crazing

updated thu 20 nov 08

VALERIE SANTARELLI on mon 1 jul 96

Hi!
I really need someone's assistance. The majority of my work is
pinched or slabbed but my pieces are relatively thin. I have been
using a white clay from Great Lakes Clay, covering with Duncan Cover
Coat and firing to 04. When they are clear glazed and fired to 06,
they all craze. What am I doing wrong? Should I bisque to 03 on the
first firing? I am very frustrated and obviously this takes away from
my pieces.

Valerie Santarelli
Valerie_Santarelli@kirkland.com

Hluch - Kevin A. on tue 26 nov 96

Having just read Jonathan Kaplan's remarks concerning the possible danger
of crazing in utilitarian pottery in the ???'s section of Nov.CM and seeing
the remarks below....I have a statement and then a few questions.

When living in Minnesota I met potters who enjoyed crazing on their
functional pots. In fact, one potter indicated as a rather dull,
lifeless pot came out of the kiln that this one will "age" well in use.
That is, the almost indiscernable crackle pattern would darken as a result
of stains from various cooking (food) sources and make it a beautiful pot.
Certainly, I own a lot of pots where this process has occurred. Not to mention
a beautiful porcelain cup made by Chris Staley who I guess outta know better.

Also while living in Kenya, I noticed that all of the earthenware cooking
pots that the villagers used darkened and sealed up (became less porous)
through continued use.

Also, I faintly remember reading from one of you on the list that one
needn't worry about anerobic bacteria....My question:

Why aren't all third world villagers who use these kinds of pots dead and
furthermore why is it that I'm not dead yet? (Or at least constantly
sick from all of my crazed pots?) ((Or am I just building a GREAT immune
system?)) Also, are all crazed pots then purely decorative and therefore
art?

Always Wondering,

Kevin A. Hluch
Frederick, MD

On Sat, 23 Nov 1996, Tony Hansen wrote:

> ----------------------------Original message----------------------------
> We are all dedicated to form and function.
> But anyone making ware for food or drink must take seriously their
> RESPONSIBILITY to produce ware with minimum leachabilty, pinholing,
> crazing, with good resistance to thermal shock and chipping, hard,
> strong, etc. Achieving these things requires study and knowledge and
> committement.
>
> --
> Tony Hansen, IMC - Pulishers of INSIGHT, FORESIGHT, Magic of Fire
> web: http://digitalfire.com/imc.html
>

Craig Martell on wed 27 nov 96

In a message dated 96-11-26 07:41:10 EST, Kevin Hluch wrote:

<< When living in Minnesota I met potters who enjoyed crazing on their
functional pots. In fact, one potter indicated as a rather dull,
lifeless pot came out of the kiln that this one will "age" well in use.
That is, the almost indiscernable crackle pattern would darken as a result
of stains from various cooking (food) sources and make it a beautiful pot.
>>

While unloading my kiln today, I spied a couple of ,shall we say, sub
standard or rather not so hot pieces. But following this line of thinking,
they should be a lot more pleasing if I can get the glazes to craze. OK, I
just went out and looked at them again and decided that craze or no craze,
they're dogs, nothing can elevate them any further than they are.

I have quite a few pieces that are crazed too. I eat off them and serve food
in them and I doubt that they will kill me or make me ill. I must also add
that some of these crazed wonders were made by me. I am really good at
making glazes that are delayed crazers. Just when you think it's safe to go
back into the kitchen "PING".
I have one glaze that I made that does not craze on anything and I use it as
a liner for a lot of functional ware. Crazed pots don't scare me but I think
that I will still try to produce good strong functional ware that does not
craze, shiver, chip etc.

Still wondering if I should add a bit more Silica to that delayed crazer,
Craig Martell-Oregon

Leslie Ihde on mon 2 dec 96

Doesn't the dishwasher render all pieces sterile after washing? Wouldn't
the only danger be if pieces of glaze actually fall off piece and you
swallow it? In fact- even then wouldn't it be likely to go right through
your
system?

Leslie
Vestal, NY

CP Dunbar on wed 4 dec 96

If we might pause to remember from the distant past a record set by
George Harrison, All Things Must Pass..............

Yes the glaze fragments will pass thru the system fine. I believe that
almost
anything will pass, nails, needles, glass, etc. It is true that someone
might have a problem rarely, but the operative word is rarely.

As long as there is nothing inherent in the glaze that would absorb
into the system for a period of from one to three days, the glaze
fragment would be passed, like a forgotton penny.

cp
--
"And she shall have music wherever my Lady goes."

cpdunbar@concentric.net

John Britt on fri 2 may 97

I have a question for all the digital glaze gurus!

We have been having a discussion about crazing and some claim you can
have a glaze that never crazes. But I am argueing that all glazes
eventually craze. Particularly with heating and cooling, (like cups for
coffee). Not in theory but in actual reality.

My reasoning is that you will always have a difference in expansion and
contraction rates because a glaze is a more of a glass than clay. As
far as clay goes, even vitreous porcelain is not a glass but toward
glass. So, consequently you should have differences in expansion and
contraction rates as you move up the continuum.

What is the real answer to the $65,000 question????

Thanks in advance for your wisdom. (Clayart has been an education.)
--
Thanks,

John Britt claydude@unicomp.net
Dys-Functional Pottery
Dallas, Texas
http://www.dysfunctionalpottery.com/claydude

John Britt on sat 3 may 97

Attention! Digital glaze gurus, we have a dispute that only you can
settle.

We have been having a discussion about crazing and some claim that you
can
have a glaze which will never, ever craze. No matter what. They claim
that if the expansion rate is the same then the two will remain
stable.

But I, on the other side, am argueing that all glazes will eventually
craze. Particularly with rapid heating and cooling, (like cups for
coffee, dinnerware etc.).

My reasoning is that you will always have a difference in expansion and
contraction rates because a glaze is a more of a glass than clay. So the
expansion rate will never be identical. Additionally, the rate that the
glaze expands will be faster because it is more of a glass (i.e. more
dense). So even though a glaze and clay body will be close to the same
expansion rate they will never be identical nor will the speed at which
they reach that expansion be the same. So, over time they will all
craze.

Also, I would imagine that if a surface is uneven, (i.e. round, oval,
textured etc.), it could not expand at a regular rate without causing
stresses on the surface of the glaze as expansion occured.

What is the real answer to the $65,000 question????

Thanks in advance for your wisdom.

--

John Britt claydude@unicomp.net
Dys-Functional Pottery
Dallas, Texas
http://www.dysfunctionalpottery.com/claydude

Craig Martell on sat 3 may 97

At 07:06 AM 5/2/97 EDT, John Britt wrote:
>----------------------------Original message----------------------------
>I have a question for all the digital glaze gurus!
>
>We have been having a discussion about crazing and some claim you can
>have a glaze that never crazes. But I am argueing that all glazes
>eventually craze. Particularly with heating and cooling, (like cups for
>coffee). Not in theory but in actual reality.

Hi John:

This crazing stuff is making ME craze! Also, I don't even know what a
Guru is so I'm fairly certain I ain't one. But I would like to respond.

In my experience with glazes, I have seen and used a number of them that do
not initially craze, and do not craze with time. Two factors are of
importance. The glaze must fit the clay within certain parameters and the
clay must be vitrified to the point that you don't get expansion due to
change in moisture content within the clay, causing it to expand a bit more,
which can put the glaze under greater tension thus causing crazing.

Regards, Craig Martell-Oregon

M Richens on sat 3 may 97

In article <3369048E.628D@unicomp.net>, John Britt
writes
>----------------------------Original message----------------------------
>I have a question for all the digital glaze gurus!
>
>We have been having a discussion about crazing and some claim you can
>have a glaze that never crazes. But I am argueing that all glazes
>eventually craze. Particularly with heating and cooling, (like cups for
>coffee). Not in theory but in actual reality.
>
>My reasoning is that you will always have a difference in expansion and
>contraction rates because a glaze is a more of a glass than clay. As
>far as clay goes, even vitreous porcelain is not a glass but toward
>glass. So, consequently you should have differences in expansion and
>contraction rates as you move up the continuum.
>
>What is the real answer to the $65,000 question????
>
>Thanks in advance for your wisdom. (Clayart has been an education.)
>--
>Thanks,
>
>John Britt claydude@unicomp.net
>Dys-Functional Pottery
>Dallas, Texas
>http://www.dysfunctionalpottery.com/claydude
Hi John,
First, I apologize if I'm telling you stuff you know.
The craze thing is all down to fit. Look at David Hewitt's excellent
explanation on
http://www.ceramicsoftware.com/education/glaze/crazing.htm

A fully formed glaze is a glass. Glasses are strongest in compression
and weak in tension..so the first thing is to make sure the expansion
coefficient of your glaze is less than that of your body.....but not too
much or the forces at the body/glaze boundary layer will cause the two
to part company in a big way.
The critical bit is to ensure that the glaze is under compression over
the entire range of use of the item. It the fit is so slight that the
compression is relaxed by boiling water then it is not going to be much
good on a teapot. You need a larger compressive force. A bone china with
a % expanion of 4 at 500 degrees C would have a glaze with a %expansion
of 3.6 at 500 as a working fit. This should not cause crazing in the
normal life of the article as long as there is no mechanical damage.

Commercial Freezer-to-Table ware gets round this problem by using a body
with very low expansion coefficient sometimes 1 to 2 % or less in order
to minimise movement. Getting workable glazes for these can be a problem
but once done there is very low risk of crazing. Some Lithium-Alumina-
silica combination have zero expansion. (beta-Spodumene and beta-
Eucryptite structures)
Hope this Helps
Max
--
Max Richens max@richens.demon.co.uk +44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.

