The clay is terra cotta. It is volatile between 04 and 02, becoming dramatically more dense and darker colored. The electronic controller was set for cone 03, but it has fired flat (right-most cone), that means the kiln went to 02 or higher. At 02 the body has around 1% porosity, pushing into the range where the terra cotta begins to decompose (which means lots of gas expulsion). The outsides of some pieces blistered badly while the insides were perfect (because they got the extra radiant heat). The firing was slow, seven hours. Too long, better to make sure ware is dry and fire fast. Another problem: The potter was in a rush and the pieces were not dry. These factors taken together spelled disaster.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
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