Lili Krakowski on thu 22 may 08
All thanks to Mel and Steve and everyone involved. You are, to this
computer illiterate, Lords of The Kilns, wizards for Harry and other
potters---magical creatures. Many thanks for putting our Free University
back together.
To get on with what ClayArt is about. Teaching, helping, furthering.
Nori has a problem. A c.6 glaze pinholes nastily every tenth firing or so.
Whottodo?
Ok. Pinholing is a nasty.
There can be a number of causes, and none can be eliminated just like that.
As Nori tells it, the problem arises every tenth firing or so, and the
"pinholes" seem less like pinholes than like small, sharp-edged craters.
As we do not know what is going on, we wire our deerstalkers to our brains,
and speculate.
Pinholes are attributed to a variety of causes, that can be summed up as
something interfering with glaze adhesion and healing.
Well, we all knew that. Ok.
The every tenth firing makes me discard the idea that higher bisquing
would help. Yes it might burn out stuff that turns to gas and so on, BUT it
would not explain the first perfect 9.
The Puzzle of the Tenth Firing: Possibilities.
1. The bisque really has been around a while. Nori fires a really
tightly packed bisque, produces enough for three or four glaze kilns, so
when she gets to 10th kiln, the bisque is dusty. Solution? Be sure to wash
the bisque well before glazing, allow enough time for it to dry before
applying glaze. Damp is ok, wet is not.
2. The kiln is dirty. Very fine dust is stirred up by the firing,
settles on the glaze. Once it has settled on the tenth firing it does not
circulate anymore, till after 9 more firings, there is more dust. A guess,
but would suggest vacuuming the kiln.
3. There is fresh kiln wash on the shelves. It does something. No idea
what, but possible.
4. Nori uses the same glaze over and over. She does NOT resieve every
time. By tenth round there are enough tiny, "invisible" bits of dust to
cause the pinholing. This happens a lot, esp. when the pots are carved, or
have handles, or galleries around or in which teensy bits of dust can hide.
Casual washing does not remove all, some gets into glaze. Resieve the glaze
every time it is used.
5. Something IN the glaze settled out. Nori mixes a new batch of glaze,
sieves it ALL, in toto, 100%, good to the last drop. This works, but as the
glaze sits there and is re-used but not resieved in toto, 100%, something
has settled out. Then by the tenth use the glaze has changed.. "Bottom of
the barrel" syndrome! Solution. Resieve down to the last drop, scraping up
anything down there, every time you glaze.
6. The tenth firing is the one made up of "leftovers." While the first
nine firings are densely packed with lots of shelves, the tenth is a
catch-up, leftover business. Fewer shelves, fewer supports, looser, uneven
sizes stacking. I forget who wrote that about 1/2 the heat goes into the
shelves, kiln walls, etc. but the more loosely packed a kiln is the quicker
it fires--though that may not be so noticeable, and the quicker it cools.
Pay attention to the kiln stacking. If it is emptier than usual, fire down
a bit more slowly. If you do not fire down, DO experiment with that.
7. Can it be the clay? Far fetched but anything is possible. Nori
opens clay bags only when she runs out of recycled clay. So kiln 1 is all
fresh clay, 2 has some recycle, 3 has even more...etc. Nori uses slips, so
the actual composition of the clay is changed as it is recycled. I think
this immensely unlikely, but theoretically, if the slips contain borax
(water soluble) there might be enough borax in the recycled clay, it rises
to the surface (as water soluble materials will) as the pots dry, and
changes the composition of the interface layer. Improbable, yes,
impossible, no.
Now the rough edges of the pinholes do suggest that the problem lies in not
enough time for the glaze to heal properly. In other words, whatever is
causing the problem might be solved by allowing more healing time. Which
makes me lean towards 5-7.
Nori: Do you fire at the same time of day, same hours all the time? Do you
keep records of firings? From length of candling/ pre-heating stage to
times you turn kiln up? Maybe there is something there....
PLEASE lettuce know what you find out. Good wishes....pinholing is a real
nasty.
Lili Krakowski
Be of good courage
Gail Dapogny on fri 23 may 08
Dear Nori,
How can I add to what Lili has written? Well, I can't; she is a
human encyclopedia of wisdom.
But to pick up on some of her suggestions:
1. Yes, Wash or sponge your bisque.
2. Vacuum: We vacuum our gas kiln every glaze firing (every week to
week and a half). More specifically, we turn on the air (plus
blower), turn on the vacuum, and one person taps the pipes (warmups
and main burners) down the pipe lengths and especially at the joints,
while the other person is in the kiln with the vacuum head held at
the various orificies. We hear lots of klinkety-klinks as things
come out. (We even vacuum our bisque kiln, although not quite so
religiously. If we leave it for too long, we get clogs in the
pipes.) Anyway, keeping your kiln clean is important!
3. Re-sieve, I can't speak for others, but I sieve 3 times when I
mix a glaze, and then sieve the amount I am using each time I glaze.
If I don't, I pay.
4. If you do have a kiln stacked a lot lighter than usual, put in
some kiln furniture: bricks, etc. to soak up some heat mass. Among
other things, this will help you to hold your temperature during the
soak, and also will keep your kiln from cooling too quickly.
5. Soak. Hold the temperature at your peak temperature. We do this
for about an hour. Some people do it successfully for shorter amounts
of time. We start it when cone 9 is halfway down, and finish it when
10 is not quite half over. Watch it carefully to keep the
temperature more or less the same throughout the soak.
