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crazing after 10 months, b-mix^5 and clear glaze ^6 --fixed it!

updated wed 1 sep 99


Carol Baker on fri 27 aug 99

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

-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

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

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

Peace, health, joy .........and no crazing to all,
Carol Baker and the Blue Cat
Scottsdale, AZ

Henry on sun 29 aug 99

Dear Carol
I for one am very grateful to you and to all the others out there who so
generously share the wealth of imformation you all have worked so hard to
learn over the years.Thankyou Thankyou Thankyou and hopefully, one day I
will be able to recipricate. Enjoying the good life here in sunny

Craig Martell on tue 31 aug 99

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).


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