|Monthly Tech-Tip |
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
In most electric periodic kilns firing schedules are programmed into electronic controllers to control the rate-of-rise, soaking time and often the cooling curve. In industry firings are very fast, optimization of every stage is absolutely critical, in hobby ceramics and small companies firings are much slower and the awareness of the need to plan and adhere to firing schedules is less. While many higher end periodic gas kilns also have electronic controllers, it is also common to manually oversee the rate-of-rise and atmosphere of the firing. The thermal history to which ware is exposed in a tunnel kilns is controlled by the speed of the ware through the kiln and control of the heat and draft in various parts of the tunnel (thus the complex firing curves that an individual piece of ware can experience in a periodic kiln are is not possible in a tunnel kiln).
This is an often-overlooked aspect of the ceramic process and yet is very important, since it relates so directly to glaze quality and body maturity. The secret to the unique properties of many special purpose ceramic products (e.g. alumina ceramics, thermal expansion failure resistant ware, crystalline glazes, porcelains), the consistency of many types of traditional ceramics and defect-free surfaces in glazes lies in the firing curve. Engineers spend a lot of time designing good firing schedules.
Schedules must account for the needs of the ware, the kiln, the environment and the budget. These include slow early heat-up to enable water to escape, reaching the desired state of maturity without cracking or other firing defects, attention to temperatures where sudden changes in body or glaze materials occur (e.g. volume changes associated with quartz, cristobalite inversion), the ability of the kiln to follow the programmed curve and the need to save energy. If well designed, it should be possible to predict the end of a firing accurately. For example, a cone 6-10 electric hobby kiln with elements in good condition should finish within 5-10 minutes of the projected. Industrial kilns, likewise, should finish within minutes of the target. The ability to predict the end is an indicator of the quality and practicality of the schedule.
Barlett Controls makes kiln controllers for hobby kilns and they ship them with pre-programmed schedules. The instruction manuals for these can be found on line by searching the model number imprinted on the front. For example, a search for "Bartlett model V6-CF" produced the following data for fast glaze and bisque firings for low and medium temperatures (their Genesis controller has the same schedules):
The slow bisque goes at 80F/hr to 250, 200F/hr to 1000, 100F/hr to 1100, 180F/hr to 1695 and 80F/hr to for a 13 hour firing to 1945F. The fast bisque slims that to 10 hours by increasing the rate-of-rise at each step. Their slow 7 hour cone 04 glaze firing goes to 250F at 150F/hr, 1695F at 400F/hr and 1945F at 120F/hr. The fast schedule has only two steps: 570F/hr to 1695F and 200/hr to 1945F. The cone 6 glaze firings use the same steps except they add 287 degrees F to the target temperature on the last two steps. There are no holds at any of these steps.
An account at insight-live.com provides an excellent environment to develop and maintain firing schedules as a part of a larger regimen of managing recipe, material and test data.
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.
The problem occurred with standard Plainsman M340, M390, M350, M370 and P300. The stonewares have porosities of 2.5%, the M370 1% and the P300 0.5%. Thus, all of these have comfortable margins for over firing. The G2926B glaze, used on all of them, does seal the surface pretty early so it will contribute to over-fired ware bloating sooner than typical. The problem here is the cone-fire modes on kiln controllers. For this kiln, the cone 6 program goes to 2236F. That's cone 7. Or more. Most people do not want to manually program their controllers. This is unfortunate since they do not benefit from the drop-and-soak PLC6DS and slow-cool C6DHSC firing schedules. And they end up over-firing their ware. Here is what many do: They cone-fire to cone 5. That produces cone 6!
First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.
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.
When we fire our two small lab test kilns we always include cones (we fire a dozen temperatures). I want the firing to finish when the cone is around 5-6 oclock. To make that happen I record observations on which to base the temperature I will program for the final step the next time. Where do I record these? In the schedules I maintain in our Insight-live.com group account. I use this every day, it is very important because we need accurate firings.
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 transparent amber, but crystallizes to a stunning silky matte when fired using the C6DHSC schedule.
Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.
This 1 gallon heavy crock was fired to cone 6 (at 108F/hr during the final 200 degrees) and soaked 20 minutes (in a electric kiln). The bare clay base should be the color of the top test bar (which has gone to cone 6). Yet, it is the color of the bottom bar (which has gone to cone 4)! That means the base only made it to cone 4. The vertical walls are the right color (so they made cone 6). It may seem that this problem could be solved by simply firing with a longer hold at cone 6. But electric kilns heat by radiation, that base will never reach the same temperature as the sidewalls!
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.
I was consistently getting the cone on the left when using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base (they consider the indicating part of the cone to be the part above the base). So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln at that point in time (with that pyrometer and that firing schedule). Our other test kiln bends the cone to 5 o'clock at 2195F. Since kiln controllers fire cone 6 at 2230 (for the built-in one-button firings) your kiln is almost certainly over firing!
