Bill Amsterlaw on sat 29 nov 97
Regarding black coring....
Ron Roy wrote:
Guess you have to define what you mean by black coring - bloating? or
different shades of gray and/or black in the body cross section.
What you are referring to (black core) is a problem which happens because
of improper bisque firing. Not enough oxygen to burn off organics between
700C and 900C.
Ok, so you want to burn off all the carbon before you hit 900C = 1652F =
cone 010. You achieve this by firing in an oxidizing atmosphere with a
slow rise from 700-900C. This could be done in a bisque-firing or a
single-firing. If you are doing a glaze firing and you have already burnt
off the carbon in a bisque firing, you can start reducing at 800C; if you
are single-firing, don't start reduction until you are above 900C. If
there is still carbon in the clay at 900C, it will reduce red iron oxide to
black iron oxide and cause premature vitrification.
Some questions about all this...
So, what's "body reduction"? When you reduce at cone 06 or so to
accomplish a "body reduction", aren't you trying to reduce red iron oxide
to black iron oxide? That would mean that there is more than one way to
reduce red iron oxide to black: You can do it with the kiln atmosphere,
too. That would mean that even if you had burnt out all the carbonaceous
material in a prior bisque firing, a heavy reduction atmosphere below 900C
could do the same thing to red iron oxide that carbonaceous material could
So, then ... Is the difference between "black coring" and "body reduction"
a quantitative one? ... or are there unique processes that apply to each
term? Is there a continuous spectrum between "black coring" and "no body
reduction"? Is "over-reduction" the same thing as "black coring"? Is it
possible to look at a broken shard and distinguish between "black coring"
and "heavy body reduction"? How much is too much? What's so bad about
black coring, anyway? If you don't get bloating, is the clay ok regardless
of its color?
- Bill Amsterlaw (firstname.lastname@example.org)