•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
Electric Hobby Kilns: What You Need to Know
Section: Firing, Subsection: General
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.
Firing an electric kiln is like using a microwave oven, right? Just slap the ware in, slam the lid, set the controller to cone fire, and take out the beautiful ware the next day. It is that simple isn't it? Not quite! If you are using a top loading hobby electric kiln for stoneware pottery, it is good to be aware of what you have. Compared to industrial electric kilns, you have something that is fragile, hard to control, difficult to maintain, fires unevenly, has little or no ventilation and is an energy hog! It has no element holders, no draft, a heat leaking lid, bulky energy-stealing kiln furniture and a toy controller. Yet hobby kilns have proven great for earthenware and slip cast ceramics that do not require tight control and they have also given many people the opportunity to get into stoneware pottery and porcelain, and even small scale manufacturing. But make no mistake, producing consistent ware will be a matter of developing a feel for what is happening inside and learning to compensate for the shortcomings of these kilns.
Consider some specific points about making these contraptions work:
- Modern electric hobby kilns have electronic controllers. This sounds impressive at first, but in some ways it is an efficient method to automate failure. You still need to think about what is the best firing schedule for your type of ware and implement it in a way the kiln hardware is capable of reproducing. And you need to monitor it well to be sure of ongoing accurate reproduction, because, in ceramics, lack of consistency in firing will certainly bring failure. Your kiln depends on the thermocouple to know what the temperature is and the elements to heat it up. But these components are cheap, they have their limits and need maintenance and replacement regularly. Mechanical sitter devices introduce more things that need maintenance and adjustment (the rod and supports bend a little with each firing). These have come to be treated as fail-safe devices (since the controller does the firing). That being said, you still need a controller. Electric firing is about consistency and control and these devices make up for a number of shortcomings of the kiln. They work reasonably well and element life is longer (much longer the lower you fire). Multiple thermocouples also enable some controllers to even out the temperature gradients in the kiln. Crystalline glazes for example were once the domain of a select few, but now they are simple because of these devices.
- Use cones. Consistently accurate firings will not happen if you do not use traditional cones. They tell you what firing you actually got, no matter what the electronic controller might say. There presence in firing after firing will tell you when the thermocouple is starting to stray or alert you to other issues. Of course, make sure the cones are set at the right angle and away from close proximity to elements (self supporting cones are best).
- Document your firing schedules and stick to them. Adjust and perfect them over time as you get to understand your process better. Using the schedule manager at Insight-live.com you can accurately predict when your firings will end (provided no part of the schedule climbs faster than what the elements can accomplish even at the poorest condition you allow them to deteriorate to). At Plainsman Clays, we have found that we can predict the end of firings from cone 06 to 11 to within 5 minutes! When you can do this you can monitor the kiln and override it if necessary (to finish early or continue if the cone has not fallen to the proper angle).
- Electric kilns fire very unevenly as a result of compartmentalization created by full kiln shelves that reach to nearly the edge of the chamber, elements of differing efficiency, uneven distribution of ware and kiln furniture, improperly sealed lid, lack of draft and the high ratio of shelf weight to ware weight. You can improve things by having a controller that compensates zones or by setting cones to be visible in peep holes on all levels of the kiln and stacking future firings with this in mind. Also, use as little kiln furniture as possible, distribute it evenly throughout the chamber, and use half shelves where practical (to prevent the compartmentalization). But even with all this, you must realize that heavy ware cannot be fired evenly no matter how long you soak the kiln. That is because electric kilns heat via radiation and the shady side away from the elements is always going to be cooler than the element side.
- In industry, the final pore water is removed quickly and evenly from ware in drying chambers that employ boiling-point temperatures and very stiff humidity controlled drafts. I'm guessing you use your kiln for this during early stages of firing! As mentioned, an electric kiln has little or no draft to remove pore and crystal water so temperatures can generate an extremely humid atmosphere which retards drying of thick pieces. If ware isn't dry when the really hot air starts to hit you know what happens! If possible, get a dedicated drier.
- Fire slower and soak at the end. Firings will be more even, consistent, ware will be better quality, glazes will have less imperfections. This is especially true if you are firing a heavy mass of ware. If you fire too quickly and do not soak at the end of the firing, the ware simply will not reach temperature (even though cones or instruments may indicate otherwise). The color of stoneware clays typically varies with temperature, often from a yellowy straw color to a grey (or light red to dark red, then brown). Thus you can compare the color of the raw clay (or clear glazed) surface on a light thin piece with a heavy thick one (or the shady side of a heavy piece with the element side). Differences in color indicate differences in maturity. Again, in tightly packed loads with heavy ware, the shady or undersides of ware simply will never reach the temperature of the element side, no matter how long you soak.
- Electric kilns really shine at low to medium temperatures where it is possible to create high strength stoneware bodies. "Stoneware" is simply dense and strong non-pourous ceramic. It is no longer synonymous with cone 10, it can be made as low as cone 1. However it is more difficult to create glazes that are non-leaching, hard and uncrazed. Matte glazes are easy to make at cone 10, but not so at medium temperatures, some key mechanisms of the high temperature mattes do not exist at cone 5-6, for example. Glazes at high temperature have lots of clay and the slurry suspends well, but at middle temperatures more flux is needed, that means less clay and more problems with the slurry and application. Materials that melt well a high temperatures (e.g. dolomite, talc, feldspar, whiting) melt poorly or not at all at middle temperatures. Boron is the king of fluxes at medium temperatures, glazes need it to melt. It is sourced from frits and natural boron materials like gerstley borate, ulexite and colemanite. Recently there has been a proliferation of books on cone 6 electric firing and we have been deluged with middle temperature glaze recipes. But that does not mean that these glazes are good or easy-to-use. Developing glaze formulation and adjustment ability and learning to understand some of the chemistry and the materials will give you more control to create what you want and what you need to really make efficient use of your electric kiln.
