Bryan sometimes warping especially using porcelain is caused by kiln shelves with marred or uneven surfaces. Your mug might “catch” on a chunk on your shelf which inhibits the piece’s movement during the glaze firing. I had this problem with cone 10 reduction firings using porcelain and it was solved when I bought advancer kiln shelves. Anyways look carefully at your shelves first before you start monkeying around with the slip recipe. And good luck!