ASTM C829-81(2022)

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

SIGNIFICANCE AND USE
3.1 These practices are useful for determining the maximum temperature at which crystallization will form in a glass, and a minimum temperature at which a glass can be held, for extended periods of time, without crystal formation and growth.
SCOPE
1.1 These practices cover procedures for determining the liquidus temperature (Note 1) of a glass (Note 1) by establishing the boundary temperature for the first crystalline compound, when 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.  
Note 1: These terms are defined in Terminology C162.  
1.2 Two methods are included, differing in the type of sample, apparatus, procedure for positioning the sample, and measurement of temperature gradient in the furnace. Both methods have comparable precision. Method B is preferred for very fluid glasses because it minimizes thermal and mechanical mixing effects.  
1.2.1 Method A employs a trough-type platinum container (tray) in which finely screened glass particles are fused into a thin lath configuration defined by the trough.  
1.2.2 Method B employs a perforated platinum tray on which larger screened particles are positioned one per hole on the plate and are therefore melted separately from each other.2  
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Jan-2022
• ICS: 81.040.10 - Raw materials and raw glass
• Technical Committee: C14 - Glass and Glass Products
• Drafting Committee: C14.04 - Physical and Mechanical Properties

Relations

• Refers: ASTM C162-23 - Standard Terminology of Glass and Glass Products
• Effective Date: 01-Oct-2023
• Refers: ASTM C162-05(2015) - Standard Terminology of<brk type="line"/> Glass and Glass Products
• Effective Date: 01-Nov-2015
• Refers: ASTM C162-05 - Standard Terminology of Glass and Glass Products
• Effective Date: 01-Oct-2005
• Refers: ASTM C162-05(2010) - Standard Terminology of Glass and Glass Products
• Effective Date: 01-Oct-2005
• Refers: ASTM C162-04 - Standard Terminology of Glass and Glass Products
• Effective Date: 01-Jun-2004
• Refers: ASTM C162-03 - Standard Terminology of Glass and Glass Products
• Effective Date: 10-Jul-2003
• Refers: ASTM C162-99 - Standard Terminology of Glass and Glass Products
• Effective Date: 10-May-1999

Overview

ASTM C829-81(2022): Standard Practices for Measurement of Liquidus Temperature of Glass by the Gradient Furnace Method is an internationally recognized standard developed by ASTM. This document outlines reliable practices for determining the liquidus temperature of glass-the temperature boundary at which the first crystalline compound forms and below which glass may crystallize upon extended exposure. Knowing the liquidus temperature is crucial for glass manufacturers and researchers because it directly impacts glass stability, processing, and performance.

Key Topics

• Liquidus Temperature: The standard defines liquidus temperature as the point where crystals begin to form when a glass is subjected to a specific thermal gradient. Accurately measuring this temperature helps determine the minimum holding temperature to avoid unwanted crystallization.
• Gradient Furnace Method: The procedure employs a controlled temperature gradient over the length of the glass sample, allowing for clear observation of the onset of crystallization.
• Methods A and B:
  • Method A uses a trough-type platinum container (tray) with finely screened glass particles fused in a thin layer.
  • Method B involves a perforated platinum tray, placing larger glass particles one per hole. This method is preferred for highly fluid glasses, as it reduces thermal and mechanical mixing.
• Thermal Equilibrium: The glass sample must remain under the temperature gradient long enough to ensure equilibrium between crystalline and glassy phases, ensuring accuracy of results.
• Microscopic Evaluation: Once treated, glass specimens are microscopically examined to detect crystal formation and determine the temperature at which it initiates.
• Precision and Calibration: The standard stipulates a repeatability of within 10°C between duplicate tests and allows for equipment calibration against recognized reference materials (e.g., NIST SRM 773).

Applications

• Glass Manufacturing: Industrial glass producers use these practices to optimize melting and forming processes, selecting appropriate operating temperatures to prevent devitrification and ensure consistent product quality.
• Quality Control: Laboratories apply these methods to characterize new glass compositions and to verify that production batches meet liquidus temperature criteria, minimizing the risk of crystallization during storage or use.
• Research & Development: The standard is essential for researchers developing innovative glass materials. Understanding the liquidus boundary enables precise formulation adjustments for improved stability and functionality.
• Product Safety and Performance: By identifying safe temperature ranges, manufacturers can improve the durability and reliability of glass products in fields ranging from construction to electronics.

