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As you know, kilns are not fired just to a temperature. They are fired to a "cone" level, which accounts for time as well as temperature. Think of it as heat absorption rather than just temperature.
Cones come in different numbers, each of which corresponds to a heating rate / temperature combination which will make that cone deform. The hottest is cone 10 that can go as high as 2381F (read more about firing to a cone and see a cone chart with temperatures).
When it comes to firing Cone 10, many people ask what is the difference between Cone 10 and TRUE Cone 10.
The difference is in how many firings you will get before you have to replace your elements. Any Cone 10 kiln should be able to fire to Cone 10 when the elements are new. And any Cone 10 kiln is sufficient for someone who fires mainly to Cone 6 or 8. But when consistently firing a kiln to its maximum cone, a couple factors should be taken into play.
Heat is created from Watts which is Voltage times Amps. A drop in eithcer of these will reduce the heating to your kiln.
Volts - Your line voltage may vary. Your voltage may be affected by how far you draw from a breaker box. Sometimes during peak loads, such as mid-day in summer when everyone is running their air conditioning, the voltage levels on the line are low. Some power companies are reducing voltages on purpose to help with the power crunch. Even whether you have a plug or direct wire the kiln can affect your voltage. The bottom line is that if your voltage is low, the kiln needs more amps to compensate or it may not be able to get to its highest temperature, or may take a long time to get there.
Amps - Every time you fire your elements they get a little weaker, or draw slightly fewer amps. In normal use elements cycle on and off throughout the firing, so kilns can compensate at first by keeping the elements on longer. But at some point they can’t compensate anymore. And at Cone 10 the kiln is typically already on for most of the time during the last few hours of firing so it is particularly taxing. At some point, in order to compensate for aging elements, the kiln needs to pull more amps. The True Cone 10 kilns are able to draw more amps, allowing you to fire more times before having to replace those elements. (Keep in mind that the same issue would occur when firing a Cone 6 kiln to Cone 6, which is why we suggest getting a kiln with a higher Cone rating than what you plan to fire. But since kilns are rarely spec’d at Cone 12 because other components limit them to 10, the True Cone 10 accomplishes the same thing. What you are really looking for is higher watt density, or higher watts per cubic foot.)
Of course, there usually are tradeoffs involved to achieve the highest temperatures. The main one is that you will need to have more power available, and some people just can’t get a larger circuit into their space. And at the larger sizes, the kiln has to be direct wired rather than have a plug, and some people insist on a plug.
Let’s take an example, the Paragon TnF24-3 which is Cone 10 and 48 amps, and Paragon Viking 24 which is the same kiln but has 60 amps. Obviously you're going to be able to fire the Viking more times before having to replace your elements. Thus we called it “True Cone 10”.
So what kilns are powerful enough if you want to fire to Cone 10? We recommend choosing between the following TRUE Cone 10 kilns that have more power for their size:
Remember that firing a TRUE Cone 10 kiln that has a higher than normal watt density requires more power going to the kiln, (more amps), which not everyone has. For example, instead of a 60 amp breaker you will need a 70 or 80 amp breaker.
The above-mentioned kilns have heady-duty electrical components or offer them as additional options. The heavy-duty electrics provide enough power (kilowatts per chamber volume) to hit Cone 10 without too much stress. They are also better insulated than an average electric kiln for higher efficiency and slower cooling.
One good way to compare different kilns is to compare their watts per cubic feet. If two kilns have the same interior space, the one with higher watts (voltage times current) will get hotter faster. 3" brick (rather than 2.5") also helps the kiln reach hotter temperatures, and is available on most kilns. Finally, adding the QUAD element option to any kiln will increase the element life considerably (3 to 4 times) - which is the biggest issue with cone 10 firing.
The number one reason people fire to Cone 10 in an electric kiln is to do crystalline glazes. If that applies to you, in addition to more power you may want APM elements (which are available for many kilns), and should consider the L&L JH Crystalline Kilns.
Some people prefer gas kilns to achieve True Cone 10 but gas kilns are expensive and difficult to install. If you are not already an expert in gas kilns, we would not recommend choosing one.
In the end, less than 5% of people with electric kilns actually fire them to Cone 10 and few do so on a regular basis. If you only fire to Cone 10 once in a while, a TRUE Cone 10 kiln is not a must. Most people fire Cone 6 in an electric kiln, because Cone 10 is a lot harder on elements and uses more power.
Also remember that you can get excellent Cone 5/6 stoneware and porcelain clay and that there are tons of great glazes available for Cone 5/6 that will deliver stunning results at Cone 6.