"Crazing" refers to the appearance of a fine network of hairline fractures in the surface of glazed pottery. These cracks do not go all the way through the pottery, and don't affect its basic structure, but they can present an aesthetic problem. Crazing may appear either immediately after firing, or over time, as the glaze ages. Crazing is a common feature of antique pottery, and is actually desirable in some ceramic styles, but is usually considered a problem.
This problem appears as a web-shaped network of fine lines on the surface of the pottery. The initial cracks are thicker and deeper. Next, smaller, finer cracks appear between them. Crazing can render food-safe glazes unsafe, and ruin the look of smooth-glazed pieces. In some extreme cases, crazing can cause small chips of glaze to come off the ceramic surface.
Immediate crazing occurs shortly after a piece is removed from the kiln, and is caused by a glaze under too much tension. When the glaze has a tighter "body" than the clay, it shrinks more quickly, resulting in a patchwork effect of tiny cracks.
Delayed crazing occurs weeks or months after the piece has been fired, and usually gives no warning. It happens most often in ceramics that have suffered from moisture seepage through stilt holes in the glaze, or because the piece was underfired and still contains some water. Expansion and contraction of the clay due to this water content causes the less expansive glaze to crack.
Many antique ceramic pieces develop crazing as they age. It's particularly common in pieces displayed on window sills, which are subject to extreme temperature and humidity changes. Crazing may also occur in older pieces that were glazed with unstable material. Items with no glaze crazing are considered more valuable than objects with heavy crazing, but, according to Gales Antiques, most older pieces suffer from this problem to some degree.
Prevent glaze crazing on ceramics by choosing glazes and clay with similar expansion rates. According to the Digital Ceramics Technical Articles database, ceramicists can adjust glaze expansion rates by using different fluxing oxides. Fire all ceramic pieces fully to ensure that no water remains inside, and hand-wash all low-temperature ceramic material to prevent water from seeping into the clay. Keep display pieces away from windows, doors, fireplaces, heater vents and other areas where temperatures or humidity levels can change suddenly.
Raku ware, ceramic pieces fired using a special Japanese technique, often sport glaze crazing and other features that would be considered flaws in conventional pottery. However, in raku firing, these features are desirable. Ceramicists cool their raku pots suddenly, resulting in fractured glazes, speckling and other unpredictable natural effects. After the cooling process, raku ware is often smoked. This results in tiny black and gray carbon particles trapped in the glaze, further emphasizing any crazing.
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