Callie and timbo pretty much have it, but I wanted to add a few more thoughts. As identified, the two main causes of bloating are 1) insufficient burnout of organic materials in the clay, (I will come back to this below), and 2) metals in the clay which will not burn out at any temperature.  These metals are mostly harmless at standard bisque temps, but at the higher temps experienced during the glaze firing these metals can melt and form gaseous pockets in the clay.  From a