dannon rhudy on thu 28 jul 05
Not to be too simplistic, but it is NOT the kilns
that make glazes craze. It is the fit between the
clay and glaze. If the glazes do not fit, then you
must either tweak the glazes (best) or find a clay
that the glazes are compatible with.
regards
Dannon Rhudy
> I've been battling with my kilns (an old Paragon A-77B and a Skutt 1018 -
> both with Dawson Kiln Sitters) to get a good glazed pot. Basically the
only
> glaze I am having any success with is Floating Blue for Cone 6 firings and
> Amaco opalescents for cone 05.
>
> Crazing is the basic problem
Carty Ellis on fri 29 jul 05
I've been battling with my kilns (an old Paragon A-77B and a Skutt 1018 -
both with Dawson Kiln Sitters) to get a good glazed pot. Basically the only
glaze I am having any success with is Floating Blue for Cone 6 firings and
Amaco opalescents for cone 05.
Crazing is the basic problem (hearing those little bells for a few days
after taking things out of the kiln) or seeing crazing appear after an hour
or a couple of days.
I'm going to focus on Cone 6 right now - back to the low-fire later when I
get the cone 6 firing technique problem solved.
The cone 6 clay body is standard 182g - couple of weeks back I asked about
Licorice blistering - Ron Roy suggested it was not hot enough - fired again
- got full cone 6 from the sitter (witness cones verified) and while it was
slightly better - no cigar. Then I looked at a pot I had glazed in
Spearmint with a bit of Waterfall Brown on the rim - initially it was
beautiful - but I looked again later and there is a lot of crazing in the
Spearmint - inside and outside the pot.
My Floating Blue stuff is not crazing.
Also a floating blue cup I re-fired (added some glaze where I had missed a
tiny spot) has small brown spots showing through the glaze -somewhat in the
manner a clay that has magnesium speckles will spot through a glaze. I
expect that "might" be the grog in the 182G - but why on the second firing
and not the first?
All in all - having a very unsuccessful time glazing/firing. Firing
technique is basically this:
In Skutt - (for Cone 6) - top open width of a shelf support lying on its
side, top peephole open - bottom element on low for 30 minutes, then top
closed and top element added on low for 1 hour. Then both to medium for 1
hour. Then both to high. Kiln shuts off with cone sitter about 4 hours
later. Kiln opened when pyrometer shows below 100 degrees - usually 18
hours or so later.
Sure seems simple. But I am not being successful. I've recently had a week
long class with Michael Connelly (is a great teacher) and have some nice
jars I want to glaze. Sure don't want to doom them to failure before I
start - so I'm only going to be firing pieces of broken pots until I find
what I am doing wrong.
I'm open for suggestions on what I may be missing. I have a bunch of crazed
stuff - and I don't want to add to it - the Garden is getting pretty full of
decorations!
Carty Ellis
Oxford, OH
Cindy Gatto on fri 29 jul 05
Hi Carty,
Crazing is from the fit between the clay and the glaze. You might want to
consider switching clays If you are using Standard clays try using #181 We used
to use that clay and we never had any problems with crazing or anything for
that matter! I can also suggest #225-a light brown and #553-buff, both ^6
clays great for throwing, handbuilding we've also had great success with raku
Give 'em a try and good luck.
Cindy Gatto & Mark Petrin
The Mudpit
228 Manhattan Ave
Brooklyn, NY 11206
718-218-9424
_www.mudpitnyc.com_ (http://www.mudpitnyc.com/)
mudpitnyc@aol.com
Craig Clark on sat 30 jul 05
Carty, first of all, I hope you are able to solve this problem
quickly. I know how frustrating it can be to loose nice work to bad
firings. Crazing, unless there is some type of odd contaminent in a kiln
which I would be hard pressed to imagine, is not caused by the type of
kiln you are using. It most likely ain't even caused by the manner in
which you are firing the kilns unless you are firing to some radical
extreme for a specific type of result. Crazing is generally caused by a
bad fit between the glazes and the clay body that you are using. Your
glaze is shrinking to much for the clay body and it develops tiny
fissures to relieve the stress. I mostly do raku and for me this is a
good thing. It is what we are after. If we want more of a crackle
pattern (crazing) we just add a bit more silica.
