Firing and fixing · Loading and unloading a kiln
Shelves, posts, clearances and what can touch what.
Bisque and glaze firings want opposite things from you: a bisque load is packed and nested, a glaze load is spaced a finger’s width from everything. Under both, the same skeleton: three posts in columns, an inch from the wall, a cone on two shelves, and nothing opened above 200 degrees. Our staff load ours; this page is for a kiln of your own.
Four steps with a tick list at each: which firing, shelves and posts, cones and wash, and unloading. Six things that go wrong, and six videos from potters loading their own kilns.
This is the short version: bisque against glaze, the posts, the cones, the unload, each with a tick list. The reasoning, the tumble-stacking notes and the questions are in the comprehensive version.
Read it, then watch it
Why this is written down and filmed, both.
Loading a kiln is rules and a feel for a three-dimensional puzzle. The rules, three posts, an inch from the wall, a quarter inch between glaze, bisque can touch and glaze cannot, cones on two shelves, are a list a page holds. The puzzle, how a production potter fits sixty mugs into a load without a wasted inch, is a thing you watch. Our staff load our kilns, so this page is written for a kiln of your own, and for members who want to know why the drop-off rules exist.
Use both, in this order
Read the rules for the firing you are loading, bisque or glaze, because they are opposite. Watch a load of that kind packed. Then load with the tick list next to the kiln.
The whole idea
Bisque is packed. Glaze is spaced. Everything sits on three posts, an inch from the wall.
Bare clay can touch bare clay, so a bisque load is stacked and nested. Glazed clay is liquid glass at temperature, so a glaze load has a finger’s width around every piece, on cookies, naked bottoms. Three matched posts in columns to the floor, the bottom shelf off the floor, an inch from the wall, a cone on two shelves, nothing opened above 200 degrees. Our staff load ours.
The five-minute version
The whole load in six moves.
If you have loaded a kiln before and you just want the shape of it, this is the shape of it. Everything below is these six moves, slowed down and with the places they fail marked.
Before the first shelf
What you need at the kiln.
Six things, and one of them is a plan. More posts than you think is the advice every potter gives after their first load.
Shelves and plenty of posts, kiln wash on the shelf tops, witness cones, cookies and catchers, gloves and a diamond pad, and a loading plan.
Step one
Decide which firing this is, because the two loads are opposites.
Bisque is packed. Glaze is spaced. In a bisque every piece is bare clay, and bare clay can touch bare clay: stacked, nested, rim to rim, foot to foot, lids on. In a glaze firing every glazed surface is liquid glass at the top, and anything it touches, it joins: a finger’s width from every neighbour, post and wall, on a cookie, naked bottom.
Loading a bisque
Stack and nest, with two rules: nothing heavy on anything thin, and nothing wedged so tightly it cannot shrink, because clay shrinks in the bisque and a bowl jammed inside a bowl cracks both. Rim to rim for bowls, foot to foot for mugs, plates in short stacks, lids on. Tumble-stacking is a skill, and Florian Gadsby’s and ClayShare’s videos below are the two best lessons in it.
Loading a glaze
A quarter inch between glazed pieces is the minimum; a finger’s width is the habit. Nothing glazed touches a post, a wall, a neighbour or the shelf; every piece stands on a cookie with a bare foot, and anything that might run stands in a catcher. Lids only if the seat is bare and waxed both sides. Jonthepotter’s video is a glaze load with the gaps shown.
Both firings share the rest: dry work only, an inch from the wall, three posts, a cone. The bisque is where things explode, because it is the firing that meets water; the glaze is where things fuse, because it is the firing that makes glass. Our what a firing does lesson is the reasoning under both.
Step two
Bottom shelf on short posts, heavy things low, three posts in columns.
The first shelf sets up every shelf above it. It sits on short posts, never on the floor, so heat moves under it and the floor bricks are protected. Three posts, the same height, an inch from the wall, and a shelf that is level when you press each corner. ClayShare’s four minutes below is exactly this shelf.
Then the load goes up in columns. Every shelf sits on three posts, never four, because three points always sit flat and four rock. Each set of three sits directly above the three below, all the way to the floor, so the weight travels down through posts and not through the middle of a shelf. Half shelves let one side of the kiln run tall while the other runs short, and a load with a few half shelves fits far more than one with full ones.
Heavy, dense pieces go low, where the shelves are strongest and the heat is steadiest. Tall pieces go in the middle of a shelf, never at the edge, because the edge is where the elements are; an inch from the wall is the rule for everything and the thermocouple gets a hand’s width. Measure the tallest piece before the shelf above it goes on, because a shelf an inch too low is a layer wasted, and a shelf an inch too high is a pot with a lid on it.
Step three
Cones on two shelves, kiln wash on the tops, and the lid down.
