Cheap mugs work best due to the inexpensive glaze used. Put mugs in COLD oven, turn oven to 450. The glaze needs to re-melt to incorporate the marker into it! AFTER oven reaches temp, bake for 30 minutes. Turn oven off. LEAVE MUGS IN CLOSED OVEN (not yelling, just important). Cool for 15 min, slightly crack oven open and let cool over night. This should properly incorporate the ink with the glaze.