A baking stone is a large, usually thick, piece of stoneware that is often recommended for bread and pizza baking. The instructions usually say to put them on the bottom or middle rack of your oven, then preheat it for an extra-long time before baking. Using a stone is said to improve the crust of both bread and pizza, but often little explanation is given as to why this is the case.
The way that baking stones work is simple. Baking stones absorb and retain heat very well, so when you put them in the oven and allow the oven to run for a while, the stones heat up. They transfer heat directly to the bottom of your bread or other baked good, as baked goods can be placed directly on the stone (or on parchment, then on the stone) so the bread never starts out on a cold surface, as it would when baked on a baking sheet. This allows bread to bake more evenly all over, resulting in a much nicer crust – particularly for artisan-type breads. Since the stones retain so much heat, they also help keep the oven at a much more even temperature and make up for some of the heat that is lost when you open the door to put something in.
You can leave your baking stone in the oven when you’re not using it, especially if it’s on the bottom of your oven. If it’s on a rack, however, you may want to remove it when baking cakes and cookies. The extra heat from the baking stone directly underneath (say, if you’re setting a baking sheet right on the stone) can cause cakes to cook faster and cookies to overbrown on the bottom, so keep that in mind if you do decide to do your regular baking with the baking stone in your oven.
You can buy baking stones at many specialty grocers and kitchen stores, as well as online. I often see unfinished quarry tiles recommended as inexpensive alternatives to commercial baking stones, although an unfinished tile is a bit more likely to become brittle and break-able over time. The thickness of the tile should not have an impact on the results, so choose a stone that fits in your oven well and is light enough that you can handle it easily.