Firing Pottery: Bisque

24 Sep

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.

  1. 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.
  2. 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.
  3. 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,

Cindy

11 Responses to “Firing Pottery: Bisque”

  1. Russ Neptune January 2, 2009 at 1:10 pm #

    Cindy,

    Once you have completed step 4, how long do you
    estimate before the kiln shuts down ( switch falls )

    Have a great New Year!!!!
    Russ

  2. cindyinsd January 3, 2009 at 4:22 am #

    Hi, Russ

    It depends on your kiln. Larger kilns take longer, and if your kiln is stacked tightly (especially if you’ve used lots of shelves), it will take longer than it normally does. In a smaller kiln, the switch may fall before you turn the elements to high. You just need to keep track of how long it typically takes. For a small kiln (like around 5 cu ft), it may take an hour. For a larger one (7 cu ft, maybe), a couple of hours. It also depends greatly on the age and condition of your kiln elements. Old elements take much, much longer.

    If you’re interested in more control, you might want to use shelf cones. those are cones that sit on the interior kiln shelves. The kiln has peepholes in several locations. For an accurate picture, the middle hole would be best, but the top hole is easiest.

    You have to place the shelf cones at least 6″ or so away from the peephole to prevent cold air from affecting them when you take the plug out to look. You need a clear line of site in front of and behind the cone all the way to the opposite wall of the kiln. The cones are very difficult to see, and you should use protective glasses. It’s a good idea to always use shelf cones, and most potters recommend using one cone cooler, the target cone, and one cone hotter. Sit them in a row. In this way, you’ll get a more comprehensive picture of your kiln’s firing behavior. I much prefer the self-standing shelf cones.

    Especially for the first several firings of your kiln, the shelf cones will be helpful if only to assure you that you’re not overfiring. When the tip of the cone bends over and nearly touches the shelf, your ware has reached the target temperature. The cone (and your ware) will continue to mature for a little while after the kiln shuts off since the heat will be retained for a time.

    You should be aware that the cones in the kiln sitter are not the same as shelf cones, since they are thinner. They will usually bend a cone cooler than the shelf cones, so if you truly wish to fire to cone 6, you should use a #7 cone in your kiln sitter. If you’re bisque firing to 06, you need a #05 in the kiln sitter. I use ^6 glazes at my work kiln, though, and fire them with a ^6 cone. I know they’re only going to ^5, but they’re student pots and I feel better not firing them too hot in any case. That way the glaze matures adequately, but is less likely to drip.

    I hope this is clear and helpful for you, Russ. If you have questions or need clarification, please let me know.

    God bless,

    Cindy

  3. martha March 16, 2009 at 5:16 pm #

    I am breaking every other bisque platter I make. It seems to crack while firing, as the glaze and underglaze are seperated at the crack. What would cause this to happen?

    • cindyinsd March 16, 2009 at 6:18 pm #

      Hi, Martha

      Has the glaze melted down into the crack, or is there a sharp division with no glaze inside the crack? If glaze has run into the crack at all, the platter cracked during heating or at peak temperature, but if there is a definite edge to the crack that has not softened much, the crack occurred during cooling. Is the crack straight or curved in one direction or is it an “s” curve?

      Cracks that occur during heating can be the result of internal cracks which were either too small to see, or had not cracked all the way to the surface of the piece. Many things could cause this: drying too fast, clay that has too much bentonite or other highly plastic clay in the mix, and wetting the platter during trimming are some of them. To prevent this from happening, dry the pieces much slower than seems necessary. You would think that such cracking would show up after the bisque firing, and sometimes it does, but not always. Obviously, do not wet the base of the platter (at all) while trimming or smoothing. If you determine that your clay is causing the problem, you can mix in some sand or grog, or alternatively, use up this clay on for small pieces with strong structural shapes and get some other clay for your platters and any other pieces you’ve been having difficulty with.

      Such cracks may also be caused by uneven trimming and thickness of the base. Platters must be left very thick during throwing to allow for trimming properly. The base and rim of the platter should be of equal thickness. I often trim a double foot ring on large platters to prevent them from slumping toward the middle. You can make plates/platters without a foot ring, but many of them will crack if you do this, and others will develop a convex bottom so that they don’t sit flat on the table. You must exercise great care to see that your platters don’t get too thin in the very center or at the edges where they begin to curve up.

      Cracks that have well-defined edges with no glaze in them occur during cooling and are most often caused by allowing the kiln to cool too quickly. Often, this kind of cracking causes the plat to crack completely in half. You have to be gentle with those platters. I never take them out of the kiln or off of a hot kiln shelf until they have reached room temperature, and I leave the peep hole plugs in during cooling.

      Of course, if glaze has become adhered to the kiln shelf, such cracks can be caused by the platter not being able to move on the shelf during shrinkage so that it pulls itself apart. If you don’t use kiln wash, this can also happen as the clay does sometimes stick a bit to the unclothed shelf. If you prefer not to use kiln wash, you can sprinkle a fine layer of sand on the shelf to prevent this from happening, but you have to be careful not to let any of the sand find its way to places it isn’t welcome–like the glazes on your other pots. Open peepholes or a kiln ventilation system will move the sand around during firing and deposit it in your prettiest bowl, so be aware.

