|Monthly Tech-Tip |
The cracks happened on heat up (since they have opened up wide). A combination of issues contribute. The kiln shelves heat-sink the wide flat bottoms, vessel walls are thick, there is some unevenness of wall thickness and only a 30-minute hold at 220F to remove glaze water from the bisque (that could have left dampness in thicker sections). Factors like these combine to produce temperature gradients within the piece. The firing schedule rose rapidly from 250-2100F (400F/hr) amplifying these gradients as it climbed. At quartz inversion these gradients produced a wave of volumetric change moving through the bisqued piece and this likely initiated a crack where stresses met at a combination sharp-contour and thickness-change, the bottom corner.
The difficulty of crying large plates increases dramatically by size and thickness. The better the mixing and wedging, the smaller the piece, the thinner the walls, the more even the wall thickness, the less porcelaneous the clay, the lower the drying shrinkage, the higher the dry strength and the more even the drying the fewer cracks there will be. The poorer the mixing and wedging, the larger the piece, the thicker the walls, the more uneven the cross-section, the more porcelaneous and smooth the clay, the higher its drying shrinkage, the lower its dry strength and the more uneven the drying the more cracks there will be. Do not dismiss the possibility that it could take a month or more to dry this type of piece.
Body Cracking and Dunting During Firing
Ceramic industry can fire much faster and deal with much heavier objects than potters can, how do they do it. The answer is they pay more attention to the basics, it is all common sense and good equipment.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down a temperature window centering on 573C.