What Actually Happens Inside the Kiln During a Cone 6 Firing
The kiln is the one part of pottery nobody gets to watch. Here is what is actually going on in there, from the first wisp of steam to the moment a little cone bends over at 2,232°F.
I'm Haley. I run Floridian Ceramics in Seminole Heights, and the question I get after almost every class is some version of "so what happens to my bowls now?" The honest answer is that they go into a steel box, I press a button, and for the next day and a half a lot of chemistry happens that I don't get to see either. I just trust it, the same way you trust an oven.
Except I've always wanted to see it. So this year we put the kiln's controller online. Our kiln is an Evenheat electric that we call Chompy Two, and whenever it's running you can watch its real temperature on our homepage. This post is the guide to what that number means. Each rung below is a stage of a Cone 6 glaze firing, and when Chompy Two is on, the rung it's in lights up.
- Room temp to 212°FWater smoking. The last of the physical water steams out of the clay. Go slow here or pieces pop.
- 212 to 900°FBurnout begins. Organic matter in the clay and the gums in the glaze burn away.
- 900 to 1,200°FChemical water leaves. The clay crystal itself gives up its bound water. From here it can never be clay again.
- 1,063°FQuartz inversion. Silica grains snap into a slightly bigger shape. The whole load grows about 1%, all at once.
- 1,200 to 1,700°FSintering. Particles start welding at their edges. Carbon and sulfur finish burning out.
- 1,700 to 2,100°FGlaze starts to melt. The fluxes get to work, the surface goes glassy and bubbles through.
- 2,100 to 2,232°FCone 6. The glaze flows smooth, the clay body vitrifies, and the little cone on the shelf bends over. Top of the firing.
- Cooling to 1,500°FGlaze sets. Mattes and crystalline glazes do their real work on the way down, not the way up.
- Back through 1,063°FQuartz inversion, in reverse. The load shrinks again. Cool too fast and pots crack with a ping.
- Below 1,000°FThe long wait. Hours of slow cooling. The lid cracks open under 200°F and we unload under 150°F.
Temperatures are typical for our kiln and clay. Cone 6 is a measure of heat work, not a fixed temperature, so the exact numbers shift with how fast the kiln climbs.
First, what "Cone 6" means
Potters don't fire to a temperature. We fire to a cone. A pyrometric cone is a little pyramid of ceramic material designed to bend over when it has absorbed a certain amount of heat. Not a certain temperature, a certain amount of heat over time, which potters call heat work. Cone 6 bends at about 2,232°F if the kiln climbs at 108°F an hour through the last stretch. Climb slower and it bends a little lower. Climb faster and it needs a bit more.
That's why the ladder above says "about." Our controller has a thermocouple in each of its three zones and it reads temperature, but what we actually care about is whether the clay and glaze got enough heat to finish. The cone is the judge. The thermometer is just a witness.
Cone 6 is called mid-fire. It's hot enough to turn stoneware clay dense and watertight and to melt the glazes we use, without the fuel bill and wear of the Cone 10 firings you'd see in a gas kiln. Almost every electric studio in the country lives here.
Stage one: the water has to leave
Your bowls felt dry when they went in. They weren't. Clay that has sat on a shelf in Tampa for two weeks still holds a few percent of its weight in water, tucked between particles. From room temperature up to 212°F the kiln is basically a very slow oven, and that water turns to steam and finds its way out through the pores.
This is the stage where pieces blow up. If the kiln climbs too fast, steam forms inside the wall quicker than it can escape, pressure builds, and a chunk of the pot leaves at speed. It's the reason we won't fire a piece that was trimmed yesterday, no matter how much someone needs it for a gift. The first hour or two of every firing is deliberately boring.
Stage two: burnout
Between about 212°F and 900°F, anything organic in the load burns. Clay comes out of the ground with bits of plant matter in it. The glaze we brushed on has gums and binders that help it stick. The underglaze you painted in class has the same. All of it turns to gas and leaves through the kiln's vent. If you've ever noticed a faint campfire smell outside a studio, that's this.
It's also where the kiln needs air. An electric kiln that's sealed tight will trap that carbon and it can show up later as gray clouds in a white glaze, or worse, as pinholes where gas tried to bubble out through glaze that had already started to melt. So the kiln breathes a little through the whole bottom half of the firing.
