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Pottery/Kiln Firing and Time vs Temperature

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Question
This is my first time firing a cone 5 glaze with my FX23 (208 volt) cress kiln. I'm using Amaco PC (cone5/6) glazes and was told that i should fire this load to cone 5 and not to fire it longer than 6hrs. My question is; how do i get it to fire to cone 5 at 6hrs? i have a kiln sitter and my bisque fire to cone 06 took 7 1/2 hrs. for my bisque fire i had my kiln set on "C". is it just a matter of heating it up faster?

Answer
Dear Tom,

    Thank you for your recent question. Your question is one of time vs temperature. When firing a kiln, no matter whether it is electric, gas or wood, they all depend on the maturing temperature of the cone being firing to. Think of a kiln like a gas or electric oven. Let's say you turn your oven on and set it for 350 degrees. You could then set a timer and find out how long it took the oven to reach the 350 degree mark. Let's say, it took 30 minutes. But you decide you want to reach the 350 degree mark in 15 minutes. In order to do this, you would need to turn the kiln up to, let's say 450 and hypothetically, the oven would reach 350 degrees in 15 minutes. Now, there may be a slight difference in the speed of the oven reaching 350 depending on if you were using gas or electric, but in general they can be tested and a suitable time and temperature rate could be reached to get the oven to 350 degrees in a specific amount of time.

    When firing a kiln, you can set the kiln to ramp (the degrees in temperature raised per hour) to reach a goal temperature. During the heating, the cone begins to change in chemical structure and weaken as it reaches the maturing temperature. So, to reach cone 5 at 6 hours, the kiln would have to ramp at over 361 degrees per hour (this does not count for slow ramp of 108 degrees per hour during the first few hours to allow steam to escape from the ware). At this rate, the ware would probably break, and it would be very hard on the kiln elements.

    The information you were given is incorrect and should not be used. To fire your kiln to cone 5, set the kiln to fire low for the first couple of hours, even if this is a glaze fire. Then begin turning your kiln up to ramp about 270 degrees per hour until the kiln reaches the maturing temperature, or the cone bends. Your glaze will not over fire as the kiln will turn off when the cone reaches maturity. If you were to not fire the kiln over 6 hours as told, and the cone does not reach maturity in that 6 hour period, then your glaze will be under fired.

    This is why cones are used, to create a fail safe method to determine the maturing temperature. The only way to speed up a firing to reach maturity is to increase the ramp per hour rate, and this is not recommended due to the amount of stress it places on the kiln elements and the risk of pots cracking or exploding.

    I hope this helps you. If you have any further questions, please feel free to contact me at any time. I am always at your service. Good luck with this firing.

Sincerely,
Ms. Ti Phillips
Earth Stoke 'N Fire Pottery Studio and Artist Retreat
www.earthstokenfire.com

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Ti Phillips

Expertise

Will answer any questions on hand building, wheel, glaze, firing. Speciality questions to include those in glaze calculation and development, firing techniques. Please do not send questions on identifying pottery. Although I would love to add this to my question topics, I have a retreat to run as well as the studio and volunteering on AllExperts, and therefore do not have the available time to research indentification and marks. Thank you for understanding.

Experience

Experience includes 30 years in pottery design and education. Have taught online and studio classes worldwide for the last 20 years. Own a pottery retreat specializing in firing techniques. Have 12 years solid experience in glaze calculation and formulation as well as problem solving in glaze chemistry. I am the first potter in the United States to have developed a complete package of pottery equipment blueprints for a studio. The blueprints include wheels, kilns, studio furniture, wedgeboards, raku kilns, slab rollers, ball mills and studio tools.

Organizations
Alliance of Pottery Artists Worldwide Association

Publications
Ceramic Industry - PPP Wyndstryder Press - Pottery Journal

Education/Credentials
University of Sciences and Art's of Oklahoma, studied under Professor Jaymes Dudding.

Awards and Honors
Potter of the year with APAWA, various awards for showmanship and design.

Past/Present Clients
Available upon request.

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