PLC6DS Firing Schedule
Cone 6 Drop-and-Soak Firing Schedule
Start temperature assumed: 75°F
||Rate to Temp °C
||60°C/hr to 121
||Drive out any remaining mechanical water
||194°C/hr to 1148
||Optionally soak here 30 minutes to clear bubble clouds
||60°C/hr to 1204
||Slow this rise or extend hold if the kiln is very densely loaded
||500°C/hr to 1148
||This step heals blisters, clears more bubbles
*Rates are expressed as "Celcius/Fahrenheit degrees", temperatures as "Degrees celcius/fahrenheit"
Use this, instead of the traditional hold-at-top schedule, where programmed slow cooling is not needed (this free-falls after 2100F whereas the C6DHSC schedule slow-cools to 1400F). Large or densely-packed kilns will slow-cool naturally.
Glazes that matte too much on slow cool (e.g. G2934) need to be cooled faster, use this schedule and pack your kiln loosely enough that it will cool more quickly.
-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. 250F, 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. If it can, consider increasing it to 400F/hr. The optional soak is to even out the heat distribution in the ware and enable the micro-bubble clouds of escaping gases of decomposition to combine and escape before the final push to top temperature (with a lower bubble population during that push). If you just need to heal blisters, the soak part of this step is not needed.
-Step 3: The push to the final temperature. Include self-supporting cone 6 frequently in firings to monitor the accuracy of your controller (and adjust the program to compensate).
- Step 4: Free-fall 100F and hold. The reason: Bubbles often just percolate during soaks at top temperature, becoming blisters in the fired ware. Holding at the lower temperature imposes the melt viscosity sufficient to overcome surface tension and burst the bubbles but still affords enough fluidity to heal them. You may need to customise this temperature for your kiln, ware and glazes. For example, if your glazes are more fluid and reactive, drop lower, perhaps 200F down (most boron glazes remain fluid down to even 1600F so experimentation may be needed). Do not go too low or the glaze could crystallize during the soak.
You must program your firing manually, there is no built-in schedule even remotely similar to this. Include a self-supporting cone 6 frequently in firings to monitor the accuracy of your controller. Adjust the temperature of step 3 to correspond to where the tip of cone 6 falls even with the top of the base (see picture below).
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.
Same glaze/body. One fired flawless, the other dimpled, pinholes. Why?
The difference is a slow-cool firing. Both mugs are Plainsman M340 and have a black engobe inside and partway down on the outside. Both were dip-glazed with the GA6-B amber transparent and fired to cone 6. The one on the right was fired using the PLC6DS drop-and-hold schedule. That eliminated any blisters, but some pinholes remained. The one on the left was fired using the C6DHSC slow-cool schedule. That differs in one way: It cools at 150F/hr from 2100F to 1400F (as opposed to a free-fall). It is amazing how much this improves the brilliance and surface quality (not fully indicated by this photo, the mug on the left is much better).
6% rutile is too much in this cone 6 oxidation glaze
Rutile variegates glaze surfaces. But it also opacifies at higher percentages. The blue effect is a product of crystallization that occurs during cooling, it is thus dependent on a slower cooling cycle, especially above 1400F. This is GA6-C Alberta Slip glaze with 4, 5 and 6% rutile. At 6% the rutile crystallization has advanced to the point of completely opacifying the glaze. At 5% the blue is still strong, even on a buff burning body. The loss of color occurs suddenly, somewhere between 5 and 6 percent. Rutile chemistry varies from batch to batch. The blue develops differently on different bodies. So do you want to play "at the edge", with 5% in the glaze, in view of these other factors and the finicky firing curve needed. Change in any of which could push it into the blueless zone?
What position should the cone be for correct firing?
Four degrees F. These are self-supporting cones, use these. I was consistently getting the cone on the left using a custom-programmed firing schedule to 2204F. However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I changed the temperature to 2200F and got the cone on the right. But don't assume your kiln fires cone 6 at 2200F, it could be much higher or lower, depending on your pyrometer.
A bubbling glaze having an encapsulated stain fixed. How?
These two pieces are fired at cone 6. The base transparent glaze is the same (G2926B Plainsman transparent). The amount of encapsulated red stain is the same (11% Mason 6021 Dark Red). But two things are different. Number 1: 2% zircon has been added to the upper glaze. The stain manufacturers recommend this, saying that it makes for brighter color. However that is not what we see here. What we do see is the particles of unmelting zircon are acting as seed and collection points for the bubbles (the larger ones produced are escaping). Number 2: The firing schedule. The top one has been fired to approach cone 6 and 100F/hr, held for five minutes at 2200F (cone 6 as verified in our kiln by cones), dropped quickly to 2100F and held for 30 minutes.
Out Bound Links
Glaze blisters are a surface defect in fired ceramic glazes. They have caused every potter and company grief at one time or another. The problem can be erratic. The blisters trace their origins to the generation of gases as particles in the body and glaze itself decompose during firing (loosing H2O,...
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 ...
As glazes melt, gases from decomposition of organics, carbonates, sulphates and hydrates are generated (if the body was glazed green, or unbisqued, many more of these gases will be present). If glazes are already melting while the gases are being generated, bubbles form and suspend in the glass melt...
- (Schedules - Related)
Plainsman Cone 6 Slow Cool (Reactive glazes)
In Bound Links
Many fluid glazes will do magic things (e.g. variegate) with the addition of rutile (usually less than 5%). The effects are often amplified when other colorants are present (especially iron). The classic rutile effect happens when a glaze melt runs in rivulet patterns. Employment of this effect is c...
Questions and suggestions to help you reason out the real cause of ceramic glaze blistering and bubbling problems and work out a solution
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
2014-03-26 - A cone 6 boron-fluxed MgO matte developed at Plainsman Clays by Tony Hansen (a link below will take you to its page there). This page contains technic...
Ulexite - NaCaB5O9Â·8H2O - Sodium calcium borate
G2926B - Cone 6 Whiteware/Porcelain Transparent Base Glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
2014-02-06 - A cone 6 transparent general purpose base recipe developed at Plainsman Clays by Tony Hansen (see link to go there below, it contains technical and mi...
By Tony Hansen