Don Jones on sun 4 may 97

>----------------------------Original message----------------------------
>At 07:06 AM 5/2/97 EDT, John Britt wrote:
>>----------------------------Original message----------------------------
>>I have a question for all the digital glaze gurus!
>>
>>We have been having a discussion about crazing and some claim you can
>>have a glaze that never crazes. But I am argueing that all glazes
>>eventually craze. Particularly with heating and cooling, (like cups for
>>coffee). Not in theory but in actual reality.
>
>Hi John:
>
>This crazing stuff is making ME craze! Also, I don't even know what a
>Guru is so I'm fairly certain I ain't one. But I would like to respond.
>
>In my experience with glazes, I have seen and used a number of them that do
>not initially craze, and do not craze with time. Two factors are of
>importance. The glaze must fit the clay within certain parameters and the
>clay must be vitrified to the point that you don't get expansion due to
>change in moisture content within the clay, causing it to expand a bit more,
>which can put the glaze under greater tension thus causing crazing.
>
>Regards, Craig Martell-Oregon

Here's a test for all you food functional potters. Pull down your favorite
sald bowl that you have used for a while and give it a tap. THUNK!
Don Jones
claysky@highfiber.com

John Britt on sun 4 may 97

Ron Roy,

Thank you for your detailed answer to the question of crazing.

One thing that struck me in your reply was the statement: "Glazes craze
easily by the way - they are just not strong under tension (being
streched)".

So you are saying that theoritically glazes can be formulated to not
craze but that is very difficult because 1. the tensile strength of
glaze is not very high 2. because of the changing nature of raw
materials and 3. the variability in each firing.

[I am reminded of a math class I once took where they said, if Mary were
traveling from point "A" to "B" and she covered half the distance each
time they moved toward "B", she would never reach Point "B". So,
theoritically, you could never get to the store to get your morning
milk. Yet in actuality we all do it. So, theoritically we can find a
glaze that doesn't craze yet no one really does (once and for all)
because of the variablity of the above?]

I have done some testing with celadons and to correct for crazing I have
increased the silica content in the glaze. It is my understanding that
increasing silica in either the clay body or the glaze decreases crazing
in the glaze due to the presence of the silica crystal, which has a
lower exp/cont rate. I have also decreased the silica mesh (from 200 to
325) under the theory that more silica will be in solution in the glaze.
I have also tried ZnO to reduce the crazing. All these have given me
successes but never a craze free glaze. (One glaze crazed after about 9
months. With only two large crazes, but they were actually more visible
that the super fine crazing that I started with.)

I have noticed more crazing if the kiln is fired fast or cooled quickly,
even in a glaze that is relatively matched to the body. I quess my next
question is why do glazes craze when thicker and why do they sometime
crawl off when thick. (Would you suggest calcined kaolin to correct
this?) The glaze is the basic: 50 custer, 30 silica , 20 kaolin. And
then, next time, I will ask how I can test my glazes to prove they will
not craze?

Original questions eh?


--
Thanks,

John Britt claydude@unicomp.net
Dys-Functional Pottery
Dallas, Texas
http://www.dysfunctionalpottery.com/claydude

Kat Berger on sun 4 may 97

As a point of discussion. All glazes will craze on the clay at one point in
time or another. It may take 100 years but it will happen. If you don't
believe this go to a museum or gallery or even look at the pictures in your
trusty World Ceramics book. Even the ancient Chinese bowls with all their
extensive testing and perfect fitting "relatively speaking" easy to calculate
clear glazes are now crazing.

I suppose that in theory one could make a glaze to fit a pot perfectly but I
don't believe anyone has done it yet.
Kat

Ron Roy on mon 5 may 97

>I have a question for all the digital glaze gurus!

>We have been having a discussion about crazing and some claim you can
>have a glaze that never crazes. But I am arguing that all glazes
>eventually craze.

Hi John,

Some may think you have to be some kind of computer nerd to understand this
stuff - not so. Neither do you need any higher education than you have -
all that is required is the desire to understand.

Yes! You can have glazes that will never craze (that's an unqualified yes
by the way.)

Let me explain why.

Perhaps a preamble is required here - an explanation of expansion and
contraction. It would be easy for someone reading the above statement (all
glazes eventually craze) to misunderstand what actually happens when clays
and glazes expand and contract. To put it in the most simple and basic way
- they both expand when heated and contract when cooled each at it's own
rate. If a piece of clay (or glaze) is measured at 20C, heated to say 100C
there will be a small increase in size. A very small amount but a
measurable amount if you have the right equipment. The point I am trying to
make is not how much it expands but that it returns to exactly the same
dimensions when cooled to 20C again.

It is also the habit of potters, when writing about this phenomenon, to use
the word expansion. What is really meant is the normal reversible
expansion/contraction of most solids when heated and cooled..

When a glaze solidifies (freezes) it can no longer adjust to the body as it
did while still pyro-plastic (sticky, wet, soft.) This usually happens in a
cooling kiln between about 700C and 500C depending on the cone the glaze is
designed for. After the clay and glaze have solidified - and remember they
are now bonded together - Two situations can occur during further cooling.

The glaze can contract, on cooling. 1: more than the clay - or - 2: less
than the clay.

If it contracts more than the clay then we get crazing. If it contracts
less than the clay then we have the opposite of crazing - the glaze is too
big for the clay. If the expansion/contraction of the glaze is too low
(winds up much bigger than the clay then you have shivering and or dunting
which is the clay actually breaking the pot.

What needs explaining is the situation called delayed crazing - the glaze
is not crazed but does so after time - a day, a week sometimes even a year.
This is the situation where the clay and glaze are close to being the same
size. The strains between body and clay eventually cause the glaze, under
tension, to break up. Glazes craze easily by the way - they are just not
strong under tension (being stretched.)

There is also the situation where there is enough compression (glaze winds
up bigger then the clay) to prevent crazing but - If the clay is not
sufficiently vitrified to exclude water then the clay will probably
re-hydrate some what, swell a bit, and make the glaze eventually craze.

Glazes under enough compression will not craze - they cannot. In order to
craze they have to somehow become smaller than the clay. This is why
freezing will help show up delayed crazing. The cooling is continued down
and may aggravate the fit enough to make the glaze craze. It also helps to
show the opposite problem - glaze too big.

There are other factors involved here as well. We all work with materials
which are variable. That means our bodies and glaze materials change - it
is possible to have a clay/glaze combination go along for years without a
craze line ever showing up and then it begins to happen. The body changed,
or the clays in it and/or the glaze materials changed - maybe you have
changed the way you fire.

Every clay body has it's own unique expansion. To expect your glazes to fit
well on each one is simply not reasonable.

There is more to this but I am way over my limit per post. If anyone is
interested in more - ask - why do some glazes only craze where thick? How
can I test my glazes to prove they will not craze. Should I adjust the
glaze or the clay? Can I mistake the opposite of crazing for crazing?

I am currently working on an article for Contact Magazine on this subject
which goes into the actual measurements of clay and glaze expansion (and
contraction of course.)

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
ronroy@astral.magic.ca

David Hendley on mon 5 may 97

>Here's a test for all you food functional potters. Pull down your favorite
>sald bowl that you have used for a while and give it a tap. THUNK!
>Don Jones
>claysky@highfiber.com
>
>Don - I have a set of stoneware dishes made circa 1975-76 that have been
through 20 years of daily use, dishwashers, and raising 2 kids. They still
ring like they did coming out of the kiln, thank you very much.
It IS difficult to keep everything technically on track, but far from
impossible.
David Hendley
Maydelle, Texas

David Hewitt on mon 5 may 97

In discussing whether or not a glaze will never ever craze, I think it
is important to know if the body is porous or not, eg is it earthenware
or stoneware or porcelain.
To quote from Hamer's 'The Potter's Dictionary of Materials and
Techniques'
'Earthenware bodies are porous and absorb moisture from the air. As
they do they expand permanently. The glaze does not expand but crazes to
accommodate the tension put upon it. This is another reason for the
crazing on earthenware and other porous bodies.'

I wouldn't say - 'never ever' - ,therefore, for a porous body.

If the body is stoneware or porcelain, however, and has been fired to
its maturing temperature and the glaze fit is initially a good, ie it is
under slight compression, then I would expect it to remain craze free.
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://www.ceramicsoftware.com/education/people/hewitt.htm

The Shelfords on tue 6 may 97

>There is more to this but I am way over my limit per post. If anyone is
>interested in more - ask

Ron -

Yes please. I think I understand why glazes craze or shiver, but I'm glad
to keep listening.

What I don't understand is whether there is any way to find out the
expansion/contraction information for various clay bodies. We are
concentrating so much on the glazes in this list, I suppose because of the
glaze calculation programs being readily available. But the glaze is only
half the story.

I'm assuming if it was as easy as entering the clay body ingredients in the
calculation program, and getting expansion numbers from that, someone would
have said so. Are there such things as limit formulas for clay bodies? As
I asked Tony also, if there are any numbers to be had, what do they mean?

>I am currently working on an article for Contact Magazine on this subject
>which goes into the actual measurements of clay and glaze expansion (and
>contraction of course.)

Looking forward to it.

- Veronica
____________________________________________________________________________
Veronica Shelford
e-mail: shelford@island.net
s-mail: P.O. Box 6-15
Thetis Island, BC V0R 2Y0
Tel: (250) 246-1509
____________________________________________________________________________

Kris Baum on wed 7 may 97

Yet another opportunity to display my ignorance!

I picked up a crackle glaze recipe off Clayart (80% gerstley borate, 20%
nepheline syenite) to try on my tiles. I fired it a little under
suggested temp (^04 instead of ^03), and it came out matt with gloss on
the thinner, high points, and with a nice crackle. I thought this had
potential!

