A proofing box (sometimes also called a “proof box”) is sealed space where a baker can control the temperature and humidity in order to proof dough under controlled conditions. Generally, the temperature of a proofing box is kept around 100 degrees F, and the humidity at about 85%. In a professional bakery, a proofing box might look like a large, walk-in cooler. By using a controlled environment, the baker is able to proof dough faster and more predicably than by relying on room temperature.
For the home baker, there are a number of ways of improvising a proofing box. Some people use Styrofoam coolers or aquariums equipped with lights or submersible heaters to produce the desired atmosphere for proofing bread. Others try to control the temperature by putting dough in the oven with the light on. I have found that a simple and reliable way is to use the microwave.
Let me first clear up any misconceptions by saying that I do not run the microwave with the dough in it — ever. I have read about proofing dough in the microwave by actually nuking the dough; this to me is anathema to the whole idea of baking homemade bread.
So, how do you proof bread in the microwave? Start by placing about 2 cups of water in the microwave and heating it for several minutes, until it boils rapidly. Allow the water to continue boiling for a minute to really fill the microwave with steam. Then, working quickly to avoid losing too much of the steam, open the microwave, move the cup off to the side, put your dough in the microwave, and close the door. You have now created a warm, humid environment in which to ferment or proof your dough.
Dough proofed in a microwave “proofing box” will rise more quickly than dough proofed at room temperature, so keep an eye on it. But avoid opening the door too often, as each time you do you will lower both the temperature and humidity. Generally, I leave the dough alone in the proofing box until the minimum time given in the recipe for fermenting or proofing, then check it to see if it has developed sufficiently. For long ferments, you might even want to remove the dough, reheat the water, and then put the dough back in.
So, there you have it. At zero expense and with minimal effort and attention, you can recreate an expensive proofing box in your own kitchen.