You may have heard experienced bakers comment that some ovens "run hot," while others "run cool." What they're really saying is that the thermostats in those ovens are unreliable, and the real temperature of your oven can be substantially different from the temperature you've set. Knowing the real temperature of your oven is crucial to a good result, so it's usually a good idea to test the oven for accuracy and hot spots.
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Testing for Accuracy
Most kitchenware stores sell simple, inexpensive oven thermometers for exactly this purpose. Typical models can either stand on an oven rack or hang from it by a hook. To test your oven, position one in the middle of the oven -- either on a rack, or dangling from it -- in a spot where you can easily read it through the oven's window. Alternatively, use a probe-type digital meat thermometer. Rest the probe on a ball of crumpled parchment paper, or the edges of a silicone baking pan, to keep it from making contact with the metal rack. Set your oven to 350 degrees Fahrenheit, and let it heat for 20 to 30 minutes.
Your oven doesn't arrive magically at its preset temperature and then stop. Instead, it heats up, shuts off and then heats up again. Check the thermometer every 15 to 20 minutes, and make a note of the temperature each time. Once you have four readings, average them out to see your oven's real temperature. If it's off by more than 5 to 10 F, raise or lower your oven's settings accordingly. If you bake a lot, it's worth testing your oven at other common temperatures as well.
Testing for Evenness
Knowing that your oven maintains the temperature it's supposed to is only half the battle. Ovens rely on air currents to distribute that heat, and it isn't always as even as you'd like. Testing your oven for hot spots is a simple process, and can save you a lot of grief in the form of scorched cookies and uneven cakes.
Select the largest baking sheet you can fit into your oven, and cover it evenly with breadcrumbs. Heat the oven to 350 or 375 F, and slide the sheet of crumbs onto the middle rack. Check the crumbs after 10 minutes, then again at five-minute intervals. If your oven is prone to hot spots, you'll see the crumbs brown more rapidly in those places.
A Toast to Hot Spots
Another way to test for hot spots is with several slices of bread. Once your oven has heated, open the door and fill the rack with a single layer of bread, leaving a few inches open at the edges -- and an inch between each slice -- for air flow. Watch the slices to see where they brown and scorch most rapidly.
Once you know where the hot spots are, you can avoid them when positioning cake pans in the oven. Sheet pans are usually too large to avoid trouble spots, but you can minimize their effect by rotating the pans midway through the recommended baking time.