[Clayart] So, Mel, how fast can you fire B -Mix?

Snail Scott claywork at flying-snail.com
Wed Sep 8 22:17:19 UTC 2021

On Sep 6, 2021, at 7:34 PM, Robert L. Johnson <impaladrive at gmail.com> wrote:
> It seems to me that there must be some limit or range on how fast one can
> fire with each clay body. I like B-mix, but I want to avoid stress cracks
> from firing too fast. (Or is that a problem?)

Not really. It is really tough to fire 'too fast’ in a vitrification firing, unless you are single-firing. I have never personally seen a fault I could attribute to firing too fast, as long as it levels off with a decent soak to reach the intended final cone. And I have never met a non-industrial electric kiln that was even capable of a ‘fast' heat rise by the standards of fuel kilns. 

Bisque or single-fire? That’s a different ballgame. Clay bodies need time to outgas properly, some more than others. Bisque slowly and let that happen! And remember, candling is essential! I candle long - then fire fast. As a rule of thumb, I candle about 4 hours for every 1/4” of thickness, IF the work seems dry already. Longer, if it’s not. Open bodies are more forgiving than ’tight’ ones, but taking time below 200F to drive off the last of the moisture is time well spent. It hardly uses any fuel or electricity, and exploded clay is the biggest waste of all. 

I have fired some nukin’ fast fuel firings (single-fire, 5 hours to ^4 with thick sculpture) that worked just fine, because the candling was thorough beforehand. It did not really get a proper burnout, though. It had some black-coring, and I suspect if I’d had glazes on it, they would have pinholed rather a lot. On the other hand, I got some unexpected and lovely surface effects that I have never gotten before or since. (Disclaimer: I do not usually fire this way.)

> And I fear quartz
> inversion--although I have never seen one up close. 

Unless you are firing to near ^10 or so, you are unlikely to develop enough crystobalite to be an issue. Also, quartz inversion  and dunting is a problem only after vitrification, not during initial heat rise (unless the work is already vitrified).  Unvitrified clay is actually pretty forgiving of that inversion. The problem is in fast cooling, not heating. As I said, I have never met an electric kiln that truly fires fast enough to be ’too fast’, and even an underinsulated kiln really doesn’t cool terribly fast, either. If you are firing mid-range electric, as is so common lately, it’s really a non-issue. (If you are actually doing fuel-fired high-fire, I assume you have a mentor; hence my blithe generalization. Not many people try to self-teach fuel firing.)

> And I've heard that a
> slow cool brings out some glaze effects that are achievable no other way.

You bet! I have glazes that you would swear are actually three different recipes, when in reality, they are the exact same glaze cooled for different length of time. Microcrystalline glazes (’true’ matte glazes) require time to develop that surface, and it takes time spent after vitrification, cooler than peak temp but still hot enough to be molten, for this to happen. (Macrocrystalline glazes absolutely require it! That’s sort of its own specialty, and another topic, perhaps.)  A glaze that is glossy if crash-cooled can have a satin surface if naturally cooled in a well insulated kiln, or a very dry matte if held for an hour halfway through cooling, and each of these could be a slightly (or very) different color. 

We often talk about how a fuel-fired kiln allows you to modify the glaze by tinkering with the atmosphere, but tinkering with the cooling rate can have equally dramatic (though different) results. 

I developed my matte glaze palette by simple trial and error, with no computer controller or even a pyrometer. In my electric kilns, I check my witness cones to choose when the glaze is fully matured., (The kiln sitter is a failsafe, not an automatic firing device, so I put in a sitter cone one number higher than my intended firing cone.) When the witness cones look right, I might (for example) turn the middle switch from ‘high’ to ‘medium’, to soak, then 20 minutes later, turn all the switches to 'medium’  and set the timer for an hour, then let the timer turn it all off. How hot is that? I dunno; I don’t care. Only the results matter.


> So, what's a potter to do? Can you do it all with a fast fire?

Fast fire, then slow cool! (Or not.)  Everybody’s clay and glaze will be different, and your intentions as the maker will differ as well, but I have never seen the need to up-fire as slowly as some people do.


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