Wanting to see what happened (and thinking that firing it higher would
make it glossy all over - or make it run right off the tile!), I threw
the same tile into a ^6 firing I was doing. It came out an extremely
stony matt with absolutely no hint of crazing.

(BTW, this was on a low-fire white grogged clay that is supposed to slump
around ^2 but that I have taken up to ^6 without any slumping whatsoever.
Believe it or not, I've been trying to get that clay to slump.)

My question: * why was the glaze glossier at lower temperature?

(I presume that the disappearing crazing was because the clay shrunk
enough more at ^ 6 that the glaze then fit. Although the clay tile
looked the same size before and after firing, I didn't do pre-post
measurements, and I guess it doesn't take too much shrinkage to alter the
glaze fit?)

===============================================
Kris Baum, Shubunkin Pottery,
mailto:shubunki@erols.com
===============================================


--
===============================================
Kris Baum, Shubunkin Pottery,
mailto:shubunki@erols.com
===============================================

Tony Hansen on wed 7 may 97

The Shelfords wrote:

> What I don't understand is whether there is any way to find out the
> expansion/contraction information for various clay bodies.

Plainsman Clays will have full expansion charts on our web site on each
of
the data sheets for our principal
clay bodies in the next few months. I will also post charts for a number
of
standard glaze recipes and you'll be able to use these as starting
points.

Can't easily calculate expansions for clay bodies. Two bodies of similar
chemistry can have totally different fired properties, two of totally
different
chemistry can have very similar firing properties. Particle size
distribution
and firing technique are the main obstacles to anaytical approaches to
expansion prediction.

----
Tony Hansen, IMC thansen@mlc.awinc.com
Plainsman Clays

Don Goodrich on thu 8 may 97

John, Kat, & whoever else has an opinion on this:
Although it may be true that all pots will craze eventually, I looked very
carefully at the 100+ Chinese brown and black-glazed pots in the "Hare's Fur,
Tortoiseshell, and Partridge Feathers" exhibit that toured last winter and
could not discern crazing on them. Granted I didn't use a microscope, but the
museum gallery was well-lighted and I knew what to look for (having made
quite few crazed wares of my own). I may conclude that either the
conservators of this collection have a way to make crazing invisible, or the
folks making this stuff 500 to 1500 years ago got the fit right and the clay
hasn't absorbed any moisture. Any other possibilities?

Don Goodrich on a cool and rainy night in Zion, Illinois
GoodrichDn@aol.com

Tony Hansen on thu 8 may 97

Kris Baum wrote:
> My question: * why was the glaze glossier at lower temperature?

There are a number of 'mechanisms' of matte glazes. One of these is
crystallization. Firing the glaze higher thins the melt and for the
right chemistry and cooling a mesh of crystals will invade the
entire surface.

=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com

Jean Stephenson on tue 30 sep 97

I'm being attacked by Murphy's Law and I need help. A customer
ordered some rice bowls except wanted them in a shiny green on white
stoneware. Made up a ^10 test glaze of Oribe and glazed test forms
made of Danish White [Laguna} stoneware and a few porcelain forms.
Fired well. Made up a big batch of Oribe and a back-up glaze of
Coleman Standard Clear With #6242 stain addition. The Oribe was
crazed on the stoneware and several pieces of the Oribe glazed
porcelain ran. The back-up glaze also crazed. The only variables
that I can think of was we opened a new bag of silica and the first
firing was done on a rainy day-the second on a sunny day. Do you
think an addition of silica would stop the crazing? If so, what
percent? It isn't a fine small crazing, but more than just a few
lines. The customer has called and I am starting to dither. Any and
all help is greatly appreciated. Jean
^10 Oribe Green
Custer Feldspar 30.91
Flint 25.32
Whiting 22.36
Talc 7.81
EPK 12.55
Bone Ash 1.05

Add:
Copper Carb 6.54

Too Old for Dithering on the S.Coast of Or.

Dana Henson on thu 20 nov 97

When a C/9-10 celadon reduction glaze crazes is it still food safe? Thanks
for any comments. Dana Henson

Tony Hansen on fri 21 nov 97

? When a C/9-10 celadon reduction glaze crazes is it still food safe?

Yes it probably is OK. However I'm note sure. Picture the size
of bacteria in your mind. Picture the width of the crack as wider
then that. Imagine a porous clay under that glaze that keeps it
nice an damp in the cracks.
Reflect on the rise of drug resistant strains of bacteria
that threaten us all.....

Another factor is that ware will be severely weakened,
easily half its potential
strength and potentially much less then that. Much more likely
someone will break the handle on a hot cup of coffee.....

--
-------
T o n y H a n s e n thansen@digitalfire.com
Get INSIGHT, Magic of Fire at http://digitalfire.com

Claymate2 on sun 30 nov 97

I was hoping someone could answer a quesion for me. I just unloaded a kiln of
gloss-fired earthenware. Everything looked great untill I got to the bottom
shelf. The pieces on the bottom were crazed only on the bottom portion of the
outside of the pieces. I glazed them on the inside first by pouring and then
dipped the top half. After that was dry, I then dipped the bottom half. The
other pieces were the same clay body and same gaze and they looked fine. I
fired to 06, all bisque was fired to 04. Any help would be greatly
appreciated.
Thanks,
Leslie

Ray Carlton on mon 1 dec 97

sounds like overfiring to me leslie..What type of kiln is it?? and have you
used the glaze before.. Is it a new batch of a favorite I don't think the
way you glazed the pots is the problem....



At 05:16 30/11/97 EST, you wrote:
>----------------------------Original message----------------------------
>I was hoping someone could answer a quesion for me. I just unloaded a
kiln of
>gloss-fired earthenware. Everything looked great untill I got to the bottom
>shelf. The pieces on the bottom were crazed only on the bottom portion of
the
>outside of the pieces. I glazed them on the inside first by pouring and then
>dipped the top half. After that was dry, I then dipped the bottom half. The
>other pieces were the same clay body and same gaze and they looked fine. I
>fired to 06, all bisque was fired to 04. Any help would be greatly
>appreciated.
>Thanks,
>Leslie
>
>
raycarlt@valylink.net.au



Ray Carlton
McMahons Creek Victoria Australia 3799

marvin s flowerman on tue 2 dec 97

Leslie:

Try glazing the inside first by pouring as you are doing. Let the inside
dry until it has no sheen. Then, perhaps using tongs, or by expanding
your fingers, dip the outside completely; by that I mean not just the
top, but the entire outside surface (assuming you intend to glaze the
outside with just one glaze.

See if that doesn't cure your problem.

Good luck.

Marvin (marvpots@aol.com)

Leslie Kinsey on tue 2 dec 97

Thanks for all the help. I think I may have figured out some of the problem.
What I think might have happened is that the pieces that I had on the bottom
of the kiln were all pieces that were fired at the community college where I
take classes. I'm not sure, but I think that they only fire to ^06 for both
bisque and gloss firing. Could it be that the clay body did not get
completely vitrified and accomplish its maximum shrinkage before I gloss
fired it?
Thanks,
Leslie

Ray Carlton on wed 3 dec 97

cone 06 should be ok leslie you can glaze fire un bisqued pots with good
results...
cheers
At 10:20 02/12/97 EST, you wrote:
>----------------------------Original message----------------------------
>Thanks for all the help. I think I may have figured out some of the problem.
> What I think might have happened is that the pieces that I had on the bottom
>of the kiln were all pieces that were fired at the community college where I
>take classes. I'm not sure, but I think that they only fire to ^06 for both
>bisque and gloss firing. Could it be that the clay body did not get
>completely vitrified and accomplish its maximum shrinkage before I gloss
>fired it?
>Thanks,
>Leslie
>
>
raycarlt@valylink.net.au



Ray Carlton
McMahons Creek Victoria Australia 3799

Clay on wed 3 dec 97

I ran a production line for 3 years in earthernware only .However I still
have a little suggestion.
High talc clay seems to solve a lot of problem in that case. Ask your
supplier see how much Talc they have.
Then cover the the whole thing with glaze & used stilts to fire them. If you
don't do that half of the time even your glaze wasn't crazing now , it will
crazed latter. Test with a household dish-washer which have about 1 hour cycle.
I find that no matter what kiln you use Top & Bottom is always 2 Cones
different(Sorry, I'm talking about my case ,if you are doing day in day out
production ,you will only allow yourself to touch the HIGH botton on your
kiln)BisQ is very critiqcal in Earthernware. you have to at list fire to
^04. So I put ^02 in the cone sitter to ensure I got ^04 in the bottom.
Even you have Unitemp Control (Orton Parts)I still recommand you fire at
list ^03.
Put underglaze after you bisq so You can have all the Beauty you wanted.
Yes, they will stick on even you put the glaze right after.If not I'll tell
you what brand I have been using.

Then go home & watch X-file will make you feel better .You find there is
more mistery out there then playing with glaze.

Ian Ian & Ian
>>----------------------------Original message----------------------------
>>I was hoping someone could answer a quesion for me. I just unloaded a
>kiln of
>>gloss-fired earthenware. Everything looked great untill I got to the bottom
>>shelf. The pieces on the bottom were crazed only on the bottom portion of
>the
>>outside of the pieces. I glazed them on the inside first by pouring and then
>>dipped the top half. After that was dry, I then dipped the bottom half. The
>>other pieces were the same clay body and same gaze and they looked fine. I
>>fired to 06, all bisque was fired to 04. Any help would be greatly
>>appreciated.
>>Thanks,
>>Leslie
>>
>>
>raycarlt@valylink.net.au
>
>
>
>Ray Carlton
>McMahons Creek Victoria Australia 3799
>

Ron Roy on wed 3 dec 97

Hi Leslie,

What the bisque fired to will have very little to do with the
expansion/contraction of the final product. I has occurred to me that it is
possible to have ware craze when it would ordinarily not. Picture this -
the ware is cooling fast - the glaze is cooling and contracting faster than
the body. It can happen if it is in a fast cooling part of the kiln - like
the bottom. You could re-fire those that crazed - in the middle or top of
your kiln - if they don't craze there then it must be fast cooling - let us
know what happens if you do now!

