise) and strain point (viscosity ~10¹⁴·⁵ poise)
Applicable to any glass composition that can be drawn into fiber form
Testing conducted in a vertical furnace with controlled heating and cooling rates
Fiber elongation measured as a function of temperature under constant load
Suitable for soda-lime, borosilicate, aluminosilicate, and specialty glass types
Used to define and validate thermal processing and annealing schedules

Applications

Establishing annealing schedules for flat, container, and specialty glass
Thermal stress analysis and residual stress control in glass products
Optical glass characterization for precision lens and prism manufacturing
Quality control of glass composition consistency during production
Research and development of new glass formulations and compositions
Validation of glass thermal properties after compositional modifications
Process optimization for glass tempering and heat-strengthening operations
Comparative evaluation of glass types for high-temperature applications
Failure analysis of glass products fractured due to thermal stress
Academic and industrial research on glass viscosity-temperature relationships

Benefits

Directly determines two critical thermal reference points from a single test
Applicable to virtually any drawable glass composition
Requires only a small fiber specimen, minimizing material consumption
Provides viscosity-correlated temperature data for process engineering
Supports accurate design of annealing lehrs and furnace profiles
Enables early detection of compositional drift affecting thermal properties
Highly reproducible results with properly drawn and conditioned fibers
Foundational data for glass viscosity curve development
Supports compliance with optical and technical glass product specifications

Test Process

Fiber Drawing

A uniform glass fiber of specified diameter is drawn from the bulk glass sample for testing.

Furnace Loading

Fiber is suspended vertically in a calibrated furnace with a standardized load attached to its lower end.

Elongation Measurement

Furnace temperature is raised and lowered; fiber elongation rate is continuously recorded as a function of temperature.

Reporting

Annealing and strain point temperatures are determined from elongation rate data and reported in °C.

Technical Specifications

Parameter	Details
Parameters Determined	Annealing point and strain point temperatures
Measurement Principle	Fiber elongation under constant load vs. temperature
Annealing Point Viscosity	~10¹³ poise
Strain Point Viscosity	~10¹⁴·⁵ poise
Fiber Diameter	Typically 0.5–1.0 mm
Applied Load	Standardized dead weight per ASTM specification
Furnace Type	Vertical tube furnace with controlled atmosphere
Reporting Units	Temperature in °C

Instrumentation Used for Testing

Vertical tube furnace with precise temperature control and uniformity
Calibrated thermocouple or platinum resistance thermometer
Dead weight loading system for constant fiber tension
Fiber elongation measurement device (optical or mechanical)
Glass fiber drawing apparatus for specimen preparation
Temperature controller with programmable heating and cooling rates
Micrometer for fiber diameter verification

Results and Deliverables

Annealing point temperature in °C
Strain point temperature in °C
Elongation rate vs. temperature curve
Corresponding viscosity values at both reference points
Comparison against target values for the glass composition
Full test report with fiber dimensions, load applied, and furnace conditions
Calibration records for furnace and temperature measurement equipment
Recommendations for annealing lehr temperature profile where requested

Why Choose Infinita Lab for ASTM C336?

Infinita Lab is a trusted USA-based testing laboratory offering ASTM C336 testing services across an extensive network of accredited facilities across the USA.

Infinita Lab is built to serve the full spectrum of modern testing needs—across industries, materials, and methodologies. Our advanced equipment and expert professionals deliver highly accurate and prompt test results, helping businesses achieve quality compliance and product reliability.

Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote

Frequently Asked Questions

ASTM C336 determines the annealing point and strain point of glass by measuring fiber elongation under a defined load across a temperature range, providing critical thermal reference points for stress relief and annealing schedule design.

The annealing point is the temperature at which glass viscosity reaches approximately 10¹³ poise, where internal stresses relax within minutes. It defines the upper boundary of the annealing range used during controlled glass cooling.

The annealing point defines the temperature at which stress relief occurs efficiently. Accurate knowledge allows manufacturers to design correct lehr temperature profiles, preventing residual stress that causes spontaneous breakage or optical distortion in finished products.

Higher silica and alumina content generally raises both points, while increased alkali oxide content lowers them. Compositional changes during manufacturing directly shift these temperatures, making routine testing essential for process consistency.

A wider gap allows more flexibility in the annealing schedule, giving more time for stress relief across the glass cross-section. A narrow gap requires more precise temperature control during cooling to avoid locking in residual stress.