[Clayart] advice on bisque firing

[Snail Scott]

> On Feb 10, 2020, at 8:16 AM, Antoinette Badenhorst <porcelainbyantoinette at gmail.com> wrote:
> 
> Snail, I am curious about 800 F? That is not the temperature at which
> silica changes, so I assume you talk about bonded water?

Yep. I am being _very_ non-exact in naming 800F as the relevant temperature.

> ...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!
> 
> 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%...


This is very true of high-fired porcelain, and it’s why I’ve described that as a separate issue. I am not convinced that it is critical in the up-firing phase, though, since the clay particles themselves are not tightly bonded together yet.  This remain true, it seems to me, for any usual low-fired bisque in both heating and cooling phases. In the cooling phase of a vitrification firing, after the porcelain is highly glassified and brittle when cooled enough to harden, this inversion certainly becomes more critical. 

Note, also, that I am referring to electric firing, and I’m unconvinced that a typical electric kiln can up-fire fast enough to make it an issue, even on maximum ramp speeds. (A fuel-fired kiln, with its potential to fire much faster and create hot zones in the flame path, is a different animal, as is an industrial electric kiln.) The rapid heat loss of the standard thinly-insulated electric kiln raises the opposite issue during cooling, and so a controlled cooling through the inversion point may well be worthwhile for high-fired work, in porcelain especially. 

It seems to me that vitrification of the object makes a crucial change in the connections between particles, as well as internally to the particles themselves. So, is there any consensus in the literature or experience out there in Clayart-land?  Does the silica lattice within unvitrified clay particles make the object as vulnerable to the stresses of quartz inversion as it will be after vitrification?

My conjectures are not as well-founded as I would like, and I don’t want to add more speculation to the ‘bad info’ pool. Bring on the facts, y’all!  Let’s science the s#*t out of this!

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