Wednesday, January 8, 2014

Loading and Firing the Kiln

When I worked for The Potters Shop, I was intrigued by a video in the library there.  It was about 30 minutes long and solely dedicated to the topic of loading an electric kiln.  "My gosh," I thought.  "What untapped secrets have I been missing all these years?  I'm going to learn some expert tips!"  Five minutes later, the coffee had perked and I sat down to edify myself.  The secret I learned?  That there really are no secrets.  Loading an electric kiln is about as straightforward as you can get.  

But, that is reassuring to know for sure, which is why such a video exists.  Because kilns are scary!  They can burn you.  (Done it.)  They can actually catch you on fire.  (I have seen that, although it hasn't happened to me personally... yet.)  They are held together by a sheet metal jacket that can cut you right down to the bone if you aren't being careful.  (I do that quite frequently.)  They are incredibly fragile.  So fragile that you can pick the bricks comprising them apart with your fingernail.  (Please don't be tempted to try that if you ever visit my studio.  Those things are a pain in the patoot to replace.)  Their electrical elements are also incredibly fragile. The kilns themselves are very expensive, as are replacement parts, which you are expected to be able replace yourself as part of routine maintenance about once a year, or the kiln will stop working entirely.  Eek!  But we potters put up with all of those challenges, because electric kilns are so easy to load and fire.  

We actually take our kiln technology a bit for granted these days.  It was kiln technology that gave the East the monopoly on porcelain production for hundreds of years after its development.  Europe hadn't figured out how to make a kiln capable of firing at the high temperatures necessary for porcelain and the rich copper red, and translucent celadon green glazes that went along with it.  The technique for making porcelain was such a valuable trade secret that alchemists traveling the courts of Europe used to claim, not only that they could make gold from lead, but also that they could make porcelain.  There is a fabulous history of the introduction of porcelain to Europe called "The Arcanum," for those who are interested.  

Long story short:  current kiln technology is amazing!  And here's how it works…

An electric kiln is pretty much a giant toaster oven.  It is made up of specialized soft firebricks.  I call them "air brick" to distinguish them from the hard type of firebrick used in your fireplace and in some other types of kilns.  Some people call them "soft brick."  As I understand it, air brick is made from clay with a lot of combustible material mixed in.  When the bricks are fired at the factory, in their kilns, (kilns from kilns!) all the combustibles burn out, leaving a brick that is full of tiny holes, with the density and fragility of a sugar cube, which you can cut easily with a hacksaw.  Ceramic is a great insulator to start with, but when you add all those holes to the mix too, you get a truly amazing material.  The inside of my kiln can withstand temperatures of 2381 degrees Fahrenheit.  Three inches of air brick is all that separates you and me from a quarter of the temperature of the surface of the sun.  And yet, the outside of the kiln only gets to about 200 degrees.  Hot enough for a drop of water to hiss if it hits the side of the kiln and hot enough to burn you, but still, how remarkable is that?  

The heat comes from electric coils that wind their way along grooves in the bricks all the way up the interior sides of the kiln.  Some kilns have more coils, also known as "elements," in the floor, or inside the lid. The elements are very brittle at room temperature, particularly after their first firing, so installing them initially is a bit nerve-wracking, and doing repairs if they pop out of their element grooves and dangle into the interior of the kiln is even worse.  And dangle they do, if you didn't put them in properly!  They expand and contract during the heating and cooling of the kiln and can escape to touch the liquid surface of a hot pot and burn themselves out.  Oops.  Time to replace that one!  $100 down the drain and there goes my afternoon…  Generally kilns need all their elements working properly to be able to achieve glaze temperatures, so if a single element breaks during a glaze firing, you may lose the entire kiln-load of pots.  

Which begs the question, "how do you know what's going on in there anyway?"  

Tool number one:  cone packs.  Pyrometric cones are made of clay that is specially formulated to melt at a precise point.  Different cones melt at different points, so if you line them up in order of melt, you can peer in at them during a firing and see how far you have progressed.  