As an aside - bodies fired below stoneware temperatures are rarely
vitrified - in fact I have never seen one - mot with the kind of raw
materials we traditionally use.


>----------------------------Original message----------------------------
>Thanks for all the help. I think I may have figured out some of the problem.
> What I think might have happened is that the pieces that I had on the bottom
>of the kiln were all pieces that were fired at the community college where I
>take classes. I'm not sure, but I think that they only fire to ^06 for both
>bisque and gloss firing. Could it be that the clay body did not get
>completely vitrified and accomplish its maximum shrinkage before I gloss
>fired it?
>Thanks,
>Leslie

Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm

Sue Hintz on sat 23 may 98

I'm just about to give up on this glaze but I love it so that I thought
asking you all will be my last attempt to get it right.

I have a glaze recipe (don't have it here with me but if it will help I can
bring it next time I email) It is a matt with 21% whiting. I added 10% silica,
20% silica, 30% silica and it still crazed. Actually the more silica I added
the closer, and more cracks I got. This is that raspberry glaze that was
shared a while back. I've been under the attitude that I could fix any
glaze with the right combination but now I'm starting to wonder. I'm firing
this on a white clay body at cone 5. I'm told by the clay producer that others
have said they have had problems with crazing on this clay. The only other
glaze I've tried on this clay is Patty's Crystal clear. This is a great glaze
but it crazed on the white. What do you all think? Give up this white clay?

pam easley on sat 22 may 99

I am currenlty using a low talc cone 06 clay from ClayArt Center in
Tacoma, WA for some community center classes I teach. It is a fine
textured white clay body, very forgiving and great to use with beginners
if it's not too soft/moist. I have been having a heck of a time with
crazing. I use glazes supposedly formulated to match this body, and have
overcome the shivering problems I encountered with ClayArt's Xtra White
low fire body. Still occasionally have some underglaze pop off when
glazed undr clear. I bisque to 04, glaze to 06. Last firing kept pinging
for days. Many of my students make functional ware, and with the advent
of this crazing I have advised them to rinse with chlorox water after
use to kill bacteria in the craze llines. Sounded like a reasonable idea
at the time...
Any suggestions? I'm about to switch to an earthenware or stoneware and
fire hotter, hoping that will alleviate the problem.
TIA for any and all advice.

Pam, in not yet warm , but getting sunnier (some days) Seattle

Paul Carlson on sun 23 may 99

>pam easley wrote:

> ----------------------------Original message----------------------------
> >I am currenlty using a low talc cone 06 clay from ClayArt Center in
> >Tacoma, WA for some community center classes I teach. It is a fine
> >textured white clay body, very forgiving and great to use with beginners
> >if it's not too soft/moist. I have been having a heck of a time with
> >crazing. I use glazes supposedly formulated to match this body, and have
> >overcome the shivering problems I encountered with ClayArt's Xtra White
> >low fire body. Still occasionally have some underglaze pop off when
> >glazed undr clear. I bisque to 04, glaze to 06.
>
> Pam, in not yet warm , but getting sunnier (some days) Seattle

Pam, I had the same problem some time ago with the talc body. Steve
Howell's advice to me was to bisque to 03 and glaze to 05. I haven't had a
single piece craze since. Also, be sure you are reaching 03 and 05
throughout the kiln. Witness cones top and bottom to be sure.

Paul Carlson
Paul Carlson Pottery

Ron Roy on tue 25 may 99

Hi talc bodies (over 50%) are better to stop crazing at the lower
temperatures. They also prevent rehydration of the fired ware. This is a
common problem with earthenware clays - water enters fired (but
unvitrified) ware during use and rehydrates and the result is expansion of
the body and delayed crazing.

RR
>> ----------------------------Original message----------------------------
>> >I am currenlty using a low talc cone 06 clay from ClayArt Center in
>> >Tacoma, WA for some community center classes I teach. It is a fine
>> >textured white clay body, very forgiving and great to use with beginners
>> >if it's not too soft/moist. I have been having a heck of a time with
>> >crazing. I use glazes supposedly formulated to match this body, and have
>> >overcome the shivering problems I encountered with ClayArt's Xtra White
>> >low fire body. Still occasionally have some underglaze pop off when
>> >glazed undr clear. I bisque to 04, glaze to 06.
>>
>> Pam, in not yet warm , but getting sunnier (some days) Seattle
>
>Pam, I had the same problem some time ago with the talc body. Steve
>Howell's advice to me was to bisque to 03 and glaze to 05. I haven't had a
>single piece craze since. Also, be sure you are reaching 03 and 05
>throughout the kiln. Witness cones top and bottom to be sure.
>
>Paul Carlson
>Paul Carlson Pottery

Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849

Web page: http://digitalfire.com/education/people/ronroy.htm

pearcelr@island.net on wed 26 may 99

<3745158e.f59ede5-@foxinternet.net> wrote:
Original Article: http://www.egroups.com/group/clayart/?start=45100
> ----------------------------Original message----------------------------
> I am currenlty using a low talc cone 06 clay from ClayArt Center in
> Tacoma, WA for some community center classes I teach. It is a fine
> textured white clay body, very forgiving and great to use with beginners
> if it's not too soft/moist. I have been having a heck of a time with
> crazing. I use glazes supposedly formulated to match this body, and have
> overcome the shivering problems I encountered with ClayArt's Xtra White
> low fire body. Still occasionally have some underglaze pop off when
> glazed undr clear. I bisque to 04, glaze to 06. Last firing kept pinging
> for days. Many of my students make functional ware, and with the advent
> of this crazing I have advised them to rinse with chlorox water after
> use to kill bacteria in the craze llines. Sounded like a reasonable idea
> at the time...
> Any suggestions? I'm about to switch to an earthenware or stoneware and
> fire hotter, hoping that will alleviate the problem.
> TIA for any and all advice.
>
> Pam, in not yet warm , but getting sunnier (some days) Seattle
>
> To Pam in Seattle:

My suggestion is to bisque at 06 and glaze fire at 04. But, if this doesn't
help I would consider firing one or two cones up from that as well.
There seems to be no justice with clay, as I have tried many,
many recipes to get a nice crackle on low temperature clay.
Ah, such is clay.

Hope I've helped...
Gabrielle
pearcelr@island.net

pam easley on sat 29 may 99

Gabrielle, thanks for the suggestion. Isn't it always the way - when you
WANT a craze, it never happens! I have been firing 06/04 and got the
crazing. Obviously a mismatch between the clay and the glaze formulas.
Last load was 03/05, as someone suggested a couple of days ago, and am
awaiting the next unload. Am keeping my fingers crossed, as all this
stuff is student's work and I hate to see them disappointed. I think
I'll change clay bodies for next year's classes ---
Pam, near Seattle

Diane G. Echlin on sat 12 feb 00

Hi folks,
I'm having a problem with crazing that I hope you can help me out with.
I'm taking a class at a local handcrafts center that has just started
bisquing at ^06 instead of ^04. I am using a lightly grogged white
stoneware body (commercial) and firing at ^10 reduction. I have been
using the same celedon glaze for several years with great results, but
this time my wares have a ton of crazing. Can this be caused by the
lower bisque firing and subsequent increased shrinkage of the clay in
the glaze firing? If that's the case, why wouldn't the glaze shiver?
There are many variables to consider, not the least of which is that
there are several new apprentices in charge of the glaze firing, and
also different people mix up the glazes. I guess what I'm trying to say
is that in this particular situation I have very little control over
what happens to my work after I throw and trim it.
TIA
Di

Sue Hintz--Salvaterra Pottery on mon 14 feb 00

Diane,
I've done a lot of studying about crazing lately and a lot of my own
experiments. I'd recomend going to the web site digitalfire.com. They have
a ton of information on this sort of thing. I would highly recommend buying
their book. The very first thing I would do is mix up a small sample of
that glaze myself and see if your batch crazes. Did you change your clay
body? I have glazes that craze on one clay body but not the other because
the expansion coef. is different for the two differnt clays. Be careful
with quick fixes like adding extra silica. If you want to e-mail me
directly to talk in greater detail I'll be happy to take a look at your
recipe.
----- Original Message -----
From: Diane G. Echlin
To:
Sent: Saturday, February 12, 2000 11:17 AM
Subject: crazing


> ----------------------------Original message----------------------------
> Hi folks,
> I'm having a problem with crazing that I hope you can help me out with.
> I'm taking a class at a local handcrafts center that has just started
> bisquing at ^06 instead of ^04. I am using a lightly grogged white
> stoneware body (commercial) and firing at ^10 reduction. I have been
> using the same celedon glaze for several years with great results, but
> this time my wares have a ton of crazing. Can this be caused by the
> lower bisque firing and subsequent increased shrinkage of the clay in
> the glaze firing? If that's the case, why wouldn't the glaze shiver?
> There are many variables to consider, not the least of which is that
> there are several new apprentices in charge of the glaze firing, and
> also different people mix up the glazes. I guess what I'm trying to say
> is that in this particular situation I have very little control over
> what happens to my work after I throw and trim it.
> TIA
> Di
>

Ron Roy on mon 14 feb 00

The stresses that lead to crazing and shivering are not developed untill
the glazes has become ridgid on cooling - the temperature of the bisque
firing has little if any effect on glaze fit.

