Compared to other basic cooking techniques, grilling is hard: the temperatures are high, timing is crucial and slight differences in the thickness or wetness of the food can dramatically affect how quickly it cooks.
Bad design choices by equipment makers — kettle-shaped grills with black interiors, for example — make it harder still. But if you're willing to do some simple arithmetic or break out a roll of foil, you can reduce the guesswork and get better performance from your grill. Similar tricks work for broiling; after all, a broiler is basically just an inverted grill.
Every grill has a sweet spot where the heat is even. You know you're cooking in the sweet spot when all of the food browns at about the same pace. In most situations, the bigger the sweet spot, the better. One notable exception is when you need to reserve part of the grill for cooking some ingredients more slowly or keeping previously cooked food warm.
If you find yourself continually swapping food from the center of your grill with pieces at the periphery, that's a sure sign that your sweet spot is too small.
You can get an intuitive feel for where the edge of the sweet spot lies by looking at the heat from the food's point of view. I mean that literally: imagine you are a hotdog lying facedown on the grill. If the coals or the gas flames don't fill your entire field of view, then you aren't receiving as much radiant heat as your fellow wiener who is dead-center over the heat source. If the falloff in the intensity of the heat is greater than about 10 percent, you're outside the sweet spot.
You can use the table below to estimate the size of the sweet spot on your own grill. The 26-inch-wide gas grill on my deck has four burners with heat-dispersing caps that span about 23 inches. The food sits only three inches above the burner caps, so when all four burners are going, the sweet spot includes the middle 16 inches of the grill. But if I use only the two central burners, which are 10 inches from edge to edge, the sweet spot shrinks to a measly 5.4 inches, too small to cook two chicken breasts side by side. I can use this to my advantage, however, if I have a big piece of food that is thick in the middle and thinner at the ends, such as a long salmon fillet. By laying the fish crosswise over the two burners, I can cook the fat belly until it is done without terribly overcooking the slimmer head and tail of the fillet.
Sweet spots are narrowest on small grills, such as little braziers, kettles, hibachis, and the fixed grilling boxes at a public parks. If the sweet spot on your grill is too confining for all the food you have to cook, you can enlarge it in several ways.
If the grill height is adjustable, lower it. Bringing the food a couple inches closer to the heat can easily expand the sweet spot by 2 to 3 inches. The effect on the intensity of the heat is less than you might expect: typically no more than about a 15 percent increase.
If your grill is boxy in shape, line the sides with foil, shiny side out. Your goal is to create a hall of mirrors in which the heat rays bounce off the foil until they hit the food. A hotdog at the edge of the grill then sees not only those coals that are in its line of sight, but also reflections of the coals in the foil-lined side of the grill.
The foil trick unfortunately doesn't work well on kettle grills because their rounded shape tends to bounce the radiant heat back toward the center instead of out to the edges. But if you can find a piece of shiny sheet metal about 4 inches wide and 56 inches long, you can bend the metal into a reflective circular ring and build the coal bed inside of it. All food within the circumference of the ring should then cook pretty evenly.
Jury-rigging a grill in this way wouldn't be necessary if grills came shiny on the inside and we could keep them that way. But, presumably because nobody likes to clean the guts of a grill, the interiors of most grills are painted black, the worst possible color for a large sweet spot. A black metal surface doesn't reflect many infrared heat rays; instead it soaks them up, gets really hot, then re-emits the heat in random directions.
Someday, some clever inventor will come up with a self-cleaning grill that has a mirror finish inside, and the sweet-spot problem will simply vanish.
HOW BIG IS YOUR SWEET SPOT?
For grills, measure the width of the coals or gas burners (including any burner caps that disperse the heat). Then measure the distance from the top of the coals or burners to the upper surface of the grill grate. Find the appropriate row in the table to estimate the size of the sweet spot, centered over the heat source. This table assumes a nonreflective grill.
To calculate the sweet spot of an electric broiler — which is the ideal vertical distance between the top of the food and the broiler element — measure the distance between the rods of the heating element. Multiply that measurement by 0.44, then add 0.2 inches to the product. For example, if the rods are 2.4 inches apart, the sweet spot is 1.25 inches from the element to the top of the food.
EDITOR'S NOTE: W. Wayt Gibbs is editor-in-chief of The Cooking Lab, the culinary research team led by Nathan Myhrvold that produced the cookbooks "Modernist Cuisine: The Art and Science of Cooking" and "Modernist Cuisine at Home." Their new book, "The Photography of Modernist Cuisine," will be released in October.