Sentence:  At this point <WSD>overfiring</WSD> begins, as is shown particularly by the volume curve, which indicates decided bloating, so that at 1450°C the clay has about the same volume it had at 1050°C before vitrification took place. At 1450°C it is, therefore, decidedly overfired. However, its excellent burning behavior is apparent from the long temperature range. 
Meaning of the word marked with <WSD> tags:  To fire at a high (or excessively high) temperature.