I made test tiles from 4 different kinds of paper clay. From left, is Gault 10 paper clay, Southern Ice Porcelain paper clay, a red-bodied paper clay, and Black Mountain sculpture paper clay.
All three paper clays except for the Black Mountain sculpture paper clay were commercially purchased. I added the paper pulp into the Black Mountain clay to make it into a paper clay variety. I made a couple of changes to the Southern Ice Porcelain paper clay by adding feldspar crystals to it for one of my earlier projects and to the red-bodied paper clay, I decided to add more paper pulp into the already made paper clay.
The test tiles were marked off in one inch intervals for a total of six inches. This will enable me to calculate the shrinkage during drying and after firing. These tiles were fired ONCE, from dry greenware to Cone 5, without going through the bisque stage. I wanted to try out this one fire approach.
From fresh clay to bone dry greenware:
All showed approximately 4 - 5% shrinkage.
After Cone 5 fire (total shrinkage, from fresh clay to Cone 5 fired):
Gault 10 Pclay - 10.9%
Southern Ice Porcelain (with feldspar) paper clay - 8%
Red-body paper clay - 12.5%
Black Mountain sculpture paper clay - 10.4%
Caveat: These test tiles were made and fired flat in the electric kiln so the shrinkage values reflect the manufacturing and testing conditions. I did not test for vertical shrinkage. Adding the feldspar crystals into the Southern Ice Porcelain paper clay may also have affected the shrinkage since this clay showed the smallest amount.
I also made small pinch forms from the above four paper clays and fired them to Cone 5 (one fire) together with the test tiles. The result from the one fire test showed that I can easily fire to Cone 5 without any problems.
Why Fire to Cone 5?
For my sculptural pieces, I really don't have to fire to Cone 10. The higher cone puts more stress on the clay structure and provide more opportunities for the work to wrap, sag, or even fall apart. Granted that at Cone 10, the clay will be vitrified and stronger.
Another good reason is it is more energy efficient to go only to Cone 5. The ability to do a one-fire with paper clay also reduces the energy consumption since I don't have to do a bisque fire.