Joseph Herbert on sun 26 aug 07
"Donna Pfledderer@Virtual Business Connection" wrote: I'm totally confused
about bisque firing, or either that I'm really hard headed and can't grasp
it."
From the description it seems there could be a couple of mis-apprehensions:
Bisque (or biscket) firing is the initial firing in a progression of at
least two firings. The level of maturity produced by the firing depends on
the ware and the intention of the first firing. In some porcelain
production the Bisque firing is the hottest firing and produces ware that is
completely vitrified. All the subsequent Glaze (or Gloss (glost)) firings
are at increasingly lower temperatures until all the various levels of
decoration are completed. This process can take several firings but the
first one is always the hottest. All the others are just melting things
that have been applied to the mature ware and do not change the underlying
vessel at all.
In Stoneware and some porcelain firing the purpose of the bisque firing is
to produce an object that is sufficiently hard to sustain handling, is
heated enough to drive off volatile components of the clay body, and porous
enough to allow the waterborne glaze powder (slurry) to be attracted to and
stick on the surface. Once the glaze slurry is applied the stoneware (or
porcelain) object is fired to maturity producing a dense shiny (usually)
object with a tightly adhered glass coating.
I guess that Earthenware firing is more like the porcelain dinnerware
process except that the "mature" earthware objects is still really porous
and the glaze firings are accomplished at a temperature very close to the
original firing temperature.
So, bisque firing can be different depending on the purpose of the firing
and the final destiny of the ware.
A couple of words about cones. It was not clear from the post if you really
appreciated the significance of the zero in front of the cone number. In
the cones usually used in this country, there is no Zero cone. There is a
cone 1 and there is a cone 01, Cone 01 (using large cones, slow fire)
matures (fully bends) at 2043 degrees F.; Cone 1 matures at 2077 degrees F.
The difference between Cone 04 (a common stoneware bisque fire cone) at 1922
and Cone 4 (rarely used as an end point cone) at 2134 is even greater.
Watch those Zeros.
So the directions about your Cone 6 clay could have been fire the bisque to
cone 06 and fire the glaze fire to cone 6.
IF you could spend some time watching an experienced person load and fire a
kiln, you might get some of your questions cleared up. Perhaps not a green
ware firing person but someone making thrown or hand built work.
Good luck,
Joe
Joseph Herbert
Technical Writer
Irving, Texas
214-725-8305 (Cell)
| |
|