Related Standards

• ASTM C162: Terminology of Glass and Glass Products - Provides definitions for terms used throughout glass standards, including terms referenced in ASTM C829.
• NIST SRM 773: Certificate for Liquidus Temperature - Offers a reference for calibrating apparatus used in liquidus temperature measurements.
• ISO Standards and WTO TBT Principles: ASTM C829-81(2022) adheres to principles for developing globally accepted standards, facilitating international trade and regulatory alignment.
• Other relevant standards in the ASTM C14 series may address specific methods or properties related to glass testing and analysis.

Conclusion

The ASTM C829-81(2022) standard is an essential resource for anyone involved in glass manufacturing, quality assurance, or material science research. By providing standardized practices for measuring the liquidus temperature of glass using the gradient furnace method, it helps ensure product consistency, safety, and optimal performance. Compliance with this ASTM standard facilitates efficient glass processing, supports technological innovation, and promotes international regulatory confidence in glass products.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C829 − 81 (Reapproved 2022)
Standard Practices for
Measurement of Liquidus Temperature of Glass by the
Gradient Furnace Method
This standard is issued under the fixed designation C829; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 These practices cover procedures for determining the 2.1 ASTM Standards:
liquidus temperature (Note 1) of a glass (Note 1) by establish- C162Terminology of Glass and Glass Products
ing the boundary temperature for the first crystalline
2.2 Other Document:
compound, when the glass specimen is held at a specified NIST Certificate for Liquidus Temperature, SRM 773
temperature gradient over its entire length for a period of time
3. Significance and Use
necessarytoobtainthermalequilibriumbetweenthecrystalline
and glassy phases.
3.1 Thesepracticesareusefulfordeterminingthemaximum
temperature at which crystallization will form in a glass, and a
NOTE 1—These terms are defined in Terminology C162.
minimum temperature at which a glass can be held, for
1.2 Two methods are included, differing in the type of
extended periods of time, without crystal formation and
sample, apparatus, procedure for positioning the sample, and
growth.
measurement of temperature gradient in the furnace. Both
methodshavecomparableprecision.MethodBispreferredfor
4. Apparatus
veryfluidglassesbecauseitminimizesthermalandmechanical
4.1 The apparatus for determining the liquidus temperature
mixing effects.
shall consist essentially of an electrically heated gradient
1.2.1 Method A employs a trough-type platinum container
furnace, a device for controlling the furnace temperature,
(tray) in which finely screened glass particles are fused into a
temperature measuring equipment, and other items listed.
thin lath configuration defined by the trough.
4.1.1 Furnace:
1.2.2 Method B employs a perforated platinum tray on
4.1.1.1 Method A—Horizontal temperature gradient, electri-
which larger screened particles are positioned one per hole on
cally heated furnace, tube type, as illustrated in Figs. 1-3 and
the plate and are therefore melted separately from each other.
described in A1.1.
1.3 This standard does not purport to address all of the
4.1.1.2 Method B—An alternative furnace detail employing
safety concerns, if any, associated with its use. It is the
pregrooved Al O cores and dual windings, as illustrated in
2 3
responsibility of the user of this standard to establish appro-
Figs. 4 and 5, and described in A1.2.
priate safety, health, and environmental practices and deter-
4.1.1.3 Equivalenttemperaturegradientconditionsmayalso
mine the applicability of regulatory limitations prior to use.
be obtained with furnaces having multiple windings equipped
1.4 This international standard was developed in accor-
with separate power and control, or a tapped winding shunted
dance with internationally recognized principles on standard-
with suitable resistances. For high precision, temperature
ization established in the Decision on Principles for the
gradients in excess of 10 °C ⁄cm should be avoided.
Development of International Standards, Guides and Recom-
4.1.2 Furnace Temperature Control:
mendations issued by the World Trade Organization Technical
4.1.2.1 Method A—A suitable temperature controller shall
Barriers to Trade (TBT) Committee.
be provided to maintain a fixed axial temperature distribution
