Sentence:  BUBBLES IN GLAZE may be caused by too heavy an application, or by severe underfiring or <WSD>overfiring</WSD>. […] LOSS OF COLOR IN CHINA PAINTING is a result of <WSD>overfiring</WSD> or mixing too much medium with the paint. Be careful not to <WSD>overfire</WSD> china colors, and if a light shade of a dark color is needed, apply the color very lightly rather than thinning too much. 
Meaning of the word marked with <WSD> tags:  To fire at a high (or excessively high) temperature.