I am baking my first loaves of sourdough and following the LaBrea Bakery book. I have a great starter that is over two weeks old and the bread looks fabulous when I put it in the oven. The problem I am having is with the baking times/temp...it is suggested in the book to start at 500 then knock it down to 450 after putting the bread in. After 5 mins, you are supposed to close the oven and leave it for a full 20 mins at 450. After 20 mins, you are supposed to cook it for an additional 25 minutes. However, when I open the door after 20 min. to check it it looks burnt. The crust on my last loaves were so tough I could barely cut through it. What am I doing wrong? I am about to cook my last loaf for the day so if anyone has a suggestion, please let me know!