ARMCHAIR POTTING.
PLASTER EARTHENWARE
While teaching at Curtin University I have experienced many disasters with my beloved students. One Friday morning a student with her eye's red from crying confessed to me while sobbing heavily "Mike, I have lost all my pots in the last firing; pots I'm supposed to have for tomorrow's exhibition that at Fremantle".
There was no time to waste as I sprung into action "Get
a clean bucket and weigh up the following ingredients:
1100 g BBR clay
600 g silica 300 mesh
300 g whiting
Total 2000g earthenware body
And also to add to the earthenware body mix 600g or plaster of Paris and mix
the whole lot together thoroughly on a large piece of butchers paper"
From there I took over.
I transferred the dry mix back into the bucket and very carefully added water to it from a slow running tap, while mixing it vigorously until I achieved the right plastic consistency. I then wedged it for five minutes on a plaster block and gave it back to my disappointed student and said "cut it in half and throw a couple pieces on the wheel - preferably shallow fruit bowls or platters and do not forget to finish them on the wheel completely" then I added "you have 30 minutes before the plaster sets."
It took a bit longer to set we could see very light steam rising from the pots as the exothermic reaction took place. and now I said "The plasters is set, the pots should be rigid you can now handle them, so use commercial color slips to decorate the top surfaces and as soon as you finish you can spray the decorations with a clear earthenware glaze and place them in the small electric kiln to ready to fire".
The kiln with pots was switched on and set to climb to 400°C in 5 hours and then at 150°C an hour up to 1100°C. The slow heating at the beginning was designed to stop any damage of cracking due to the plaster decomposing - losing part of its water at 70°C and also losing the rest of the water of crystallisation at approximate temperature of 230°C. No blow up was expected from evaporation of water of plasticity; the body was reasonably open with normal plastic ingredients.
The kiln was still very hot late Saturday morning but not hot enough to stop my student burning her fingers trying to remove her treasures from the kiln. I wish you could have seen her face smiling from ear to ear.
Please note the special mixed (plaster-earthenware) can be
fired to earthenware temperature only. Do not fire to stoneware above 1200°C.
The anhydrous calcium sulphate would decompose into calcium oxide and sulfur
trioxide.
Calcium oxide is a powerful flux at stoneware temperatures and bloating and
pyroplastic deformation could take place.
Cheerio for now.
Cheerio for now
Mike Kusnik