The cones are all exactly the same shape and size, and the bottoms are cut at a slight angle, so that they sit leaning forward a little bit.  This encourages them to bend in a graceful and predictable arc as they begin to soften.  If the firing continues much past the softening stage, they will melt into a complete puddle and potentially run all over the floor, shelves, and pots in your kiln.  Big mess.  

I have three cones in my cone packs for a glaze firing, but just one for a bisque firing, since that needs much less precise monitoring.  They stand up neatly, all adjusted to the exact same angle, with their bases pressed into a little finger of soft clay.  A little extra clay at the front of the cone pack with a little fingerprint dimple in it keeps the first cone from touching (and sticking to) my kiln shelves during the firing.  After six to eight hours of a glaze firing, the first cone will soften and melt to tell me that my kiln is nearly finished and I had best start checking on it every 10 minutes or so.  For my clay and glazes, this is cone 4.  The second cone is cone five.  That one will soften and bend in a curve like a birch tree with a young Robert Frost swinging from it.  When the tip of the cone touches the pad of clay holding it, I can say that my firing has "attained cone 5."  It doesn't count until the tip touches.  For my clay and glazes, getting to cone 5 means, really, pay attention now!  Could be five minutes, could just be two, until you need to turn the kiln off…  The last cone, cone 6,  is the "guard cone."  It is there to let me know if I have gone too far.  For my clay and glazes, cone 6 should be just beginning to soften when I turn off the kiln.  It should look like a tree swaying slightly in the wind, but not like one blowing in a hurricane.  I have a little chart of "degrees of cone bend" taped to the wall next to the kiln and for each firing I record exactly what the degree of bend is on that last cone, so I can compare one firing to the next.  I like cone 6 to have 20 to 30 degrees of bend.  If I worked in porcelain, I might fire to cone 9 or 10.  If I worked in earthenware, might fire to cone 3.  It all depends on the clay and glazes you use.  

You may have noticed that I haven't talked about temperature during this discussion of cones.  That's because cones don't measure temperature.  They measure heat work.  Heat work takes time into account, as well as temperature.  Think of cookies.  You don't just bake them at 350 degrees.  You bake them at 350 for 10 minutes.  It's about the time at 350, not just the temperature.  And depending on how big your "rounded teaspoons" are, and how many cookies you put on the pan, and how thick your cookie sheets are, and what material they are made of, and how cold the cookie dough was when you started, it may take 10 minutes, or it may take 15.  If your oven runs a little on the cool side, maybe you are actually baking at 300 degrees instead of 350.  The cookies will still get done, but they will need a little longer.  With cookies, you can turn on the oven light and peek in at them to see if they are done.  With a kiln raging away at 2000 degrees, you don't exactly want to open the lid to check on things.  You'd lose your eyebrows.  So there are small "peep holes" at intervals in the side of the kiln, just big enough for you to see a row of cones.  The peep holes are closed with "peep hole plugs," or "peeps" for short, to keep the heat in.  These are made of, you guessed it, more clay!  

What you care about is not time, or temperature, but the relationship of the two as it relates to the specific mass of clay that you happen to be firing.  How golden brown are these 12 cookies after 10 minutes?  Since it's a complex relationship, with the temperature changing over the course of the firing, and it is hard to measure temperature really accurately at such extreme ranges anyway, it's easiest to put  something in the kiln that will melt after a certain amount of heat work, and peer in at that.  A cone pack.  

It is possible to take the temperature of a kiln, however.  Special high temperature thermometers called "pyrometers" can be used.  I don't like to depend on them to make any important decisions, since, again, it's the heat work that you really care about, but there are certain types of glazes that require more complicated firings schedules where keeping tabs on the temperature can be helpful.  Crystalline glazes are heated and cooled at very specific rates during different periods of the firing to achieve big snowflake-like crystals growing in the glaze.  There just isn't room for enough cones in a cone pack for all the adjustments you need to make for those.  Potters who work with crystalline glazes may use cone packs for the most important transitions in the firing, and do the math for temperature and time for the others.  The advent of computerized kilns has greatly simplified this.  

