The editors of Cook's Illustrated take their BBQ seriously, and they're sharing their secret to uniform cooking: an aluminium baking pan with water, placed opposite the coals.
It's a hand-me-down trick they have used for a long time, but they recently tested it with three BBQ grills: one without a pan, one with just a pan, and a third with a pan filled with 3 cups of water. Spreading bread slices over the cooler side of each grill, they found that the grills with the pans toasted the bread uniformly while the grill without the pan charred the bread on one side. So why does this happen?
Even putting an empty aluminium pan in the grill will significantly drop its temperature. Aluminium is very efficient at absorbing heat, allowing it to lower the temperature inside the grill by about 30 degrees and to even out hot spots. Water added to the pan captures more heat, helping drop the temperature even further, but it's mainly the metal pan that's responsible for the change. So if you ever put an empty pan in the grill to catch drips, be aware that it will lower the temperature as well.
So if you've been using an aluminium pan to catch the juices dripping from your meat (and thus avoid a grease fire), you've already been doing something right.
Guarantee Perfect Grilling with a Disposable Baking Pan — Here's Why [Cook's Illustrated]