Note: This article is from Jesse Wiseman Hull’s website, now archived here.
Wednesday, February 6th, 2008
There are many good kilns available, but choosing one to suit your needs can be difficult. Take note that simply because a kiln boasts a “Rated to Cone 10″, “Energy Efficient” or “Heavy Insulation” label, it does not necessarily mean it is the best choice. As a matter of fact, that last one can even cause you a good deal of grief.
I believe a good analogy to use toward an electric kiln, is that of a boat. This may seem like an odd comparison at first; however, consider that both require power to operate against resistance, and since neither has brakes, it is up to the user to compensate for the dissipating momentum. This “momentum”, in the case of a kiln, translates to residual heatwork .Firing a heavily insulated kiln then, is comparable to piloting a heavier and less agile boat –for even after a pyrometric cone achieves the desired bend, it can easily drop well past that perfect arch if your kiln releases heat too slowly.It is also important to realize that automated controllers don’t make this any easier to deal with. A controller can stop supplying power to the elements, but it cannot make a kiln cool any faster than it’s ability to release heat. The user still has to figure out how to compensate, and then program the controller accordingly.I believe the best way to get the kiln that is perfect for your firings is to either build it from scratch, or start with a commercially manufactured shell of the size needed. Whichever choice you go with, the options to consider adding should focus on the need for accuracy and account for the components that will wear out the fastest.The variables below are those that led to designing the kiln I fire.
Variable 1 - The ability to heat:The kiln should at least have the ability to climb all the way to it’s top-rated temperature at 270°F/hr –without lagging. To accomplish this, the kiln needs enough power. I would suggest erring on the side of an “overpowered” design, for if your kiln lacks the ability to ramp fast when new, it will surely be difficult to reproduce results after the elements and other components age even slightly.The correct match between that power and the heating elements must also be calculated. Whether you go with standard or heavier gauge wire, it will have to be factored into a good watt density. As each kiln is going to be different, this subject gets much more involved than this post allows for, so I suggest speaking to a qualified individual (contacts I’ve had good experience with include: L&L, Geil, Skutt, Euclid, Kanthal).APM’s are often said to be the best choice for high temperature work. The long holds (”heat soaks”) used to grow the crystals in a glaze cause “grain growth” in standard element wire, thus weakening the metal. The thought is that since APM’s possess a different structure developed through a sintering process, they aren’t as susceptible to the damage produced by long holds at high temps.Having said this, my kiln currently runs with longer lengths of 12 gauge Kanthal A1 elements. This wire has proven more resistant than thinner gauges to the effects of higher temperatures, as well as the chemical attack occurring from clay and glaze during the firing. These elements currently have over 60 ^11-12 crystalline firings on them, show no visible signs of degradation, and perform with the same efficiency as they did when they were new.-Also, A-1 wire is about 1/3 the price of APM. Beyond power and a good watt-density, the kiln will need adequate insulating properties to retain the heat. Notice that I chose the word “adequate”…
Variable 2 - The ability to cool:Heavily insulated kilns heat well, because they hold heat well. This fact is often used to qualify them as “efficient”. But such a kiln’s Achilles’ heel resides in the inability to release heat at higher temperatures. I do understand that this goes against the current industrial tendency to add more and more insulation, but I hold that the value in a kiln that allows for a rapid diminishing of heatwork after achieving the desired cone value is extremely underrated.In 2006, Diane Creber passed on to me a wonderful collection of literature on crystalline glazes, including some of David Snair’s notes & letters. This information, along with my own correspondence with Derek Clarkson and others, showed a common preference in using small, high-powered kilns capable of completing the first and second phase of the firing (the climb to peak & descent to the first hold ) in a short time frame. This, in my mind is a truer example of “efficiency”. I have fired several types of clay and glaze in kilns of various sizes, and there’s nothing like relying on a kiln capable of completing the entire firing program as it is actually set by the user.Many of the more spectacular glazes I’ve worked with require a very accurate firing. With crystallines in particular, my goal is to heat the glaze well past its melting point, possibly maintain a hold to dissolve silica, zinc, and excessive nuclei, then quickly halt run-off by descending into the next ramp. As all of this must occur within a very small window of time, excessive residual heatwork is the last thing you want. Variable 3 - Structural Integrity at High Temperatures:It seems obvious to choose kiln parts that are designed for the temperature and type of firings intended. But this is something that is overlooked often enough.At it’s core, a kiln is simply a strategic stack of insulating firebrick. These bricks, cut to fit and usually wrapped in a metal skin, make up “the kiln shell”.K23 brick is the current standard for most hobby/studio kilns. If you are only going to ^6-^8, then it’s a suitable choice; however, both ^9 & ^10 (as high as 2381°F, depending on your rate of rise), occur at or beyond the recommended range of k23. The hotface , or the side of the brick exposed to the interior of the kiln, can shrink and crack if exposed to temperatures beyond the rating (e.g., k23≈2300°F). Keep in mind that this will occur even if you are only firing that hot occasionally, or if you are firing to a slightly lower temperature and holding there to achieve the equivalent level of heatwork.In designing my own kiln, I selected k25 brick, because I like the option of firing hot… often bending ^11-12.My firings show four important benefits with regard to K25:
The k25 brick is less fragile than k23.
The k25 brick performs extremely well from 2000°F to peak without deforming…
…it then allows for fast cooling from the top temperature down to 2250°F.
It still, however, releases heat slower through the middle to lower phases (1500-500°F), when dunting and crazing typically occur.
Variable 4 - Accuracy:Along the same line of using brick rated to your needs, consider the ill-advised use of Type-K thermocouples (rated only to ≈2200°F), which still come as standard equipment in many high-fire kiln models. The option of using protective sheaths on Type-K’s can create a lag in temperature readings. My suggestion is to always go with a thinly sheathed Type-S thermocouple for any firings above^6.Refer to the post on Studio Kiln Thermoucouple Calibrations.If you’re replacing a kiln of roughly the same size, go with your notes on how that kiln fired –both up and down. Did one section typically lag behind during the ascent and/or descent? Now would be the time to add insulation to that particular section, or to the floor or lid. Remember to start with the least amount you need however, and add more only where it’s necessary… otherwise, you’ll be right back in the same (big) boat, so to speak. Variable 5 -Efficiency & Energy Consumption:An argument against building a kiln as described here, is that the result seems under-insulated, and to require more power. It may therefore appearing inefficient by modern standards.What I am focusing on here, is a small to medium sized kiln meant for studio use, when reproducing accurate levels of heatwork is important. If your goal is to run several large kilns on a daily or weekly basis, and you are firing glazes with a wider heatwork tolerance, then the few dollars you save per firing is certainly a significant consideration.On that note, and to qualify my view toward energy efficiency, I refer you to the relevant post on the actual power consumption of an electric kiln . Related Links: New L&L JD18-JH