Some potters also "crash cool" their kilns, either to achieve a certain effect with their glazes, or just to get the kiln unloaded and ready to use again faster.  They pull one or more peeps out, or even prop the lid of the kiln open, while their kiln is cooling.  This is a tricky process however.  Different clays and glazes, and the bricks of the kiln itself, all expand and contract at different rates at different temperatures as they heat and cool.  It's possible to crack your pots if you cool them too quickly using this technique.  It's also possible to crack your kiln bricks or shorten the life of the coils.  Potters who do this are very experienced and are working with familiar glazes.  There are certain temperatures at which they can crash cool for a while, and other temperatures at which they need to have the peeps and lid closed to prevent the pots from cracking as they pass through quartz inversion.  These potters keep pyrometers in their kilns during cooling and watch like a hawks.  

The other way to know what's going on in your kiln is to keep an eye on the kiln "atmosphere."  As your kiln heats, it will begin to glow, first a dull red, then cherry red, then orange, then yellow, and finally white hot.  The color correlates with an exact temperature.  With experience you can pull a peep, peer into the kiln, and know what the temperature is.  Since the cones only start to fall during the last hour of what may be an eight-hour firing, I keep a chart of color vs. temperature next to the kiln so I can know how I am progressing.  It helps me spot problems quickly.  If the bottom and middle sections of the kiln are glowing bright yellow, and the top is still only dull red, that is a tip-off that I may have blown out a coil and should stop the firing before I waste any more electricity.  It also lets me know whether I have time for a leisurely lunch upstairs in the kitchen, or I will need to run back down to the studio every five minutes to check for falling cones.  This kiln has attained cone 08, which means my bisque is ready to turn off.  I can see my bent cone through the peep and the bright orangey-yellow atmosphere is just what I expect for this temperature.  

The last way to know what is going on in your kiln is a good kiln log and experience with consistent firings in your kiln.  There are a number of variables that determine how long a kiln will take, but the more of them you control for, and the better records you keep, the better you can predict what will happen in your next firing.  

Because I take good notes, I know that my kiln will finish faster if I start it at 6:00 a.m. than if I start it at 8:00 a.m., because voltage available through the power lines actually varies over the course of a day.  The way our power grid is set up in this country there are slight brown outs during times of peak demand.  Most customers will never notice, but if you run a kiln you will see the decrease in available power as a slightly slower firing rate during peak demand hours.  

Every kiln has a sweet spot, an ideal set-up.  Take notes on every aspect of your firing so that you can determine the best running conditions for your particular kiln.  And then keep taking notes, so that you will notice changes over time that indicate a failing coil or relay, a change in the voltage available from your electricity provider, or changes in the quality of your materials.  Relying on your gut intuition and memory may lead to decent firings eventually, but you'll achieve consistent results much faster if you have thorough notes to look back on that allow you to isolate for a particular variable when you have a question.  Be a good little scientist.  Don't trust what you think you know.  Trust the data.  I know I need a taller shelf at the bottom of the kiln for optimal heat circulation, but how tall?  What happened last time I tried a 10 inch shelf there?  In a kiln that was packed to the brim?  On a 90 degree day in June?  With good notes from last June, I can answer that question pretty exactly.  

So, that was a ton of background information, but how do you actually load an electric kiln?  

Kilns are loaded with sets of ceramic posts separating sets of ceramic shelves that hold the pots.  The walls in a house are built to transfer weight directly to the foundation.  In the same way, an initial short set of posts are located precisely over the kiln's foundation:  the metal kiln stand that holds it off the floor.  This first set of stilts is usually a half inch, or an inch tall, serving to lift the bottom shelf off of the kiln's delicate floor.  

