Standard Test Method for Softening Point of Glass

SIGNIFICANCE AND USE
This test method is useful to determine approximately the temperature below which the glass behaves as a rigid solid in glass-forming operations and for a control test to indicate changes in composition. It has been found useful for specification acceptance and for providing information in research and development work with glass.
SCOPE
1.1 This test method covers the determination of the softening point of a glass by determining the temperature at which a round fiber of the glass, nominally 0.65 mm in diameter and 235 mm long with specified tolerances, elongates under its own weight at a rate of 1 mm/min when the upper 100 mm of its length is heated in a specified furnace at the rate of 5 ± 1°C/min.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

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Publication Date
31-Mar-2008
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ASTM C338-93(2008) - Standard Test Method for Softening Point of Glass
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: C338 − 93(Reapproved 2008)
Standard Test Method for
Softening Point of Glass
This standard is issued under the fixed designation C338; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3.1.2 Furnace Stand—A means shall be provided for sup-
porting the furnace so that the fiber hangs below it. This stand
1.1 This test method covers the determination of the soft-
must be provided with a leveling device such as three screws.
ening point of a glass by determining the temperature at which
The stand shown in Fig. 1 is convenient when used with either
a round fiber of the glass, nominally 0.65 mm in diameter and
a cathetometer or a telescope and scale.
235mmlongwithspecifiedtolerances,elongatesunderitsown
3.1.3 Heating Rate Controller—Suitable controls shall be
weight at a rate of 1 mm/min when the upper 100 mm of its
provided for maintaining the furnace heating rate at 5 6
length is heated in a specified furnace at the rate of 5 6
1°C/min.
1°C/min.
1.2 The values stated in SI units are to be regarded as
NOTE 1—A continually adjustable transformer has proved effective for
standard. No other units of measurement are included in this controlling the heating rate.
standard.
3.1.4 Temperature-Measuring Equipment—The furnace
1.3 This standard does not purport to address all of the
temperature shall be measured with a calibrated Type R or S
safety concerns, if any, associated with its use. It is the
thermocouple and a calibrated potentiometer capable of mea-
responsibility of the user of this standard to establish appro-
suring the true temperature of the furnace within 60.2°C. The
priate safety and health practices and determine the applica-
cold junction shall be maintained at 0°C by means of an ice
bility of regulatory limitations prior to use.
bath.An alternative acceptable means of temperature measure-
ment is the use of a potentiometer to oppose the thermocouple
2. Significance and Use
electromotive force. This potentiometer shall be set at a
standard setting for the type of glass being measured, and
2.1 This test method is useful to determine approximately
galvanometer deflection shall serve as a means of obtaining
the temperature below which the glass behaves as a rigid solid
relative temperatures, the deflection of the galvanometer hav-
in glass-forming operations and for a control test to indicate
ing been calibrated. Also acceptable for temperature measure-
changes in composition. It has been found useful for specifi-
ment is a solid-state digital thermometer that is capable of the
cation acceptance and for providing information in research
accuracy specified.
and development work with glass.
3.1.5 Fiber-Elongation Measurement Equipment—The fiber
3. Apparatus elongation shall be measured by a device capable of measuring
the position of the end of the fiber within 0.02 mm throughout
3.1 The apparatus for determining the softening point of
the entire elongation period.
glass shall consist essentially of an electrically heated resis-
tance furnace, a furnace stand, a device for controlling the
NOTE 2—Suitable devices that have proved effective for measuring the
heating rate of the furnace, equipment for measuring the
elongation are cathetometers, projection magnifiers, and telescope and
scale combinations.
temperature of the furnace, and equipment for measuring the
elongation rate of a fiber of glass suspended in the furnace.
3.1.6 Timer—A timing device with a least count and accu-
3.1.1 Furnace—The furnace shall conform in all essential
racy of 1 s shall be used.
respects to the requirements shown in Fig. 1.
4. Preparation of Test Specimens
4.1 The fiber specimen used for the test shall meet the
This test method is under the jurisdiction of ASTM Committee C14 on Glass
following requirements:
and Glass Products and is the direct responsibility of Subcommittee C14.04 on
Physical and Mechanical Properties.
4.1.1 It shall be round.
Current edition approved April 1, 2008. Published June 2008. Originally
4.1.2 It shall be smooth and shall contain no void spots or
approved in 1954. Last previous edition approved in 2003 as C338 – 93 (2003).
DOI: 10.1520/C0338-93R08. foreign matter.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C338 − 93 (2008)
FIG. 1 Details of Softening Point Furnace
C338 − 93 (2008)
4.1.3 Its average diameter shall be 0.65 6 0.10 mm and the 6.2 Again, cool and hold the furnace about 20°C below the
maximum diameter shall not exceed the minimum diameter by expected softening point and insert the fiber in the furnace by
more than 0.02 mm over the entire length of the fiber. placing the bead end in the furnace sample holder. Check the
4.1.4 It shall be 235 6 1 mm in length, not including the top fiber to be certain it is hanging free in the center of the furnace
bead. Test fibers conforming to these requirements may be and relevel the furnace if necessary. Standardize the potenti-
drawn by attaching a clean sample of the glass under test ometer
...

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