Occurs during heat-up of bodies on first fire. All clay bodies contain some water not removed from the drying process. Heating greenware too fast through this range is the most common cause of cracking (ware can even explode if the water cannot escape fast enough through the pores). On the up-cycle, kilns are often held at around 120C. Even though this is beyond the boiling point of water, it has been proven practical and even necessary to remove most of the pore water before continuing with the firing.