over the length of the furnace.
4.1.2.2 Method B—Arheostatshallbeusedtosupplypower
totheouterwinding.Aseparaterheostatandcontrollershallbe
These practices are under the jurisdiction of ASTM Committee C14 on Glass
and Glass Productsand are the direct responsibility of Subcommittee C14.04 on
Physical and Mechanical Properties.
Current edition approved Feb. 1, 2022. Published March 2022. Originally For referenced ASTM standards, visit the ASTM website, www.astm.org, or
approved in 1976. Last previous edition approved in 2015 as C829–81(2015). contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
DOI: 10.1520/C0829-81R22. Standards volume information, refer to the standard’s Document Summary page on
From NBS Research Paper RP2096,Vol44,May1950,byO.H.GrauerandE. the ASTM website.
H. Hamilton, with modification and improvement by K. J. Gajewski, Ford Motor Available from National Institute of Standards and Technology (NIST), 100
Co., Glass Research and Development Office (work unpublished). Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C829 − 81 (2022)
NOTE 1—See A1.1 for further description.
1. Outer shell (stainless steel) 7. Outer protection tube
4 5
2. End plate (Transite) 8. Sil-O-Cel insulation
3. End plate (quartz) 9. Control thermocouple (platinum/rhodium)
4. Stand 10. Heating element wire
5. Inner protection tube 11. Specimen tray
6. Heating element tube
FIG. 1 Liquidus Furnace (Method A)
Material: 26-gauge stainless steel
FIG. 2 Liquidus Furnace Shell (Method A)
Millimetres
No. of Turns
A: 6 turns—4.8 mm spacing
B: 13 turns—9.5 mm spacing
C: 5 turns—6.4 mm spacing
D: 24 turns—4.8 mm spacing
FIG. 3 Recommended Liquidus Furnace Winding (Method A)
usedfortheinnercorewinding.Thebasicfurnacetemperature within 0.5°C. In addition to control thermocouples, MethodA
level is achieved by controlling power to both inner and outer requires an unshielded supported thermocouple for insertion
core windings. The slope of the gradient is achieved by into the furnace chamber to determine temperature gradients,
adjusting power input to the outer core winding only. The and Method B requires five thermocouples mounted in the
establishedtemperaturegradientisthenmaintainedbycontrol- specimen support fixture as shown in Fig. 6. An alternative
ling power to the inner core winding only. method is to attach (spot weld) the thermocouples to a fixed
4.1.3 Temperature-Measuring Equipment— Furnace tem- platinum or platinum alloy plate which supports the tray or
peratures shall be measured with calibrated Type R or S perforated plate. A solid-state digital thermometer capable of
thermocouples in conjunction with a calibrated potentiometer, the measurement accuracy specified may be used for tempera-
orothercomparableinstrumentation,capableofmeasurements ture measurement.
C829 − 81 (2022)
NOTE 1—See A1.2 for further description.
1. Stainless steel shell 7. Inner heating element tube
2. End plates (Transite ) 8. Perforated platinum tray
3. End seals (Fiberfrax ) 9. Mullite tube of riding device
4. Insulating cover (Fiberfrax ) 10. Alumina spacers
5. Refractory or Sil-O-Cel insulation 11. Controlling thermocouple
6. Outer heating element tube
FIG. 4 Liquidus Furnace (Method B)
FIG. 5 Liquidus Furnace Heating Cores (Method B)
NOTE 1—Hottest thermocouple positioned at forward edge of cut-away section of mullite tube.
FIG. 6 Specimen Support Fixture (Method B)
4.1.4 Microscope—Amicroscopecapableofresolutionofat 4.1.5.3 Reshaping die for trough-type boats (see Fig. 8).
least 5µm at 100× is required. A petrographic microscope is 4.1.5.4 Stainlesssteelmortarandpestle.(The stainless steel
preferredforeaseofcrystalidentificationunderpolarizedlight. must be magnetic.)
4.1.5 Additional Equipment for Method A: 4.1.5.5 Sieve,U.S.Standard,No.20(850µm)withreceiver
4.1.5.1 Laboratory stand to support thermocouple horizon- pan.
tally (see Fig. 7). 4.1.5.6 Small horseshoe magnet.
4.1.5.2 Trough-type platinum boats (see Fig. 8 and Annex 4.1.5.7 Glass vials with covers.
A2). 4.1.5.8 Graduated measuring rod.
C829 − 81 (2022)
6. Procedure
6.1 Method A—Fill to one-half to three-quarters full two
specimen trays that are free of cracks, pits, or adhering glass
with the crushed glass specimen. Distribute evenly over the
length of each tray. Place the filled trays in the furnace, one on
either side of the maximum temperature point, and locate so
thattheircentersareatthepredeterminedgradienttemperature
level corresponding to the liquidus temperature, if known.
