introduction: is crazing just a crack in the glaze or a crack in pottery's safety?
while there's a time and place for the unique aesthetic of crazing, such as in decorative pottery that's not intended for food use, it's important to distinguish between artistic intent and functional safety. this post isn't about bashing intentional crazing, which can add character and beauty to ornamental pieces. instead, we'll focus on understanding why, when it comes to tableware and other food-related pottery, crazing is a concern that goes beyond mere appearance.
section 1: what is crazing?
crazing refers to a network of fine cracks that can appear on the surface of glazed pottery. these cracks are usually superficial, affecting only the glaze layer, and do not necessarily indicate a structural failure of the piece. crazing is often easily visible, forming a distinct pattern that can sometimes be felt with a fingertip. since crazing isn't always immediately apparent, potters should consider using a magnifying glass to inspect their foodware pieces as part of their quality control routine, ensuring any defects are identified before the items are delivered to customers.
section 2: why crazing makes pottery not safe for food
bacteria growth: the intricate network of cracks in crazed pottery can harbor bacteria and other microorganisms, posing a health risk. these tiny crevices are difficult to clean thoroughly, allowing bacteria to accumulate and potentially contaminate food.
difficulty in cleaning: due to the fine nature of the cracks, it's challenging to ensure all residues and bacteria are removed during cleaning. this issue is particularly concerning for items that are frequently used and washed, like dinnerware.
potential for leaching: crazing can raise the risk of leaching, which occurs when chemicals from the glaze or clay body migrate into food. this concern is heightened if the pottery is crafted with glazes or materials not safe for food contact. therefore, it's advisable to consistently use food-safe glazes for foodware.
section 3: causes of crazing
thermal expansion mismatch: crazing often occurs when there's a mismatch in the coefficient of thermal expansion between the clay body and the glaze. this mismatch creates stress in the glaze during temperature changes, leading to cracking. for more advanced potters who create their own glazes, it's crucial to calculate the coefficient of thermal expansion for both the clay and glaze to ensure compatibility and reduce the likelihood of crazing. this practice, while more complex, is essential for those committed to custom glaze creation.
inappropriate glaze fit: choosing a glaze that doesn't align well with the clay body can lead to a poor fit and crazing. it's essential for potters to ensure the glaze expansion properties match those of the clay. this is where creating test tiles becomes invaluable. by making glaze test tiles using your specific clay body, you can assess how a glaze will adhere to your clay before applying it to the final piece. this step can save you both time and disappointment if the glaze doesn't perform as expected.
rapid temperature changes: exposing pottery to sudden temperature changes, such as moving it from a hot oven to a cold surface, can cause crazing due to thermal shock. I include a note about this in the care instructions for each of my pieces online, as it's an easy mistake that can ruin an otherwise perfect item, and it's not always something that people might think of.
aging: over time, environmental factors and repeated use can lead to crazing as the materials in the pottery expand and contract with temperature and moisture changes.
section 4: crazing vs crackle
it's important to distinguish crazing from crackle glaze. while both crazing and crackle glaze involve a network of fine lines, the key difference lies in their intention and impact: crazing is a term often used for an unintended and potentially problematic result in pottery, compromising its safety for food use, whereas crackle or crackle glaze is a deliberate artistic technique, designed to create a distinct and controlled pattern without affecting the pottery's functionality.
conclusion
crazing is more than just a superficial crack in the glaze; it's a significant crack in the safety of pottery, especially when used for food. this phenomenon raises serious concerns regarding bacteria growth, cleaning difficulties, and the potential for harmful substances to leach into food. by understanding and addressing the causes of crazing, potters can ensure their creations are not only aesthetically pleasing but also safe for everyday use. so, while crazing might add a certain character to pottery, it's clear that when it comes to food safety, it's an issue that cannot be overlooked.
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