Longer glaze firing can have an effect either way - by making the bond
between clay and glazes stronger and lowering the amount of free quartz in
the clay. Cristobalite can also be increased with longer firings. All of
these factors can be used to improve (or upset) clay/glaze fit. Most glazes
can be adjusted to change their coeficient of expansion and contraction.

If kilns are opened too soon, the glaze cooling faster than the body, can
promote crazing.

No I don't think it's the difference in bisque temperature.

>----------------------------Original message----------------------------
>Hi folks,
>I'm having a problem with crazing that I hope you can help me out with.
>I'm taking a class at a local handcrafts center that has just started
>bisquing at ^06 instead of ^04. I am using a lightly grogged white
>stoneware body (commercial) and firing at ^10 reduction. I have been
>using the same celedon glaze for several years with great results, but
>this time my wares have a ton of crazing. Can this be caused by the
>lower bisque firing and subsequent increased shrinkage of the clay in
>the glaze firing? If that's the case, why wouldn't the glaze shiver?
>There are many variables to consider, not the least of which is that
>there are several new apprentices in charge of the glaze firing, and
>also different people mix up the glazes. I guess what I'm trying to say
>is that in this particular situation I have very little control over
>what happens to my work after I throw and trim it.
>TIA
>Di

Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849

Paul Taylor on mon 14 feb 00

Dear Di


By raising the biscuit temperature The body is more fluxed therefore
less absorbent also the surface will be smoother because the clay particles
are closer together .

So several things could be happening. The glaze body layer may have been
made thinner because no glaze materials have been absorbed into ,and around,
the particles on the out side of the pot, and therefore the large buffer
zone, which was helping your glaze fit, is no longer there.

These particles that lodge themselves in the biscuit are the less
refractory because they are small, and probably not siliceous.

Although stoneware glazes are for all intent and purposes insoluble
they are not completely. When the soda from some of the glaze slop leaches
into the water it would previously have been absorbed into the lower fired
biscuit. It is now all on the surface.

All these fctors could be lowering the expantion of the glaze.

The other thing that comes to mind is that your silica bearing materials
could be from a newly opened bag that contains 15% water for health reasons
, lowering your silica and shrinking the glaze.


You could be making the glaze thicker to compensate for the extra run
off the harder biscuit.

All these could be contributing in a small way to your crazing problem.


If it was my glaze I would also suspect the absentminded dumbo that
mixed the glaze normally yours truly.

Best of luck

Paul T
----------
>From: "Diane G. Echlin"
>To: CLAYART@LSV.UKY.EDU
>Subject: crazing
>Date: Sat, Feb 12, 2000, 4:17 pm
>

>----------------------------Original message----------------------------
>Hi folks,
>I'm having a problem with crazing that I hope you can help me out with.
>I'm taking a class at a local handcrafts center that has just started
>bisquing at ^06 instead of ^04. I am using a lightly grogged white
>stoneware body (commercial) and firing at ^10 reduction. I have been
>using the same celedon glaze for several years with great results, but
>this time my wares have a ton of crazing. Can this be caused by the
>lower bisque firing and subsequent increased shrinkage of the clay in
>the glaze firing? If that's the case, why wouldn't the glaze shiver?
>There are many variables to consider, not the least of which is that
>there are several new apprentices in charge of the glaze firing, and
>also different people mix up the glazes. I guess what I'm trying to say
>is that in this particular situation I have very little control over
>what happens to my work after I throw and trim it.
>TIA
>Di
>

mel jacobson on tue 21 mar 00

this is from a friend.
second one.

The other day I followed the thread on "crazing". There are a lot of
messages about crazing through the years. I haven't had time to look at
all of them but I did look at about 15 messages. What I noticed was that
they all follow the same line of reasoning- that crazing is undesirable on
utilitarian pots because it would be "unsanitary". I was amazed that
nobody offered another perpective on this subject.
In Asia they use crazing for effect, on plates, bowls, you name
it. Nobody seems to be concerned about the effects of crazing on their
health there. I suspect that there is actually no evidence on becoming
sick from eating on a dish that has a crazed glaze. The amount of
unsanitary substances that we would ingest from exposure to what is
absorbed from hairline cracks in a bowl is probably not even measurable.
In fact one could turn the whole fear thing around by surmizing
that the minute amount of germs one is exposed to because of crazing is
actually beneficial- causes one to build up resistance to the particular
germ. Homeopathy is based on this principle. I've been eating out of
crazed dishes all my life and I've hardly ever been sick.
Finally, I must admit that I like crazing. In fact I use information
about what causes crazing and how to cure it so that I can make just the
right size crackle pattern to be what I consider to be pleasing. Then I
smoke the pots in cone 07 reduction to bring the crackle out with a
beautiful golden patina.
In Japan (not just Japan) where they value their pottery and where they
have a keen sense of art and esthetics they use crackle as artistic
expression. If there is a store with Japanese pottery near you check out
all the crazed domestic ware.
Bill Boyd
Galiano Island, B.C. Canada
minnetonka, minnesota, u.s.a
http://www.pclink.com/melpots (website)

Jean Cochran on wed 22 mar 00

Dear Mel:

For several years I had mucho trouble with my glazes containing gerstley
borate crawling. Have it pretty much licked now (going up a little
slower during the first part of the firing for one thing).

Anyway, I was in the St. James Court Art Show in Louisville, KY and was
almost out of pots on Sunday, HAD to take a couple of boxes of crawled
pots. I was unloading boxes and crying because they were so ugly and
what is happening? People are buying my pots as I am unloading. I was
just astounded at the remarks "Oh, look, here is a little window!" The
glaze would just peal back and it would look like little blue and green
windows or tiny framed pictures. I had people try to order sets of
dishes from me, but oh, no, I don't like it. It's a mistake to me. But
these glazes are very desirable and beautiful to others. Go figure.

Jean Wadsworth Cochran
Fox Hollow Pottery
New Haven, KY

Sheron Roberts on wed 22 mar 00

------------------
What I know about crazing is
zilch=21
I do know that it happens on my pots
to the point of frustration. The year my
pottery class was to begin studies on
glazing and all the fun stuff that it involves,
our guru/teacher quit, as I have mentioned
here before. So what I have learned has
been from books by Daniel Rhodes and
others.
It is with relief that I read the posts that
indicate crazing may not be such a bad
thing. I know that I ate from my mother's
and grandmother's tables all my life and
many of the dishes and serving
bowls had crazing.
I would also add that for years my Dad
sold stainless steel cookware for a living.
He raised six head of children cooking supper
for other people. Part of his demonstration
involved taking a piece of cookware from the
kitchen he was working in on a particular
night and boiling water in it. This cookware
was always aluminum. The heating would
open up pores in the pot and all the previously
trapped =22stuff=22 from years of use would be
released into the water. (Just as it would if a
person were cooking food in the pot).
He would pour this water into a clear drinking
glass and set it where all could see. During
the rest of the demo and supper you could see
the water begin to congeal and turn gray. He
would instruct the husband or wife to check it
in the morning and call him. He knew what they
would find, that the =22stuff=22 had etched the glass.
The point obviously being that for years we have
eaten things, bacteria, food deposits, Teflon, etc.
that erode off into our food, or secretes from the
very cookware we use. That is unless of course,
you use stainless steel. hahahaha
I have told friends about this and they just shrug
and say, you gotta go sometime.
This doesn't mean I will give up looking for a
crazeless clear glaze or try solving the problem
myself. It just makes the time till then less
disappointing.
Sheron in NC
PS Dad also sold bone china sets. He would set a
teacup on the floor bottoms up and place one foot on it,
all the women would gasp, Dad was a big guy, and
then stand on it balancing all his body weight
on that one little cup. Not once did a cup break.
Good stuff=21

-----------------------------------------------------
Click here for Free Video=21=21
http://www.gohip.com/freevideo/

Craig Martell on thu 23 mar 00

Sheron commented:

>What I know about crazing is
>zilch!

That's OK Sheron. Look at all the learning you have to look forward to.

There are some sites online that you can check out. Lawrence Ewing, who
wrote the Matrix glaze calc program has some craze control strategies in
the Matrix website. Also check out Digital Fire's web site. Tony Hansen
has some stuff about crazing that will help you unnerstan if you would like
to delve into this. It takes a bit of learning and investigation but we
can all understand what this irritant is and learn ways to contol the
problem. One of the worst things about crazing is that it weakens the
ware. The glaze craze lines will penetrate and score the clay/glaze
interface, thereby weakening the pot.

regards, Craig Martell in Oregon

Paul Taylor on thu 23 mar 00

Dear Bill

You are about to get fifteen messages to explain the difference between
crazing and crackle.

However I would agree with you on the health issue. I am concerned that
worrying about barium and lithium traces , crazing et all, is bad for ones
health.