Most kilns can use either whole shelves, which are round, octagonal or ten-sided, and fill an entire layer of space, or half shelves, which split the shelf along its diameter and allow you to stagger shelves if you have pieces of different heights.  I almost never use half shelves, because the shape means they tend to sag in the middle after a few firings, and since I'm a production potter, I make a LOT of pots.  I always have a whole shelf's worth of pots the same height, so there's no need for half shelves.  Whole shelves do warp a little bit, but not nearly so badly.  

Whatever kind of shelves you use, it is important to flip them regularly to help minimize warping.  Firing a large platter on a warped shelf can mean a warped or cracked platter.  I fire my bisques with the shelves kilnwash-side down, since kiln wash isn't necessary for a bisque.  I also use shelves that are rated for cone 10 firings, even though I only fire to cone 5.3.  Good shelves are expensive, but even the cheapest ones are rather pricey, so buy what you really need from the start and you won't have to pay to replace them.  They'll outlast the kiln itself if you take care of them.  

Each shelf, whole or half, is supported by three stilts.  Ever go to a restaurant where your table rocked and you had to wedge a sugar packet under one leg to even it out?  I bet it had 4 legs.  Unless everything is perfectly level, four-legged tables rock.  Three-legged tables don't.  Neither do three-stilted shelves.  For each shelf, the three stilts supporting it must line up exactly with the three stilts supporting the shelf below so that weight transfers evenly down through the stilts to the kiln stand and the floor.  At the high temperatures of a glaze firing, if a stilt isn't lined up with the one beneath, it can warp the shelf it's on pretty badly.  

It's very important to consider heat circulation through the kiln when loading to assure an even firing.  Remember that heat rises, so if you had no shelves or pottery at all in the kiln, you might expect the top of the kiln to be a little hotter than the bottom, with currents of hot air moving through the kiln producing additional hot and cool areas.  Once you put in shelves and pots, though, you change the natural convection pattern of the air in the larger space.  If you think about this from the start, you can adjust your firing set up to even out the temperature from top to bottom, so that all shelves in the kiln heat evenly.  Since the bottom of the kiln will naturally be cooler, you want the bottom shelf to have tall stilts on it creating a large, open space with several rows of kiln elements heating that area directly.  I like a 9 or 10 inch shelf on the bottom section of my particular kiln for my particular firings, but this varies from kiln to kiln, so you will need to experiment and take notes.  

Shelves, stilts, and pots not only block convection, they also soak up heat energy.  The greater the mass of pottery and kiln furniture you have in a load, the more energy it will take to reach your desired cone, and the longer your firing will be.  Let's say you had a big order of plates to fire and decided to pack a kiln with as many shelves as it would physically hold to fire all those plates at once.  Say a shelf, a three-inch stilt, a shelf, a three-inch stilt… all the way to the top of the kiln.  You would be wonderfully efficient in your use of space, but the kiln would never make it to glaze temperature.  All those thick shelves would soak up heat energy as fast as the coils in the kiln could pump it out.  You can get away with that sort of thing for a bisque, but not at stoneware or porcelain glazing temperature.  

For a bisque firing, pots can be stacked on top of each other, foot to foot, foot to rim, or rim to rim to save space, but they should always be stacked in such a manner that the weight at the top transfers evenly to the pots at the bottom, or you risk cracking and warping.  I like to place the largest pots first and then work my way down to the smallest for each shelf, to be sure I fit things as efficiently as possible.  

Nothing should ever hang over the edge of a shelf.  That's an invitation to warping for bisqued pots and glazed pots alike.  It's just too hot right next to the elements in the walls.  Putting pots right on the edge of a shelf is fine, but leave the gap between the shelf and the wall of the kiln clear for air circulation.  
There's always room in the next kiln load if something didn't fit this time.  