Stage three: the point of no return
Here's the part I find genuinely amazing. Somewhere between 900°F and 1,200°F, the clay crystal itself falls apart. Kaolinite, the mineral that makes clay clay, has water built into its structure, not sitting between the particles but chemically bonded inside them. At this temperature that water is driven off, and what's left is no longer clay. You could grind it to powder, add water, and it would never be plastic again. Your bowl is now a ceramic, permanently.
On the way through, at 1,063°F, there's a small moment called quartz inversion. The free silica in the clay body flips from one crystal arrangement to another, and in doing so it gets about one percent bigger. One percent doesn't sound like much until it happens to every grain in the kiln at the same instant. A good controller slows down here, both going up and coming back down, because this is the temperature where pots crack if you rush them.
Stage four: sintering
From roughly 1,200°F to 1,700°F the clay particles begin to weld to each other at their edges. Nothing melts yet. Think of a pile of sugar cubes that have been left in a humid room and fused into a block. The bowl gets harder and rings when you tap it, but it's still porous. This is as far as a bisque firing goes. Our bisque is Cone 05, a little under 1,900°F, and we stop there on purpose: a bisqued pot is strong enough to handle but still thirsty enough to drink up a coat of glaze.
Stage five: the glaze wakes up
A glaze is glass that hasn't happened yet. It's a mix of silica, which wants to be glass, alumina, which keeps it from running off the pot, and fluxes, which are the ingredients that persuade silica to melt at a reasonable temperature instead of 3,000°F. From about 1,700°F the fluxes get going. The dry powdery coating on the pot slumps, goes shiny, and starts to boil. Really. Gases from the clay underneath push up through it, and for a while the surface looks like the top of a pancake, full of tiny craters.
The last few hundred degrees are about giving those craters time to heal. As the glaze gets more fluid it flows back over the holes and smooths itself out. That's why the end of a firing matters so much more than the middle, and why we don't race it.
Stage six: Cone 6
Around 2,232°F the cone on the shelf bends over, the controller sees the heat work it was programmed for, and the kiln shuts off its elements. At this moment the glaze is a thin layer of liquid glass. The clay body has vitrified, meaning enough of it has turned glassy to seal the pores. That's what makes your mug hold coffee. Stoneware at Cone 6 absorbs under two percent of its weight in water, which is the difference between a mug and a flowerpot.
Chompy Two usually gets here eight to ten hours after I press start. Then comes the part people don't expect.
Stage seven: the way down is half the firing
The kiln doesn't open at the top. It cools for fourteen hours or more, and a lot of what makes a glaze look the way it does happens during that descent. Glossy glazes freeze more or less as they are. Matte and satin glazes form their texture as tiny crystals grow in the cooling glass between about 1,900°F and 1,500°F. Cool too quickly through that window and a matte glaze comes out glossy, which is a surprise nobody wants on a commission.
Then the load passes back through 1,063°F, and quartz inversion runs in reverse. Everything shrinks that one percent again. Glaze and clay shrink at slightly different rates, and if the fit between them is wrong you get crazing, the fine web of cracks you sometimes see in old dishes. If the whole piece is cooled unevenly you can get dunting, a clean crack with a little ping sound that I have heard exactly once and never want to hear again. So the kiln takes its time.
Under 200°F I'll crack the lid. Under 150°F I unload. A pot that comes out too hot can still crack from the shock of room air, and after thirty hours of patience that would be a silly way to lose it.
Why your intro bowls go through this twice
In your class you threw two bowls and painted them with underglaze. After they dry and get trimmed, they go through a bisque firing to Cone 05, which gets them through the steam, the burnout and the point of no return, and leaves them hard and porous. Then I dip them in clear glaze and they go through the whole ladder again, this time to Cone 6, so that glaze turns to glass over your painting. That second trip is the one that makes them food safe and dishwasher proof. It's also why "two to three weeks" is the honest answer when you ask how long it takes.
Watch it happen
If the kiln is on right now, the homepage shows its real temperature and the ladder on this page shows which stage it's in. It updates every few minutes straight from the controller. I check it from my couch more than I'd like to admit. Next time you book a class, remember that a week or so afterward, that number on the homepage is your bowls.
Floridian Ceramics is a working pottery studio in Tampa's Seminole Heights neighborhood, run by Haley Melcher, a fourth-generation Floridian. Beginner wheel classes, multi-week workshops, private events and memberships. 7110 N Florida Ave, Tampa, FL 33604.