Regards Paul T
----------
>From: mel jacobson
>To: CLAYART@LSV.UKY.EDU
>Subject: crazing
>Date: Tue, Mar 21, 2000, 5:58 pm
>

>----------------------------Original message----------------------------
>this is from a friend.
>second one.
>
>The other day I followed the thread on "crazing". There are a lot of
>messages about crazing through the years. I haven't had time to look at
>all of them but I did look at about 15 messages. What I noticed was that
>they all follow the same line of reasoning- that crazing is undesirable on
>utilitarian pots because it would be "unsanitary". I was amazed that
>nobody offered another perpective on this subject.
> In Asia they use crazing for effect, on plates, bowls, you name
>it. Nobody seems to be concerned about the effects of crazing on their
>health there. I suspect that there is actually no evidence on becoming
>sick from eating on a dish that has a crazed glaze. The amount of
>unsanitary substances that we would ingest from exposure to what is
>absorbed from hairline cracks in a bowl is probably not even measurable.
> In fact one could turn the whole fear thing around by surmizing
>that the minute amount of germs one is exposed to because of crazing is
>actually beneficial- causes one to build up resistance to the particular
>germ. Homeopathy is based on this principle. I've been eating out of
>crazed dishes all my life and I've hardly ever been sick.
> Finally, I must admit that I like crazing. In fact I use information
>about what causes crazing and how to cure it so that I can make just the
>right size crackle pattern to be what I consider to be pleasing. Then I
>smoke the pots in cone 07 reduction to bring the crackle out with a
>beautiful golden patina.
> In Japan (not just Japan) where they value their pottery and where they
>have a keen sense of art and esthetics they use crackle as artistic
>expression. If there is a store with Japanese pottery near you check out
>all the crazed domestic ware.
> Bill Boyd
> Galiano Island, B.C. Canada
>minnetonka, minnesota, u.s.a
>http://www.pclink.com/melpots (website)

Kathryn L Farmer on fri 24 mar 00

According to "Pottery" by Tony Potter, crazing= fine hairline cracks all
over the glaze, caused by the glaze shrinking more than the clay. Suggested
solutions:
* Biscuit fire to a higher temperature
*Try a different glaze
* Don't open the kiln door until it has cooled.

Kathryn

----- Original Message -----
From: "Craig Martell"
To:
Sent: Thursday, March 23, 2000 6:13 PM
Subject: Re: Crazing


> ----------------------------Original message----------------------------
> Sheron commented:
>
> >What I know about crazing is
> >zilch!
>
> That's OK Sheron. Look at all the learning you have to look forward to.
>
> There are some sites online that you can check out. Lawrence Ewing, who
> wrote the Matrix glaze calc program has some craze control strategies in
> the Matrix website. Also check out Digital Fire's web site. Tony Hansen
> has some stuff about crazing that will help you unnerstan if you would
like
> to delve into this. It takes a bit of learning and investigation but we
> can all understand what this irritant is and learn ways to contol the
> problem. One of the worst things about crazing is that it weakens the
> ware. The glaze craze lines will penetrate and score the clay/glaze
> interface, thereby weakening the pot.
>
> regards, Craig Martell in Oregon

iandol on fri 24 mar 00

------------------
My friend Roger Klienig of Laura has made a feature of crazing for all of =
the
past eighteen years. Pleased to say his work sells wholesale and is =
collected
and exported far and wide.

Currently some of the kitchen ware speciality shops are importing a white =
crazed
ware which is unusual. The crazing is patterned into whorls which flow =
around
the pots. They are not small either, some examples are over two feet high. =
As
might be expected, the sales assistants know nothing.

Ivor Lewis.

Ray Aldridge on mon 27 mar 00

At 06:13 PM 3/23/00 EST, Craig wrote:
>----------------------------Original message----------------------------
>to delve into this. It takes a bit of learning and investigation but we
>can all understand what this irritant is and learn ways to contol the
>problem. One of the worst things about crazing is that it weakens the
>ware. The glaze craze lines will penetrate and score the clay/glaze
>interface, thereby weakening the pot.
>

Just to add to what Craig says above, a pot with a properly fitting glaze
will be also be much stronger than one with a crazed glaze because the
glaze will be in slight compression, which substantially increases the
pot's resistance to rupture. The compression, as I understand it, helps
distribute any impacts to the entire piece. Ron could probably explain
this much better than me, but it's another advantage of good glaze fit.

Ray

Aldridge Porcelain and Stoneware
http://www.goodpots.com

Ron Roy on tue 28 mar 00

Just some further thoughts on crazing.

I don't mind eating out of crazed pots and do it almost every day -
but it is a fault and can be fixed most of the time. It does weaken
ware and from that standpoint it is a fault.

I can imagine there could be a problem with storing certain foods in
ware already contaminated - surely the bacteria already in the
crazing would have an effect on spoilage time?

As to sales - I can imagine the effect badly stained would have on
some people. If you are selling dinnerware and it is crazed - I am
sure many would find the effect so distasteful they would not return
to buy more.

RR
Ron Roy

93 Pegasus Trail,
Scarborough, Ontario
Canada. M1G 3N8

Tel: 416-439-2621
Fax: 416-438-7849

terryh on wed 29 mar 00

what's so distasteful about crazing? haven't we ever seen any "kan-nyu
seiji" (crackle celadon)? i saw a few contemporary examples shown in robert
yellin's web page.
http://www.japanesepottery.com/Potter_Spotlight/S__Minegishi/s__minegishi.ht
ml
and as for hygienic of cracklles, well, i worry more about carcinogens in
processed junk food and gene-altered new crops and .... i can always wash my
tea pot (grin).
Terry Hagiwara
terryh@pdq.net
http://terryh.lookscool.com
http://www.geocities.com/SoHo/Cafe/3755

Natalie vL on mon 2 oct 00

Hello fellow cermacists!
I am new to Clayart and am hoping that someone will have an answer to a
problem I have been having with some large tiles that will be placed
outside. We are using a cone 6 white bodied clay (formulated commercially)
with a commercial underglaze and a clear matt glaze. Clear glaze has
crazed on some of the tiles and I am hoping to find a way to seal them so
they will still be usable for outdoors and freezing and thawing. We
already have some ideas for how to avoid this problem in the future.
However, I would like to salvage these other tiles. We have been trying
something called "creeping crack sealer" and it seems to work in some areas
and not in others. Any ideas about why this happens and how we could fix
this problem would be approcoated.

Thanks in advance for the help!
Natalie

Lily Krakowski on fri 6 sep 02

I am astonished that the clay body has not been mentionned in the
discussion of crazing on low fired bodies. I have encountered clay bodies
on which most of my perfectly good standard glazes craze--especially after
torture tests (heating and cooling rapidly, soaking the whole pot in water
overnight, the dishwasher etc.

That is why it is SUCH a good idea when starting out to have several clay
bodies and test one's glazes on them all, and pick the body that accepts
them best. I still keep bodies I do not use (Goodness, does that sound
kinky or does it?) around just to test glazes on.



Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389

Be of good courage....

Burness Speakman on mon 9 jun 03

I use Cushings cone 6 white clay body. It takes a matt glaze very well but
I have trouble using gloss and clear giving fine crackles. Any suggestions?

The Harris Family on mon 25 aug 03

I have a friend who is using the following recipe on Cone 04 bisque. =
The clay is high fire clay called 1/2 and 1/2 which is half porcelain. =
The problem is crazing on a Cone 6 firing. She really likes this glaze =
and does not want to use a different but would like to know how to alter =
this glaze to make it fit the piece better. After much reading, we =
decided to add more Silica, but how do you know how much to add and =
does she need to add/remove some chemicals. The recipe is as follows:

900 grams Nephyline Syenite
500 grams Gerstley Borate
380 grams Silica
100 grams EPK
80 grams Rutile
30 grams Cobalt
20 grams Iron Oxide
15 grams Soda Ash
15 grams Bentonite.

She would like to adjust the glaze to fit the clay or will longer or =
shorter firing make a difference.

Thanks for your response.

Tony Hansen on tue 26 aug 03

Here is an article I wrote on why adding silica is often
the worst strategy for fixing crazing. This applies
especially to glazes like yours, with that much
Nepheline Syenite I do not think any amount of silica
will dilute its expansion increasing effects.

http://www.digitalfire.ab.ca/cermat/education/4.php

This article has some other methods that are much better.

-------8<--------
I have a friend who is using the following recipe on Cone 04 bisque. =
The clay is high fire clay called 1/2 and 1/2 which is half porcelain. =
The problem is crazing on a Cone 6 firing. She really likes this glaze =
and does not want to use a different but would like to know how to alter =
this glaze to make it fit the piece better. After much reading, we =
decided to add more Silica, but how do you know how much to add and =
does she need to add/remove some chemicals. The recipe is as follows:

900 grams Nephyline Syenite
500 grams Gerstley Borate
380 grams Silica
100 grams EPK
80 grams Rutile
30 grams Cobalt
20 grams Iron Oxide
15 grams Soda Ash
15 grams Bentonite.

She would like to adjust the glaze to fit the clay or will longer or =
shorter firing make a difference.

Thanks for your response.

______________________________________________________________________________
Send postings to clayart@lsv.ceramics.org

You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/

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--------
Tony Hansen, Digitalfire Corp.

David Hewitt on wed 27 aug 03

A quick run through of this recipe on my CeramDat indicates that your
recipe has a C of E of 4.357 x10-6/oC English & Turner and that
increasing the silica to 680 grams this figure reduces to 3.826.

The simplest thing to try is to do a line blend with increasing amounts
of silica, say additions in 100 gram steps.

The more you alter the recipe the more chance you run of altering its
nature in other respects, so my thinking would be to try this simple
test first rather than consider changing the other ingredients until
this becomes necessary.

David
In message , The Harris Family writes
>I have a friend who is using the following recipe on Cone 04 bisque. The clay
>is high fire clay called 1/2 and 1/2 which is half porcelain. The problem is
>crazing on a Cone 6 firing. She really likes this glaze and does not want to
>use a different but would like to know how to alter this glaze to make it fit
>the piece better. After much reading, we decided to add more Silica, but how do
>you know how much to add and does she need to add/remove some chemicals. The
>recipe is as follows:
>
>900 grams Nephyline Syenite
>500 grams Gerstley Borate
>380 grams Silica
>100 grams EPK
> 80 grams Rutile
> 30 grams Cobalt
> 20 grams Iron Oxide
> 15 grams Soda Ash
> 15 grams Bentonite.
>
>She would like to adjust the glaze to fit the clay or will longer or shorter
>firing make a difference.
>
>Thanks for your response.

