Lili Krakowski on fri 18 aug 06
Having corresponded with Fred Parker a bit off-list, I decided
a small discussion of clay might be an idea.
Clay is found in nature and varies from place to place as much
as all else found in nature will. Even in one fair-sized area the clay h=
ere
may be more suitable for potters than the clay over there. Nothing to be
amazed at--the same applies to trees of a species which do better on this
side of the hill than that one...
When clay is found and dug it is cleaned--stones and roots, and what not=
.
And then, for industrial use, of which pottery is only a teensy part, it =
is
ground and sieved and otherwise refined.
In this country Kentucky, Tennessee, Georgia, and Ohio seem to be the mai=
n
clay producers. There are several varieties of clay from Kentucky, and
several from Tennessee and, several from Ohio. In other words, a general
geographic location will yield a bunch of different sub-categories of cla=
y.
Canada produces some clays, I do not know if they are exported into the
US.(I mean the raw clays. I know both Tucker and Plainsman claybodies can=
be=20
bought in the US.)
Over time, clay mines disappear. Either they run dry, or sub-divisions =
are
built over them, as neighboring cities expand. And so Monmouth, Perrine,
Dalton, Jordan, and Fetzer disappeared, as more recently did Albany Slip =
and
Blackbird/Barnard which, while clays, were mainly used in slips and glaze=
s.
It needs to be added that many of the feldspars once used to make these
clays into usable clay bodies also have disappeared.
The books will explain the differences between types of clay...clays that
stayed put for millions of years, and others that were washed down mounta=
ins
into rivers where they bedded down. Clays that are close to the surface,
and clays deep in the ground.
But when it all gets to the studio, a potter can opt to buy clays in the
bag and mix her own clay bodies, or to buy mixed, ready to use, clay bodi=
es
from a producer. A hybrid solution is for a potter to develop a clay bod=
y
she really really likes and order it premixed and prepared from a produce=
r.
Producers only have available to them clays and
other materials they can get. Some clay bodies include imported
clays and spars. Others are made entirely from clays and other materials
"locally" found. (This, I assure you, is reflected in price.)
The clay producers mix up clay bodies with certain goals in mind. First
would be the temperature at which the clay body will mature, and be good =
and
durable. Then there is color, shrinkage, and texture to consider. I lik=
e
extremely plastic bodies, and want no more than 1.5% porosity at c. 6, an=
d
no more than 12% shrinkage. I would gladly have less shrinkage but that
does not seem to exist in very plastic bodies. Someone who made tiles mi=
ght
prefer a body that is extremely non-plastic. I like very smooth bodies, =
a
sculptor might want a very coarse one. You get the idea.
All clay-body manufacturers of good repute produce excellent bodies. A
friend had trouble getting just what she wanted for her work, and ended u=
p
wedging three bodies together. But such experiences are rare,
and I need to add she had only one supplier available to her.
As shipping costs are huge these days, only those who can pick up a suppl=
y
while visiting Mom anyway can afford to buy from a distant supplier. And=
=20
one is
well advised to buy a selection of clays from the dealer most convenientl=
y
located to one's studio, and try out what works best. I have often said
I wish all clay suppliers offered sample kits of all their bodies for a
specific cone range. I gather some now do.
With experience, the potter learns to modify clay bodies. I use both a=20
white
stoneware, and a brick red one. Each for specific work.
The scraps of both are mixed at random and
used for most of my production/functional work. Because my pieces
are individual, and decorated with slip, it does not matter if this week'=
s
shade is replicated exactly next week. When I need a grogged body I eith=
er
add sand, or (coarser) grog.
Good producers will tell one what shrinkage to expect. And a proper
slip will fit nicely whether the pot is fired to a hot c.5, a good c.6
and probably c.7. A proper slip fits over a number of cones. Problems
that arise with slips generally come from poor inter-action between the b=
ody
and the slip. That can be remedied easily with testing of the straight l=
ine
blend variety. ( The PMI site on the Internet gives several good slip=20
recipes)
It should be noted that clay manufacturers cannot, and do not, guarantee
that the same MATERIALS AND INGREDIENTS go into their bodies
this year as ten years ago.A manufacturer with replicate as closely
as possible a specific body sold under a specific name,
providing the same characteristics, but not
necessarily with the same materials. See sub-divisions above!
One caution from Someone Who Got Badly Burned. I was invited years ago t=
o
take part in an invitational show. I decided to use a white body with
cobalt decoration. I called the place I was buying my clay from and aske=
d
for a white, magnesium (talc) free body. I was firmly told that X was it=
.
I bought it, trustingly. The cobalt turned purplish. When I complained =
I
was told the employee had been mistaken. And, horrible person that I am,=
I
snickered a few years later when that supplier went belly-up. Deal with
clay suppliers who are trustworthy. Have no hesitation to interrogate ab=
out
ingredients, and better yet, get a sample to run through your "mill".
I have found both Rhodes and Cardew informative and useful about clay. T=
hey
will acquaint you with what types of clay do, what additives do, and so o=
n.
There is absolutely no point in taking anyone=92s word about clay. A fri=
end=20
will tell you =93Oh, my goodness, Chimpanzee Mush is the greatest clay bo=
dy=20
ever!=94 Fine. Love your friend, trust your friend=85but do NOT take ANY=
ONE=92S=20
word about a clay body. YES about its firing characteristics, NO NO NO=20
about how it feels to your hand, how it works for you, and so on. Get=20
samples of potential clay bodies, check them out for yourself. (You know=
=20
how a girlfriend will croon she met The Most Perfect Man? And what a je=
rk=20
he was? Same with clay=85)
The shrinkage and absorption figures the supplier lists are not necessari=
ly=20
identical with what you get in your studio. (You don=92t want another=20
tedious tirade about THAT do you?) So do yourself a favor and make a=20
shrinkage bar and do the absorption test for each sample you try.
Buy a 6=94 plastic ruler that has centimeters marked on it IN RELIEF. I =
get=20
mine at the sewing counter/knitting supply store. But I am sure other=20
places also sell these. Roll out a piece of your sample clay, pat it=20
flattish to about 3/8" thickness, squoosh the ruler into it so as to impr=
int=20
the centimeters. Release the ruler, being careful not to deform the cla=
y,=20
poke a hole at a free (unmarked end) of the bar (so you can hang it up=20
later) and inscribe the name of the clay on it. When this bar ultimately=
is=20
fired to your temp. the bar will show the shrinkage of that clay. A hand=
y=20
reference at all times as when you are making things to size.
The absorption test you know.
Lili Krakowski
Be of good courage
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