Ceramicware Defects Handbook/Crazing
Crazing
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Descriptions: It is a network of fine cracks spreading throughout the glost ceramic wares. If the pottery body is porous, crazing permits the moisture to enter into pottery articles (such as earthenware) making the ware unhygienic for dining purposes. Some of the craze lines is so fine that it is usually undetected with normal visual inspections.
Solutions:
- The crazing line is usually found more on thicker glaze areas. Therefore, one of the most obvious solutions is to reduce glaze thickness.
- Underfiring in glost firing (Temperature must be over 1100C) as the glaze obtain body thermal expansion that will be sufficient to avoid crazing as with most cases of glaze. If craze appears, refiring the crazed ware should clear the problem provided the glaze can withstand high firing temperatures
- Insufficient heatwork as the firing time is reduced making the glost firing not giving enough heatwork to the ceramic ware. The usual standard firing cycles is around ramping of 100C-120C/hour should suffice to reduce crazing in ceramic ware
- A Combination of low expansion body/high expansion glaze may cause crazing at normal glazes. For body, consider adding cristobalite to induce body expansion. For the glaze, to reduce expansion, consider adding zircon (5% amount) into the glaze to reduce thermal expansion and increase hardness. Also consider replacing soda, potash and/or alumina with zirconia, magnesia, ,zinc oxide or boric oxide.
- Heat shock also causing crazing. Therefore to reduce crazing, it is ideal to open up the kiln only when the kiln is around 150C and below.