Bisque Firing TipsEach time you load your kiln and fire your ware, the geological processes that formed our raw materials in the first place are recreated! That is a pretty cool thing. So today’s tech tip will focus on the physical changes clay undergoes just during bisque firing.
Many potters view bisque firing as just another step in the long process of producing clay work, however, many important events occur during this stage:
Breakdown of organic material
Burnout of sulfur and carbon
Tightening of pore structure
While pots are drying on the ware rack, the atmosphere is absorbing their moisture. Depending on the humidity level, this could take a few hours to several days. A good way to judge if pots are dry enough for bisque is to put the clay up to your cheek. Coldness means that it is still too wet and it should dry some more. Keep in mind that even when the pots feel bone dry there is still physical water in the clay that needs to be released slowly.
The first stage of a bisque firing is the water smoking phase. The heat from the kiln drives off the remaining water still present in the pot. You want to drive off the majority of this water before the kiln reaches the boiling point of 212°F. Otherwise the steam from water boiling inside the clay can be so intense and forceful that it will blow up your pots. Most digital electric kilns have a preheat function that holds the kiln at approximately 180°F. The length of the hold time depends on the thickness of the work. For thin, thrown pottery 1 hour is usually sufficient. Large sculptural work could require up to 12 hours. Check for water vapor by putting a mason jar over a peephole and see if it gets foggy. If so, hold the kiln longer before going any hotter.
Once past the water smoking phase, organic material in the clay starts to break down. Some clay, particularly ball clay and fireclays, can contain organic matter like super fine bits of old roots and twigs. Our materials do come from the ground after all! These vegetative materials start to decompose from 350-400°F. A lot of the raw materials used to produce a commercial clay body are refined and air floated. So while today's potter has relatively clean clay there is still organic material that has to break down.
Around 415°F there is a small expansion of the ware in the kiln. It is very slight and the pots can generally handle the change in expansion without cracking. It is good to know that the shelves and posts also expand slightly. The fact that the pots and kiln furniture expand together helps keep the ware safe.
The next phase of the firing is the most exciting! It is called ceramic change because it is the point of no return for a pot. Starting around 660°F and continuing to 1300°F, the chemical water bound in the raw materials' crystal structure is liberated and the clay particles start to sinter. This water is different from the pore water released during the water smoking stage. A dry bag of ball clay will have a 10-15% water content that is part of the clay's crystal structure. That starts to be released after 660°F. At this stage there is less concern about pots exploding due to escaping steam. Since the pore water has evaporated, the clay has a more open network that allows the chemical water to be removed easily.
An equally important part of ceramic change is the transformation the clay undergoes. It is going from a dry, easily crumbled material to a more permanent form. Remember, we are basically making rocks in our kilns and this is when the action starts. When the kiln reaches 1000°F the pot would be sintered and hard, but also quite porous and easily chipped. The porous nature of clay during this stage allows it to pass though silica inversion and handle the expansion without cracking. Sintering continues as the pore space between clay particles diminishes. It will eventually lead to fusion, then vitrification of the clay into a dense, watertight material. Kind of like a rock.
The next step is the burnout phase. All clays contain carbon and sulfur to varying degrees. It is a natural part of its make up. When the kiln reaches 1290°F, carbon and sulfur start to combine with oxygen and release as a gas. The burnout phase continues for several hundred degrees and it is a good idea to slow down the firing during this phase. Some potters will hold the kiln for an hour or so if they have large or thick work. Also make sure the kiln is well ventilated so the volatile gases can easily escape. Carbon should be burned out by the time the kiln reaches 1650-1700°F. Sulfur should as well, but with some clays it can continue releasing until 2100°F.
If these gases are not released during the bisque, they can cause problems with the glaze firings. Bisque loads that are stacked super tight, i.e. bowls nested 3 or more deep and kilns with poor ventilation, can trap some of the volatile gases. This can lead to all kinds of headaches: pin holing, bloating and black coring. Firing too quickly can seal the surface of the clay enough so that gases get trapped in the core of the pots. When in doubt about the speed of firing, always err on the slow side.
The final stage of a bisque firing starts the clay’s vitrification process. Around 1470°F, fluxes in the clay body start to get excited and fuse. This fusion creates a tighter pore structure within the clay body as it grabs hold of the clay particles. The size of the pot is shrinking as the pores in the clay are shrinking. As the temperature gets hotter, the fluxes get more active and weld the clay particles together creating a durable material that is still porous. Optimal bisqueware is easily handled without breaking but still porous enough to accept glaze. If the vitrification process keeps going, like if your kiln overfires, the clay will eventually fuse enough that it turns into a viscous glass. After all, clay and glazes are made of the same things-just in different ratios.
So when do you stop the firing? Most potters bisque fire from cone 06 to 04 and hold the target temperature for 5-10 minutes to allow the kiln to even out. How you plan to glaze the pieces can determine how high to bisque. If you are working with low-fire clay and glazes, bisque one to two cones hotter than what you want to glaze to. There is less of a chance for glaze problems because the clay has released its carbon and sulfur. For example, if you plan to use Earthen Red with glazes that mature at cone 06, bisque fire to cone 04. For high fire work, like cone 6-10, it is pretty much standard practice to bisque between 06-04.
The last thing to consider for bisque firing is the cool down cycle of the kiln. When the kiln shuts off the temperature drops dramatically. The expansion the clay underwent during the firing is reversible and the pots begin to contract at an even, steady rate. The exception to this is the silica inversion that occurs at approximately 1060°F. Around that temperature, the pots suddenly shrink and then continue on their merry way until they undergo another sudden contraction at 440°F. It is essential to allow the kiln to cool naturally through these temperature ranges. If the kiln is cooled too quickly or if there are drafts from open peepholes, the pots can crack from the sudden stress. Once the temperature falls below 200°F, it is okay to crack the lid.
The transformation of sticky plastic clay to a dense permanent material is endlessly fascinating. I hope you feel that you have a better understanding of what is going on inside your bisque kiln. What takes place during a glaze firing adds a whole other dimension of physical and chemical change!