[Clayart] advice on bisque firing

[Snail Scott]

> On Feb 8, 2020, at 3:23 PM, Antoinette Badenhorst <porcelainbyantoinette at gmail.com> wrote:
> ...nothing other than slow through the silica conversion periods as it goes up. Beware at 220 C (can you convert; I am not at my computer) and 573 C ...
>> On Feb 8, 2020, at 4:59 AM, carol at knighten.org wrote:
>> I'd like suggestions re a bisque firing… 



In my experience, the most critical phase of firing is candling. Keep it below the boiling point for a few hours at least, for normal-thickness thrown work, and longer for thicker pieces. When in doubt, overdo it. Candling takes only about the same power consumption as a 100 watt lightbulb; it doesn’t cost much or take effort, so why not play it safe? Past the boiling point, you can take it up pretty fast. Few electric kilns are actually powerful enough to go too fast, anyway.   I have fired a lot of student work of dubious thickness in old one-speed electric kilns (just an on/off switch - no variable power levels or ’ramp speed’), and I’ve never blown anything up doing it…just candle thoroughly; everything else is secondary. 

Many people are concerned about the burnout of the molecularly bonded water of hydration around 800F, ad advocate slowing the firing down for this, but I have never met anyone, ever, who could attribute any actual flaw to firing too quickly through this range. Nobody. There may be fuel-fired kilns that could go fast enough to be an issue, but I’ve fired those pretty dang fast too, with seeming impunity. And as I mentioned above, I doubt that any standard electric kiln is capable of going too fast, even if set for maximum ramp speed or ‘’hi’ as soon as candling is complete.

I have also never followed the custom of slowing down through the quartz inversion temperature range during a bisque firing. Near as I can tell, most clay bodies don’t develop the problematic forms of silica at typical bisque temperatures (or even midrange-stoneware temps), so why bother? Even for a vitrification firing, it’s only the cool-down that is likely to cause inversion issues, and only for high-fire bodies. 

Getting adequate burnout shouldn’t be a problem if you go to ^04 as you intend. For lower cones, a soak near peak temperature is not a bad idea, generally. Porcelain is much cleaner’ than clay bodies full of ‘dirty’ secondary clays, so it’s not likely to be a big issue for you either way, so choose your bisque temp based on the desired hardness and absorption.

Snail Scott
claywork at flying-snail.com
www.snailscott.com