Plainsman Cone 6 Slow Cool for Reactive Glazes

Start temperature assumed: 75°F
Step Rate to Temp °C Rate
°F/hr
To °F Hold Accum
Time
1 60°C/hr to 104 108 220 60min 2:20 Longer soak if ware is heavy or thick
2 194°C/hr to 1146 350 2095 15min 7:56 Climb faster is ware is thin
3 60°C/hr to 1201 108 2195 15min 9:07 Slower climb if ware is thick, keep soak here short if possible
4 277°C/hr to 1043 500 1910 30min 10:11 Drop quickly to 1910F
5 83°C/hr to 760 150 1400   13:35 Slow cool to 1400F
*Rates are expressed as "Celcius/Fahrenheit degrees", temperatures as "Degrees celcius/fahrenheit"

This is for a Cone-Art 2818D kiln with an exhaust fan on at start of firing.

Use for defect free rutile glazes that need to develop the blue coloration during cooling.

Include self-supporting cones 5, 6 and 7 frequently in firings to monitor the accuracy of your controller. Adjust the temperature of step 3 for each kiln you have.

Alberta Slip GA6-A cone 6 base glaze slow cooled

GA6-A Alberta Slip base glaze (80 Alberta Slip:20 Frit 3134) fired with Plainsman slow cool cone 6 firing schedule on Plainsman M390 iron red clay. If this is cooled at normal speed, it fires to a glossy clear amber glass with no crystals.

Cone 6 rutile floating blue effect lost. Then regained.

Left: What GA6-C Alberta Slip rutile blue used to look like. Middle: When it started firing wrong, the color was almost completely lost. Right: The rutile effect is back with a vengeance! What was the problem? We were adjusting firing schedules over time to find ways to reduce pinholing in other glazes and bodies. Our focus was slowing the final stages of firing and soaking there. In those efforts the key firing phase that creates the effect was lost: it happens on the way down from cone 6. This glaze needs a drop-and-soak firing (e.g. cooling 270F from cone 6, soaking, then 150F/hr drop to 1400F).

Plainsman iron red clays with rutile blue Alberta Slip glaze

Cone 6 mugs made from Plainsman M350 (left) and M390 dark burning cone 6 bodies. The outside glaze is Alberta-Slip-based GA6-C rutile blue and the inside is GA6-A base (20% frit 3134 and 80% Alberta Slip). That inside glaze is normally glossy, but crystallizes to a stunning silky matte when fired using the schedule needed for the rutile blue (cool 100F and soak, slow cool to 1400F).

Rutile blue glaze effect completely lost! A temporary solution.

Left: 4% rutile in the Alberta Slip:frit 80:20 base. This glaze has been reliable for years. But suddenly it began firing like the center mug! Three 5 gallon buckets of glaze (of differing ages) all changed at once. We tried every combination of thickness, firing schedule, clay body, ventilation, glazing method on dozens of separate pieces with no success to get the blue back. Even mixed a new batch, still no color. Finally the 'crow bar' method worked, 0.25% added cobalt oxide (right mug). It is identical ... amazing. It is not the same mechanism to get the color and it is not exactly the same, but worked while we figured out the real issue: the firing schedule (the secret turned out to be cooling, soaking, then slow cooling to 1400F).

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By Tony Hansen

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