Firing green ware straight to cone6 in Clay and Glaze Chemistry Posted December 5 · Edited December 5 by terrim8 · Report reply I used to do this a lot in my outdoor gas kiln. It is correct to use a glaze with a significant quantity of clay in the recipe. Just go by the book- slow warm up - (its called candling) even overnight then start the regular firing in the morning. I used recipes specifically for once firing - some even went on as a slip right after trimming. Dry things really well. Everything has to shrink and fit together at the same rate.