I turn all handles to face the middle of the kiln for a bisque.  Handle joints are likely places for a mug to still be slightly damp.  If there is a little trapped moisture there, it will turn to steam as the kiln heats and explode the handle off of the pot in a spectacular fashion.  If the handle is turned inward, it's less likely to send little chips of clay flying into all the element grooves.  

Beginning students spend a lot of time worrying about trapped air bubbles causing explosions in their work.  In my experience, most exploded beginner pots are actually wetness blow-ups, because beginner work is often thick, and unevenly so.  What seems to be a completely dry pot is actually only dry on the walls; the inch-thick bottom is hiding moisture.  You can tell a wetness explosion by its truly spectacular nature.  One little air bubble may cause one little *pop!* and then, the air having escaped, no more problem.  A wet section in a pot however is usually distributed throughout a large volume of clay.  It goes !!!BOOM!!! and scatters pieces of clay shrapnel onto every shelf, into every pot, knocks handles off of other pots…  You get the idea.  

If a chip of exploded clay lodges in an element groove, it will melt there and burn out the element during the next glaze firing.  Picking these chips out with tweezers, vacuuming them out, taking the kiln apart entirely to be sure you got all the pieces…  all of these options are a royal pain and if you miss just one little shard you'll have ruined your next firing.  Best not to fire anything that could possibly be damp, and turn handles inward just in case.  The feet of pots always dry slower than the rims, so I turn everything upside down while it dries, and also in the bisque, to give the pots as much air circulation as possible.  This also has the benefit of preventing warping.  

For a glaze firing, all pots need to be right-side up, since the glazed rims would cement the pieces down to the kiln shelves.  Nothing can touch anything else, or it will be stuck together permanently.  Because glazes expand and bubble a bit during the firing, it's good to leave a bit more space between pots than seems strictly necessary.  I like to be sure I could put a finger between two any two pots at their closest point just in case.  

A layer of kiln-wash made up of clay, alumina hydrate, and silica sticks to the shelves just enough to stay put, but not so much that it won't flake off easily.  In the case of a minor glaze drip, you can scrape off the kiln wash, drip and all, before the next firing.  Any kiln wash that falls into a glazed pot on a shelf below will leave a permanent chalky flake in the finish, so it's important to keep it well away from the edges of shelves and to handle the shelves carefully.  The whole point of the stuff is that it flakes easily, so it's easy to knock a little chip loose.  

As I load the kiln, I place cone packs where I will be able to see them easily through each of the peep holes.  Sometimes this means propping a cone pack up on a short stilt to get it even with the level of the peep.  In addition to the cone packs, which tell me when it is time to switch off the power, many kilns also have backup devices that will turn the kiln off for you.  Always use these solely as a backup.  Your brain knows more about the appropriate time to turn off a glaze firing than these mechanical or computerized aids, and they need frequent recalibration to stay accurate.  On the other hand, in case something goes horribly wrong, it's good to have a backup for safety's sake.  My kiln has two mechanical backup switches.  

The first is a kiln sitter.  This is a mechanical switch operating on the principles of the lever.  A little baby-sized cone, just like the ones in my cone packs is placed in between a set of prongs inside the kiln.    This cone should always be one cone higher than the cone I actually want, so it doesn't accidentally turn my kiln off too early.  Remember, I want to trust my brain to make the decision of when to turn off the kiln.  The quality of my results depends on getting that decision just right.  Since for my firings, I want cone 5.3 or thereabouts, so I put a cone 6 in the kiln sitter.  If I am not paying attention, and my kiln over-fires, at least it will only be by a little bit.  Some of the pots may be salvageable.  If that cone melts, it drops a lever, which releases a weighted tab, which swings smartly down and clicks the off button on the kiln's control box.  Cute eh?  Just like a game of mousetrap.  Except, having only two moving parts, it works dependably every time.  But because I'm dealing with temperatures of over 2000 degrees, there is also a backup to the backup.  Can't be too careful!  

