Beth

Beth’s Kiln Schedule

Long Bisque Fire

Cone 08

Sculpture loaded in the kiln wet with shrink slab.
Allowed to dry in the kiln at* least* 48 hours- preferably a week.
During this drying period, I also have the lid propped open and a fan blowing in through the propped lid for good air circulation
Kiln is then programmed to rise:

Drying	Rate 1	50° F /hr	150° F	48 hour hold
	Rate 2	50° F /hr	200° F	24 hour hold
	Rate 3	50° F /hr	250° F	12 hour hold
	Rate 4	50° F /hr	300° F	8 hour hold
	Rate 5	50° F /hr	350° F	8 hour hold
	Rate 6	50° F /hr	400° F	6 hour hold
	Rate 7	50° F /hr	460° F	4 hour hold
	Rate 8	50° F /hr	600° F	1 hour hold
Heating	Rate 1	100° F /hr	1000° F	No Hold
	Rate 2	50° F /hr	1250° F	No Hold
	Rate 3	150° F /hr	1780° F	No Hold
Cooling	Rate 4	150° F /hr	1250° F	No Hold
	Rate 5	50° F /hr	1000° F	No Hold
	Rate 6	100° F /hr	600° F	No Hold

So what is actually happening in the kiln?

Below I have compiled a brief description of the different phases of the firing and what is happening to the clay at different temperatures.
*Disclaimer! I collected this information from multiple sources, including Daniel Rhode's Clay and Glazes for the Potter, Val Cushing's Ceramic handbook, and a lot of surfing the web for or glaze and clay calculations. I have yet to find 2 sources that agree absolutely on the exact time and temperature that certain events occur, but that is due largely to the fact that these processes are a function of both Time And Temperature. This is merely a rough guide and some rules of thumb that I use when creating a firing schedule. I am constantly searching for a better understanding of this process, and I will keep updating as I discover new things.

Temperature in Deg C	Temperature in Deg F	Reaction
110	212- 230	Water smoking period. Atmospheric “pore water” is driven off. Clay object should now be dry. Most ‘explosions’ happen between 150-300 ° F. A good 24-36 hour soak at 200 ° F will usually dry most work sufficiently For really thick pieces (solid forms), I usually soak at 150 ° F for 24 hours, and then go to 200 ° F for 48 hours
150-200	300- 400	This is the point where thick pieces will often blow-up if they are going to. I try to go as slowly as 25 ° F/per hour through this point, often with a 3-6 hour soak at 300, 350, and 400 for solid work. This is because The center of the clay mass is often lagging as much a 150 ° F behind what the thermocouple says! The “film” water is being driven off during this period (up to 400 ° F), and the slowest temperature increase of the bisque should take place here.
220	428	Cristobolite inversion. Varying reports say there can be up to a 4% change in volume. This can cause significant cracking, especially in clay bodies that contain calcined material or are going through multiple firing.
300	450-572	Most organic matter now burned out. Paper spontaneously combusts at 451° F. I usually go slowly from 450-600 to give the bottom of the sculpture time to outgas and burn off the newspaper. I usually have a fan blowing in through the top peep hole and air exiting through the bottom peep up until the end of this part of the firing.
350-600	660-1112	Final dehydration period. Chemically combined H2_O given off up to a temperature of 600° F, and after firing clay to 1112 ° F or higher, clay cannot be slaked down again. This is the point at which the ceramic form is the most fragile.
450	842	Clay is calcined.
573	1063	Free quartz changes from alpha to beta form- a reversible 2% volume increase. Go at a very gentle 50° F/per hour from 1050° F through 1250 ° F to avoid most cracking.
600	1112	Clay just becoming ceramic. 1150-1200 plugs in.
800	1472	Sintering begins. Clay particles are fusing together
980	1796	Quick shrinkage. Formation of mullite begins, giving the body strength. Biscuit-firing completed for most clays
1000	1832	Melt in the clay pulls the particles together, causing further shrinkage. Soft earthenware glazes melted. Stoneware reduction can begin.
1100	2012	Hard earthenware glazes melted.
1200	2192	Cristobolite begins to be formed. Vitrification of red clay.
1300	2372	Most stoneware firings completed between 1250^oC – 1300^oC
COOLING
1200	2192	Glazes become increasingly viscous. Crystallization in glazes down to 0oC Rapid cooling of kiln down to 800^oC can take place to help the quality of glazes and colour of clay body.
1000	1832	Clay body becomes solid.
800	1472	Rapid cooling not safe below this temperature.
600	1112	Dunting (cracking) danger to clay, particularly if kiln cooled unevenly. Glaze begins to craze if it does not fit the body.
573	1063	Quartz inversion from beta to alpha.
220	536- 428	Cristobolite inversion. Sudden Alpha to Beta conversion happens at this point with about a 1.5% to 4.0% expansion This is usually only an issue during cooling and with clay that have a high percentage of Free Silica.

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