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Electric Hobby Kilns: What You Need to Know

Section: Firing, Subsection: General

Description

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.

Article Text

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:

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.

Pictures

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 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.

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.

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By Tony Hansen




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