Pottery cone numbers describe how much heat work a firing reaches, using small ceramic cones that bend at a known point. Numbers with a leading zero, like cone 06 or 04, are low fire and get cooler as the number gets bigger, while plain numbers like cone 6 or 10 are mid and high fire and get hotter as they rise. Clay and glaze labels list a cone so you can fire them at the right range.
Pick up almost any bag of clay or jar of glaze and you’ll see something like cone 6, cone 04, or cone 5 to 6 printed on the label. If you’ve ever wondered what those pottery cone numbers mean, and why some have a zero in front, you’re in very good company. It’s one of the most confusing bits of studio jargon for beginners.
At Le Studio Art Space in Mordialloc, we explain cones to new members in their first few weeks, because understanding them makes choosing clay and glaze so much easier. This guide breaks down what a cone actually is, how the numbering works, and how to read a label with confidence.
What A Pyrometric Cone Actually Is
A pyrometric cone is a small, slender pyramid made from a carefully controlled ceramic mix. Each cone is formulated to soften and bend once it has absorbed a particular amount of heat. When the tip bends over until it’s level with the base, the cone has reached its target.
The idea dates back to the late nineteenth century. German ceramicist Hermann Seger developed an early cone system in the 1880s, and in the United States Edward Orton Jr. began producing cones in the 1890s. Orton cones are still the standard reference most potters use today, including in Australia.
Potters place cones in the kiln where they can see them through a spy hole, or where they can check them after the firing. These are often called witness cones. Many potters set out three cones together, one a step cooler, one at target, and one a step hotter, to see exactly how the firing went.
Some kilns also use small cones inside a device called a kiln sitter, which switches the kiln off when the cone bends. Modern electronic controllers use temperature sensors instead, but they’re programmed around the same cone numbers. Our guide to the parts of a kiln explains how these pieces fit together.
How Pottery Cone Numbers Work
Pottery cone numbers run along a single scale, but the way they’re written trips everyone up. The lowest cones have numbers with a zero in front, such as 022, 018, 06, and 04. For these, a bigger number means a cooler firing, so cone 06 is cooler than cone 04.
Once you pass cone 01, the zeros disappear and the numbers count upward in the normal way. Cone 1 is hotter than cone 01, cone 6 is hotter than cone 1, and cone 10 is hotter again. A simple way to remember it is that the zero numbers count down towards cone 1, and the plain numbers count up from it.
Potters group cones into three rough ranges. Low fire usually covers around cone 06 to 04 and is common for earthenware, paint your own pottery, and many bright glazes. Mid fire, around cone 4 to 7, is popular for stoneware in community studios. High fire, around cone 8 to 10 and beyond, is traditional for porcelain and some stoneware.
As a rough guide, at a medium heating rate Orton charts list cone 04 at around 1063 degrees Celsius, cone 6 at around 1222 degrees Celsius, and cone 10 at around 1285 degrees Celsius. Those are reference points rather than exact targets, for reasons we’ll come to shortly.
Why Cone Numbers Matter For Clay And Glaze
Every clay body and glaze is designed to mature within a particular cone range. At the right range, clay becomes strong and dense, and glaze melts into a smooth, stable surface. That’s why manufacturers print a cone number on their labels.
Fire a clay too low and it stays weak and porous, which can make mugs leak or chip easily. Fire it too high and it can warp, bloat, blister, or even slump into a puddle on the kiln shelf. Our article on why pots crack in the kiln covers some of the related problems.
Glaze behaves the same way. A cone 6 glaze fired at cone 04 will usually look dry and rough because it hasn’t melted. A cone 04 glaze fired to cone 6 may run off the pot, bubble, or change colour dramatically.
The key rule is matching. Your clay and your glaze should both suit the cone your kiln will reach. Once that’s sorted, most of the guesswork disappears from choosing materials.
Heat Work Versus Simple Temperature
Here’s the part that surprises a lot of people. A cone doesn’t simply measure temperature. It measures heat work, which is the combined effect of temperature and time.
