Butter is sometimes used for frying and sautéing. It has the advantage that its largely saturated fats are resistant to being broken down by heat, and so don’t become gummy the way unsaturated oils do. It has the disadvantage that its milk solids brown and then burn around 250°F, 150° below the smoke point of many vegetable oils. Adding oil to butter does not improve its heat tolerance. Clarifying does; butter free of milk solids can be heated to 400°F/200°C before burning.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee. By permission of Scribner, a division of Simon & Schuster, Inc.