--
David Hewitt
David Hewitt Pottery
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ.
Tel:- +44 (0) 1633 420647
Fax:- +44 (0) 870 1617274
Web:- http://www.dhpot.demon.co.uk

Ron Roy on tue 2 sep 03

I'm way behind but better late than never - maybe.

I have lower the expansion of this glaze and that should work for most
clays - you will know when you try it.

Let us know if it works.

630 grams Nephyline Syenite
580 grams Gerstley Borate
430 grams Silica
240 grams EPK
80 grams Rutile
30 grams Cobalt
20 grams Iron Oxide
15 grams Soda Ash
15 grams Bentonite

Total - 2040

RR

>In message , The Harris Family writes
>>I have a friend who is using the following recipe on Cone 04 bisque. The clay
>>is high fire clay called 1/2 and 1/2 which is half porcelain. The problem is
>>crazing on a Cone 6 firing. She really likes this glaze and does not want to
>>use a different but would like to know how to alter this glaze to make it fit
>>the piece better. After much reading, we decided to add more Silica, but
>>how do
>>you know how much to add and does she need to add/remove some chemicals. The
>>recipe is as follows:
>>
>>900 grams Nephyline Syenite
>>500 grams Gerstley Borate
>>380 grams Silica
>>100 grams EPK
>> 80 grams Rutile
>> 30 grams Cobalt
>> 20 grams Iron Oxide
>> 15 grams Soda Ash
>> 15 grams Bentonite.
>>
>>She would like to adjust the glaze to fit the clay or will longer or shorter
>>firing make a difference.
>>
>>Thanks for your response.

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

bill edwards on fri 11 nov 05

Copy this/ Read it in bits and pieces. It just too
long a read for one setting.

There's probably never going to be a conclusion to
this because the dynamics of people, time and change
usually come back to re-haunt the old houses our clay
spirits live in.

Safety - That would be my first priority. So if a pot
that I was going to cook from was crazing I would
approach it as scientifically as possible regardless
or aesthetics or otherwise. First we need to have
factual information of harm from the event, I have
never found any conclusive evidence to back the theory
up and too many years and too many people have been
there and done that. Your pottery could jump off the
table and run away with the spoon if we reached for a
theory without scientific evidence that something
magical just happened, but we have to prove it.

Fact - Crazed wares based on my R&D and after reading
countless sanitary ware lab reports as well as
finishing McKenzie College myself many scholastic
events ago for food service technology and service,
led me further into the materials being weakened
moreso than anything else. But then again... Some
crush testing showed variations in that based on
material such as clay and how high the pieces were
fired. Hince I gather that some of the argument for
Earthenware could be regarded as valid based on
testing under pressure even if porosity remained
partly questionable. In other words, I can see where
some well produced Earthenware could easily surpass
some of the poorer formed stonewares in impact ability
due to many variations in firing and clay choice or
rather materials the clay would be refined with. You
can bet I have seen ^ 10R glazes that I wouldn't dare
eat off of. You can see some of these difference in
time periods on Earthenware pottery in museums due to
clay location and perhaps processes they used in close
realation to their specific time periods. Different
cultures, different techniques, same use. Vessles for
food and carrying water. It changed from location to
location, some better and some for the fragmented
worse. We do have to consider a 3200 year old jar
found in China's Ganzu province and made in
Earthenware and still shows details worthy of notation
don't we? So its a tad hard for me to discount those
results. If any of my stoneware survives for 3200
years, I think I may have done pretty well.

Water will go through porus materials. That could be a
better argument. Over time and if heated back and
forth it could very well cause more crazing and
eventually destroy the wares. This would also have to
take into consideration if the glaze had good merit
and how much of the surface is that glaze protecting
and how much drying in between cooking cycles is that
piece seeing.

Use a pressure cooker with a test tile to help you
speed up some of your testing. Find an autoclave if
you need faster results. You can really get more bang
for your buck and reduce the time it takes by not just
boiling your wares but putting them under pressure at
the same time. Let it cool if you use a pressure
cooker till you can remove the jiggler and then
submerge it in cold water. Run some dye over it once
it has dried and then rub it off. Any cracks, crazing
or otherwise?

My bottom line and I still look for a bottom line, is
to try to get the best possible fit I can between
glaze and clay body. Reduce crazing on foodwares as
much as possible keeping in mind that over-lapping
glazes can hamper this if not carefully studied given
the choices used. I see many out there taking it for
granted that its fine to use so and so's glaze because
it is inert but we never know what they are going to
do with it. Add 10 X's the ammount of metal oxides it
might can hold, fire it much different than what the
originator of the glaze intended it for. And while
thinking about it, ALL glazes leach in some amount.
Please show me a record of zero return on any recipe
with as little as 0.25% of any metal oxide so I can
use that data to further my knowledge. Its a matter of
trying to fit as much colorant as possible in
different glaze bases as possible without having an
over abundance or saturation point where it losses
enough oxides to surpass regulatory standards based on
whichever level of government we wish to believe.
California or some other country may have different
sets of rules. So its rather important to not go
beyond the regulatory processes with any material we
use in order not to open Pandora's Box and get the
attention we don't deserve nor desire.

While trying to think clearly, (I have a really nasty
case of poison ivy and haven't slept well for several
days and am a tad cranky) the more important issue
would be in not having lead or cadmium in my glaze, I
don't use lithia or barium myself but you can do
whatever it is that you like. Reduce other leachates
to a bare minimal. Reduce crazing and where food
contacts the surface go for glossy clean finishes. On
the outside I sometimes get a little artistic with
aesthetic touches and will use a satin and if it needs
to match a plate, I might add a tinge to the outter
lip since I know that few people are going to place
their food on the lip of any plate for any long term
use. But I don't over-load even those glazes since
most food service glazes can be made safer and more
durable and bright. Jars, jugs and vases give me more
freedom and I will explore on these wares a little
differently. It doesn't mean I get hog wild but I do
explore. After all, I would cook bread from a flower
pot and perhaps someone else might use a vase to hold
their wine. And yes, I also own some lovely crazed
wares that are old and still very much usefull! (I
will not cook in them)I will serve in them however.

Make good strong attachments and handles where
possible. We don't want anyone getting burned from a
weak piece of pottery where boiling liquids or grease
is involved. Reduce crazing where food contact for
cooking is concerned, not as much for serving already
cooked but still warm food but moreso for assurance
your not going to wear the food. Wash your dishes
please, its just good manners. But I don't believe you
will need to get out your microscope anytime soon and
do a swab test and culture for your dishes. If you do,
just make sure you check the rest of your house and
have a comparison study to check against. If your
harboring a nasty bug in one place, your most likely
going to find it somewhere else living the high life
without the Miller.

So technically speaking, wouldn't it also merit good
design practices in your pottery and getting down a
thickness for food bearing cookery or pottery as well
as limiting the metal contents to stay within the
confines of regulatory status as well as reduce
crazing as often as possible so as not to cause injury
or a dunting dish to pop in your hands while its still
hot?

I once made a very beautiful and very large platter a
long time ago with some pretty ^10 R glazes that I
didn't know the chemistry for nor the clay. I was so
proud of that piece. I had it for a few years and
placed in a safe place in my daughters bedroom high up
on a cabinet. (I would have paid several hundred
dollars for a piece like that) She and I was in there
talking and we both heard what sounded like a zipper
and a ping, I jumped up just in time to see my
wondeful platter make its self become two twins almost
identical in halves. The platter had not been touched
in months but the compression eventually got the best
of it and left me with the worst. It happens, we can
reduce that now because we know how. I can only
imagine had I been using it and it doing the same in
my hands while holding a load of hot greasy food or
liquid. As I said before, we use our pottery, I work
only in ^6 Ox now with a few tests in ^10R for spot
checking myself on occasion. (I am gaining anything in
particular from ^6, I am just there and found I like
it and I have pushed the limits there for some years)
Also forgive this very tired and long rambling post
from me, I think I will slack it off again once I get
over the itching and can use the adrenaline for active
duty at Horseshoe or in the pottery studio. I really
missed my run today but there's no way I can stand to
run on a few hours sleep and blisters taking over my
body.







Bill Edwards
Edmar Studio and Gallery
302 South Main St (Shipping)
POB 367 (Mailing)
Camp Hill, Al. 36850
http://apottersmark.blogspot.com/

"Those that matter don't mind, and those that mind don't matter"




__________________________________
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mel jacobson on mon 14 nov 05

i just want to remind potters of the many
years of work that ron roy has put in to teach
us that crazing is a flaw.

it does not have to be that way.

it will not kill anyone.
but, like so many things that happen with
glaze...we know how to stop crazing.
added silica for starts. balanced glaze formula.

poor, cheaply made commercial dishes will craze.
just a fact.

if you are using crazing as a design technique...wonderful for you.

but, it is a glaze flaw. and the only folks that will
notice are other potters. and we can be very critical.
i don't want ron looking at my work through a magnifying glass
and say....`tsk, tsk, hmmmmmmmmmmmmmm.`
from mel/minnetonka.mn.usa
website: http://www.pclink.com/melpots

steve graber on mon 14 nov 05

crazing can make for a very nice design, but has anyone seen attractive pin holing? i see commercial dishware with pin holes in places like Cost Plus, "trendy" chic stores.

granted people can eat off a leave or piece of bark, so cleanliness isn't always an issue.

see ya

steve



mel jacobson wrote:
i just want to remind potters of the many
years of work that ron roy has put in to teach
us that crazing is a flaw.

it does not have to be that way.

it will not kill anyone.
but, like so many things that happen with
glaze...we know how to stop crazing.
added silica for starts. balanced glaze formula.

poor, cheaply made commercial dishes will craze.
just a fact.

if you are using crazing as a design technique...wonderful for you.

but, it is a glaze flaw. and the only folks that will
notice are other potters. and we can be very critical.
i don't want ron looking at my work through a magnifying glass
and say....`tsk, tsk, hmmmmmmmmmmmmmm.`
from mel/minnetonka.mn.usa
website: http://www.pclink.com/melpots

______________________________________________________________________________
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Scott Paulding on mon 14 nov 05

i dunno mel, is it a defect if you do it on purpose? to me, saying this
definitevely is like saying that loose throwing is a defect or that its
sloppy or that it is incorrect technique.

its a matter of personal aestethic/preference/choice.

honestly, some of the most amazing celadon/porcelain vessel combinations i
have ever seen are crazed like mad. it just looks good on some forms.