A cone 11 oxidation firing schedule used at Plainsman Clays (maintained in our account at insight-live.com). Using these schedules we can predict the end of a firing within 5-10 minutes at all temperatures. We can also link schedules to recipes and report a schedule so it can be taken to the kiln and used as a guide to enter the program.
This is a cone 04. It is bent too much, the kiln has over-fired a little (cone 03 was also bent somewhat). The built-in firing schedule goes to 1945, that would be much more over-fired than this was (and the built-in ones do not soak, drop-and-soak or slow cool). It only takes a minute to edit the program I made, all I have done is drop the step-three temperature to 1930 (it was 1935). I adjust my schedule fire-up-to temperature as needed, I cannot imagine not doing this.
This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soak PLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls). For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.
So many glazes appear as they do because of the firing schedule (especially the cooling curve). Imagine getting an awesome result out of a kiln and not knowing (or being able to replicate) the exact firing schedule that produced it. This device reads and records the temperature once per minute. It is a Raspberry Pi computer with camera, Wifi and custom software I am developing. It costs about $100 (with the Lego case and GoPro compatible gooseneck mount). Because the device is a full-power Linux web server I can login, will be able to see the list of schedules and download any into my Insight-live.com account. And then I can link that firing it to photos of glaze test results!
Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). To actually go to cone 10 we need to manually create a program that fires higher than the built in cone-fire one. Determining how high to go is a matter repeated firings verified using a self supporting cone (regular cones are not accurate). In our lab we keep notes in the schedule record in our account at insight-live.com. And we have a chart on the wall showing the latest temperature for each of the cones we fire to. What about cone 6? Controllers fire it to 2235, we put down a cone at 2200!
Put the pots in, select a cone, press start. It is time to rethink that approach! Seriously. The Bartlett Genesis kiln controller is standard equipment on hobby and production electric kilns now. It is not meant to be run like a toaster! Good glazes are about much more than recipes, they are about firing schedules. None of the built-in "toaster schedules" have hold times on any segments, drop-and-hold sequences or controlled cools. Or even fire-to-cone accuracy. Yet such are a must for defect-free glazes, enhancing the effects of reactive glazes that must develop crystallization or variegation or firing accurately. It is easy to program: Tap the blue edit button to edit a program, tap a column of any segment to edit its value. Tap a segment number to delete or duplicate it. Google "bartlett genesis controller" for videos on creating and editing a schedule.
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.
It was put into the kiln before it was dry (from glazing). The kiln was fired fairly fast (without using a drop-and-hold firing schedule). These glazes have significant boron, they melt early and seal the surface. But water vapor can remain until surprisingly high temperatures. And it needs to get out. So it finds a discontinuity in the glaze cover and vents and bubbles out there. That leaves these defects that even a drop-and-soak and slow-cooling did not heal.
These are the inside uppers on two mugs made from the same clay with the same clear glaze. The one on the left was fired in a large electric kiln full of ware (thus it cooled relatively slowly). The one on the right was in a test kiln and was cooled rapidly. This glaze contains 40% Ferro Frit 3134 so there is plenty of boron and plenty of calica to grow the borosilicate crystals that cause the cloudiness in the glass. But in the faster cooling kiln they do not have time to grow.
This is important to understand that when looking at our firing schedule charts. “Degrees Fahrenheit” is a measure of the temperature of something. For example, 212F is the boiling point of water (the equivalent of 100C). "Fahrenheit degrees" are the divisions on the thermometer, there are 170 of them between the freezing and boiling point of water, for example (32-212, while there are 100 celcius degrees for the same span). "Fahrenheit degrees" are thus measures of change-in-temperature, not what the temperature is. In firing schedules, that is what we are talking about, how many degrees should the kiln rise during each step.
Refers to the practice of slow-heating a kiln during early stages to give mechanically-bound water a chance to escape.
A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower.
A type of ceramic glaze made by potters. Giant multicolored crystals grown on a super gloss low alumina glaze by controlling multiple holds and soaks during cooling
All types of ceramic are fired in a kiln to cement particles together to produce a hard and water and temperature resistant product.
In ceramics, this is the period in the kiln firing where the final mechanical water is being removed. The temperature at which this can be done is higher than you might think.
Borate glazes, those fluxed with the oxide B2O3, are the most common type used in ceramic industry and hobby for low and medium temperatures.
Bartlett V6-CF hobby kiln controller instruction manual
Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware
Electric Hobby Kilns: What You Need to Know
Electric hobby kilns are certainly not up to the quality and capability of small industrial electric kilns, but if you are aware of the limitations and take precautions they are workable.
Manually program your kiln or suffer glaze defects!
To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.