Everything You Need to Know About Firing a Kiln

Firing a kiln can be an exciting and terrifying experience. The vision of fire truck sirens and billowy smoke is enough to keep you awake at night. Will I burn down the school? Will one hundred student artworks explode into smithereens? Why did they not teach me how to do this in my college ceramics class!?

This post is a compilation of my advice and step-by-step guidance on firing a kiln in a classroom setting. If you prefer a video format, check out my YouTube video here that has all of this information as well as me time lapse loading for a bisque fire.

You are not alone.

The first thing to remember is that you are not in this alone. A smart and capable electrician installed your kiln and made sure all the electrical components were up to code. A well-trained district employee set up your kiln room and ventilation. A verified company manufactured your kiln, along with thousands others, and most companies have a great customer support programs. If you are lucky like I am, you also have the support of several other art teachers in your district. Don’t be afraid to call, text or email a buddy art teacher for advice or venting (insert kiln ventilation joke here….).

Kiln Best Practices

Before we jump into the nitty-gritty, here is a list of my kiln best practices. These tips will save you from a lot of stress.

Load you kiln slowly. Anytime I rush to load a kiln the same day I plan on firing it I break at least one piece. Try explaining that to a twelve year old…
Test fire, test fire, test fire! Whether it’s new glazes or a new kiln, try it out with one of your demo pieces to make sure you can vouch for the results to your audience of eager artists.
NEVER fire kiln when you are not there. And keep it unplugged!
Wait to fire pieces until they are truly ready. Don’t rush the drying! Teach your clay unit early in the semester so you have appropriate time for the process.

Why Does Clay Explode in the Kiln?

The two main culprits of kiln explosions are moisture and air pockets.

Trapped moisture in clay heats up and causes steam, which causes clay to explode and crack. Clay needs ample time to dry and larger pieces take longer. Time of year, climate, humidity and air flow in your classroom or work space all affect dry time. Setting pieces in front of a fan will help, but can also lead to cracking and damage because smaller pieces dry faster. Pieces of clay that stick off the base (think handles) will dry quicker and will become very fragile. My best advice is plan your timing carefully. There is nothing worse than not leaving proper time for your students’ clay pieces to dry and having several explosions when fired. Not that I know this from experience…

I like to allow a full week for dry time. Keep finished pieces on open shelves and not in closed cabinets. If you are making flat pieces like tiles or relief sculptures, don’t stack them. They will retain moisture longer if clay is touching clay. Laying each piece on a piece of cardboard or newspaper will help flat pieces dry evenly and quicker.

Clay will also explode if there is a trapped pocket of air inside the clay body. Wedging clay is the best practice for ensuring all air pockets have been released. Many clay brands are great at having clay wedged and without many air pockets. When recycling your clay, you are responsible for wedging it into proper form. This takes time, and upper body strength, and it’s a great skill for students to pick up while helping out their over worked art teacher.

Working with small pieces at a time is the best way to ensure there are no air pockets. Have students wedge in the palm of their hand for small pieces like coils and pinch pots. This does take practice because students have the tendency to overwork their clay until it dries and is hard to work with. If the pieces are hand sized, two to three squeezes does the trick in my experience. For pinch pots I do check each one for air pockets before we move on to the next step. If it’s too thick I can work around with my hands until I feel comfortable there won’t be any grenades going off in my kiln.

Loading A Kiln

Once clay pieces are finished, it is time to load the kiln! Greenware (or unfired clay) can touch in the bisque firing, however you need to leave about a one-inch space between the kiln walls and the clay pieces itself. This will ensure heating coils are protected.

Loading a kiln is a lot like working a puzzle or playing tetris. It takes strategy and careful movements. Which unfortunately, are not two of my strengths. I load the kiln the day before I plan to fire it. On the fast setting my kiln fires at Cone 05, or about 1,900 degrees, in seven hours. I press start when I arrive to work and by the last class of the day it is beeping that it’s finished.

Click here to watch me time-lapse load my kiln!

When firing a kiln for the first time, cones and temperatures can be really stressful. Your kiln should have a manual that describes the temperature and time for each cone. If you are not the first art teacher at your school the manual may be lost in the Narnia of your supply closet. Most kiln companies like Skutt and Amaco have digital manuals on their websites. A google search of “kiln cone chart” will give you pleeeeenty of resources with a bunch of scary numbers.

