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 gina@theceramicshop.com, or send them to our head tech Rachel at rachel@theceramicshop.com.

In ceramics, zero DOES have value!

This blog post is intended for anyone new to the field of ceramics and pottery - it's just a quick explanation of how to interpret cone-speak. 

When I first got into pottery, I have to admit that a very specific (yet very important!) corner of pottery jargon tripped me up -- the use of the word 'cone' to describe heat accumulation. I seem to recall thinking that saying you fired a kiln to cone 6 was kind of like asking someone the time and having them tell you 'banana'. Until I understood cone calibration, there was just a total disconnect between the words and the meaning.

So why the blog post? To reiterate a very important point that is VERY important to those new to ceramics to grasp -- that Cone 06 and Cone 6 are NOT the same thing.  See that '0'? That makes all the difference, and this blog post explains why.

In the first place, a cone is exactly what it sounds like - a small, cone-shaped object that is made from ceramic materials that have been formulated to melt or slump at a particular accumulation of heat -- here, heat being the interaction of temperature and time. These cones have been assigned standardized numbers that correspond to a particular accumulation of heat. 

Self-supporting cones, sold in boxes of 50.

These cones bend when they melt, which tells you - the person firing the kiln - just how hot the kiln is inside. Cone 06 (say it like 'cone oh six) melts at 1830 degrees F when fired at a rate of 150 degrees per hour, where cone 6 (say it 'cone six') melts at a higher temperature - 2232 degrees F - when fired at the same rate. So, there IS a significant difference as indicated by the inclusion of that '0'.

 

The slumped cone of the left is a Cone 08, which melts at a lower point than 

Cone 06 (center) and Cone 8 (right) 

So that all makes sense, right? Basic concept, basic explanation -- so why dedicate a whole blog post to this? Well, one funny thing about the commercial ceramic industry is the fact that many clays and glazes tend to be formulated for two very popular firing ranges -- Cone 06, and Cone 6. 

For anyone who works in the low-fire clay and/or glaze range, that roughly translates to Cone 06. Popular low-fire glaze lines, like Mayco Stroke & Coats, Duncan's Pure Brilliance clear glazes, and Amaco's Liquid Gloss series are all formulated to be fired around Cone 06. 

Another very popular firing range is mid-range, or right around Cone 6. Amaco's Potter's Choice line, for example, is a pretty well-known mid-range line. 

So while it's just sort of an interesting coincidence that the two cone ranges that happen to be standards within the industry sound an awful lot alike, it does need to be mentioned again that 'Cone 06' and 'Cone 6' are NOT interchangable! If you fire a low-fire (or, say, something in the Cone 06 range) clay or glaze at a mid-range (Cone 6) temperature, you run the risk of drastically OVER-firing (and likely ruining) your work. You'll see low-fire clays slump and melt when too much heat is applied; low-fire glazes can run right off of your piece and even wreck your kiln shelving!

Check out the image above, taken from Joe Kowalczyk's Adventures in Kiln Repair blog. That disgusting mass you see up there is actually over-fired clay -- yep, that's what can happen when you muddle '06' and '6'. Yikes!

Mixing up the cones in the opposite direction is not typically quite so destructive, but you can still end up with a lot of weird-looking pottery. When you fire a mid-range glaze intended for Cone 6 at a much lower temperature, like Cone 06, you UNDER-fire your pots. This can result in chalky, un-vitrified surfaces that are often just not very attractive.

This example of underfired work comes from the blog Paul the Potter. See how the surfaces are dull and chalky? Because the kiln didn't get hot enough to melt the glassy elements in the glaze that would form a slick, gloss surface, the end result just looks kind of unfinished. Classic underfiring -- and pretty much what you might expect when you place a Cone 6 glaze in a Cone 06 firing (although, it should be noted, the image above was the result of a much more nebulous firing issue). Of course, with under-fired work, you can always just re-fire it at the proper range, but it's still a net loss of time and energy.

So! In conclusion, know your firing ranges for the clays and glazes you use. Know that the small '0' you see on glaze instructions should definitely not be ignored! Know there is a difference between the low-range Cone 06 and the mid-range Cone 6. I'll leave you with a handy, techy-looking firing chart, so this post looks even more official: