Firing of pottery can be accomplished in many ways, whether in a campfire, a pit, a wood or oil or gas kiln or, more commonly for hobby and small professional potteries today, by electricity. My expertise is almost entirely in the realm of electrical kiln pottery in the cone 6 or mid-firing range, so that’s what I’ll be talking about.
Firing is another of those deceptively simple parts of making pottery and ceramic art. You take the stuff to a ceramics shop; they fire it for you; you pay the bill–right? Well, maybe you’d like to know just a bit more about the process, especially if you’re thinking about getting your own kiln or if you’re charged with operating one for your school or art center. As mentioned in earlier posts (please read them if you need this info), there are two firings–one for bisque ware and the second for after the glaze has been applied. This post is primarily about bisque firing, though I will mention the glaze kiln occasionally.
Before I talk about firing, do make sure your kiln is adequately vented. Venting of kilns, especially in settings where people live and work, is absolutely essential. It’s not all that expensive, so do it right. An open window and a table fan are not good enough. Ask your supplier of ceramic equipment to help you figure it out. You’re better off not to do pottery at all than to fire an unventilated kiln. In addition, make sure the kiln room isn’t going to get hot enough to trigger your building’s automatic sprinkler system. As you can imagine, this tends to irritate the very administrators who, only a few weeks ago, assured you that no additional measures were necessary. (I haven’t experienced this, but I know someone who has.) A proper ventilating system takes out toxic or irritating fumes, but will not keep your kiln room cool. Both of these factors must be addressed before beginning to fire your kiln.
Though I’ve given you some actual temperatures in my earlier posts, firing isn’t quite that simple. Potters use “pyrometric cones,” which are little cone-shaped pieces of ceramic material that have been calibrated to slump or bend at a certain point in the firing. Here’s a link to a short Wikipedia article on pyrometric cones. If you’re not familiar with the use of cones, I recommend you read it–plus, it’s got good pictures. The slumping point of cones doesn’t correspond with any exact temperature. Many kilns don’t even have thermometers (or more accurately, thermocouples). All they have are devices called “kiln sitters,” which use a mini-cone or a mini pyrometric bar as a prop to keep the circuit that turns the kiln on closed (that is, connected). When the cone bends, the switch falls, opening the circuit and shutting off the kiln.
Just to take a little of the mystery out of it for those of you who’ve never seen one, here’s a photo of a kiln sitter.They’re all more or less similar.
- At the top left is a dial that sets how many hours you are planning to allow the kiln to run. This is a switch and will turn the kiln off when it winds down to zero. You would typically set this dial to run maybe a half hour longer than you think the firing will take based on your past experience with the kiln.
- At the top right is a hooking device. It’s a little lever whose weighted tail extends through the wall of the kiln and into the firing chamber. It is propped by a mini-cone or mini-bar of pyrometric material so that the inside end will fall down, raising the outside end up when the cone or bar slumps.
- At the bottom right, the little squared ladder-shaped thingy is your second switch. It’s hinged at the top. To start the kiln, you swing it up and hook the hook thingy (#2) onto it (with a cone in place inside the kiln). It doesn’t look like it in this picture, but there’s a bar across the end for the hook to hook onto. The silver dot that’s just above it in this photo is a button that you push to start the kiln. It won’t start, though, unless you’ve dialed in some time on #1.
Two factors cause cones to bend, both of which affect the pottery similarly. The first is, of course, temperature. Second is “heat work,” which is effected by a combination of temperature and the duration of the temperature in the ware. Just as you can cook a casserole at 250 degrees or 350 degrees (250 will take longer), you can fire a pot completely though the temperature within the kiln may not have reached the target–it just held longer, causing the heat work to get done, though at a lower temperature. If that all sounds too complicated, all you need to know is that, when the cone bends, the pottery is done (or if it’s not, you need to adjust your kiln sitter or change to a different cone (or change your clay or glaze). For more information on this, I recommend Orton Ceramics, which manufactures most of the pyrometric cones used in the USA.
You should know that a mini-cone or bar designed to is roughly equivalent to a shelf cone of the next higher denomination. In other words, though they’re made of the same stuff, they bend at a cooler temperature than their big brothers because they’re such thin, frail little things.)
More modern kilns are controlled by computer devices that use thermocouples to measure the temperature within the kiln. At the high temperatures within a kiln, normal thermometers can neither give a reading nor survive. Thermocouples aren’t all that accurate, but they’re consistent. There are more accurate ways to measure temperature within a kiln (by analyzing the color of the light, among other things), but they’re unnecessary for home or hobby potters and tend to be pricey.
Even if you have a computer controlled kiln, you should still use cones. Cones are a more relevant way for potters to measure the effects of temperature and time on ceramic ware because cones are made of ceramic material. You’ll need shelf cones–get the self standing ones–they’re worth it. By experience, you’ll learn how far you want your cone to bend over for the type of clay and/or glaze you’re using. When the tip of the cone touches the shelf, it has reached its calibrated “doneness,” but the melting will continue during any “soak” time you’ve programmed into the firing schedule, so you want to start the soak period before this happens. And your particular glaze may actually mature at a lesser or greater degree of cone melting. So, experience and experimentation are necessary.