A suggestion is to run a series of tests, no more than a few at a
time, that will determine what does and does not work. Make some quick
test tiles by taking a small handful of clay and scrunch it down to form
a self supporting form or just bend a few very small slabs to do the
same thing.
Take your test tiles and one by one test your glazes until you
figure out what the problem is. Try to limit the number of radicals in
the equation. Keep very detailed notes of everything, from how you
intially quickly clean the tiles before glazeing to where they are in
the kiln, the firing cycle, cool down cycle, type and batch number of
the clay body, etc. The thing to avoid is ruining any more of your good
work. Use test tiles not pots.
I have used most of glazes that you are having troubles with. I use
WaterFall Brown consistently and have wonderful results. It is a
dynamite glaze! If you discover that you are having crazing problems
with one or more and not the others then once again look at the
compatabilty with your clay body and/or the way you are mixing your
glaze. There may be a mistake in the glaze mixing. How much of each
glaze do you mix at a time? Have you used the same source for your glaze
components? Are you using same glazeing technique on all or your pieces?
Are all of your glazes completely dry before you begin your firing? A
good rule of thimb is to glaze your pots one day and then fire the next.
These are all important things to methodically consider.
After a quick re-read of your post I noticed that you say you have
the top of your kiln open at the beginning of your firing. Have you
already bisque fired your pots or are you trying to do a single fire
from bone dry ware? If you have already bisque fired your work then
there is no reason to crack the lid. If you haven't then there may be
other problems with the glazes in that I don't know whether they will
work for single fire applications or not. Generally speaking, unless
specifically stated otherwise, most glazes are designed to go on bisque
fired pieces.
I also noticed that you mentioned that Ron Roy offered up a few
suggestions. Catagorically and without hesitation I can state that you
should listen to and follow his directions to the letter. He is among
the best in the business. His knowledge of glazes, clay and firing is
encyclopedic.
Your firing cycle seems to be quite fast. Try this.....With lid
closed, bottom element on low....30 minutes later middle elements on
low......30 minutes later top elements on low (this is assuming that you
have one of the older kilns that manually adjustable elements controls)
Let it run for a coupla hours and then turn all three sets of elements
to medium. Let it run for another coupla hours and then turn all three
sets of elements to high. What has worked for me is to take at least
eight hours to reach cone 6. Then I fire down. This will take three to
four more hours depending upon what type of glaze results I am after.
The firing down cycle is critical with the Water Fall Brown. For a
definitive explanation of this consult Matering Cone 6 Glazes by Ron and
Jon. In my opinion it is the best text available for an understanding of
successful electric kiln firings.
Make certain that you are using a witness cone. Not the kiln sitter.
Make certain that the tip of your cone is just touching your cone pat
when you reach your high temp; Let it soak for a bit at that point
before you begin your fire down.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
Ron Roy on sat 30 jul 05
HI Carty,
Now that you have detailed your firing cycle I am sure all you need to do
is fire slower - or at least slow down towards the end of your firing - to
give your glaze a chance to smooth out and heal over any bubbles - which
are normal as glazes go through certain stages of melting.
It is the main problem with sitters - race up to the end and shut off -
just not a good idea with the majority of glazes.
My general rule is one hour for every 100C (200F) which means about 12
hours for a firing. There are places where you can go faster and even some
where you can go slower but that is my recommendation.
You will probably also find that some of those crazing glaze will stop
because they get enough time to bond properly with the clay.
If you still find our glazes crazing I can make adjustments easily to lower
the expansions.
RR
>In Skutt - (for Cone 6) - top open width of a shelf support lying on its
>side, top peephole open - bottom element on low for 30 minutes, then top
>closed and top element added on low for 1 hour. Then both to medium for 1
>hour. Then both to high. Kiln shuts off with cone sitter about 4 hours
>later. Kiln opened when pyrometer shows below 100 degrees - usually 18
>hours or so later.
>
>Sure seems simple. But I am not being successful. I've recently had a week
>long class with Michael Connelly (is a great teacher) and have some nice
>jars I want to glaze. Sure don't want to doom them to failure before I
>start - so I'm only going to be firing pieces of broken pots until I find
>what I am doing wrong.
>
>I'm open for suggestions on what I may be missing. I have a bunch of crazed
>stuff - and I don't want to add to it - the Garden is getting pretty full of
>decorations!
>
>Carty Ellis
>Oxford, OH
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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