Think of it like baking. A cake in a slightly cooler oven for longer can come out much the same as one baked hotter for less time. Ceramics work similarly. A slow firing can bend a cone at a lower peak temperature than a fast one, because the clay and glaze have had longer to react.
That’s why cone charts list different temperatures for different heating rates. Orton’s own charts show, for example, that cone 6 bends at a noticeably lower temperature when the kiln climbs slowly than when it climbs fast.
For a potter, this means that cones are often a better guide to what’s happening to the ware than the number on a digital display. Many studios keep using witness cones even with modern controllers, because the cone shows how the pots actually experienced the firing.
What Competing Guides Leave Out About Pottery Cone Numbers
Most explanations of pottery cone numbers stop at the chart. They rarely explain how a shared studio decides which cone to fire to, which is something every new member needs to know.
Community and teaching studios usually pick a standard firing range and build everything around it. Many choose mid fire, around cone 5 or 6, because it gives strong, durable stoneware with lower energy use and less kiln wear than high fire. Their clay and glaze stock is then chosen to match.
So what if your glaze label doesn’t match the studio firing? Don’t simply put it in and hope. Ask the studio first. Some glazes work across a range, such as cone 5 to 6, but a glaze made for cone 04 might run in a cone 6 firing and damage kiln shelves or other people’s work.
Studios with a separate low fire schedule may be able to fire a mismatched piece in a different load. If not, it’s often better to choose a glaze from the studio’s range. Our firing service team can tell you which cones we fire to.
It’s also common for clay and glaze to list different numbers for the bisque and glaze firings. A clay might suggest bisque at cone 04 and glaze at cone 6, which is normal and simply means two separate firings. Our guide to kiln firing temperatures explains how the two fit together.
Reading Pottery Cone Numbers On A Label
Let’s put it into practice. A label that says cone 06 to 04 means the product is designed for low fire, so it suits earthenware clay and a low temperature kiln. It probably won’t suit a stoneware studio firing to cone 6.
A label reading cone 5 to 6 is mid fire. This is common for stoneware clays and glazes used in community studios, and it’s a range many beginners end up working in.
A label reading cone 10 is high fire. Some cone 10 clays can still be used at lower cones for specific purposes, but it’s best not to assume. Check with your supplier or studio before using it outside its stated range.
If a label shows a wide range, like cone 06 to 6, it usually means the product works at both low and mid fire, though the colour or finish may vary. Trying a test tile before glazing a whole piece is always worth it. If you’re buying materials for the first time, our guide on where to buy pottery clay can help, and our workshops and classes include plenty of hands-on practice with real firing schedules in our ceramic studio.
Still unsure which cone your clay or glaze needs? Ask about firing ranges and our firing service at Le Studio Art Space in Mordialloc, and we’ll help you match your materials to the right kiln load.
Frequently Asked Questions
Why is cone 06 cooler than cone 6?
Cones with a leading zero sit on the low end of the scale and count down towards cone 1. Cone 06 is a low fire cone, while cone 6 is a mid fire cone and much hotter.
What cone do most pottery studios fire to?
Many community studios fire to mid fire, around cone 5 or 6, for durable stoneware. Studios differ, though, so check with yours before buying clay or glaze.
Can I use a cone 6 glaze on cone 04 clay?
Usually not well. Firing hot enough for the glaze can damage the low fire clay, and firing low enough for the clay leaves the glaze under melted.
Do I really need witness cones if my kiln has a digital controller?
Many potters still use them, because cones show the actual heat work your pots received. They’re especially useful for spotting a kiln that’s firing hotter or cooler than expected.
Does Le Studio Art Space fire low fire and mid fire work?
Our firing service team can tell you which cone ranges we currently fire and whether your piece suits a load. It’s always best to ask before glazing so your materials match.
This article is provided for general educational purposes by Le Studio Art Space. Cone temperatures are approximate and vary with heating rate, so always follow your clay and glaze manufacturer’s guidance.