I鈥檝e discovered that glasses with a thick base crack around the bottom when cleaned in the dishwasher. Why is this?
鈥� When I was a child helping wash dishes, my mother would tell me not to rinse hot glasses with cold water because they might break. I later learned that glass has poor ductility, so stress created by different thicknesses of the material contracting by different amounts can cause it to fracture.
Glass, like many substances, expands and contracts with temperature. Glasses of variable thickness are more likely to break with thermal shock because the thin parts will heat or cool faster and thus expand or contract faster than thicker parts. Dishwashers use water at a much higher temperature (between 70 and 75 掳C) than you would use to hand-wash dishes (between 40 and 45 掳C). When the hot water in your dishwasher hits a cold, thick-bottomed glass, it causes differential expansion, then it cracks.
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Some people use hot water to defrost car windscreens. For the same reason, they may have found it to be a shattering experience. Older windscreens are more likely to break because chips and scratches act as nucleation points for fracturing.
David Muir Edinburgh, UK
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This article appeared in print under the headline 鈥淐lean break鈥