Glass Testing

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ASTM C158

Standard Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture)

Flexural strength is determined in three-point or four-point bending on bars of rectangular or circular cross-section. Between ten and thirty specimens per type/brand is recommended by ASTM.

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ASTM C336

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.

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ASTM C338

Standard Test Method for Softening Point of Glass

The softening point of a glass is defined as the temperature at which a round fiber, nominally 0.65 mm in diameter and 235 mm long, elongates under its own weight at a rate of 1 mm/min when the upper 100 mm of its length is heated at a rate of 5°C/min. 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.

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ASTM C598

Standard Test Method for Annealing Point and Strain Point of Glass by Beam Bending

The annealing point of a glass is defined as the temperature at which a 3-point loaded beam, nominally 3 to 4 mm in cross section on a 50 mm span, elongates under a load of between 0.2 kg and 1.0 kg at a rate determined by the span, load, and moment of inertia 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. Representative specimens, nominally 3 to 4 mm in cross section and 75 mm in length, are required.

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ASTM C657

Standard Test Method for D-C Volume Resistivity of Glass

Surface and volume electrical resistivity of glass are determined by DC, as well as by AC methods. Specimen dimensions and electrode configuration are determined on an individual basis.

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ASTM C829

Standard Practices for Measurement of Liquidus Temperature of Glass by the Gradient Furnace Method

The liquidus temperature is the maximum temperature where equilibrium exists between the amorphous glass and its primary crystalline phase. A sample of about 30 gm is crushed to -20 mesh and placed in a platinum boat with dimensions of 0.5" x 0.5" x 6.0". The glass specimen is held at a specified temperature gradient over its entire length for a period of time necessary to obtain thermal equilibrium between the crystalline and glassy phases. The specimen is quenched and viewed with a microscope to determine the glass/crystalline interface location and corresponding temperature.

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ASTM C965

Standard Practice for Measuring Viscosity of Glass Above the Softening Point

The viscosity of glass above the softening point is determined using a platinum alloy spindle immersed in a platinum crucible of molten glass. The crucible is placed in a vertical tube furnace capable of 1600°C. The viscometer is mounted above the tube furnace and is capable of measuring from 1.5 to 5.0 Poise (log10). About 400 gm of glass is necessary for the test. After the data is collected, it is fit to a Fulcher equation to describe the viscosity/temperature relationship.

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ASTM C1350M

Standard Test Method for Measurement of Viscosity of Glass Between Softening Point and Annealing Range
(Approximately 108 Pa·s to Approximately 1013 Pa·s) by Beam Bending (Metric)

The viscosity of glass from 10 to 15 Poise (log10) is determined by measuring the rate of viscous bending of a loaded glass beam. Representative specimens, nominally 3 to 4 mm in cross section and 75 mm in length, are required.

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