- Can you fire even lower? Since it is possible to produce stoneware as low as cone 1 (weak ware can be made at cone 10 using immature bodies or ill-fitted glazes). On the one hand, there is not as much knowledge about low fire glazes, they are more difficult to fit to bodies, there are fewer natural materials that melt that low and there are fewer mechanisms that create interesting glazes. On the other hand, there are tons of frits that melt at low temperatures. Low fire glazes are brighter and stains work well. There are methods to variegate the glazes chemically, artificially and physically. If you have control of your bodies and glazes, you could move downward over time, adjusting current glazes and introducing new ones. Some industries have achieved dramatic reductions in cost by doing this.
- Make sure the lid of your kiln is sealing well all the way around. When changing elements, do them all at once when possible. Also, make sure the switches are well ventilated or they will overheat and fail. If your kiln has a solid metal top on the switch housing (with no vents or metal grate to allow air passage), it is a prime candidate for burning out switches.
- Electric kilns are fragile, so treat them with care. When changing elements, you can minimize brick damage by turning the kiln on for a few minutes, then unplugging it and pulling the otherwise brittle elements out while they are hot and flexible. When installing, read the instructions carefully and use good crimp or screw connectors. It is really important to make sure the elements are not stretched into the groves, they must sit in there under no tension and be bent to go into the corners well. If not, they will sag out during the firings. High quality electric kilns for industry have always been available and their element mounting systems are far superior to the way electric hobby kilns work, but we make do! Of course the proper functioning of these kilns depends on maintaining them well, especially the elements and the thermocouples. It is very common to find badly decomposed thermocouples and potters wondering why their glazes do not look right.
- Ventilation Systems: These not only make kilns safer to use but they introduce draft, a key to improved fired results (if the interior of the kiln can be kept at even temperatures throughout). When products of decomposition are removed quickly during firing (especially carbon, sulfur) results are much better and problems are fewer.
Hobby kilns are not so bad after all. Like so many other things in ceramics, limitations can be compensated for by experience and care. And if you are serious, take a look at an industrial
grade electric kiln, they are amazing. Still, although we do not really like to think about it, cone 10 in a gas kiln is still king. A quality gas kiln with an electronic controller is a marvel and no electric kiln will ever touch the quality of ware it can put out.
Firing schedules at insight-live.com
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.
The difficulty of vitrifying the base of heavy stoneware
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!
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).
These two pieces will not mature to the same degree in a firing
Soak the firing 30 minutes to mature the mug and the planter will not mature. Soak 2 hours for the planter and the glaze may melt too much and the clay be too vitreous. This is a troublesome issue with electric kilns. Furthermore, they employ radiant heat. That means that sections of ware on the shady side (or the under side) will never reach the temperature of those on the element side no matter how long you soak.
Resurrecting and old kiln by replacing elements
Replacing the elements in a old test kiln turns it into a new kiln! Relays are also checked. Notice how the elements are bent and pushed well into the corners. If this is not done properly, they will pull out of the corners after it is fired a few times.
How to get more accurate firings time after time
When I fire our two small lab test kilns I always include cones (I 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.
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.
A test kiln with firing controller: A necessity.
Every potter should have one of these. This one has a separate electronic controller, newer ones have it built-in. Start with one of these and then graduate to having a large, second kiln. Ongoing testing is the key to constant development of product and quality.
Can this 5 lb thick walled bowl be fired evenly in an electric kiln. No.
When electric kilns, especially large ones are tightly packed with heavy ware, the shady or undersides of the pots simply will never reach the temperature of the element side, no matter how long you soak. In this example, the inside of this clear glazed cone 6 bowl has a flawless surface. The base is pinholed and crawling a little and the surface of one side (the shady side), the remnants of healing disruptions in the melt (from escaping gases) have not smoothed over. The element side is largely flawless like the inside, however it is not as smooth on the area immediately outside the foot (because this is less element-facing). Industrial gas kilns have draft and subject ware to heat-work by convection, so all sides are much more evenly matured.
Out Bound Links
In Bound Links
Bubbling occurs in some clay bodies (especially those made from coarsely ground raw materials) if they are over fired; others just melt without bloating. Aggravating conditions that produce bloating include the presence of mineral particles (e.g. sulfates) that generate gases during the the firing s...
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 ...
The practice of holding the kiln at final firing temperature for a period of time. This is usually done to mature the clay (especially if the firing was fast) and give the glaze opportunity to flow and heal imperfections. Soaking in a firing schedule should normally be accompanied by a slower approa...
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
A firing where the atmosphere inside the kiln has sufficient supplies of oxygen to react with the glaze and clay body surfaces (and thus produce the colors characteristic of this). Electric kilns are synonymous with oxidation firing. However these kilns lack the air flow of their gas counterparts. S...
By Tony Hansen
Copyright 2003, 2008, 2015 https://digitalfire.com, All Rights Reserved