Some older kiln models require you to use an actual cone to get the most accurate temperature reading. I am going to make an embarrassing confession: I have not once ever in my years of college education or in my own classroom used an actual cone.

Some of you may be judging me, others may be relieved. I have always used an electric kiln. I am an impatient Millennial (ugh). Why bother with a weird little cone when my kiln will be flashing the actual numbers for me digitally? I would never admit this to the serious and hardcore potters that I know. (Unless they are reading this…) I just believe in keeping things simple. Things are complicated enough in my tornado-of-a-public-school-classroom.

Bisque Firing

A bisque fire is the first firing of your clay pieces. It is when greenware clay transforms into ceramics. Clay must be 1,000,000,000% dry before it can be fired. This is called “bone dry”, and as the name implies, describes clay that is dry as a bone.

If your clay feels cold to the touch it is not kiln ready. If using white clay, bone dry clay will be almost all white. It needs to be all the same light color and not splotchy like in the “in progress” example above.

Clay that is extra thick will take longer to dry. I always use “the rule of thumb” with my students. Clay pieces that are larger then the width of your thumb are dangerous! There may be trapped moisture inside that is hard to detect.

Once clay pieces are bone dry, it is time to fire the kiln! I low fire all of my pieces at Cone 05. I have also had the same success firing to Cone 04 which is a slightly higher temperature. Luckily, in the art education world, firing a kiln does not have to be super complicated. Unless you are highly specialized in ceramics, you will probably only ever do a low fire for both glazing and bisque. I taught at a magnet high school for the arts (Go Cavaliers!!!) in my former life, and only did a high firing a couple times for a special project which required the ceramic pieces to withstand the elements in an outdoor mosaic mural.

Every kiln is different, but the basics are the same.

Plug in the kiln
Press “Cone” and select your number. (Remember, Cone 05 is very different than Cone 5!)
Hold (I always press 0)
Preheat (Nope, wanna start right away!)
Speed (I have been using fast, but medium is a safer speed.)
Start….then the current temperature will start to blink and the kiln will start making a clicking to indicate the coils are heating up.
Close the lid and wait!

When your digital kiln has reached the programmed temperature it will alert you by beeping or making some sort of alarm. My kiln will then flash the peak temperature, which is key to knowing if your kiln mis-fired or not. I have had a kiln break and reach only 1,000-1,200 degrees. The clay pieces had not transformed to bisque and could not be glazed. Ugh.

When the kiln shuts off read your temperatures and then it is safe to unplug. DO NOT OPEN IT! The coils will still be glowing and dangerous for your eyes. Also, the sudden rush of cold air can cause clay pieces to break. Planning is key, so allow for a night to pass before opening up your kiln to unload it. You can prop it open an inch or so to allow for a quicker cool down. I never use this feature of my kiln because I only fire it from morning to when I’m leaving for the evening.

Glaze Firing

Loading pieces for a glaze firing has a few key differences. Glazes can trail and ruin your kiln. Using a kiln wash and star stilts should help protect your kiln and kiln furniture (shelves and support stilts) from permanent damage. Depending on the piece, you may or may not encourage students to glaze the bottom of their artwork. If they do, make sure you use star stilts to prevent contact with your kiln. They will be stuck to your kiln shelves forever otherwise!

Glazes are made with glass particles, among other ingredients, that when heated become a hard, shiny glassy surface. If two glazed pieces are touching, they will touch forever. A Dremel tool or a glass saw might help solve the problem, but your glazed pieces will never be the same.

Timing is key with a glaze firing because it usually takes more than one load to fit all the pieces without touching. I have had great success doing both firings at Cone 05 with the type of glazes I purchase for mass classroom use. If you are a more specialized ceramics teacher you may end up playing around with high fire and luster glazes. Just make sure you purchase clay that is suitable for high fire temperatures!

It is possible to fire both glazed pieces and bisque ware in the same load. If I have an absent student and need to fire just a piece or two of bisque ware I have been known to add it to a glaze load. CAUTION: If it explodes there will be tiny fragments of bisque STUCK ALL OVER THE GLAZED PIECES- and it ain’t pretty! It’s best to do them separately, but I make desperate bad decisions all the time in the name of procrastination.

For a full tutorial on glazing, check out my video below:

Glazing is just one of many surface decoration options. If you are short on time or budget, here are two other techniques that don’t require a second firing of your kiln.