Your kiln controller can tell you the temperature. Witness cones tell you what the pottery actually experienced.
That difference matters.
If you are getting inconsistent glaze results, wondering whether your kiln really reached Cone 6, or testing a new firing schedule, learning how to read witness cones can give you information that a digital controller cannot provide on its own.
For U.S. home potters and small studios, witness cones remain one of the simplest ways to check whether a firing delivered the expected heatwork.
What Are Witness Cones?
Witness cones are small, specially formulated ceramic cones designed to bend when they receive a specific amount of heatwork.
The important word is heatwork.
A kiln does not fire pottery based on temperature alone. Time and temperature work together during a firing. As a result, two firings can reach a similar peak temperature while producing different results.
Witness cones provide a physical record of what happened inside the kiln.
For example, a Cone 6 firing does not simply mean that the kiln display reached a particular number. Instead, the clay and glaze experienced a combination of temperature and time that produced a certain amount of heatwork.
That is why experienced potters still use cones even when their kilns have sophisticated digital controllers.
Why Should You Use Witness Cones?
Digital controllers are extremely useful, but they are not perfect.
A controller measures temperature at the location of its thermocouple. Your pottery sits elsewhere in the kiln.
Because of that, the temperature near the thermocouple may not be exactly the same as the temperature experienced by ware at the top, middle, or bottom of the chamber.
Kiln loading also changes airflow and heat distribution.
Element condition can change over time.
The firing schedule can affect heatwork.
Even the amount of pottery inside the kiln can influence how the firing behaves.
Witness cones give you another layer of information.
If your Cone 6 witness cone bends properly, you have evidence that the kiln delivered approximately the intended heatwork at that location.
How Do You Read a Witness Cone?
Reading a witness cone becomes much easier once you know what you are looking for.
A cone should normally bend gradually toward the shelf as it reaches its intended heatwork.
A properly fired target cone is often described as bending to approximately a 5 o’clock position.
However, you should not judge a firing from the target cone alone.
A useful setup normally includes three cones.
The first is one cone below your target.
The second is your target cone.
The third is one cone above your target.
For a Cone 6 glaze firing, that means Cone 5, Cone 6 and Cone 7.
This arrangement gives you much more information.
If Cone 5 is down while Cone 6 is just beginning to bend, your firing may have reached the lower part of the expected range.
If Cone 6 is properly bent while Cone 7 remains mostly upright, the firing is behaving much closer to the intended Cone 6 heatwork.
When both the target and hotter cone are significantly bent, you may have fired hotter than intended.
What Does an Underfired Cone Look Like?
An underfired cone remains relatively upright.
The cone may show a small amount of bending, but it has not reached the heatwork required for its normal end point.
Suppose your glaze requires Cone 6 but the Cone 6 witness cone barely bends.
That result should make you investigate before blaming the glaze.
Check the firing schedule first.
Then look at the kiln load, element condition, thermocouple, ventilation and other variables that could affect firing performance.
A single underfired cone does not automatically mean your kiln is broken.
Instead, treat it as useful diagnostic information.
What Does an Overfired Cone Look Like?
An overfired cone bends well past its normal position.
If your target Cone 6 is lying significantly farther forward while Cone 7 is also bending heavily, the kiln may have delivered substantially more heatwork than expected.
That can create problems.
Glazes may become excessively fluid.
Clay can become over-vitrified.
Pieces can deform.
Certain glaze surfaces may change dramatically.
Therefore, an overfired cone deserves attention, especially if the same pattern appears repeatedly.
Where Should You Put Witness Cones?
Placement matters.
A cone placed directly beside the thermocouple may tell you less about the rest of the kiln than a cone positioned within the ware area.
For a first test, many potters place witness cones where they can be viewed through a peephole or kiln observation system while still representing the firing environment experienced by the pottery.
Larger kilns may benefit from multiple cone packs.
Try placing cones at different levels if you are investigating uneven firing.
For example, a cone pack near the bottom can be compared with another near the middle and another near the top.
The results can reveal whether different areas of the kiln are receiving different amounts of heatwork.
That information is particularly valuable when the same glaze behaves differently depending on shelf position.
Why Cone Placement Matters in a Full Kiln
An empty kiln can behave differently from a loaded kiln.
A full load absorbs heat and changes how energy moves through the chamber.
Consequently, testing a kiln completely empty may not tell you exactly how it behaves during normal production firing.
For your first few firings, keep notes.
Record the firing program.
Record the cone pack location.
Record the type and amount of ware.
Then compare the witness cones after the firing.
Over time, these records can become a useful map of how your kiln behaves.
Can Witness Cones Replace a Kiln Controller?
No.
Witness cones and digital controllers serve different purposes.
The controller runs the firing according to the programmed schedule.
The witness cone provides physical evidence of the heatwork received at its location.
Think of the controller as the system directing the firing and the cone as a verification tool.
Using both gives you a stronger understanding of what happened.
That combination is especially useful when you are troubleshooting glaze defects or testing a new firing schedule.
Cone 6 and Cone 06 Are Not the Same
This mistake causes serious problems for beginners.
Cone 6 and Cone 06 are completely different firing ranges.
The zero matters.
Cone 06 is a low-fire cone.
Cone 6 is a much hotter mid-range firing.
Therefore, always check whether your clay body and glaze call for Cone 06, Cone 04, Cone 6 or another firing range.
Never assume that adding or removing the zero is simply a formatting difference.
When buying firing supplies, double-check the cone number before ordering.
The Kiln Depot carries Orton pyrometric cones for pottery firing and provides cone information for potters who need to match firing materials to their kiln schedule.
How Witness Cones Help Troubleshoot a Kiln
Witness cones become particularly useful when something suddenly changes.
Imagine that your Cone 6 glaze normally looks glossy and well melted.
One firing produces a noticeably different surface.
Before changing the glaze recipe, look at your firing evidence.
Did the target cone bend normally?
Did the cone below it bend more than usual?
Did the hotter cone also bend?
Was the cone pack positioned differently?
Was the kiln loaded differently?
Those answers can help separate a glaze problem from a firing problem.
Similarly, if the bottom shelf consistently underfires while the middle shelf looks correct, multiple cone packs can help confirm that you have a heat distribution issue.
That is much better than guessing.
How Often Should You Use Witness Cones?
There is no reason to make every firing unnecessarily complicated.
However, witness cones are particularly useful when you are:
- Setting up a new kiln
- Testing a new firing schedule
- Troubleshooting inconsistent results
- Replacing kiln elements
- Checking an older kiln
- Changing how you load the kiln
- Seeing unexpected glaze results
- Comparing different shelf positions
For a new kiln, regular cone checks during the early firings are especially valuable.
Once you understand how the kiln behaves, you can use cones strategically whenever something changes.
Keep a Firing Log
A simple firing log can make witness cones much more useful.
Write down the date.
Record the kiln model.
Note the firing type and target cone.
Write down the firing program.
Record where the witness cones were placed.
After the kiln cools, record how each cone bent.
Add notes about unusual glaze results or problems.
After several firings, patterns become easier to see.
You may discover that the top shelf consistently fires cooler than the middle, or that a particular loading pattern produces more even results.
That information is far more valuable than relying on memory.
Frequently Asked Questions
What does a properly fired witness cone look like?
A properly fired target cone should bend gradually toward the shelf, generally reaching the commonly used endpoint around the 5 o’clock position.
Should I use three witness cones?
Yes. Using one cone below, the target cone, and one cone above gives you a clearer picture of how much heatwork the firing delivered.
Can I use witness cones in an electric kiln?
Absolutely. Witness cones are widely used with electric pottery kilns to verify heatwork and investigate firing consistency.
Why did my kiln reach the programmed temperature but my cone look underfired?
Peak temperature does not tell the entire story. Heating rate, firing time, load size, thermocouple location and other factors can affect the heatwork received by the ware.
Where should I place witness cones?
Place them where they can represent the conditions experienced by your pottery and where they can be viewed safely according to your kiln manufacturer’s instructions. Multiple cone packs can be useful when investigating temperature differences.
Do I need witness cones for every firing?
Not necessarily. They are especially valuable when testing, troubleshooting or verifying a kiln. Many experienced potters also use them routinely because they provide useful firing evidence.
Final Thoughts
Learning how to read witness cones is one of the simplest ways to become more confident with kiln firing.
Your digital controller tells you what the kiln was programmed to do.
Your witness cones help show what actually happened at their location inside the chamber.
That distinction can save pottery, glaze materials and plenty of frustration.
Start with a three-cone pack.
Watch how the cones bend.
Record the results.
Then compare those results with your glaze surfaces and clay bodies.
After a few firings, you will begin to understand your kiln rather than simply trusting the number on its display.
For potters building a reliable home studio, that knowledge is worth far more than another setting on a controller.
