Sentence:  [W]hen the enamel is thick and a little <WSD>underfired</WSD> it will be lighter in color and have a pitted surface. Going further, extremely thick areas that are well <WSD>underfired</WSD> will be blistered, cracked, or bubbly. 
Meaning of the word marked with <WSD> tags:  Fired at a low (or excessively low) temperature.