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When you hear people referring to the temperature that they are firing their kiln to, you will probably hear them talking about using cones, or perhaps firing ‘to a cone’, of a particular number. This may have left you wondering, what are cones in Pottery? In this article, I look at that question…
Cones are pieces of ceramic that help you gauge whether a kiln has reached sufficient temperature and whether the pottery will have been fired the correct amount. Cones measure ‘heat-work’, which is a combination of the temperature reached, and the time it took to become that hot.
What are Cones in Pottery?
Cones in pottery also called pyrometric cones or witness cones, are cone-like pieces of ceramic that are designed to help you fire your pottery to the right temperature.
However, cones do not just indicate the level of temperature that has been reached, they also help you gauge if the temperature has increased at the optimal rate, and if the kiln has been at the right temperatures throughout the firing schedule for the correct amounts of time.
During firing a piece of pottery undergoes chemical and physical changes. These changes will take place a different times, depending on how quickly the temperature is changing in the kiln. Cones help you gauge when the pottery has undergone the changes needed to have turned into ceramic.
The Pottery Cone Range
There is a range of cones, from those that measure low-temperature firing to those at higher temperatures. The lower end of the chart starts at cone 022, which is about the lowest cone available for firing pottery. The chart goes all the way up to 14.
The higher up the chart the higher the firing temperature. There are some cones above the 14 mark. For example, Orton cones go up to 42, but most potters stay within the 022-14 range. Cones between 13 and 42 are normally used for industrial products.
Whilst 022 might sound like a higher cone number than 014, this is incorrect. The cone numbers that start with a zero (eg 014) are effectively negative numbers. The zero is rather like a minus sign. 022 is lower than 020.
When the cone numbers get to 1 and above, then they can be thought of as positive numbers. So, cone 6 is higher than cone 1. And so on. This can cause some confusion because potters new to firing can, for example, get cone 05 mixed up with cone 5. In reality firing to cone 05 is a much lower temperature than firing to cone 5.
Cones in Pottery can be Visualized on a Spectrum:
What are Different Cone Ranges Used For?
Very Low Fire Cone Range
This range is used for overglaze decorations, like luster glazes and enamels. Luster glazes are applied to pieces that have already been glazed. Also, luster glazes come in a number of finishes, including gold, silver, platinum, and mother of pearl.
Low Fire Cone Range
Bisque fires are normally done between 04 and 06. 06-04 is sometimes referred to as the bisque scale. Earthenware clay is normally fired in this range.
Mid-Fire Cone Range
Mid-fire range – Many pieces are fired at cone 6 because most electric kilns go to cone 6. Glazes fired at the mid-range work through a process of oxidation. In oxidation firing, oxygen interacts with the glazes to create very bright colors.
High Fire Cone Range
High fire range – Glazes fired at the high range work through a process of reduction. In reduction firing, oxygen is prevented from interacting with the glaze.
Approximate Temperatures for Different Cone Firing Ranges
| Temperature range | Cone Range | F | C |
|---|---|---|---|
| Very Low Range Low Fire Range Low Mid Range Mid Fire Range High Fire | 022-013 012-02 01-3 4-7 8-14 | 1112-1566 F 1623-2048 F 2079-2134 F 2167-2264 F 2305-2530 F | 605-850 C 882-1120 C 1110-1145 C 1165-1210 C 1260-1390 C |
The above information suggests that cones are categorized only according to the temperature that is reached. However, firing to a particular cone is not just a matter of firing at a particular temperature, it is about how quickly you get to that temperature too.
If you fire a piece too quickly, it can crack or become damaged. That is why firing schedules state how quickly the temperature should increase. Also, if you speed through a firing, without giving the clay enough time to be fired at a given temperature, then it does not undergo the necessary chemical and physical changes that are required and the resulting piece of pottery may be of poor quality.
Pottery that is fired at a higher temperature, and is therefore further along on the cone spectrum will be harder and more resilient than pottery that is fired using a lower cone number.
How Do Pottery Cones Work in a Kiln?
Cones in pottery are designed to start melting and wilting at certain temperatures. They have chemical and physical properties that determine their melting point. Each cone number is correlated with a different melting point.
Different clays and glazes are made to fire at different temperatures, therefore the cones that you use to fire a piece will depend upon the materials you are using.
So, for example, if you are doing a bisque fire, you would most likely use a cone between 04 and 06.
