Manually program your kiln or suffer glaze defects!

To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.

B. Glazes

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There is no other way to do a drop-and-hold firing but manually program your computer controller. If you are using the built-in firing schedules in kilns then your glazes are not the quality they could be!

What is drop-and-hold? It is a firing that holds at the top temperature only long enough to even out the heat, then drop one hundred degrees (or what is appropriate for your glazes) and hold the temperature there (e.g. half an hour)

Why should you do this? Crystalline glazes. Rutile glazes develop much better. But there is a much more important example. If you are using bodies that contain particles that produce gases during firing these have to escape. If they are generated after the glaze melts, they have to bubble through it. If the latter is not able to heal a pinhole will result, if it partially heals you could have a blister and if it almost heals a dimple. Materials in the glaze itself also gas, some quite late in the firing. The problem with soaking at top temperature is that reactions that produce gases are ongoing, bubbles just keeping coming. Some bodies have so many particulates that gas during latter stages of firing that it is almost impossible to fire defect-free glazes on them. This is particularly true with low temperature terra cotta bodies. But using this technique completely solves the problem! You might think that the glaze will not be fluid and mobile enough to smooth out but most glazes, especially those with boron, begin to melt far lower than you might think. Ideally one should try different drop-and-hold temperatures to arrive at one that is a good compromise between sufficient glaze melt mobility and maximum reaction shut down.

Links

Glossary	Bisque Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it.
Glossary	Firing Schedule Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Glossary	Kiln Firing All types of ceramic are fired in a kiln to cement particles together to produce a hard and water and temperature resistant product.
Glossary	Pinholing Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
Glossary	Soaking The process of holding a kiln at the final temperature (or at other temperatures) to enable the heat to penetrate the ware or to effect or complete a glaze or body reaction
Glossary	Drop-and-Soak Firing A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower.
Troubles	Glaze Blisters Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
Troubles	Glaze Pinholes, Pitting Analyze the causes of ceramic glaze pinholing and pitting so your fix is dealing with the real issues, not a symptom.

Manually programming a Bartlett V6-CF hobby kiln controller

I document programs in my account at insight-live.com, then print them out and enter them into the controller. This controller can hold six, it calls them Users. The one I last edited is the one that runs when I press "Start". When I press the "Enter Program" button it asks which User: I key in "2" (for my cone 6 lab tests). It asks how many segments: I press Enter to accept the 3 (remember, I am editing the program). After that it asks questions about each step (rows 2, 3, 4): the Ramp "rA" (degrees F/hr), the Temperature to go to (°F) to and the Hold time in minutes (HLdx). In this program I am heating at 300F/hr to 240F and holding 60 minutes, then 400/hr to 2095 and holding zero minutes, then at 108/hr to 2195 and holding 10 minutes. The last step is to set a temperature where an alarm should start sounding (I set 9999 so it will never sound). When complete it reads "Idle". Then I press the "Start" button to begin. If I want to change it I press the "Stop" button. Those ten other buttons? Don't use them, automatic firing is not accurate. One more thing: If it is not responding to "Enter Program" press the Stop button first.

By Tony Hansen
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