The second backup is a limit timer.  Basically an egg timer that turns off the power for me.  I set it for 15 hours at the start of every firing.  My firings never take 15 hours, but if I were lying unconscious on the floor and unable to turn off the kiln myself, and the kiln sitter also failed, after 15 hours the timer would click the kiln off before it had a chance to burn the house down around me.  The pots would likely be a runny mess by then, but still.  Nice for my peace of mind.  

Okay, I lied.  There's a third safety backup.  It's a checklist and a stiff set of studio protocols.  There are mechanical failures, and then there are human failures.  I'm undoubtedly the weakest link in the chain.  The most likely cause of error is me, not my equipment.  So, just like airline pilots, I'm not allowed to work when I'm tired.  I have strict personal rules about when I am allowed to load kilns and when I am allowed to fire them.  I don't break the rules.  Even to get a final order done.  Even during the busy season when I don't get a single day off for three months straight.  I'm allowed to glaze pots when I'm tired.  I'm allowed to throw pots when I'm tired.  But loading a kiln when I'm tired is a great way to get the kiln sitter set incorrectly, or forget to include a cone pack.  By the time I reach the loading and firing steps of the process I have invested about 40 hours of work in a kiln load.  I can't afford to screw up.  So I go to bed and finish loading early in the morning.  

And doing certain steps of the pottery process is just plain dangerous when you are tired.  I can't afford to take time off from throwing because I have sliced my hand open, pulled my back, or given myself a nasty burn during loading or firing.  Best not to do those jobs when you are at any less than your best.  It saves money and time in the long run.  

A checklist next to the kiln has saved me on countless occasions from missing an important safety step when starting the kiln.  You are never too smart for a good checklist.  The smart only look smart because they accept their own fallibility and put systems in place to compensate.  I always check and double check the list before I walk away.  It is placed between me and the way out of the studio, so I can't walk upstairs for a cup of coffee without noticing it.  Did I open the windows to allow fresh air in and prevent carbon monoxide build-up?  Is the ventilation fan on to suck fumes out?  Is the house heat off to avoid accidentally sucking fumes into my living room if the ventilation system should happen to fail?  Is the limit timer set properly?  Is the kiln sitter set properly?  Did I close all the peeps back up?  Have I left anything flammable anywhere near the kiln?  

My kiln is in my studio, which is in the basement of my house.  There is a good argument to be made for having the kiln in a separate kiln shed, all on its own, like the separate kitchen buildings of ye olden days.  There is less exposure to kiln fumes.  And if the worst should happen, and there is a fire, it is less likely to take out your home and studio.  But it is easiest for a situation like that to occur when you aren't paying good attention to your kiln.  Forgetting to check on a kiln is astonishingly easy when it's not right in front of you, and a fire can spread very quickly.  I have plentiful carbon monoxide detectors, smoke alarms, and fire extinguishers in case of the worst, a good ventilation system, and three exits from my studio, but most importantly, I have me, right there, with the kiln directly visible from my seat at the potter's wheel, ready to stop a problem before it gets out of hand.  If you are building a new kiln room, I strongly recommend a glass door and windows into the kiln room that allow you a clear line of sight to your kiln from anywhere in your studio as an important safety precaution.  

So, we're loaded, we turn on the button and set the timer.  We check the checklist and turn on the ventilation.  It gets hot in there.  We watch for 6 to 8 hours.  We turn it off when the cones are just right.  And then… we wait.  The kiln cools slowly for the next 12 hours or so.  It's important not to rush to open it, or again, you can crack your pots.  That's another reason I stick to a strict firing protocol.  If I always start the kiln at 6:00 a.m., it will be ready to open by 6:00 a.m. the next morning.  I'm not tempted to open it early.  It will still be glowing red hot when I go to bed and I'm not likely to abandon my pillow at 2:00 in the morning to go unload.  The schedule saves me from my own curiosity.  


  1. Such a wonderful and detailed read. Thank you for posting your professional process! Love this... and you. There, I said it. :)

  2. Haha! Thanks, my fellow crazy potter!