Whether its a smart phone, a car that parks itself, or the latest in sportswear, advances in technology bring changes in the way we do things every day. The same can certainly be said of the computer controller, and firing a kiln. Where the kiln sitter once dominated the market, now the computerized kiln has come to dominate the way that we fire kilns. The level of control ranges anywhere from simple shutoff at a pre-determined cone value, all the way to atmospheric type, flame and draft-rate controls, that you can monitor from across the country. We might begin to ask ourselves “what with all the bells and whistles that come with a computerized kiln, are cones even necessary anymore?” Frankly, I believe that the pyrometric cone is essential to the ceramic studio. This month’s Tech Tip Tuesday is about the humble witness cone.
Pyrometric cones have been around a LONG time. Orton has been making and selling cones since 1896. Originally, cones were arranged into a cone pack, and placed in a kiln where one could observe them during the firing. When the tip of the target cone bends to the 4 o’clock position, then you know the kiln has hit that cone
Cone packs are made using three (or sometimes more) cones to let the operators know when to do certain thinks in the kiln. For standard ‘shutoff’, the three cone method is used. The first cone is the Guide Cone, the middle is the Firing Cone, and the third is the Guard Cone. The Guide Cone will bend first, and alert you that its almost time to shut the kiln off. The Firing Cone lets you know when the kiln has hit the target value, and the Guard Cone is there to ensure you don’t over-fire.
Making Cone Packs
Cone packs are not difficult to make, its a simple wad of clay, but there are some rules that you should follow:
If you hold a cone upright against a table, you will notice that it leans at an angle. This angle is the proper mounting angle in a cone pack, and care must be taken to ensure that this angle is maintained. For those of you who want to know, the angle is 8 degrees.
When pushing the cone into the wad of clay, the height matters. If you push the cone too far into the cone pack a mere 1/4″, it will cause the cone to read off by more than half a cone!! Large cones should be pushed into the clay such that exactly 2″ remains above the cone pack. Junior cones can be used, but they should have 15/16″ exposed above the clay. USE A RULER to get it right.
Make sure you make your cone packs IN ADVANCE, and let them dry completely before firing. Make a bunch at a time, to ensure you always have the cone pack you need for that firing.
What if I don’t want to make a cone pack?
For Junior cones, there are wire holders available, for Large cones, there are clay cone-packs available Both are reliable, but cost more than the clay you would use to make a cone pack. For my money, I just make the pack.
Alternately, you can buy Self Supporting cones, and just set them on the shelf. The box costs more, and you get fewer cones in it, but the convenience is certainly worth it.
So why should I use a witness cone if I have a computer controller?
Thermocouples can only measure temperature. Cones measure heat-work. Computers use a mathematical algorithm to determine when the appropriate amount of heat work has been done. As the thermocouple starts to age, its reading starts to roam away from true. Eventually the kiln will under-fire, even though the computer thinks it is firing appropriately. Having a witness cone where you can see it will allow you to verify the kiln has hit cone before it is too late to do anything about it.
Cones also protect against OVER-firing as well. When the cone has bent to 4’oclock, you know it is time to shut the kiln off…regardless of what the computer thinks.
Lastly, cones can let you know more about what is going on in your kiln than just the amount of heat work. If your cone is bloated, black, snapped, or otherwise shows something strange, it can be analyzed to determine what else is going wrong.
So how often should I use witness cones in my computer controlled kiln?
At first, I recommend using witness cones about every 5th firing…or during a firing if the ware is super critical. Once you start to see variance between what the cones are telling you, and what the computer is telling you, then use them every firing. At that point, place them on the outer edge of the shelves (where you can see them through the spy hole), AND in the center of the kiln as well.