I've made several breads using the Mark Bittman recipe. They've all come out wonderfully . . . except each has been, to varying degrees, burned on the bottom. I've been using Le Creuset pots, at 450 degrees. The only one that wasn't really too dark on the bottom was one that I took out after 15 mins uncovered, and that one I thought could have used a little more time for the rest of the crust. Any suggestions?