The controller reads a thermocouple. The cone reads the firing. A witness cone bends when it has received a certain amount of heat over time, which is what a glaze cares about. One set on the bottom shelf and one on the top tell you whether the kiln fired evenly and told the truth.
A set is three cones: the one you fire to, one below it, and one above. The lower one bending first says you are close; the target bending says done; the upper one staying upright says not over. A cone pack in every load is how a potter knows the elements are ageing before the pots say so, and it is the first thing to look at when a load comes out wrong.
Kiln wash goes on the top face of every shelf, never the bottom or the edges, and gets patched whenever a drip has been ground off. A washed shelf lets a small drip chip away; a bare shelf is a permanent home for it. Then the lid down, the peepholes plugged the way the kiln’s maker says, and the schedule set. Our kilns fire cone 05 for bisque and cone 5½ for glaze; yours fires whatever your clay and glaze were made for.
Step four
Unloading has rules of its own: under 200, gloves, and a tap on every foot.
Nothing opens above 200 degrees Fahrenheit. Crack the lid at 250, open it at 200, unload at 150. A kiln opened hot hits its load with cold air, and clay and glaze contract at different speeds; the pings are crazing, and the crack is a plate that could not take it. Patience is free and plates are not.
Then gloves, because a warm kiln is still warm, and a tap on every foot as it comes out: a glazed pot that picked up a grain of sand, a kiln wash flake or a neighbour’s drip has a foot that catches on a table, and the diamond pad, wet, takes it off in a minute. Read the cones before you move the shelves. Drips get ground off shelves and the wash patched before the shelf goes back in.
Bisque comes out chalky, rings when tapped, and goes straight to a shelf, not a wet table. Glaze comes out with a naked foot that might have a bump on it and a glaze that might have done something the diagnostic in our glaze problems lesson can name. Then the shelves get vacuumed or wiped, the posts sorted by height, and the next load starts with the bottom shelf.
Watch it done
Now watch six people load six kilns.
Watch this one, then go count your posts.
None of these potters work here or know we exist. Between them they cover the whole load in three minutes, a bisque packed tight, the bisque tricks, a glaze load with its gaps, the bottom shelf, and stacking bisque in full. Nothing but the still frame loads until you press play.
Shelves, posts, clearances, cones and the lid in three minutes from a kiln maker. The other five videos are in the comprehensive version.
When it goes wrong
Six ways it goes wrong, and what to do about each.
The three failures that happen on the first load.
Pick the one that looks like your kiln. The first three are spacing, water and airflow. The last three are posts, walls and patience.
Two glazed pots fused together or to the shelf
They touched, or a glaze ran. In a glaze firing every glazed surface is liquid glass at the top, and anything it touches, it joins. A quarter inch is the minimum gap.
A finger’s width between every glazed piece and its neighbour, its shelf post and the wall. Naked bottoms on cookies. A catcher under anything that runs.
Our fused pottery lesson. Sometimes a tap separates them; usually the diamond pad does the rest, and the shelf gets ground and rewashed.
A piece exploded and took its neighbours with it
Water. A piece that was not dry all the way through, or was thick and hollow with no vent, turned its trapped water to steam. It is never air, and it is always a bisque firing.
Every piece room temperature to the cheek before it goes in. Pinholes in hollow forms. A slow first ramp on a bisque, candling overnight for anything thick.
Shards out, shelves vacuumed, the neighbours inspected for chips, and the next load gets a slower start.
The top fired hotter than the bottom, or the middle underfired
Airflow and mass. A kiln packed solid in the middle with no gaps for heat to move, or a bottom shelf on the floor, or a lid propped wrong, fires unevenly. Elements age unevenly too.
Gaps between shelves and the wall, staggered posts, the bottom shelf on short posts off the floor, and heavy dense pieces low. Witness cones on more than one shelf tell you what actually happened.
Read the cones. Refire the underfired ones. If it is the elements, that is a repair, not a loading problem.
A shelf wobbled or tipped
Posts not matched in height, or not stacked one above another through the load, or a warped shelf on three posts that did not agree.
Three posts per shelf, the same height, placed directly over the three below them so the load runs in columns to the floor. A warped shelf gets flipped each firing or retired.
Unload it and start that shelf again. A shelf that tips at temperature takes a load with it.
A pot touched the elements or the thermocouple
Loaded too close to the wall. A pot against an element ruins the pot and can break the element; a pot against the thermocouple lies to the controller about the temperature.
An inch from the wall for everything, and a hand’s width around the thermocouple. Tall pieces in the middle of a shelf, never at the edge.
The pot is probably fine if it was bisque and probably fused if it was glaze. Check the element for damage before the next load.
You opened it too early and things cracked
Thermal shock. A kiln opened above about 200 degrees Fahrenheit hits the pots with cold air, and glaze and clay contract at different speeds. The ping you hear is crazing, and the crack you see is a plate that could not take it.
