GLENDA WILSON on mon 21 apr 97
Dear Clayarterss:
I continue to have trouble with crazing on my small porcelain pins. I'm
using Amaco high fire HF-9 clear glaze on polar porcelain. I paint on
Velvets (amaco) and get great colours. I'm thinking that the glaze is on too
thick and that's what's causing the crazing. However, if I try and brush
the clear glaze on, my underglaze colours run together.
Should I add water to the glaze and keep dipping? If it's not the amount of
glaze I have on the porcelain what else could it be that's causing the crazing?
I bisque at 04
I glaze fire at 6
Thanks for any help you can give me, Glenda from Minnesota
Robert Kittel on tue 22 apr 97
At 02:50 PM 4/21/97 +0000, you wrote:
>----------------------------Original message----------------------------
>Dear Clayarterss:
>
>I continue to have trouble with crazing on my small porcelain pins. I'm
>using Amaco high fire HF-9 clear glaze on polar porcelain. I paint on
>Velvets (amaco) and get great colours. I'm thinking that the glaze is on too
>thick and that's what's causing the crazing. However, if I try and brush
>the clear glaze on, my underglaze colours run together.
> Should I add water to the glaze and keep dipping? If it's not the amount of
>glaze I have on the porcelain what else could it be that's causing the crazing?
>
>I bisque at 04
>I glaze fire at 6
>
>Thanks for any help you can give me, Glenda from Minnesota
The crazing is caused by improper glaze fit not the application. Since you
are using a ready mix glaze i would solve the problem by fritting the
existing glaze. Try ferro frit 3249, start at an addition of 2.5%, test. if
you still have crazing move up by 2%. Dont go beyond 10%.
Test by placing a finished piece in an oven, heat to 250, promptly remove
and place in cold water. If there isnt any crazing after this test then you
should be ok
Bob
David Hendley on tue 22 apr 97
At 10:50 AM 4/21/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Dear Clayarterss:
>
>I continue to have trouble with crazing on my small porcelain pins. I'm
>using Amaco high fire HF-9 clear glaze on polar porcelain. I paint on
>Velvets (amaco) and get great colours. I'm thinking that the glaze is on too
>thick and that's what's causing the crazing. However, if I try and brush
>the clear glaze on, my underglaze colours run together.
> Should I add water to the glaze and keep dipping? If it's not the amount of
>glaze I have on the porcelain what else could it be that's causing the crazing?
>
>I bisque at 04
>I glaze fire at 6
>
>Thanks for any help you can give me, Glenda from Minnesota
>
>Glenda -
Crazing is caused because the glaze and the clay are not compatable.
During cooling the glaze contracts more than the clay, thus the stress
cracks in the glaze. That's it. Variables such as under or over-firing,
glaze thickness, or other small variables can make a difference, but it
comes back to glaze fit.
If you formulated your own clay and glaze you could adjust one or
the other to achieve a proper fit. If you want to use off the shelf
products you need to experiment with different combinations to find
compatable mates.
If you want to learn more, consult some glaze calculation books.
David Hendley
Maydelle, Texas
Karen Gringhuis on tue 22 apr 97
Glenda - My studio mate uses a mouth atomizer to blow clear
glaze onto her pins. Maybe this would give you a lighter coat.
Karen Gringhuis
Sharon Miranda on wed 23 apr 97
I used a commercial clear glaze for my (lofire) stuff and it worked really
well - for awhile- then started to crfaze. I called the company (in this
case Laguna) and talked to the techies who were very nice and helpful.
Turns out it was the thickness of the glaze that was the culpril. I thinned
it down, brush it on and am back in business (the glaze surely thickened up
over time). If you don't want to brush it on, because of smearing I
suggest you spray the pieces with a cmc solution or spray starch , then you
can brush it on. Or just dip in the thinner glaze. I strongly suggest
calling the company who made the glaze.
Sharon
*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*
Sharon LaRocca-Miranda *
92 Morgan Street *
Oberlin, Ohio 44074 *
Sharon.Miranda@oberlin.edu *
. . . . . . . . . . . . . . . .*
* * * * * * * * * * * * * * * * *
d zcldomlze
David Hewitt on wed 23 apr 97
Hello Glenda,
I guess that your basic problem is that the glaze does not exactly fit
your clay body. If a glaze is going to craze for this reason it will be
more obvious if the glaze is applied thickly. I am not familiar with
polar porcelain, but porcelains often present crazing problems as they
generally have a low coefficient of expansion. The most common remedy is
to modify the glaze so that it has a lower coefficient of expansion. Do
you know the recipe of the glaze you are using?
As to remedies in general and alternatives to modifying the glaze
recipe,
may I suggest that you look at the article on Crazing which you will
find on the IMC site. Start with the URL given below at the end of my
signature and click on 'Calculating Crazing'.
Hope that this helps.
David
In message , GLENDA WILSON writes
>----------------------------Original message----------------------------
>Dear Clayarterss:
>
>I continue to have trouble with crazing on my small porcelain pins. I'm
>using Amaco high fire HF-9 clear glaze on polar porcelain. I paint on
>Velvets (amaco) and get great colours. I'm thinking that the glaze is on too
>thick and that's what's causing the crazing. However, if I try and brush
>the clear glaze on, my underglaze colours run together.
> Should I add water to the glaze and keep dipping? If it's not the amount of
>glaze I have on the porcelain what else could it be that's causing the crazing?
>
>I bisque at 04
>I glaze fire at 6
>
>Thanks for any help you can give me, Glenda from Minnesota
>
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://ceramicsoftware.com/education/people/hewitt.htm
Carter T. Seaton on thu 24 apr 97
Glenda:
I too use Amaco HF 9 and had some crazing. Amaco says to use their
porcelain 65 which is formulated to match the shrinkage rate of the
glaze or vice versa. I bought some,. but haven't tried it yet. Also,
if you wait till the underglazes are fully dried, a few days depending
on the weather, the overglaze will not cause it to run. Don;t thin the
HF 9 brush it on straight out of the jar. Use 2-3 coats and it will do
better.
Tony Hansen on thu 24 apr 97
Sharon Miranda wrote:
> I used a commercial clear glaze for my (lofire) stuff and it worked really
> well - for awhile- then started to crfaze...
> Turns out it was the thickness of the glaze that was the culpril.
I'm not so sure. The thicker glaze could be simply revealing a problem
the
thin glaze was hiding. Glaze fit means the glaze fits. If simply making
it a little thicker causes crazing there are stresses just waiting to
craze the thin glaze when the piece is thermally tested. Those same
stresses can cut the strength of your ware by 200% easily. The real
culprit is that the thermal expansion of the glaze is not compatible
with the clay. The recipe probably needs adjustment.
--
=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com
INSIGHT5/Magic of Fire II demos at www.ceramicsoftware.com
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