When the cone starts to melt then you know that your kiln has started to reach the temperature at which the clay will have been sufficiently fired.
In short, the numbers on the cone refer to the point at which a clay or glaze has matured. Maturity means that the clay has become sufficiently hot over a particular period of time to become dense and hard. The density and hardness are caused by vitrification and sintering. The cone number will also be related to the point at which the clay or glaze melts, which happens if the pottery is overheated.
Do You Always Have to Use Pottery Cones?
Cones measure if sufficient heat has been put into the clay to either bisque fire or glaze the. Therefore, if you have a computerized kiln, the computer will be able to calculate whether the right conditions have been met.
As stated above, the right condition is not just that a particular temperature has been met, rather the right conditions include how quickly that temperature was reached.
This determines whether the clay had a chance to undergo the necessary chemical and physical changes as the heat increased.
Although an electric kiln with a computerized programming function may not need cones as a matter of course, cones can still be useful if the potter is having difficulty achieving the kind of effect they are looking for.
Perhaps the potter has set the kiln and the ware isn’t coming out as they had hoped. There may be a problem with the kiln. Perhaps an element is not functioning as it should. Positioning cones in the kiln can give the potter more information about what is happening during the firing. Importantly, this means that they can troubleshoot, make adjustments and address the problem.
By contrast, wood burning and gas kilns do require cones as a matter of course, to determine whether the clay has been sufficiently fired.
What are the Different Kinds of Cones in Pottery?
Broadly speaking there are three different kinds of pyrometric devices, they are:
- Cones
- Bars
- Rings
Cones
Cones are three-sided pyramids made from clay with a particular composition. They can either be large, self-supporting cones, or they can be small cones that are positioned in a row.
Large and Self-Supporting Pottery Cones are Free-Standing
They measure if enough heatwork has been done to the clay or glaze to have reached maturity. Once the cone has bent over enough so that the tip is just about touching the surface of the kiln, then this indicates that the piece has been sufficiently fired.
Small pottery cones
However, potters also use small cones stuck into a piece of clay or a cone support beside one another in a row. If four cones are being used, then they will be of different values, placed in descending order. So, for example, if a potter is firing to cone 6, they would have a row of four cones, starting with one higher than cone 6 and then descending to cone 4.
This means that 4 cones sit in a row in the kiln, going from cone 4 up to cone 7. As the temperature of the kiln increases, the cones of lower value will start to bend. Eventually, when the right temperature for a cone 6 firing is reached, the cone 6 will start to bend too.
Once the cone is bent over with the tip of the cone just about touching the shelf and appears to be flattened, then the right temperature has been reached. Cone 7 might also have started to show signs of bending too.
Firing is about heat absorption, so the right temperature has to be maintained for long enough for the cone to fall. Normally a cone will fall if the right temperature has been maintained for around 20 minutes.
Potters either make their own stands, by pressing the small cones into a piece of clay, or it is possible to buy cone stands into which the cones are placed.
Bars
Bars are square and are held across two supports. When the kiln starts to heat up the bars start to bend and sag. They are often used in a device called a Kilnsitter. A Kilnsitter is a mechanical device in the kiln. Once the bar has sagged enough, indicating that the clay will have reached maturity it triggers the Kilnsitter to turn off the kiln.
Rings
Another way of measuring heatwork is to use a ring. These are hollow rings that contract during firing. The amount by which it contracts indicates how much heatwork has been done to the pottery ware. A gauge measures how much the ring has contracted.
Some potters state that the most accurate way to gauge if enough heatwork has been done is to use a large self-supporting cone that is positioned very close to the pottery ware that is being fired.
What are Pottery Cones Made of?
Cones are made of ceramic materials. They also contain fluxes. Fluxes are materials that lower the high melting point of the glass-forming components in the clay. The more flux a cone contains the more quickly it will bend. Therefore, cones with different numbers will contain different amounts or ratios of flux.
The contents of the cone are designed to mimic the behavior of different clays or glazes.
Final Thoughts
Stated briefly, to answer the question of what are cones in pottery? Cones are pieces of ceramic, that can be either large and free-standing or supported in clay or a stand. Each cone has a different numeric value. These values correspond to the point at which they will melt.
These melting points, when used in a kiln will indicate if the clay or glaze being fired has had sufficient heat work (energy absorption) to have transformed it sufficiently. The desired changes depend on the kind of clay being fired and the purpose of the fire, for example, whether it is a bisque fire or a glaze fire.