This is a lesson for anyone out there using a pyrometer.
I’ve been using a pyrometer for the last 5 years and I have come to I love them! There can be problems with the cones in a soda firing and the pyrometer can be a great backup to the cones. Around c.6, the residual soda in the kiln can start to vaporize. Sometimes this soda can hit the cones and “freeze” them temporarily. Then all of the sudden, it “unfreezes” and the cones fall quickly. Then sometimes the soda can flux out the cones prematurely. Sometimes the cones on the top melt prematurely and the cones on the bottom freeze. And sometimes they are fine and go down when they are supposed to go down. By using both the cones and pyrometer, I feel like I am not firing totally blindly. Neither is perfect, but when used together, I have enough information to comfortably fire with.
There are 2 types of thermocouples that potters use, type J and type K. Type J is rated to about 2000′ F, and type K is rated to almost 2500′ F. The pyrometer is the same for both thermocouples, you just have to push a little button to switch from one type to the other. For the beginning of the firing, if you have it accidently pushed to type J when using type K, it will read normally. Then all of the sudden, your perfect firing seemingly goes crazy. For a couple of minutes, you think something is wrong, even though everything was going along just fine. Don’t make any adjustments, don’t play with the damper, don’t turn up the gas or air. Just check the pyrometer and make sure that the setting for what type of thermocouple you’re using is correct. Nothing has actually happened to your firing, it’s just that the gadget wasn’t working properly (user error). I teach people pretty regularly how to fire kilns and so I get to see that certain “mistakes” are common. This is one of those common mistakes.
I had been told by a couple people last month that they were having a problem at the end of their firings. After all the soda was in and the kiln was continuing to climb, something weird kept happening. The kiln dropped 15 degrees for basically no reason. Adjustments were made and the kiln started to climb back up. It was odd; I had never seen this happen before. Then it happened 2 or 3 times in a month. I chalked it up to firer error (just being honest here). Then I fired. I was having a great firing. I was within 2 degrees top to bottom from c.010 on. The soda went in at c.9 and the kiln continued to climb. Nearing the end, I thought I had about 20 minutes left before the kiln was to be shut off. I checked the kiln for what I thought was the final check, and it had dropped 15 degrees! What happened? This couldn’t possibly be a firing error if it happened to me (hee hee). I managed to finish off the firing and the next day I talked with master kiln builder, Donovan Palmquist. His thoughts on this weird problem was that basically the thermocouples were fried. What was happening is that the pyrometer was acting totally fine up to about 2300’F. Then it was starting to freak out at about 2300’F. If the thermocouple is exposed to soda, then that can really cause trouble. I use a ceramic protection tube to encase the thermocouple so it isn’t exposed to the soda, so I wasn’t quite sure why there was suddenly any trouble. When I unbricked the kiln I discovered the problem. There was a crack in the protection tube, but it was in the first 9″ so it was hidden by the brick. Once I pulled it out I found where the soda was getting in. New thermocouples were wired up. Problem solved! When I fired on Wednesday, it went up without trouble. Luckily I always keep a couple of extra thermocouples and protection tubes on hand for when these problems come up. (That was a lesson that I had learned early on in my pyrometer experience.)
One other way a misreading can happen is that the little prong end of the thermocouple that is plugged into the pyrometer might not be in quite all the way (it doesn’t snap in, so it’s not as easy to tell if it’s in all the way). If it’s not in all the way, it can give you a reading that is close to normal, but not quite right. Always check the wires and attachments when the kiln seems a little bit off. Wiggle them around until they “calm down” and settles on a number.
The moral of this story is to enjoy your thermocouples and pyrometer, but always be skeptical. It’s a sensitive piece of electronics that you’re using in and next to a very hot kiln. Before you freak out and make all sorts of adjustments, calm down and listen to your gut instincts with some logic thrown in.
For another very important studio lesson, check out Keith’s tip from earlier this week.