Under 200 degrees, ideally under 150, before the lid opens more than a crack. Peeking with the kiln at 400 is how plates split.
Whatever cracked is cracked. The rest of the load is fine. Close the lid and wait next time.
How it works here
At The Clay Hole, our staff load and unload every kiln.
This page is written for potters with a kiln of their own. Here is what happens to your work between the drop-off table and the pickup shelf, and why the rules are what they are.
Three things to know here
Our staff load and unload every kiln; members put dry, finished, paid work on the drop-off tables and collect it from the pickup shelves. Every drop-off rule is a loading rule from this page: dry to the cheek, naked bottom on a cookie, a catcher under anything that runs, a ticket. Bisque is cone 05 and free; glaze is cone 5½ at five cents a cubic inch, paid before drop-off.
The drop-off table
Dry to the cheek, because a damp piece explodes in the bisque and takes its neighbours. Naked bottom on a cookie, because a glazed foot fuses to a shelf. A catcher under anything that runs. A ticket, so it comes back to you. Every one of those is a loading rule from this page, seen from the other side.
Why staff load
A kiln load is dozens of members’ work packed by someone who does it every day, and one badly placed pot can cost everyone on the shelf. Loading is a craft of its own, and here it is a staff craft. This page is so you know what they are doing and why your piece came back the way it did.
Firing and pickup
Bisque is cone 05 and free. The glaze firing is cone 5½ at five cents a cubic inch, $4.80 minimum, paid before the piece goes on the drop-off table. Fired work comes back to the pickup shelves; take it home within two weeks, because the shelves are shared.
Your own kiln
If you have one, every rule on this page applies to it, and the videos are from potters loading theirs. Pots thrown here from our clay can go home to your kiln; pots for our kilns are made from our clay, because a body we do not know at cone 5½ is a body that can spit on somebody else’s work.
Questions people actually ask
The ones that come up every week.
What is the difference between loading a bisque and loading a glaze?
Bisque is bare clay, and bare clay can touch bare clay: pieces can be stacked, nested, rim to rim, foot to foot, a bowl inside a bowl, as long as nothing heavy sits on anything thin and nothing is wedged so it cannot shrink. Glaze is liquid glass at temperature, and nothing glazed can touch anything: a quarter inch from every neighbour, post and wall, on a cookie, with a naked bottom. Bisque is packed; glaze is spaced.
How far apart do things go?
A quarter inch between glazed pieces is the minimum and a finger’s width is the habit. An inch from the kiln wall for everything, in both firings. A hand’s width around the thermocouple. Between shelves, an inch above the tallest piece on the shelf below.
How do posts work?
Three per shelf, never four, because three points always sit flat and four rock. The same height as each other. Placed directly above the three posts on the shelf below, so every column of posts runs straight to the floor and the weight goes down through the posts, not the shelves. Half shelves let you stagger heights on each side.
Can lids fire on their pots?
In the bisque, yes, always, so they shrink together and fit. In the glaze firing, only if the seat is bare on both sides, waxed and cleaned, so the lid does not fuse to the pot. Many potters glaze fire lids on for fit and accept the wax work that takes.
What are cones for?
Witness cones are small pyrometric cones set on the shelf that bend when they have received a certain amount of heat over time, which is what matters to a glaze. The controller reads a thermocouple; the cone reads the firing. A cone on the bottom shelf and one on the top tell you whether the kiln fired evenly, and a cone in every load is how a potter knows the kiln is telling the truth.
When can I unload?
Under 200 degrees Fahrenheit, and under 150 is kinder. Crack the lid an inch at 250, open it at 200, unload at 150. A kiln opened hot pings and cracks its own load, and the pots that survive it craze weeks later. Patience is free and plates are not.
Who loads the kilns at The Clay Hole?
Our staff, every load, bisque and glaze. Members put finished, dry, paid work on the drop-off tables and pick it up from the pickup shelves, and the whole of this page is what happens in between. It is written for potters with a kiln of their own, and for members who want to know why the drop-off rules are what they are.
Related lessons
Six lessons that sit next to this one.
Three to read next.
A kiln load is drying, catchers and glaze fit seen from the shelf. These are the pages on either side.
That is the whole load
Bisque packed. Glaze spaced. Three posts in columns. Cones on two shelves. Nothing opened above 200.
Your first load will have a shelf an inch too low and a mug you forgot to give a cookie, and the second will have a plan taped to the kiln. That is how everyone learns it, including the people who load ours. If you never own a kiln, now you know why the drop-off table asks what it asks.
If you want the reasoning, the stacking notes, the cone detail and the answers to the questions everybody asks, switch to the comprehensive version at the top. Otherwise, go count your posts.
Written for potters everywhere. Where our studio does things differently, the page says so.
Which version would you like to print?
Both print clean, with the videos turned into labelled boxes you can search for later.