A glass baking dish almost always cracks for the same reason: thermal shock. Part of the glass heats or cools rapidly while the neighboring area does not, creating stress that can break the glass. Placing a hot dish on a cold counter, pouring cold liquid into it, or setting it on a wet cloth after removing it from the oven can create that stress suddenly. The dish may shatter rather than simply crack.
Glass expands when heated and contracts when cooled. The problem is that different parts may change temperature at different rates. When an oven-hot dish touches a cold counter, the surface in contact with it contracts within seconds while the inside is still hot and expanded. The resulting stress can exceed what the glass can withstand. Kitchen glass bakeware is made from two types of glass: borosilicate glass expands very little when heated and is notably resistant to thermal shock; soda-lime glass is less expensive but more likely to break under the same temperature difference.
There are several common situations that can cause breakage: placing a hot dish on a wet counter, marble, or metal surface; adding cold water or liquid from the refrigerator to hot food in the dish; moving a dish from the refrigerator directly into a preheated oven; or taking a hot dish straight to the sink to wash it. In each case, the cause is a sudden temperature difference in one direction. Placing a glass baking dish directly on a stovetop burner can also cause breakage; an oven dish is not designed for stovetop use, and its bottom cannot withstand concentrated heat. Thawing a glass dish from the freezer by running warm water over it creates the same kind of temperature shock. Glass cannot withstand a sudden difference in either the heating or cooling direction.
Safe use comes down to a few habits. Set a hot dish on a dry wooden trivet or folded dry towel; use a trivet whenever the counter is cold or wet. Do not place an empty glass dish in the oven; an empty dish is most vulnerable to direct heat. Let a dish taken from the refrigerator sit on the counter for 20–30 minutes before putting it in the oven. Wash it only after it has cooled completely. Adding a little liquid to the bottom of the dish helps distribute heat evenly and reduce stress. Put the glass dish in the oven only after the oven has preheated to the desired temperature; do not leave it inside while the oven preheats.
Another less visible factor is old scratches. Fine scratches left by forks, knives, or steel wool can concentrate stress, and glass often breaks at those points. Do not cut food in glass dishes or clean them with steel wool. A dish with a chipped edge, a visible crack, or a whitened scratch on its surface should never go back in the oven; it may shatter the next time it is used. When stacking dishes, place a cloth between them; glass rubbing against glass can cause scratches.
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