Thursday, May 1
Cones before and after
I thought this might be interesting for any non-potters who may be reading this. What you see in this photo are cones. Cones are used to visually determine what is known as heat work. What this means is that, it is not enough say, to say that I am firing to cone 6 so shut the kiln off at 2190 degress or so. There are variables that can affect the final temperature.
If you were to fire very slowly, you will most likely reach glaze maturity at a lower temperature, say 2184 degrees or if firing very rapidly, you might have to go to 2232 degrees to reach the same glaze maturity. It's not precise, there is no absolute temperature when your work is "done". This is where cones come in.
Cones are formulated to bend or melt at the same rate as your glazes. In the photo above (albeit backwards, I set mine facing the other direction in the kiln) the blue cone is cone 5, the pink cone is cone 6, and the orange cone is cone 7. So what happens is the kiln fires merrily along. As it approaches the point where cone 5 starts softening, I start paying attention.
I stop Seinfeld or whatever else I was doing and start watching what is going on in the kiln. (I have a computer on my kiln so I fire more or less on autopilot according to what I have programmed in, but I ALWAYS pay attention closely towards the end and am present in the end so I can manually shut the kiln down if necessary). Cone 5 is the warning cone. It means you are getting close to temperature. Cone 6 is the cone I am firing to. I want it to bend to at least 9:00, although for my taste, 8:00 or 7:00 is even better. Cone 7 is the witness cone or safety cone. You do not want this cone to bend too much. I like for cone 7 to just begin to get soft, because that tells me that cone 6 is fully mature, which means my glazes are fully mature.
It is a fine dance between the two sometimes. Cone 7 goes too much, and so do your glazes and then you have a runny mess on your shelves which will keep you out of your studio for months from the depression of it all . . . but I digress. The colored cones in the photo are what they look like before firing, and then they turn white during the firing. It is not a good idea to fire without cones. Lots of people do it. To save money, out of laziness because they don't want to make cone packs, whatever. There is no substitute for a visual cue to what is going on in your kiln. Use cones when you fire. Period.