As I mentioned, bone day ware still has some moisture in it, from the humidity of the air. It may also have moisture from not having dried completely, especially if your pottery has any especially thick parts. I start my bisque kiln by programming it to hold the temperature of 180 degrees Fahrenheit for several hours. If I have monster pots inside, I might hold the temperature for as much as 6-8 hours. This might not be necessary, but there’s a lot of work in those big pots and I don’t like them exploding. If you’re using a kiln sitter, you just can’t fire big, thick pots. If you’ve figured out how to do it, let me know–I’d like to hear. The kiln (even with only one element set to low and the lid propped open a foot) will heat to above the boiling point of water, the water remaining in the thick walls of your pots will flash into steam, and you’ll have a broken pot. Since I live and have my studio at an elevation of about a mile, I keep the pots at 180 degrees rather than 200 degrees. The boiling point of water is lower up here in the clouds.
So, step 1: Place a mini-cone into the kiln sitter. You can adjust firing temperature slightly by sliding the cone to the right or the left. Closer to the small tip, the cone will bend sooner; closer to the large base, it will take longer, resulting in a hotter firing. If you’re using a mini-bar, the adjustment isn’t possible, but your firing results will be more consistent. Unless you have only thin, light pots, I recommend “candling” the kiln before firing it. Prop the kiln lid open about a foot and fire with one element set to low for a day–maybe 8 hours.
If you have a computer controller, place three self-standing shelf cones where you’ll be able to see them through a peep hole. If you’re going for a ^06 firing, you should have an 07, 06, & 05 cone (of these, the 05 is the hottest cone). Place them in a row horizontal to the middle peep hole, at least 6″ away, with no pots behind or in front of them to obscure your line of sight. You can set the kiln to hold at a temp around ten degrees below boiling for several hours–adjust this time depending on how thick your pots are. Go to step two for your next setting.
Step 2: If you “candled” the kiln as I recommended in step one, you might want to simply turn off the kiln when you leave and close the lid. You can start it back up in the morning. When you’re ready to fire, make sure the lid is well-closed and all peephole plugs are in. Turn all elements to low. Fire this way for two hours.
If you have a computer, you will have set the kiln to fire at 200 degrees an hour up to 1100 degrees (this is where chemically combined water is exhausted). Go to step three for your next setting.
Step 3: Turn the elements up to medium and hold there for two hours.
For the computer controller, you will have set the kiln to fire at 300 degrees an hour up to your chosen peak temperature. For bisque firing, this is usually cone 06, or around 1860 degrees Fahrenheit. Go to step four for your next setting.
Step 4: Turn your elements up to high. When the switch falls, you’re done. Make sure you check the kiln to be sure the sitter shuts it off properly–you’ll want to keep records of how long your kiln typically takes to fire. Kiln sitters can get stuck, so this is important. Never fail to check (in a timely manner) that the kiln has shut off. Kiln sitters are prone to occasional failure, and it’s possible that your timer knob could also fail, particularly if the kiln gets hot enough to damage it. If the switch has fallen, the kiln is off, though you might want to disconnect the circuit by switching off the breaker to make extra sure.
Set your computer controlled kiln to hold the peak temperature for 30-60 minutes. This allows impurities (such as organic matter and sulfur), which could cause bubbles in your glaze later, to burn out. Check the kiln to make sure it shuts off after your soaking period, then just let it cool.
If you’ve placed shelf cones where you can see them through a peep hole, you can put on some kiln gloves (or heavy leather gloves), take out the peephole plug and immediately place it securely on the lid. It will be glowing orange. Wearing a good pair of sunglasses, look into the kiln and try to discern the shadows of the cones to see whether any of them have begun to bend. You should have your target cone in the center, and when its tip has touched the shelf, your firing is complete. Don’t forget to replace the plug when you’re done. You lose more heat than you might think doing this, and it’s hard to see the cones, but it does help you to figure out what temperature you should set the controller to fire to. It’s usually way different from the theoretical temperature. If you absolutely can’t see the cones, just try to adjust your kiln’s temp by looking at the cones after the firing. If your target cone is a pile of goo, you can back off the temp maybe 30-40 degrees. It’s a matter for experimentation and trial and error.
Above all, make sure you or some other responsible person can be present when the kiln is reaching its peak temperature. It’s important, despite all the fail-safes built into modern kilns, that you make sure the kiln shuts off as it has been told to do.
Let the kiln cool until the pots and shelves can be handled bare-handed. Any hotter than this and you risk breaking your pottery by taking it out too hot, thus exposing it to thermal shock. (Or simply dropping it because it’s burned your hands even though you were wearing thick leather gloves–yes, I’ve done both of these things.)
For both the kiln sitter and the computer controller, shelf cones are helpful (see step one for the computer kiln). They give you a truer picture of the conditions inside the kiln needed to attain your desired results. Pots that exhibit glaze faults such as pinholes in the glaze may need a longer soak at the peak temperature or a slightly higher bisque firing temperature. Clay should burn off all its gasses during the bisque firing–bubbles in glaze indicate that this hasn’t happened (or that the pots were dusty when the glaze was applied). To soak a kiln with a sitter, you have to prop up the switch, which isn’t recommended. However, if you do it, bring a chair and sit there and watch the switch for a half hour, then un-prop it. Do not, under any circumstances, leave a propped switch for even a few minutes. You will forget about it and melt down your kiln.
Pots that haven’t been bisque fired hot enough may take on too much glaze, causing the glaze to run and stick to the shelf. The solution to this is to either fire the bisque hotter or to thin out the glaze slurry.
There’s certainly more to firing than this, and if you have suggestions or questions, please feel free to contribute. It sounds daunting, and many books have been written on this subject, but don’t get scared off if you’re a novice. You’ll get the hang of it really quite quickly, though you’ll likely never stop learning new things about firing.
God bless and happy potting,