Friday, August 22, 2014

Can I get a witness?

Here at The Ceramic Shop, we receive calls on a daily basis from customers who have a very wide variety of firing issues. Below, a few examples:

"My glazes don't look right."

"One shelf of my work is bisqued, but one barely looks fired at all."

"The colors burned out of my decals."

And so on. We're always happy to help you troubleshoot -- particularly when it comes to the tricky world of firing issues! -- but when we get these questions, regardless of the source of the woes being clays, glazes, decals, or something else entirely, we usually start with the same question:

"What do your witness cones look like?"

More often than not, the voice on the other end of the phone line goes silent, before confessing that no, there weren't any witness cones in the firing. And with that, we can offer a few suggestions, but our true ability to troubleshoot is hobbled by this oversight. We wind up telling customers the same thing: Try the same firing again, this time with witness cones -- THEN, we might be able to tell you more. 

 Sorry, man. 
I just need more than you can give.

With the proliferation of digitally-operated kilns, it's very easy to rely on what seems to be a 'high-tech' set-up to gauge the overall atmosphere inside of your kiln. And hey, we've all done witness-cone-free firings ourselves! We know how that goes -- you load up your kiln with the same bisque configuration that you've done 30 times before and just let the digital program run or the sitter cone drop. And honestly, most of the time, that works just fine -- but it's not a great practice. Because when something does go wrong -- and as a kiln is a piece of equipment with finite capabilities, at some point, it will -- pinpointing exactly what your issue may be will be that much more convoluted. Keeping witness cones in all of your firings can also point to problems as they develop -- issues that you might not notice until they become, well, bad. Here's an example:

Say you have a kiln with a digital controller. The kiln has four elements, and they're all brand-new. You have nothing to worry about, right? Those elements will last through a great many firings! So you bisque and glaze your work using the pre-set programs on your kiln, and everything looks fine. Your work is properly bisqued and your glazes are coming out with similar results from the top of the kiln to the bottom. Why bother taking the extra step to place a witness cone on each shelf, right?

Then one day, you bisque a piece for a friend. Let's say this friend isn't the most talented sculptor; let's say they made something a little on the heavy side. You let it dry, you set a cautionary pre-heat on your kiln, but still, when you press that 'start' button, your fingers are crossed.

Two days later, you open the kiln to a ceramic massacre. Little bits of (now-fired) sherds are just everywhere; aside from the central explosion, you can see clay has been flung far and wide in your kiln, with several little chunks becoming embedded in your elements. Yikes. You call your friend and break the news through gritted teeth, and then you get to work cleaning your kiln. You sweep up the busted pieces, and then you thoroughly vacuum everything -- floor, elements, all of it. Maybe you cry for a minute. Once the mess is cleaned, though, you do your best to forget it.

A few more firings happen. Everything seems fine.

Finally, breakdown occurs. You open a glaze firing, and three out of your four shelves look fine. The fourth, however, is just an underfired, chalky mess. How could this happen? Your elements are practically new!! Is this just a one-time firing hiccup, or is there a larger underlying problem, like a busted relay or a faulty control panel? Oh, if only you had a way to just know!

If you had been placing cones in your kiln all along, you probably would have noticed the cone closest to the (now-busted) element appearing less melted with each subsequent firing. This would have been your first clue about the nature of the issue -- clearly, you had an element that was weakening. In this case, maybe a little fragment of your friend's long-ago busted pottery went unnoticed, nestled against that element, causing a hot spot to develop with each subsequent firing until it eventually led to the failure of the element. 

Granted, this is a pretty specific scenario, but the funny thing is, many of the cases we encounter here at The Ceramic Shop are. And while that doesn't necessarily mean that all kiln issues are totally avoidable, using witness cones can give you one giant clue as to what, exactly, needs tweaking on your kiln. In our firings, we like to use Orton's self-supporting cones -- they stand up on their own, so they are very easy to just pop on each shelf of your kiln. 

If you do have any questions about your kiln, or firings of any kind, we're happy to answer them! Give us a call at 215-427-9665, and one of our talented techs will be happy to chat. You can also email any technical questions to myself at, or send them to our head tech Rachel at

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