Monday, November 21, 2011
Testing for Thermal Shock and Crazing
Coffee Mugs and Cups are particularly vulnerable to thermal shock and crazing, as hot liquid is poured into them, then when finished they are quickly rinsed under cold water. I am concerned that my pieces remain beautiful for years, and are food safe so I have devised this simple test that I run on each batch of fired cups.
First I fill the cup with ice cubes and water. I bring my kettle up to a full boil, and quickly pour out the iced water and fill with boiling water. I leave in for around 1 minute, then replace the boiling water with ice cubes and cold water again. I repeat this process, and finally leave boiling water in the cup for several minutes. I then carefully examine the interior for any signs of small hairline cracks.
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Do the mugs ever crack or explode? Or is it always hairline cracks? (Is that different than crazing?) My favorite mug has tons of what I think is crazing, but it doesn't seem to be structurally compromised. I'm curious as to what I should search for because I'm forever switching between iced and boiling hot liquids. Thanks.
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