crafts - Glaze mismatch

On Thu, 15 May 2008 18:10:33 -0500, Sue Roessel Dura
wrote:

Hi Folks!

My sister wanted a bright red pot and I don't have a good cone 6 red. I noticed
that a mojica red that I had to go over white at cone 6 claimed to come out at
cone 05 as plain red so I tried it in a bisque firing. It was very pale over
the white at cone 6. Now I have a beautiful red pot with bisque fired cone 6
clay... Can I use this as a 'real' pot - dishwasher and food safe? Can I fire
it to cone 6 and retain the red color?

Thanks for your ideas ;-) Sue

There are a couple of issues here. I don't know how the porosity of
the bisqued cone 6 compares to a normal low-fired body... I'd guess it
is more porous. But even normal low-fire clay is porous enough that
you should probably avoid microwave use, even if you have made a
valiant effort to glaze it all over. Some moisture eventually gets
inside and can get dangerously hot in a microwave.

Another issue is whether the bisqued body has a thermal expansion that
is compatible with the low-fire glaze. I simply don't know the
answer. You may find that the glaze crazes or shivers after a while.
Crazing is probably not a big deal, despite concerns about bacterial
infestations... plenty of old "Grandma-ware" is crazed, with no ill
effects. But shivering would be serious, since it can give off
dangerous glass shards. However, based on my limited experience with
low-fire glazes on cone-6 clay that is only fired to cone 1 or 2, I'd
say that crazing is the more likely outcome.

What you might want to do is just leave that piece in the studio for a
few weeks or months and see what happens. Then if there is still no
sign of shivering, you can feel more comfortable using it. If you see
crazing, it's actually a good sign since it means that it's not going
to shiver later. You can look for crazing under a hand lens, perhaps
after pouring hot, dark tea into it and letting it stand for a while
before draining and examining.

As for the food safety of the glaze itself, that should be no problem
if it was originally claimed to be food-safe. The main concern would
be lead. If you don't know otherwise, you should probably assume that
it contains lead. I personally prefer not to mess around with leaded
glazes, but many folks do, and more have done so in the past. But at
least avoid using it with acid foods and drinks.

As far as firing to cone 6, not only are you likely to lose the red
color, but you are likely to lose the glaze itself due to running off
the pot.

Best regards,

Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!