Jon Pacini on fri 1 feb 02
Gary wrote-------When clay with grog is viewed under an electron microscope
even the grog
we potters usually use has star like patterns of cracks radiating out
from each piece of grog after firing. Some think this eventually leads
to cracking and failure of ovenware do to a connecting of the dot
theory.
Greetings all---- That makes sense to me. The clay is going to shrink and
nothing is going to stand in it's way. It's one of those, 'Physical Laws of
Nature', things.
If you take a raw, plastic, 'pure clay' that shrinks at 20% and add enough
grog to it to bring the shrinkage down to a managable 12%, the clay part of
the mix still wants to shrink at the 20% rate. But because of the non
shrinking obstacles in the way, in our case "grog", it can't. So then it
ruptures giving those star like patterns. If you heat it and cool it often
enough the little stars become bigger stars and finally you get the "Big
Bang".
Ideally, you could bisque fire some grogless body, grind it up and add it
to the raw mix, then during the bisque firing it would all equalize.
During the glaze firing, the body and grog would all shrink at the same
rate and fuse together as one homogenous structure.
Large Manufacturers who blend their own clay use hammer mills to make grog
from their "wasters". I don't see it being feasible on any smaller level. An
interesting exercise maybe, but a heck of a lot of work for the average
potter.
To answer a related question from Paul, lets look at some grog materials
that are commonly used------
Pure silica sand introduces silica into the body, altering the COE which
can play havoc with glaze fit, primarily causing shivering. With enough flux
it can be fused into the body, but if not, it can also lead to dunting
problems. The plus is, it's usually very clean as far as any speckling goes
and if you're having glaze crazing problems, it can solve those.
Feldspathic sand will soften and give as the body shrinks. It will also
fuse into the body matrix, a plus for the use of feldspathic sand. However,
if feldspathic sand is fired too hot, it can actually get squeezed out of
the body like toothpaste and ends up as tiny beads on the surface of the
pot. It also tends to speckle in white bodies.
Buff grog can be ground industrial firebrick or calcined fireclay and is
primarily used by the makers of firebrick as an additive to their bodies or
in fluid bed furnaces. Because many stonewares are a large percentage of
fireclay, this can be a good material. Especially if your body is made from
the same fireclay. The grog is usually calcined at 2700*f though, so it
never really integrates into the body.
Red brick grog is just that, ground common red brick. Once again, if your
clay is made from similar minerals you've got a good material to use.
Ione Grog once upon a time was calcined kaolin grog, now it is a buff/gray
fireclay grog that is sized to specific mesh sizes.
Molochite is a calcined English kaolin that is ground and sized. Great for
English kaolin based porcelains.
Kyanite when calcined becomes Mullite, they are primarily used in
refractory bodies. The granular sizes could be used as 'grog' I suppose, but
because the grains are cool little crystal shapes they might be rough on the
hands. These two minerals would however, impart good thermal expansion
characteristics for Raku and oven/flame ware.
So many minerals and so little time------
Jon Pacini
Clay Manager
Laguna Clay Co
Where I am getting my due regarding my smartass remarks about freezing
weather a few weeks ago. This morning it was 30*f on my porch and the snow
is half way down the San Gabriels. Does anybody out there know where I can
get a thermal Hawaiian shirt???
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