Step two: The Proof is in the Sponge

Several hours before you plan to make your dough, you need to make a sponge. If you are making bread in the morning, start your sponge the night before. If you are making bread in the evening, start it that morning. What's a sponge? It's nothing more than a bowl of warm, fermented batter, and now that you already have a starter, it'll be easy to make your sponge.

• Take your starter out of the fridge. Pour it into a large glass or plastic bowl. Meanwhile, clean the jar thoroughly. Remember, anything you don't remove will be part of your fermentation process next time around. I usually run mine through the dishwasher; if you wash it by hand, rinse it out with boiling-hot water.
• Add a cup of warm water and a cup of flour to the bowl. Stir it well, and put your sponge in a warm place for several hours. This process is called proofing, which is just another word for fermenting.
• Watch for froth, and sniff. When your sponge is bubbly and has developed a white froth, and it smells a little sour, it is ready. The longer you let the sponge sit, the stronger the flavor will be.

Proofing-times will vary. Some starters can proof up to frothiness in an hour or two. Some take six to eight hours, or even longer. You'll get a sense of how long yours takes, through trial and error. If you're going to bake in the morning, set your sponge out to proof overnight.