Standard Test Method for Crazing Resistance of Fired Glazed Ceramic Whitewares by a Thermal Shock Method
The resistance to crazing of fired glazed whitewares is determined when subjected to thermal stresses. Specimens are initially heated at 250°F and water quenched. This cycle is repeated three times. If crazing does not take place, then the three cycles are repeated in 25°F increments up to 450°F. Failure is defined as the temperature at which crazing is observed. Testing of five specimens is suggested by ASTM.
Unless there is a proper match between the expansions of the glaze and the body, all glazed whitewares may contain residual stresses from the firing that bonded the glaze to the body. In addition, whitewares are increasingly subjected to thermal stresses in service. Hence, an important use criterion for a glazed whiteware is adequate resistance to repeated abrupt thermal changes. In most cases, the result of inadequate resistance to thermal shock is the appearance of a craze pattern in the glaze. This craze pattern is visible by inspection with oblique lighting and application of a suitable ink or dye.
This test method is applicable to vitreous whitewares that have negligible crazing as a result of moisture expansion. For nonvitreous and semivitreous bodies, refer to Test Method C424.
1.1 This test method covers the determination of the resistance to crazing of fired, glazed, ceramic whitewares when stresses residual after glost firing may cause a tendency to craze, such stresses being induced by factors other than moisture expansion.
1.2 This test is not intended to induce moisture expansion, which fact should be kept in mind if the materials to be evaluated may exhibit moisture expansion.
Note 1-Test Method C424 covers a method for determining resistance to crazing induced by moisture expansion. Its use is generally confined to testing nonvitreous and semivitreous ceramic whitewares because these products may be subject to such expansion. For whitewares with negligible moisture expansion (such as vitreous and impervious ware), the thermal shock method described herein is generally to be preferred.
Extracted, with permission, from ASTM C554 Standard Test Method for Crazing Resistance of Fired Glazed Ceramic Whitewares by a Thermal Shock Method, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the standard may be purchased from ASTM International, phone 610-832-9555, e-mail: firstname.lastname@example.org, website: www.astm.org.
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