Scope:

ASTM C338-93 is a test method that measures the ‘flow points’ or temperature at which the viscosity of glass reaches 10⁵ poise. A glass fiber of 0.65 mm diameter and 235 mm length begins to elongate under its own weight at the rate of 0.1mm/min as the upper 100 mm of the glass fiber is being heated in a particular furnace at the rate of 5 ± 1° C/minute. ASTM C338-93 helps identify the unique changes in glass composition and thus is essential to maintain the desired composition. It also helps to evaluate the melting point of the glass.

Procedure:

ASTM C338-93 uses a flow point apparatus to identify and evaluate the glass’s softening point (flow point). The device has the following components: glass sample, current contactor, small incandescent lamp, plastic flag that provide active load, platinum-rhodium heating coil, transformer, photocell, variac, voltmeter, amplifier, and clock. The transformer powers the platinum-rhodium heating coil. The variac actuates 50 Volts to it, and the reading is taken from the voltmeter. A plastic flag (partially blackened) is attached to the given sample that provides the active load. It hangs between the photocell and the small incandescent lamp and thus blocks the light. The signals produced can be studied on the amplifier that receives signals from the photocell.

Specimen Size : 

For ASTM C338-93, a glass fiber of 0.65 mm diameter and 235 mm length is used.

Data:

The data is interpreted from graphs produced on the amplifier.

Conclusion:

ASTM C338-93 is a test method that measures the ‘flow points’ or temperature at which the viscosity of glass reaches 10⁵ poise.