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 1146
||Optionally soak here 30 minutes to clear bubble clouds
||60°C/hr to 1201
||Slow this rise or extend hold if the kiln is very densely loaded
||555°C/hr to 1146
||Use this step if you need to heal blisters, clears more bubbles
*Rates are expressed as "Celcius/Fahrenheit degrees", temperatures as "Degrees celcius/fahrenheit"
The idea of this schedule is to heal bubbles that normally just percolate during soaks at top temperature (these become blisters in the fired ware). This schedule also adds an option to reduce clouding caused by high populations of micro bubbles. The reasoning for why this works is explained in the steps below.
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. 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 cones 5, 6 and 7 frequently in firings to monitor the accuracy of your controller. Adjust the temperature of this step for each kiln you have.
- Step 4: Use this step if you need to heal blisters. Free-fall 100F and hold. The reason: The lower temperature imposes 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 tempertaure this 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). But do not go too low or the glaze could crystallize during the soak.
The advent of kiln controllers has made soak-and-rise and drop-and-soak schedules like this possible for even hobbyist, with them you can now fire defect-free surfaces on coarse grained bodies.
This same principle can also be applied to low temperature glazes, especially those using raw material melters like Gerstley Borate and ulexite.
Same cone 6 glazes. Same clay. Why is the one on the right pinholing?
These are thick pieces, they need time for heat to penetrate. Both were soaked 15 minutes at cone 6 (2195F in our test kiln). But the one on the left was control-cooled to 2095F degrees and soaked 45 more minutes. Pinholes and dimples are gone, the clay is more mature and the glaze is glossier and melted better. Why is this better than just soaking longer at cone 6? As the temperature rises the mineral particles decompose and generate gases (e.g. CO2, SO4). These need to bubble through the glaze. But on the way down this activity is ceasing. Whatever is gassing and creating the pinholes will has stopped by 2095F. Also, these are boron-fluxed glazes, they stay fluid all the way down to 1900F (so you could drop even further before soaking).
White spots and blisters in a high zircon glaze at cone 6
This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down.
What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.
Rutile blue glazes: Love the look, hate the trouble to make it
A closeup of a cone 10R rutile blue (it is highlighted in the video: A Broken Glaze Meets Insight-Live and a Magic Material). Beautiful glazes like this, especially rutile blues, often have serious issues (like blistering, crazing), but they can be fixed.
Carbonate gassing can cause glaze blisters
An example of how a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.
Serious blistering at low fire: How to fix it
An extreme example of blistering in a piece fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay applied to a bisque piece. Is LOI the issue? No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone 04. Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at 100F below cone 03. Doing that increased the viscosity of the glaze melt to the point that it could break the blisters (held by surface tension) while still being fluid enough to smooth out the surface.
Why is the clay blistering on this figurine?
This is an admirable first effort by a budding artist. They used a built-in cone 6 program on an electronic controller equipped electric kiln. But it is over fired. How do we know that? To the right are fired test bars of this clay, they go from cone 4 (top) to cone 8 (bottom). The data sheet of this clay says do not fire over cone 6. Why? Notice the cone 7 bar has turned to a solid grey and started blistering and the cone 8 one is blistering much more. That cone 8 bar is the same color as the figurine (although the colors do not match on the photo). The solution: Put a large cone 6 in the kiln and program the schedule manually so you can compensate the top temperature with what the cone tells you.
Blistering in a cone 6 white variegated glaze. Why?
This glaze creates the opaque-with-clear effect shown (at cone 7R) because it has a highly fluid melt that thins it on contours. It is over fired. On purpose. That comes with consequences. Look at the recipe, it has no clay at all! Clay supplies Al2O3 to glaze melts, it stabilizes it against running off the ware (this glaze is sourcing some Al2O3 from the feldspar, but not enough). That is why 99% of studio glazes contain clay (both to suspend the slurry and stabilize the melt). Clay could likely be added to this to increase the Al2O3 enough so the blisters would be less likely (it would be at the cost of some aesthetics, but likely a compromise is possible). There is another solution: A drop-and-soak firing. See the link below to learn more. One more observation: Look how high the LOI is. Couple that with the high boron, which melts it early, and you have a fluid glaze melt resembling an Aero chocolate bar!
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.
Underglazes at low fire are brighter than at medium temperature
Medium temperature transparents do not shed micro bubbles as well, clouds of these can dull the underlying colors. Cone 6 transparents must be applied thicker. The stains used to make the underglazes may be incompatible with the chemistry of the clear glaze (less likely at low fire, reactions are less active and firings are much faster so there is less time for hostile chemistry to affect the color). However underglazes can be made to work well at higher temperatures with more fluid melt transparents and soak-and-rise or drop-and-soak firing schedules.
Blistering in a high gloss cone 6 glaze fired at cone 7R
The boron and zinc fluxes make the melt of this glaze highly fluid at cone 7R. That comes with consequences. Notice the Al2O3 and SiO2 in the calculated chemistry. They are at cone 04 levels. The significant ZnO increases surface tension of the melt, this helps bubbles form at the surface (like soap in water). Al2O3 and SiO2 could be added (via more clay), this would stiffen the melt so the large bubbles would be less likely to form (this glaze melts so well that it could accept significantly more clay without loss in gloss). A drop-and-soak firing is another option, in this case a drop of more than 100C might be needed (see the link below to learn more).
A black engobe transforms the floating blue glaze over it
M340 stoneware fired to cone 6 (drop-and-hold schedule). The L3954B engobe fires deep black (it has 10% Mason 6600 black stain instead of the normal 10% Zircopax). It was applied inside and partway down the outside (a much less messy process than using a black clay body). They were bisque fired and glazed inside using the base GA6A Alberta Slip amber clear (using Frit 3195). The outside glaze is Alberta Slip Rutile Blue (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers). To learn more about how to make the engobe and start making black pots click "Product Data Sheets" at PlainsmanClays.com and go to the section on Medium Temperature.
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 1400C. 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?
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.
Deep, deep blue without any cobalt. How?
These have to be seen to be believed, it is the deepest, richest blue we have ever produced. This is Plainsman M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt (not raw) Umber instead of the normal 10% Zircopax) was applied inside and partway down the outsides (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue. Firing schedule: Cone 6 drop-and-soak.
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 is often erratic in nature and counter intuitive measures may be needed to resolve it (we have a page in the trouble-shooting section dedicated to this def...
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...
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, ig contains technical and mi...
By Tony Hansen
•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com