Marcia Kindlmann on tue 9 apr 96
Subject: bloating porcelain - the outcome
Just unpacked my kiln and, sadly, am wondering if I must now say
goodbye to my favorite cone 10 throwing porcelain. Awhile back I
wrote to Clayart when I first experienced bloating with this body,
asking for thoughts on possible causes. Various ideas emerged and
- again - thanks for all your help and encouragement. BTW, strictly speaking,
this problem should be called _blebbing_ not bloating
(thank you Ron); more about that later.
Here's what happened after my initial round of tests: I persuaded
the supplier to send me a sample from another batch, hoping that
the problem-batch was a fluke and I could then exchange it for
a new batch. But alas the sample presented the same problem. My
testing was designed to give the clay every possible chance to be
good -- i.e. be sure I had located the problem correctly; I was
really hoping it was not in the clay.
Here's how I tested: Made 3 bowls, each of a different batch of
Miller 550 porcelain--one batch from 1994 that had never bloated,
another batch delivered early '95 that never had bloated, and
the third batch the one that was being questioned. Marked each
of these bowls on the bottom with a letter corresponding to the
production code on the 50-lb box. Made several of these sets of
3, each thrown as nearly to same size and thickness as hand-throwing
will allow. Glazed these sets with the same series of glazes I had
always used, glazing all at the same time, from same buckets.
Placed each set side-by-side on my kiln shelves, equal distance
from elements. Placed various sets in various spots (upper, lower)
in the kiln. (an 8 cu. ft electric). Fired the kiln the same
way I usually do. Did this for a couple firings.
The results were dismayingly consistent: In each side-by-side
group of identically-glazed bowls, the two production batches that
had not bloated in the past, again did not bloat. The batch being questioned
did bloat (both the one that I have 400 lbs. of, and
the new sample).
The supplier states that they are not aware of any changes in the
raw materials nor have they changed the formula. But I have begun
to learn from several of you, in Clayart and in private posts, that suppliers
may not readily announce a change in raw materials (this
comment made without reference to any particular supplier). I had
been using Miller 550 porcelain since l986, BTW, and it had been
completely consistent, from batch to batch, until these recent
experiences.
Now what could be happening here? Ron Roy had suggested that this
body is getting overfired and starting to break down, producing gases,
and that a glaze can promote the melt. A slight change in raw
materials can push the body over the edge, since porcelains are so
close to the edge already. Thus the term blebbing--as distinguished
from bloating. Bloating says Ron (and you can also read about this
in Hamer) is the blistering that's caused by carbonaceous matter in
the clay grabbing oxygen from Fe2O3 in the body, making CO & CO2 gas
which lifts the clay in blisters once the glaze has sealed its surface,
and leaving FeO in the body, which is a powerful flux, causing body
to melt prematurely. Typically this occurs in _stonewares_ when
the bisque has been too fast between 1290 F - 1650 F, or 700 C - 900 C. But
in porcelain -- if it has no ball clay -- there'd be so little
iron that bloating in this sense wouldn't be the cause of body blisters. Thus
the term _blebbing_ instead. (say slowly, "bleb.") Lovely word.
Onomatopoetic, isn't it?
Anyway--for all those who helped with trouble-shooting and wondered
what was the outcome, I am still working with the supplier to get a porcelain
that works for my glazes. Miller has suggested trying
their 560, which I'll be doing soon, since they say that grolleg
bodies (eg. their 550) are more sensitive to the range of variables
in different applications. I think that my own tests suggest my
glazes were over-fluxing the present 550 when previously they did
not. If there's any explanation, it would seem that a shift in raw materials
in this 550 is making a difference with my glazes but
evidently not causing problems for others. (true?)
Now if you are wondering what sort of glazes are on those blistered
bowls of mine--I dip the bowl in a dark iron-bearing glaze, one of
several such, and then on the inside swirl a milky-clear glaze over it, which
brings out different colors in each of the various iron-glazes
I use on different bowls.
I'm firing to cone 10 almost halfway down, between 2 and 3 oclock.
The bisque is slow and with a peephole open, and BTW I've never had
any bloating (or black coring) with any of several suppliers' various
high-fire stoneware clays.
I've learned that Miller, and no doubt other suppliers, does test
each of their production runs, making a bar to get the shrinkage
percent and look at color. The testing is done without any glaze.
Then if a customer is having particular problems, further testing
of that customer's clay may be done.
Stay tuned--I hope to find a porcelain that throws as easily as
Miller 550.
BTW I keep referring to Miller, from old habit; as you know Miller
was bought by Laguna, but the clay bodies that Miller used to make
are still being made in Skaneateles NY while Laguna has their own
collection on the west coast.
Marcia Kindlmann
--in Connecticut where gigantic snowflakes are falling slowly and the
spring peepers are keeping mum.
peter pinnell on wed 10 apr 96
Hi Marcia-
It sounds like Ron is right. The symtoms you describe are consistent with
an overfired body. Its hard to say without seeing the body recipe, but
here's one possible cause: poor mixing. You might ask the supplier if
they are adding any extra flux such as whiting, talc, or wollastonite.
Its not unusual for one or two percent of these to be added to a
porcelain to get a glassier, more translucent body. If these aren't
mixed in VERY well, they could creat pockets of overfired (almost
glaze-like) clay.
Do you notice any white specks in the clay?
It can't hurt to ask.
Good luck.
Pete Pinnell
P.S. Here's a test you might to: Take some trimming scraps from the
"blebbing" porcelain, dry and slake it to mush, then drill it up well.
Dry it out on plaster, then throw another bowl. If you now get no
blebbing, then its most likely the mixing.
Pelly123@aol.com on wed 10 apr 96
I picked up a ton of clay at Miller last week...you might want to note that
Laguna is changing all of the Miller clays over to their numbering system...
Pelly
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