The enamel coating of a Dutch oven will crack or shed if exposed to high heat directly. This issue is simply because the enamel should not be in contact with "dry heat" when heat comes in contact without a layer of oil or butter as an absorbent conductor. If you want to preheat a Dutch oven, you can pour some oil or place some butter in the bottom of the pan before placing it in the oven or on the stovetop.