The advent of kiln controllers has made soak-and-rise and drop-and-soak schedules like this possible for even hobbyists, enabling defect-free surfaces on even coarse-grained bodies.
The idea of this schedule is to heal bubbles and defects they cause. These often just percolate during soaks at top temperature. This schedule also adds an option to reduce clouding caused by high populations of micro bubbles.
You have to program this manually. See the link below on how.
-Step 1. Drying needs to be complete because the next step proceeds rapidly. Extend the soaking time if your ware is thick or heavy or the kiln is densely loaded. 240F, although above boiling point, is not enough to fracture ware, but needed to completely dry it.
-Step 2 climbs quickly, your kiln may or may not be able to maintain the rate. The optional soak is to even out the heat distribution in the ware for final approach to top temperatures. And to enable micro-bubble clouds (gases of decomposition) to agglomerate and escape before last stage.
-Step 3: The push to the final temperature. Include a self-supporting cone frequently in firings to monitor the accuracy of your pyrometer and adjust the program accordingly. Slow the rate-of-rise if kiln is densely packed.
- Step 4: Use this step to heal blisters. Free-fall and hold (at cone 03 fall 100F, at cone 06 fall 50F). The reason: The lower temperature increases the melt viscosity sufficiently to overcome melt surface tension and burst the bubbles (but still affords enough fluidity to heal them). You may need to customize temperature this for your kiln, ware and glazes. 50F higher if glazes are crystallizing, lower if they are still blistering.
- Step 5: Slowly cool down to 1400F. This will improve glossy glaze surfaces, removing remaining surface defects. With this slow cool glazes can crystallize, so this step will need to be removed if you don't want that.
|1||60°C/hr to 115C||108°F/hr to 240F||60min||2:31||Drive out any remaining mechanical water|
|2||250°C/hr to 982C||450°F/hr to 1800F||15min||6:14||Adjust this to top-temp minus 100|
|4||555°C/hr to 982C||999°F/hr to 1800F||30min||6:44||Rapid fall and hold|
|5||83°C/hr to 760C||150°F/hr to 1400F||9:24||This reduces glaze defects even more|
|3||60°C/hr to 1037C||108°F/hr to 1900F||15min||14:17||Adjust this to your top firing temp|
These are the same glaze, same thickness, Ulexite-based G2931B glaze, fired to cone 03 on a terra cotta body. The one on the right was fired from 1850F to 1950F at 100F/hr, then soaked 15 minutes and shut off. The problem is surface tension. Like soapy water, when this glaze reaches cone 03 the melt is quite fluid. Since there is decomposition happening within the body, gases being generated vent out through surface pores and blow bubbles. I could soak at cone 03 as long as I wanted and the bubbles would just sit there. The one on the left was fired to 100F below cone 03, soaked half an hour (to clear micro-bubble clouds), then at 108F/hr to cone 03 and soaked 30 minutes, then control-cooled at 108F/hr to 1500F. During this cool, at some point well below cone 03, the increasing viscosity of the melt becomes sufficient to overcome the surface tension and break the bubbles. If that point is not traversed too quickly, the glaze has a chance to smooth out (using whatever remaining fluidity the melt has). Ideally I should identify exactly where that is and soak there for a while.
These are cone 03 porcelains with G2931K clear glaze base plus Mason 6021 encapsulated stain (which is very expensive). The one on the left has 15% stain added. The one on the right is slightly less dense red but has only 10% stain plus 2% zircon (zircopax). Notice the zircon has smoothed the surface also, this is a known benefit of using it with encapsulated stains. The 2% addition is not enough to opacify, the color remains deep and translucent. But the surface is still not completely smooth and glassy as is the base transparent glaze on its own. This can be improved using a drop-and-hold firing schedule.
Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.