Sentence:  <WSD>Underfiring</WSD> results when the kiln doesn't reach the cone temperature at which your glaze matures. <WSD>Underfiring</WSD> can be caused by improper calibration of kiln sitters or computer controllers, firing a kiln too quickly so that not enough time is allowed for the glazes to develop properly, or by kiln elements that are worn and need to be replaced. 
Meaning of the word marked with <WSD> tags:  The state of being insufficiently fired or underfired.