      Another big question is what your cracks look like. Cracks that occur along the edge of the plate where the side begins to curve upward can be caused by trimming too thinly or by too sharp a change of direction. Off center cracks that are straight or slightly curved often indicate bad clay (too plastic). Cracks straight through the center are usually the result of drying stresses, not compressing the base sufficiently, too thin trimming, and cooling too quickly.

      As for “compressing” the base, this refers to a process during throwing that involves running your fingers or a tool from the center of the bottom, firmly and slowly, toward your right knee, evenly pressing down on the base as you go. (Obviously, if you’re throwing left-handed, you move the other direction.) Many potters insist that it is impossible to compress clay by this means, and they’re probably right, but this technique is usually referred to as “compressing”, and if you neglect to do it, your pots’ bases will frequently manifest “s” cracks at some point in the drying or firing process.

      I hope this helps, Martha. If I’ve not answered your question, or if I’m unclear anywhere, please let me know.

      God bless, Cindy

  4. Paula Odell March 17, 2010 at 4:10 pm #

    Hello Cindy,
    I was so happy to find your site this morning!
    I had a problem as I unloaded the kiln and can not seem to figure out what happened.
    The pitcher that was fired was perfect on the outside. Red intricate design and red with white dots. The inside, however, is puzzling…..solid red inside with dots, the dots are melted, as if too hot, but the color cloudy. The handle also has melted dots, but the red is clear. The cloud-ness makes me believe it didn’t get hot enough, but the dots inside and on the handle that melted are a real puzzle!! This is 04 bisque fired perfectly and the decoration is an 06 product and I have never experienced this before. The other pieces in the load were all great.
    I would so appreciate any light you might be able to shed on this little dilemma.
    Bless You, Cindy !!!!
    Paula

    • cindyinsd March 17, 2010 at 7:18 pm #

      Hi, Paula

      It sounds like you’re working with low-fire glazes, which is an area I have almost zero experience with. I can tell you, though, that enclosed pieces such as pitchers (and even tall bowls sometimes) achieve different levels of heating on the inside and the outside. A lot can also depend on where in the kiln the piece was placed. I try to put any particularly large and/or enclosed pieces in the center. A soaking period or slow rise at the top end of the firing can help achieve even heating. As to why some of your glazes seemed melted and other parts immature, I wish I could be more help. As I said, I’m unfamiliar with low-fire glazes. I do hope this is of at least a little help to you.

      Blessings, Cindy

  5. alice July 28, 2012 at 10:52 pm #

    Hi Paula, Thanks for such a wonderful site! It is so helpful!! and great for clueless newbies like myself!!

    I have a couple of questions any advice is greatly appreciated!

    I bisque fired to cone 06 for a white talc clay body . its firing range is 06-2 is on package.

    ..now I want to glaze the clay, can I use 05-04 glaze? I have v series amaco velvet underglaze,

    I really appreciate any advice you can offer.

    Thank you!

  6. Cindy July 28, 2012 at 11:21 pm #

    Hello, Alice

    I’m not especially familiar with low-fire pottery as most of what I do is ^6. However, typically with low-fire pottery, the bisque is a little bit hotter than the glaze firing. So . . . it sounds like you haven’t done that badly. You should probably have bisqued to ^04 or ^03 so that you could glaze fire to ^05 or ^04.

    The reason for this is that low-fire glazes are designed not to shrink. If you fire hotter during the bisque than during the glaze firing, the clay won’t shrink any during the glaze firing and your glaze will fit properly.

    If you’re making decorative pieces, I guess I wouldn’t worry too much about it. I’d just go ahead and fire to the lowest recommended temp for the velvets (^05) and see what happened. Velvets are typically intended for decorative use in any case, unless you’re going to apply a functional glaze over them.

    However if you’ve worked too hard to be in the mood for taking a chance, you can re-bisque your pieces to ^04-^05 and then do your glaze firing one cone cooler (^05-^06) than the bisque temp you chose.

    It’s always hard for me to keep my mind wrapped around the numbers in the below 1sies, but remember that the higher the number after the ^0, the lower the temperature will be.

    Blessings, Cindy

    • alice July 30, 2012 at 12:11 am #

      Thank you Cindy!

      I am applying a functional glaze over the piece F 10
      so that it is dinnersafe.
      I worked very hard on it, so I can re bisque to 04 then apply glaze and fire again at 05? and that is okay?

      Thank you! I really appreciate your reply!! so kind of you !

      • alice July 30, 2012 at 12:25 am #

        so I can use the v series velvet lead free underglazes and then add the F10 clear glaze to make it dinnersafe, then fire to 05 and it will be okay? just making sure, I understand correctly. thank you!!

  7. Cindy July 31, 2012 at 3:15 am #

    Alice,

    I’m sorry, but I don’t know enough about low-fire glazes, or about Amaco glazes in particular to advise you on this. I’ll bet though, if you go to the Amaco site they’ll have a q&a section or an e-mail address where you can write to find out the answers to your questions. Alternately you could go to the shop where you bought the glazes, assuming they’re knowledgeable (which often they will be). My guess would be that this will render the item food safe, but I have to tell you that I honestly don’t know for sure.

    Good luck finding what you need to know — I’m sure you’ll be able to turn up the resources in one of those places.

    Blessings, Cindy

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