GSM_ENT on mon 30 aug 99
Hi Carol!
Normally crazing is caused by thermo-shock. In lab tests the Orton
Foundation has found this fenomena as late as 1 whole year after the firing.
The key here is to allow the kiln to completely cool off to room temperatura
before opening it. Crazing could also be caused by lack of body fit -that is
the greenware was not fired to maturity (cone). Orton has lots of
information. They accept members to the foundation for only $20.00 annual
fee.
Cordially,
Manuel R A "Tony" Diaz Rodriguez
MAJ., US Army (Ret.)
Master Kiln Repair Technician
PS: I present the Orton Firing Seminar which can also be purchased in video
form. Check you local ceramic/potter store or contact the Orton Foundation.
-----Original Message-----
From: Carol Baker
To: CLAYART@LSV.UKY.EDU
Date: Friday, August 27, 1999 1:36 PM
Subject: Crazing after 10 months, B-mix^5 and clear glaze ^6 --Fixed it!
>----------------------------Original message----------------------------
>Hi Clay lovers,
> With help from the "experts" I think I've fixed the crazing problem
with
>the ^5 B-mix and the ^6 clear glaze over stains and underglazes. I think
the
>problem was mainly due to under firing. I went back and looked at my old
>melted cones and there was a lot of, shall we say, "variation". I did not
>understand what a difference this could make in the vitrification of the
>finished piece, nor did I understand heat work, as opposed to, temperature.
>I've been doing a lot of homework!
>
> Here's my advice to other beginners:
>-TEST, TEST,TEST, before you start selling your work. The kiln gods WILL
get
>
> you!
>-Keep good records. I thought I was, but now I'm also recording the cone
>melt pattern for each firing.
>-Put cones on each shelf in each firing. I did this when I first started,
>but thought I didn't have to do it every time. At least, for now, I
will
>continue. I thought it was just the bottom shelf that was cool, but I
>think it also depends on how the kiln is loaded. Sometimes the bottom
was
>equal to others.
>-Watch the cones come down. On some glazes, a very slight variation can
>change the look of the glaze.
>-Fire slowly and fire down.
>-Be patient. Each mistake teaches you something you didn't know before.
>-Best of all - there is help out there - ClayArt!
>
>The Tests:
>The glazes below were each tested in two ways and all survived.
>(1) Finished pieces were frozen and then plunged into boiling water - 5
>times. Thank you, Ron Roy.
>(2) The same pieces were then soaked in water for I week.
>
>The Firing:
>These glazes were fired with a ^7 bar in the kiln sitter, and the kiln was
>turned to low for 2 hours after ^6 was half way down. This is a small
Skutt
>BUT tomorrow I get my big, big, new, shiny Skutt with all the "bells and
>whistles". Now I get to start testing again. I think I'll be calling for
>help with this controller. YES - I will still remember to use cones.
>
>The Glazes:
>Each of these worked well, with varying characteristics.
>
>(1) Ron Roy Clear ^6 Oxidation
> Whiting 16.50
> G. borate 18.00
> G200 feldspar 32.00
> EPK 14.00
> Silica 19.50
> 100
>
>This glaze is a bright, high gloss, clear - easy to clean up drips - hard
>surface - most pinks and reds hold at ^6 - most blues fluxed. Ron
suggested
>adding EPK 50/50 with stains. This helped and worked with underglazes, but
>did not completely correct.
>
>(2) David Woods Clear ^6 Oxidation
> G200 Feldspar 30
> Dolomite 7
> Whiting 16
> EPK 17
> Silica 30
> 100
>
>This glaze is a soft, slightly translucent, but still clear glaze. It is
>especially beautiful with blues and purples, and all pinks and reds held.
>Some greens changed slightly. It was, at first, difficult to apply and
very
>dusty. All drips and overlaps showed. Experimented with additions of CMC,
>bentonite, and Epsom salts. The Epsom salts made the difference, and it
now
>has a hard surface, easy to clean up. Apply thin.
>
>(3) Jean Richter from Lana Wilson's book Clear ^6 Oxidation
> G. Borate 47
> Flint 27
> Kaolin 20
> Alumina hydrate 3-10 - used 6
> ?
>
>This is a very interesting glaze. It is clear, bright, and high gloss, but
>some greens and tans burned out. Blues were grayed. I am not going to use
>this with my colors, but I played around with it over some other glazes and
>produced some quality effects. It is a keeper.
>
>Sorry this is so long, but after all this work I thought maybe I could help
>out another beginner. I'm also a teacher so everything gets detailed.
Some
>of you had asked about underglazes at one time. I've done a lot of
testing,
>so if you want ideas about good colors maybe I could help. Let me know.
>
>Thanks of course to all who helped. I think I have thanked you all
>personally, so I won't go into undying gratitude here. Your help is
greatly
>appreciated.
>
>Peace, health, joy .........and no crazing to all,
>Carol Baker and the Blue Cat
>Scottsdale, AZ
Carol Baker on wed 1 sep 99
Hi Craig and all,
I am the one, a beginner, who started this discussion on aftercrazing, and
with the help of many Clayarters I learned about vitrification. The
interesting thing about the moisture absorption is that I live in Arizona.
No humidity right? Wrong. The pieces that crazed were fired in October when
it was very dry. The crazing started all at once - as soon as our monsoon
storms hit in the summer bringing on high humidity. That is the only way
these pieces could have come in contact with moisture. Ron Roy did
re-formulate my original glaze to include more silica, but the original glaze
passed the freeze/boil tests. It only crazed after I soaked it for a week.
Guess it must have been the vitrification. I am continually
fascinated/frustrated by the magic of this craft. Good thing I'm a
continuing adult learner!
CBaker
Joan & Tom Woodward on thu 2 sep 99
OK, now I have another question. What do you mean by the "end point cone?"
I fire cone 5 on clay that's cone 4-6 and I use glazes that are 4-6 but also
some that I think are designated "6." Is any of this problematical?
Thanks! Joan
-----Original Message-----
From: Craig Martell
To: CLAYART@LSV.UKY.EDU
Date: Tuesday, August 31, 1999 9:15 AM
Subject: Re: Crazing after 10 months, B-mix^5 and clear glaze ^6 --Fixed it!
----------------------------Original message----------------------------
Manuel R A "Tony" Diaz Rodriguez wrote:
>Normally crazing is caused by thermo-shock. In lab tests the Orton
>Foundation has found this fenomena as late as 1 whole year after the
firing.
>The key here is to allow the kiln to completely cool off to room
temperatura
>before opening it. Crazing could also be caused by lack of body fit -that
is
>the greenware was not fired to maturity (cone).
Hi:
Actually, the primary cause of crazing is an incompatability in expansion
rate between the clay and glaze. The glaze is of significantly higher
expansion than the clay and is streched over the claybody following the
quartz inversion. The glaze will then fracture. Thermal shock only
hastens the inevitable and is not the actual cause. Delayed crazing, which
may happen up to a year or several years is caused by the body not being
totally vitrified, and expanding due to moisture absorbtion. The resulting
body expansion fractures the glaze. Not firing to the end point cone would
usually guarantee delayed crazing. Lack of body-glaze fit is caused by the
body and glaze formula being incompatable in regard to expansion. Wait, I
already said that! :>)
regards, Craig Martell in Oregon
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