Jon Pacini on sat 16 feb 02
-----Original Message-----
From: Cindi Anderson [mailto:cindi@cindi-anderson.com]
Sent: Thursday, February 14, 2002 11:55 AM
To: Jon Pacini
Subject: Clay Bodies
Cindi asks:
Hi Jon (...Jon is the clay manager at Laguna)
We had some questions come up that maybe you could answer.
Can you explain Laguna's thinking in labeling all (well, almost all)
mid-fire clays Cone 5 even though some are very porous at Cone 5. For
example, Dover White is listed as "excellent for dinnerware", but absorption
is a much to high 8% to be used for dinnerware. I suspect this does better
at Cone 6, and maybe even better at Cone 7. But then why is it labeled a
Cone 5?
Your question regarding the absorption level of Dover got me to thinking
what my aims were when I was developing that particular body years ago. I
went on a hunt overnight to look for my notes and review them.
At the time (1983) Westwood Ceramic Supply didn't produce any ^5 white
bodies other than Hagi Porc. and I wanted a white stoneware @ ^5 for dinner
ware. At the time we were having trouble with Hagi Porcelain in that the
high concentration of Neph Syn. (it's major flux) was causing it to harden
up like a brick. I was trying to stay away from that problem and it limited
the fluxing power from that source. I had to make some tough decisions
regarding glaze fit and how my alternative flux choices would impact that
and production costs. The end choice became the body that has been produced
ever since, what many potters like yourself regard as quite open, but at the
time was about average for a ^5 stoneware, 6%.
Since then, changes in the fluxing action of the sands now available, have
altered the absorption rate downward to approx. 4%. The 8% that is in the
catalog reflects tests done some years ago and I will address that issue
below. But first---- You have stated that 8% is too high for dinner ware,
that is going to be quite a shock to all the earthenware dinnerware
manufacturers on the planet who's clay bodies are closer to 12% absorption.
Nearly eight months ago I started a systematic test on absorption levels of
all the various bodies that Laguna produces. Partly to correct errors that
may have been made in prior testing and recording, and partly because
mineral drift does cause variations in clay performance. The end result of
this testing will be a revised chart in our next Catalog which is due out
this summer.
As for labeling clays using a Standardized system, (^06,^5, ^10, ect),
there are a couple of factors that account for this. One being inertia,
Laguna inherited a system from Westwood Ceramics and other suppliers that
goes back to at least the 1940's.
Another factor is that most of the clays we produce were not developed by
us, but have come as existing bodies with histories which tend to funnel
them once again into the narrow categories of ^06, 5 and 10.
Each of the 140 or so stock clays that we produce can be fired over a wide
range of temperatures. The tough part is determining what criteria you use
to define that range.
If you fire a Porcelain below a certain point it will dunt because it does
not develop enough strength to get through quartz inversion, above a certain
point it turns to milky glass and slumps. Is the firing range of this
porcelain defined by these events? You may say this is an extreme example,
but two potters, one who pit fires and one who wants vitrified tiles, will
push this same clay to these extremes. And, it may work equally well for
both of them.
Can I justify putting in the catalog that this porcelain has a firing
range of 1350-2400*f ? Will it perform to the expectations of the majority
of potters who test the extremes of this range? The answer to both of these
questions is no. The majority of potters who test these extremes will have
difficulties.
What you can really expect from any particular body is that it will work
for what it is recommended. Dover was designed to work as a 'open'
dinnerware body, not a vitreous one. I have dinnerware from back in '83 that
was made from Dover that has gotten extensive use over the years and it has
held up quite well. It may not meet everyone's particular expectations, but
then it was designed by a potter (me) to fit that potters (mine) specific
needs. That it is still in production 20 yrs later, when so many other
bodies are not, is a statement regarding the marketplace more than my
ability to develop the perfect clay. After all-- if a clay doesn't fulfill a
specific niche, and this is the important part, SELL WELL, then production
of that body stops.
It would be great to test every clay body available at every temperature,
using every forming technique and catalog it, but I'm not at all sure that
the feat could be accomplished. The system we use is a very simplified
one," These clays, work under these conditions, 99% of the time". You of
course, are welcome to use the clays we produce for whatever application
works best for you. We have 140 stock choices and experience in developing
over 2000 specials if one of the stock bodies doesn't work for you.
Regarding some general philosophy in the production of clay, that since I'm
rolling along here should be stated-------We produce a number of clays that
the formulations go back to the 1930's, 40's and 50's. Other than replacing
obsolete minerals with modern ones we don't generally substitute or adjust
clay formulations. It has been our philosophy to stick with the minerals
that the formulas call for unless they prove unsuitable. All minerals that
are mined from the ground vary and therefore raw color or other small
changes take place regularly. This is a fact of life with Ceramics. If I
were to try to adjust clays on a daily basis the ramifications would be
catastrophic. Any substitution of minerals may cause other unknown fallout
in that body. In other words, the changes that I make to solve one potters
difficulty might cause that clay to be unusable for ten others. The criteria
that I use for Quality control is my 30 yrs experience in using and
producing clays, the advise I garner from the 100 plus years experience of
my fellow staff members, Plus one heck of an extensive reference library
that I refer to Often!!!
Cindi asks---
Could you provide a list of Cone 5 category clays that you know should not
be fired at Cone 6? Some people are having trouble with bloating and
brittleness at Cone 6 due to overvitrification, and it would be nice to know
which clays have no problem at Cone 6 and which should be avoided at Cone 6.
There are a couple of factors in firing that will aggravate bloating in
clays that are approaching total vitrification. One is speed of firing the
other is the amount and timing of reduction. Both of these factors if
extreme or ill timed will not allow the gasses in the clay to escape by the
time the body is vitreous, thus bloating.
The only ^5 clay I would say definitely cannot be fired over ^5 is wc395
Porcelain 5, its range tops out at ^5. Two clays that have proven
susceptible to difficulties over ^5 or when reduced are WC 866 Electric
Brown and WC 391 B-3 Brown. Their high metallic oxide percentages are quite
active fluxes especially when reduced. The rest have at least a Cone leeway
on the top end and that is of course dependent upon your firing technique
and what you are trying to produce.
If you under fire clay bodies, absorption goes up. Plus under firing
aggravates glaze fit problems. This is because firing temperature as well as
technique determines what the COE of any given body will be as much as the
minerals it's made out of. If your glazes craze and your absorption is up,
you will end up with leaky pots.
I hope that this ramble is of some help. I know this must be a tiresome
statement to some of you out there, but-- the best thing you can do after
purchasing clay from a description in a catalog is to test -test- and test
some more. If anyone has any questions regarding Laguna Clays please feel
free to contact me directly at jpacini@lagunaclay.com
Jon Pacini
Clay Manager
Laguna Clay Co
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