Towards the end of last year, one of my glazes developed a pinholing problem. This glaze would make an occasional pinhole before, but suddenly the problem became much more prevalent. As soon as my holiday sales were over, I began trying to fix the problem.
Here's an example of too many pinholes. I pushed the contrast, and oversharpened the photo, in order to make them more visible.
A common cause of pinholing is that a glaze is being applied too thickly. This is an opaque white glaze, and it’s true I had to apply it thickly in order to get the right opacity. So my first attempt at solving the problem was to add more opacity to the glaze, so that I could apply it less thickly. On the test tiles I was firing, it seemed to work. No pinholes on the test tiles.
The glaze has another nice feature, which is that it is slightly runny. By applying the glaze less thickly, I lost the runny quality. So my next step was to add the runniness back. I tried increasing the flux so it would melt more, and I tried decreasing the stabilizer. Both approaches yielded promising results, according the the test tiles. However, it still didn’t look as nice as the previous glaze.
I had been using test tiles that were about a year old. Around this point in my testing, I ran out of tiles, and made a few dozen more. On the very next test firing, the pinholes came back.
SON OF A BITCH. It wasn’t the glaze. It was the clay.
In hindsight, it became perfectly clear. I first noticed the pinholes while setting up my display at the PMA Craft Show in November. About three weeks before that, I had bought a new supply of clay. The pots for that show were the first pots made with the new supply of clay.
All of the test tiles I had fired up to that point (in this photo) went in the trash. Useless.
Another common reason why pinholes form is because the glaze is being overfired. I had not considered that option, because I had not changed my clay, glazes, or firing schedules. I use two clays, and mix them together. And now I understand that one of them has changed. It is now maturing at a lower temperature, and my firing schedule was now “too hot” for the clay. The overfired clay was overfluxing my glaze, and causing the pinholes.
I solved the pinholing problem by lowering the final temperature of firings. But just like all things in ceramics, it’s never as simple as that. For starters, I learned that if I lowered the temperature too much, another one of my glazes, a semi-matte, started to look too dry. This means that my range of acceptable firing temperatures had gotten much smaller.
Here comes the next problem. One of my kilns (named “Dr. Evil”) was bought in 2003. My other kiln (named “Number 2”) was bought in 2013. They are the same model kiln, but during that ten year gap, the kiln went through several design changes, and the digital controller also went through several upgrades. So really these are two different kilns. Before this problem arose, I knew that Number 2 fired hotter than Dr. Evil, despite having a lower set point for the final firing segment. I was not tuned in exactly to how different the firings were, because it never mattered before. This means I had to figure out a new firing schedule not once, but twice, one for each kiln separately.
Many test firings later, I figured out that the right answer was to lower Dr. Evil’s final set point by 7 degrees, and Number 2’s final set point by 15 degrees.
Then came the next problem. It is now important to get my kilns to fire as evenly as possible from top to bottom. Again, it is normally not considered a problem to have as much as a half-cone difference within a kiln load. But it’s a problem for me now. The solution for this has two prongs: learning how to calibrate thermocouples, and changing the way I stack shelves in the kiln.
Calibrating thermocouples is a fairly straightforward process. I never had to do it before. Now I will calibrate them when they’re new, and use witness cones in every 10th firing or so, and calibrate them as needed as they get older.
Changing my shelf stacking plans took quite a few test firings. My entire stacking strategy before consisted of using a minimum of 6.5 inch posts on my bottom shelf. 6.5 inch posts allow the second element from the bottom to barely be under the next shelf. The bottom zone is the hardest for the kiln to heat, and I thought that was as much help as I could give the bottom shelf. But I was wrong. Using 6.5 inch bottom posts still made the middle zone fire 10-20 degrees hotter. The answer was to increase the bottom posts to 8 inches tall, keep the space at the top of the kiln 7 inches tall, and pack four or five shelves into the middle zone. It felt extreme to use such tall spaces at the top and bottom, but I’ll be darned, it works! Now Dr. Evil can keep all zones within 4 degrees of each other. Number 2, with its newer controller, can keep them exactly even.
Did you know that the temperature at the middle of a kiln shelf is cooler than at the edge of the shelf near the elements? If you don’t place your witness cones at a consistent distance from the elements, your cones will give you unreliable readings. I know that now.
And here comes the next snafu! That’s right, I’m not done with those yet. During the 2 months that I was trying to solve these problems, I had relay failures in both kilns, at different times. I should have seen it coming. Both of the relays that failed were past the age where they could fail at any time. And I had been doing so many test firings, one after another, it’s not that surprising that I pushed them over the edge. A relay failure meant that I had to wait at least two days for new ones to be delivered. Which meant my testing had to screech to a halt, during a time period where the clock was ticking loudly, thanks to the upcoming ACC Baltimore show.
(I am now keeping a set of three relays on hand. Next time a relay fails, it won’t cause a delay.)
I was continuing to produce pots for the show, while conducting all of these firing tests. The longer it took to figure out all of these various issues meant the pots were piling up, waiting to be fired. There were several points along the way when I contemplated dropping out of the show, thus forfeting almost $1600 in non-refundable show fees, because I didn’t know if I would get the pots done.
In the end, I got all of the issues figured out almost 2 weeks before the show, which gave me enough time to get everything fired. I was firing non-stop for days, and the kilns turned out one perfect glaze load after another. I did not drop out of the show.
Dr. Evil is still firing less consistently than Number 2. This might be due to its older controller, but also because its elements and thermocouples are about 80 firings old. Roughly two-thirds of their lifespan. Thermocouples naturally get less reliable as they age. Number 2’s components are almost brand new. Dr. Evil will get new elements and thermocouples next summer or early fall. I can gauge then how much more consistent the kiln can be, if at all. Anyhow, the amount of variation in Dr. Evil’s current results is small enough to make good pots. These are the witness cones I am getting from both kilns now. Dr. Evil produces cones within this whole range. Number 2 produces cones that consistently look like the third and fourth cone.
There are so many details to my new kiln firing “rules” that I knew I should write them down so I don’t forget. I made this small poster to hang near the kilns.
After a bumpy 2 months, I am tired. I wish I could take a little time off. On the bright side, I am nearly out of pots, thanks to a great weekend at ACC. So I need to get back to work, whether I feel like it or not. Because I am still the worker. Am I upset that one of my clays changed its composition? No, because there are many valid reasons why a manufacturer would need to tweak their formula. And they don’t need to ask for my permission first. As a person who relies on pots for income, and relies on manufacturers to make the clay, it’s my responsibility to expect these changes to happen. And to be adaptable when it does. When you’re the boss, you don’t get to blame your problems on others.