Fire Thick Cast Ware
Splitting or breaking of pottery inside the kiln usually occurs on larger, thicker wares. They can appear as long, clean, body cracks with sharp edges that could be vertical, horizontal or spiral. It can exhibit itself as simple hairline cracks or ware can fracture into pieces. Some cracks occur during cooling (cooling dunts) but can occur during the heating process (heating dunts). See our separate article for more information on this topic.
To help identify the type of split/crack, it is important to examine the piece. On a glazed piece, if both the pottery and glaze have a clean edge, then the crack occurred upon cooling thereby suggesting the possibility of too rapid or uneven cooling. If the glaze edge is rounded and has flowed into the crack, then the crack occurred during the heating process.
Here are some possible causes:
Inadequate drying: If an item is not adequately dry, it will crack during firing. At the item heats up, water will convert to steam which will cause pressures/stresses.
Firing too fast: Ensure the ramp times are reduced and make sure firings are longer otherwise the item will break during firing. A slower firing will reduce carbon burnout issues too.
Opening the kiln too early: Keep the kiln closed during the cool down process to avoid thermal shock cracks.
Inadequate heat/air circulation: Using a combination of props and stilts, plate cranks or setters will help heat circulation around the larger item. Use a combination of props and stilts (or a plate crank) as this will promote even heat distribution during the heat and cool down sections of the kiln firing process. In our experience, the bisque needs to be raised by at least 1" off the kiln shelf.
Typically, the top of the item will cool much faster than the bottom (because the bottom has the heat from kiln shelf keeping it warm).
Suggestion: raise the item by 1" using a combination of props and stilts (or by using plate cranks - see images on right) as this will promote even heat distribution during the heat and cool down sections of the kiln firing process. Remember that flat or heavy items may “sink” if not appropriately supported during the kiln firing. Do not open the kiln too early - wait until the temperature drops below 100°c.
Rapid increases in temperature may cause heating dunts.
Suggestion: Slow down the heating process (i.e. from 0°c to 600°c, lower the ramp time to 100°c increase per hour or hour and half ). Stack wares carefully to promote uniform heating, cooling and shrinkage. Keep taller items in the centre of the kiln shelf away from direct heat.
Orton Ceramics have provided a PDF document outlining how to achieve the best results CLICK HERE to view.
Cracking and Warping problems? Orton Ceramics have the answers CLICK HERE to view their PDF document.
Cracking and Thermal Shock problems? Orton Ceramics have the answers CLICK HERE to view their PDF document.