ASTM C338 Standard Test Method for Softening Point of Glass

Get Certified ASTM C338 Testing for Reliable Glass Thermal Property Evaluation

ASTM C338 testing helps manufacturers accurately determine the softening point of glass. Accurate softening point readings are beneficial for process optimization, thermal processing, material consistency, and manufacturing high-quality glass products with predictable thermal characteristics. 

• Measures the softening temperature of the glass materials
• Determines the temperature at which the glass reaches the specified deformation during controlled heating.
• Evaluates the viscosity-related behavior at high temperature.
• Facilitates the processing and forming of glass.
• Provides accurate and repeatable softening point measurements for quality control and material evaluation.
• ASTM C338 applies to many types of glass products such as soda-lime, borosilicate, optical, laboratory, and specialty glass. 

What are the Uses of ASTM C338 Test Standard?

ASTM C338 is used to determine the softening point of glass and to assess the thermal processing properties of glass in a wide variety of ways. The results are used to optimize manufacturing processes, improve product quality, and verify the thermal performance of glass materials. Common applications include:

• Measurement of softening point of glass materials.
• Determining the softening point of different glass compositions for material comparison and process control.
• Assisting with quality control in glass production.
• Setting up appropriate forming and processing temperatures.
• Comparing the thermal characteristics of different glass formulations.
• Assisting manufacturers to make glass products that can maintain the same thermal performance. 

What is Softening Point Testing in ASTM C338?

Softening point testing determines the temperature at which glass reaches the specified deformation corresponding to its characteristic viscosity under the conditions defined in ASTM C338. The temperature is a reference point in glass processing and manufacturing operations, and it represents the glass’s characteristic viscosity (about 10⁷·⁶ poise). 

Why is Softening Point Testing Important for Glass?

The softening point is an important thermal characteristic that affects the glass-forming, shaping, and high-temperature properties of the glass. Knowledge of softening temperature enables manufacturers to set appropriate processing conditions, assess thermal stability, compare various glass compositions, and ensure reliable service during processing and use. 

How Does ASTM C338 Measure the Softening Point?

ASTM C338 test method that determines the softening point of a glass by measuring the temperature at which a specified 235 mm portion of a round fiber of the glass, nominally 0.65 mm in diameter with a specified tolerance, elongates at a rate of 1 mm per minute in the upper 100 mm under the effect of its own weight in a specified furnace at a heating rate of 5 °C per minute with a tolerance of 5 °C per minute.

Common Challenges and Troubleshooting

Some factors that can affect the accuracy of ASTM C338 measurements include improper specimen preparation, specimen dimension variations, inaccurate furnace temperature control, too fast a heating rate, contamination, and deformation observation errors. Homogeneous specimens and calibrated temperature measurement systems, controlled heating rates, and continuous deformation monitoring are used to enhance repeatability.

ASTM C338 Testing Technique, Process, and Data Collection

Glass specimens are prepared to the required dimensions, cleaned, and then put in a calibrated high-temperature furnace. Technicians raise the furnace temperature at the specified rate and monitor the specimen for deformation throughout the test. The temperature at which the specified deformation is observed is noted as the softening point. A test report will generally contain the identification of the specimen being tested, the dimensions of the specimen, the heating rate, furnace conditions, the softening point temperature, the observations made during the test, and details of the calibration of the laboratory furnace.