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
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