[Clayart] advice on bisque firing

[Antoinette Badenhorst]

Snail, I am curious about 800 F? That is not the temperature at which
silica changes, so I assume you talk about bonded water?

As Ron explained, there are high mounts of silica in porcelain clay
bodies. The pure addition is sometimes as high as 30%. Add to that all
the other silica from kaolin, sometimes ball clay and soda/potash
feldspar and you have a force of up to 70% silica to recon with! There
is often very little clay in it, which makes many believe you can fast
fire porcelain at any time. the clay body is open, with  particle size
10 x larger that most other clay bodies, which allows for moisture to
go fast.

When silica goes through the inversion phases,up or down, the crystal
latices change. On the up a sudden expansion of 3 % happens.  On the
down it shrinks suddenly with 1%. All the other things that potters
learn about candling and slow up to get rid of the different kinds of
moisture are important, but porcelain is a different animal. If it was
not for the silica lattice expansion/shrinkage and organic materials
that become trapped in glass, it would have been true that you can
fast fire porcelain; especially small and thin ware.

As far as the clay itself is concerned, it is harder to compact the
small amount of clay present (which is often short in any case) with
all the other non plastic ingredients, so there is already a stress
factor present. Many times I end up with small stress cracks which
cannot be explained in any document out there. That happens especially
when I push the limits of the medium, which is at best self torture!

As potters we only learn the basics of silica, from experience I know
that silica research is a deep and unknown field for most of us. So,
the very few times that I follow the book, is when it says go slow
through the silica  inversion periods.
As far as organic materials is concerned, many potters end up with
black spots in the clay and blame the clay body without realizing that
it is organic materials that may be trapped in the clay.

Best wishes,
Antoinette Badenhorst
www.porcelainbyAntoinette.com
www.TeachinArt.com


> On Feb 10, 2020, at 5:58 AM, Snail Scott <claywork at flying-snail.com> wrote:
>
> 

>> 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
>