Record the location of the trays in the furnace. Either the
single- or the double-core furnace may be used. Modify the
FIG. 7 Thermocouple and Support (Method A)
double-core furnace design to accommodate two samples by
providingtworidingdevicesandmeansforinsertionfromboth
ends of the furnace.
4.1.5.9 Stainless steel tongs.
4.1.5.10 Other minor items as described in the text. 6.2 Method B—Use one or two perforated specimen trays
4.1.6 Additional Equipment for Method B: that are free of cracks, pits, or adhering glass. Using the
4.1.6.1 Riding device for simultaneously holding and posi- pointed stainless steel tongs or tweezers, select chips of the
tioning multiple thermocouples and a perforated platinum tray. samplefromtheNo.12(1.70mm)sieveandplaceoneineach
This device is provided with leveling screws, a means for of the drilled holes in each tray. Position a tray in the cut-away
lateraladjustment,andapositivestopforpreciselylocatingthe sectionofthemullitetubeontheridingdevicewiththedouble
boat and thermocouples within the furnace. The device shown rowofholesforward(towardthehotend),andtheforwardend
in Fig. 9 meets these requirements. of the tray indexed precisely over the most forward of the five
4.1.6.2 Perforated platinum trays (see Fig. 10 and Annex thermocouples against the forward edge of the cut-away
A2). section, as shown in Fig. 4. An alternative method is to move
4.1.6.3 Stainless steel mortar and pestle. thefurnaceintopositionaroundafixedtray.Onesampleinone
4.1.6.4 Sieves, U.S. Standard, No. 8 (2.36mm) and No. 12 tray supported by one riding device may be tested in the
(1.70mm) with receiver pan. double-core furnace. Two samples may be tested simultane-
4.1.6.5 Glass vials with covers. ously by modifying the furnace design to provide for insertion
4.1.6.6 Stainless steel pointed tongs. frombothends.Carefullyfeedtheridingdevicecontainingthe
4.1.6.7 Other minor items as shown in illustrations and tray into the furnace until the prepositioned stop plate is
described in the text. contacted. Close the end opening of the furnace around the
riding device with suitable insulation.
5. Preparation of Test Specimens
6.3 Treatment Time—Leave the specimens in the furnace
5.1 Select a mass of glass of approximately 70g. Break the
until equilibrium between the crystal and glassy phases is
sample into pieces of a size that will fit into the mortar. Clean
established. The time required is a function of the glass
the sample with acetone, rinse with distilled water, and dry.
composition. Twenty-four hours is sufficient for many glasses,
Clean the mortar and pestle, sieve, and magnet in the same
butsomeglassesmaytakedaystoreachequilibrium.Complete
manner (Note 2). Crush the sample, using the mortar and
crystallization of the specimen indicates insufficient tempera-
pestle, by using a hammer or other suitable means.
ture in heat treatment. Total lack of crystallization indicates
insufficient time or excess temperature.
NOTE2—Fromthispointon,contactwithbarehandsorothersourceof
contamination must be avoided.
6.4 Temperature Gradient—Determine the temperature gra-
5.2 Method A—Pour the crushed sample onto a No. 20
dients over the lengths of the specimens at the end of the
(850µm) sieve. Retain the material not passing the sieve and
heating period just prior to removal from the furnace.
repeat the crushing procedure until all the glass has been
6.4.1 Single-Core Furnace—Establish a temperature profile
reducedtoasizetopassthroughthesieveintothereceiverpan.
over the length of each tray by using a traveling unshielded
With the test specimen still in the pan, move the magnet
Type R or S thermocouple supported horizontally as near the
throughout the specimen to remove magnetic fragments that
top of the trays as practical and centered over their widths.
may have been introduced during crushing. If not to be tested
Start the probe at the hotter end of each tray, toward the center
immediately, place the specimen in a covered glass vial or
of the furnace, and make successive temperature readings
other suitable container.
along the tray length at ⁄2-in. (12.7 mm) intervals. Allow the
thermocouple temperature to stabilize in each position as
5.3 Method B—Pour the crushed sample onto a No. 8
indicated by constancy of temperature over a period of time.
(2.36mm) sieve fitted over a No. 12 (1.70 mm) sieve and
Record the temperature of each thermocouple position to the
receiver pan. Retain only that part of the sample not passing
nearest 1°C as related to tray position, and
...