-scott

--- mel jacobson wrote:

> i just want to remind potters of the many
> years of work that ron roy has put in to teach
> us that crazing is a flaw.
>
> it does not have to be that way.
>
> it will not kill anyone.
> but, like so many things that happen with
> glaze...we know how to stop crazing.
> added silica for starts. balanced glaze formula.
>
> poor, cheaply made commercial dishes will craze.
> just a fact.
>
> if you are using crazing as a design technique...wonderful for you.
>
> but, it is a glaze flaw. and the only folks that will
> notice are other potters. and we can be very critical.
> i don't want ron looking at my work through a magnifying glass
> and say....`tsk, tsk, hmmmmmmmmmmmmmm.`
> from mel/minnetonka.mn.usa
> website: http://www.pclink.com/melpots
>
>
______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>


"Do you realize...that happiness makes you cry?"
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Bill Merrill on wed 23 jul 08

Many potters buy their clay already made and only know the working
qualities of that clay body. =20

Sure, you can do shrinkage tests or water of plasticity tests if you dry
out some of the wet pre made clay.=20

So, with regard to crazing , the only thing we can adjust are the
glazes. To make a glaze fit the body we=20

May need to also adjust the clay. Crazing is a matter of making the
body and glaze fit like a glove to avoid=20

crazing.

=20

It may be easier to adjust the clay body to alleviate crazing than
adjusting the glaze.

Adding flint to the clay can help, which sounds contradictory, adding
silica to a glaze is also a remedy for crazing.

Check out the crazing chapter in "Clay and glazes for the Potte"r by
Daniel Rhodes for a complete explanation.

=20

If you buy your clay pre made, you may always face problems of crazing
when you use different glazes. =20

Cooling a kiln too fast can also cause crazing or dunting. So many
clays and glazes, so little time!!

=20

Antoinette Badenhorst on thu 24 jul 08

Bill, it help some if one can get more data from the dealer about the clay. Knowing the shrinkage rate help some. Crazing is seldom fixed by potters. I salute those potters that do take the time and trouble to solve their crazing problems.

Is it possible if the whole clay and glaze making process is not in your hands?

If someone asks me about food safety, it is always a difficult subject to discuss.

--
Antoinette Badenhorst
www.clayandcanvas.com
www.studiopottery.co.uk


-------------- Original message ----------------------
From: Bill Merrill
> Many potters buy their clay already made and only know the working
> qualities of that clay body.
>
> Sure, you can do shrinkage tests or water of plasticity tests if you dry
> out some of the wet pre made clay.
>
> So, with regard to crazing , the only thing we can adjust are the
> glazes. To make a glaze fit the body we
>
> May need to also adjust the clay. Crazing is a matter of making the
> body and glaze fit like a glove to avoid
>
> crazing.
>
>
>
> It may be easier to adjust the clay body to alleviate crazing than
> adjusting the glaze.
>
> Adding flint to the clay can help, which sounds contradictory, adding
> silica to a glaze is also a remedy for crazing.
>
> Check out the crazing chapter in "Clay and glazes for the Potte"r by
> Daniel Rhodes for a complete explanation.
>
>
>
> If you buy your clay pre made, you may always face problems of crazing
> when you use different glazes.
>
> Cooling a kiln too fast can also cause crazing or dunting. So many
> clays and glazes, so little time!!
>
>

Ron Roy on thu 31 jul 08

If all your glazes were crazing - changing the body would make sense - if
some were not crazing you would run the risk of throwing your non crazing
glazes into too much compression if you made the clay contract more during
cooling.

Better to learn how to correct crazing and shivering - or find someone who
does - it just makes so much more sense.

RR


Bill Merrill said -

>It may be easier to adjust the clay body to alleviate crazing than
>adjusting the glaze.

Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0

Digital Studio on mon 17 nov 08

I have been using Burke's Celadon (Cone 6, Oxidation, Glossy) that I
found in the archives on a porcelain that I also found in the archives.
It crazes a lot on this porcelain, but not commercial porcelain casting
slip.

Here is the glaze recipe:
Feldspar 58
Whiting 17
Silica 14
Ball Clay 6
Zinc Oxide 5
Add:
Copper Carbonate .5


And the porcelain that it crazes on:

E.P.K. Kaolin 40
Ball Clay 5
Nepheline Syenite 30
Gerstley Borate 5
Flint 20
Add:
VGum-T 3%

In Hyperglaze, the expansion is 8.19. I have increased the Silica, but I
haven't been able to change the recipe enough to get the expansion lower
than 7.8. I don't know what else to change to fix the crazing.

Michael McDowell on wed 19 nov 08

Kendra at Digital Studio is asking for suggestions on how to further lower
the coefficient of thermal expansion of her recipe for Burke's Cone 6
Celedon:

Here is the glaze recipe:
Feldspar 58
Whiting 17
Silica 14
Ball Clay 6
Zinc Oxide 5
Add:
Copper Carbonate .5

You say that you have tried increasing silica but that it has been of
limited success. It's not really clear that you have actually tested this
altered formula, or just were unimpressed by the numeric results in your
glaze software. It could be that you only need to make a marginal adjustment
in thermal expansion to stop the crazing. Looking at your recipe without
actually plugging the numbers into my glaze software I see two main
strategies that you might try. Whiting is your source for CaO. You could
replace some of the CaO with MgO and that would bring down thermal
expansion. This could be achieved by using dolomite in place of some of the
whiting. Or reduce whiting and add talc, which adds both magnesia and
silica.

The other immediately apparent approach would be to substitute some of the
feldspar with Spodumene. Spodumene is a non-soluble source of Lithia, which
can substitute for the highly expansive potassium and sodium in your generic
feldspar. I don't recommend that you rely on this very heavily though.
Lithium can start giving unexpected results if larger amounts are used. Of
the three fluxes Lithium, potassium, and sodium, let lithium be no more that
20% of the total and you should be alright. And steer clear of lithium
carbonate as a source of lithium, use spodumene or Petalite.

Michael McDowell
Whatcom County, WA, USA
michael@mcdowellpottery.com
http://www.McDowellPottery.com

May Luk on wed 19 nov 08

Hello Digital Studio;

I use a version of Burke's Celadon. (I made fine adjustment using Currie Gr=
id for cone 8, but it's essentially the same thing) My glaze also craze on =
porcelain sometimes, but not on stoneware.=20

Ron Roy has a fix for me using Spod but the glaze becomes slightly milky. O=
n a whim, I substituted a different spar from Custer to Kona F4 (same amoun=
t) I have decreased the sub-total Alkalis and the Si: Al ratio and it seems=
to work with my small batch test (1 kg). I have not done any large batch t=
est (4 kg) just yet.=20

I use OM4 for ballclay BTW.

Hope this helps

May=20
Brooklyn NY


--- On Tue, 18/11/08, Digital Studio wrote:

> From: Digital Studio
> Subject: Crazing
> To: CLAYART@LSV.CERAMICS.ORG
> Date: Tuesday, 18 November, 2008, 4:48 AM
> I have been using Burke's Celadon (Cone 6, Oxidation,
> Glossy) that I
> found in the archives on a porcelain that I also found in
> the archives.
> It crazes a lot on this porcelain, but not commercial
> porcelain casting
> slip.
>=20
> Here is the glaze recipe:
> Feldspar 58
> Whiting 17
> Silica 14
> Ball Clay 6
> Zinc Oxide 5
> Add:
> Copper Carbonate .5
>=20
>=20
> And the porcelain that it crazes on:
>=20
> E.P.K. Kaolin 40
> Ball Clay 5
> Nepheline Syenite 30
> Gerstley Borate 5
> Flint 20
> Add:
> VGum-T 3%
>=20
> In Hyperglaze, the expansion is 8.19. I have increased the
> Silica, but I
> haven't been able to change the recipe enough to get
> the expansion lower
> than 7.8. I don't know what else to change to fix the
> crazing.=0A=0A=0A

Lynn Goodman Porcelain Pottery on wed 19 nov 08

I apparently sent this when clayart was down, so here it is again:


What feldspar are you using--it's not specified in the recipe. That
could make a very big difference. If you're using a soda spar you
could try switching to a potash spar. One seat-of-the-pants thing you
could try, and which I do pretty regularly, is adding magnesium in
some form (10% frit 3249 at the expense of feldspar, or maybe some
talc or magnesium carb on top of the recipe). It won't be the same
glaze, as the magnesium will change the color response, but it may
end the crazing problem without having to recalculate.

Lynn


On Nov 17, 2008, at 11:48 PM, Digital Studio wrote:

> I have been using Burke's Celadon (Cone 6, Oxidation, Glossy) that I
> found in the archives on a porcelain that I also found in the
> archives.
> It crazes a lot on this porcelain, but not commercial porcelain
> casting
> slip.


Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com