When I teach new firers, I often suggest that they think of the soak
as like a batch of lasagne: it needs to bake for a certain period of
time (same temp of course) in order to have the cheese melt down
nice and smooth. Too little and the cheese is kind of uneven.
However, too much baking at that temperature and the cheese will over-
bake (over-fire) and start to blister. Overfiring is another cause
of the sharp craters you describe.
Best of luck, and keep us posted.
Gail in Ann Arbor
P.S. And as for Mel: Yeaaaaaaaaaaaa!
Gail Dapogny
Ann Arbor, Michigan
gdapogny@umich.edu
http://www.claygallery.org/
http://www.pottersguild.net/
On May 22, 2008, at 9:51 AM, Lili Krakowski wrote:
> All thanks to Mel and Steve and everyone involved. You are, to this
> computer illiterate, Lords of The Kilns, wizards for Harry and other
> potters---magical creatures. Many thanks for putting our Free
> University
> back together.
>
> To get on with what ClayArt is about. Teaching, helping, furthering.
>
> Nori has a problem. A c.6 glaze pinholes nastily every tenth
> firing or so.
> Whottodo?
>
> Ok. Pinholing is a nasty.
>
> There can be a number of causes, and none can be eliminated just
> like that.
> As Nori tells it, the problem arises every tenth firing or so, and the
> "pinholes" seem less like pinholes than like small, sharp-edged
> craters.
>
> As we do not know what is going on, we wire our deerstalkers to our
> brains,
> and speculate.
>
>
> Pinholes are attributed to a variety of causes, that can be summed
> up as
> something interfering with glaze adhesion and healing.
> Well, we all knew that. Ok.
>
> The every tenth firing makes me discard the idea that higher bisquing
> would help. Yes it might burn out stuff that turns to gas and so
> on, BUT it
> would not explain the first perfect 9.
>
> The Puzzle of the Tenth Firing: Possibilities.
>
> 1. The bisque really has been around a while. Nori fires a really
> tightly packed bisque, produces enough for three or four glaze
> kilns, so
> when she gets to 10th kiln, the bisque is dusty. Solution? Be
> sure to wash
> the bisque well before glazing, allow enough time for it to dry before
> applying glaze. Damp is ok, wet is not.
> 2. The kiln is dirty. Very fine dust is stirred up by the firing,
> settles on the glaze. Once it has settled on the tenth firing it
> does not
> circulate anymore, till after 9 more firings, there is more dust.
> A guess,
> but would suggest vacuuming the kiln.
> 3. There is fresh kiln wash on the shelves. It does something.
> No idea
> what, but possible.
> 4. Nori uses the same glaze over and over. She does NOT resieve
> every
> time. By tenth round there are enough tiny, "invisible" bits of
> dust to
> cause the pinholing. This happens a lot, esp. when the pots are
> carved, or
> have handles, or galleries around or in which teensy bits of dust
> can hide.
> Casual washing does not remove all, some gets into glaze. Resieve
> the glaze
> every time it is used.
> 5. Something IN the glaze settled out. Nori mixes a new batch
> of glaze,
> sieves it ALL, in toto, 100%, good to the last drop. This works,
> but as the
> glaze sits there and is re-used but not resieved in toto, 100%,
> something
> has settled out. Then by the tenth use the glaze has changed..
> "Bottom of
> the barrel" syndrome! Solution. Resieve down to the last drop,
> scraping up
> anything down there, every time you glaze.
> 6. The tenth firing is the one made up of "leftovers." While
> the first
> nine firings are densely packed with lots of shelves, the tenth is a
> catch-up, leftover business. Fewer shelves, fewer supports,
> looser, uneven
> sizes stacking. I forget who wrote that about 1/2 the heat goes
> into the
> shelves, kiln walls, etc. but the more loosely packed a kiln is the
> quicker
> it fires--though that may not be so noticeable, and the quicker it
> cools.
> Pay attention to the kiln stacking. If it is emptier than usual,
> fire down
> a bit more slowly. If you do not fire down, DO experiment with that.
> 7. Can it be the clay? Far fetched but anything is possible.
> Nori
> opens clay bags only when she runs out of recycled clay. So kiln 1
> is all
> fresh clay, 2 has some recycle, 3 has even more...etc. Nori uses
> slips, so
> the actual composition of the clay is changed as it is recycled. I
> think
> this immensely unlikely, but theoretically, if the slips contain borax
> (water soluble) there might be enough borax in the recycled clay,
> it rises
> to the surface (as water soluble materials will) as the pots dry, and
> changes the composition of the interface layer. Improbable, yes,
> impossible, no.
>
> Now the rough edges of the pinholes do suggest that the problem
> lies in not
> enough time for the glaze to heal properly. In other words,
> whatever is
> causing the problem might be solved by allowing more healing time.
> Which
> makes me lean towards 5-7.
>
> Nori: Do you fire at the same time of day, same hours all the
> time? Do you
> keep records of firings? From length of candling/ pre-heating
> stage to
> times you turn kiln up? Maybe there is something there....
>
> PLEASE lettuce know what you find out. Good wishes....pinholing is
> a real
> nasty.
>
>
>
>
>
>
>
>
> Lili Krakowski
>
> Be of good courage
>
>
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