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: