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Drop-and-Soak Firing


Rather than soak (or hold) a kiln at top temperature during a firing, the concept with a drop-and-hold is to approach the top temperature slowly (and only hold for a few minutes) and then drop quickly (by 100-200F) and hold the temperature there instead. At that temperature the increased viscosity of the melt is able to overcome for surface tension that holds bubbles in place. The result is smoother, defect free services. Although your glaze manufacturer may not indicate that this type of firing is helpful to finished product quality, your clay body manufacturer may not agree.

These schedules are often accompanied by a continued controlled drop in temperature through the phase where the melt solidifies (e.g. Down to 1400F). This improves the appearance of those that variegate and crystallize.

The temperature of the hold is often a matter of experimentation. Glazes with plenty of melt fluidity can stay remarkably fluid for hundreds of degrees below the firing temperature. The lowest possible hold temperature at which the glaze has sufficient fluidity to heal defects is best.

The benefits of this type of firing schedule make it worthwhile to learn how to program your own kiln controller. Controllers can remember schedules you enter so you only have to do it once. At the same time, we recommend becoming cognizant of the accuracy of your controller by verifying it with cones. Manually-entered programs are the only way to compensate for this inaccuracy (for example, your controller may think that 2180 is cone 6, but it is actually 2195).

Drop-and-hold schedules need to be accompanied by other best practices for success. For example, make sure ware is completely dry. Apply glazes as evenly as possible (usually by dipping). Maintain your dipping glazes at the correct specific gravity and viscosity. Use an engobe over the body for functional surfaces that will have a white or clear glaze finish.

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.

Manually programming a typical electric hobby kiln electronic controller

Manually programming a typical electric hobby kiln electronic controller

I enter (and tune) programs manually and document them in my account at insight-live.com. This controller can hold six, it calls them Users. Whatever program I last entered or 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" (my cone 6 test bar firing program). Then it asks how many segments: I press Enter to accept the 3 (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.

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By Tony Hansen




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