Louise Jenks on tue 16 sep 97
I have been having problems with a relatively new, to me, glaze. I got it on
Clayart months ago and another version from Claytimes.
IE 27 A/O
Nep Sy 67
Dolomite 10
Kaolin 8
Silica 9
Zinc oxide 6
Plus copper carb 1.5
cobalt ox .25
SHINY SOFT GREEN w pink flecks
above recipe with Copper carb 3%
When I tested these glazes several months ago they were shiny and lucious. I
bisque at ^06 and fire at^6 in an electric kiln.
I use a brown stoneware that has lots of Redart and some ball clay and
recently recieved a new shipment. The supplier says the ingredients have not
changed. [ I do have a few pounds of old clay that I have just made a few
pots of to see if it is the clay.]
The glazes, instead of consistent shine, have dull, matt black or brown
patches (mostly on the insides of bowls, plates etc. Sometimes there are
yellow crystal patches. In doing research, I found in Hammer that dolomite
can be adversly affected by sulphur, which is present in Redart. I have found
no other reference to any other ingredient that may cause this problem.
So, my question to you all, is:Instead of bisquing to ^06 to drive out the
sulphur, would ^05 do it? I don't want to go much higher because I am afraid
it will affect my glaze applications.
Thanks in advance,
Louise, in Cincinnati
Louis Katz on wed 17 sep 97
=3C/NOFILL=3E
------------------
Magnesium Sulfate does not decompose until 1124. Magnesium Carbonate
decomposes below red heat. It is possible that the Magnesium Oxide is
combining with the sulpur dioxide and forming MaGnesium Sulfate and
that this is goofing your glaze. I wouldn't know myself. Redart has
proven to me to be reliably unreliable and it seems a good shoulder to
pin any problem to.
Sulphur compounds can be very stable and simp=5Bly using a higher
bisque, unless much higher, is unlikely to help with them. Early
reduction seems to help with the volatilization of sulphur compounds,
not oxidation, but I should leave this to R.Burkett as he filled me in
on this.
I would also be suspicious of firing cycle changes, or kiln stacking
density. Anything with crystals forming or flecks, could be changed
easily by cooling speed.
Louis
Craig Martell on wed 17 sep 97
Louise wrote:
>----------------------------Original message----------------------------
>So, my question to you all, is:Instead of bisquing to ^06 to drive out the
>sulphur, would ^05 do it? I don't want to go much higher because I am afraid
>it will affect my glaze applications.
Hello Louise:
Try holding the temp at 06 until the sulphur odor is gone and maybe a bit
longer to be sure. It is sometimes beneficial to "soak" a bisque load
because it takes more time to drive off the sulphur if a lot is present in
the claybody.
The unpleasant and unhealthy part of all this is that it ain't good for you
to be inhaling sulphur to detect its presense, so don't linger too long in
the kiln room. But once you establish the appoximate soak time needed, you
won't have to do it again.
Regards, Craig Martell-Oregon
Ron Roy on wed 17 sep 97
Hi Louise,
This looks like the kind of glaze that will settle out badly because of all
that Neph Sy - so make sure it is stirred well - look for hard glaze in the
bottom of your glaze bucket. This glaze would do better if it were
reformulated with ball clay and some bentonite. If that didn't keep it up
then the next step is an addition of epson salts.
As for firing the sulpher out - I suggest slowing down your bisque firngs -
especially between 600 and 900 C to give the sulphur and organics time to
get out.
>----------------------------Original message----------------------------
>I have been having problems with a relatively new, to me, glaze. I got it on
>Clayart months ago and another version from Claytimes.
>IE 27 A/O
>Nep Sy 67
>Dolomite 10
>Kaolin 8
>Silica 9
>Zinc oxide 6
>Plus copper carb 1.5
> cobalt ox .25
>SHINY SOFT GREEN w pink flecks
>above recipe with Copper carb 3%
>When I tested these glazes several months ago they were shiny and lucious. I
>bisque at ^06 and fire at^6 in an electric kiln.
>I use a brown stoneware that has lots of Redart and some ball clay and
>recently recieved a new shipment. The supplier says the ingredients have not
>changed. [ I do have a few pounds of old clay that I have just made a few
>pots of to see if it is the clay.]
>The glazes, instead of consistent shine, have dull, matt black or brown
>patches (mostly on the insides of bowls, plates etc. Sometimes there are
>yellow crystal patches. In doing research, I found in Hammer that dolomite
>can be adversly affected by sulphur, which is present in Redart. I have found
>no other reference to any other ingredient that may cause this problem.
>So, my question to you all, is:Instead of bisquing to ^06 to drive out the
>sulphur, would ^05 do it? I don't want to go much higher because I am afraid
>it will affect my glaze applications.
>Thanks in advance,
>Louise, in Cincinnati
Ron Roy
Toronto, Canada
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
rballou@mnsinc.com on thu 18 sep 97
Hi Louise,
I used this glaze for about 3 years with my students. It has a very narrow
firing range, possibly due to its low silica content (55%). We used it on a
variety of clay bodies (porcelain and stoneware) and experienced similar
problems on all of them. Underfired, the patches you've described have a
metallic look, reminding me of glazes that have been overloaded with
colorant. The particular balance of flux to glass former in this glaze is
very delicate and a few degrees can make a big difference in the appearance
of the glaze. The low silica makes it difficult to form a stable glass. Too
low and the colorants don't integrate into the melt, too high and the
flecks on the surface are lost. Most of your patches are on the insides of
pieces where it is typically cooler. But this glaze is suspect as far as
being food safe goes, anyway. I don't think you can modify this glaze
without losing its special effects.
We dealt with the settling problem with a combination of bentonite and
FLOCS, a commercial glaze additive available from Axner. Worked pretty
well, though most students stayed away from the glaze because it was so
unpredictable. Failures could be successfuly refired, most of the time.
Sometimes, the glaze crawled, as well. You may ask, "Why did we continue to
use this glaze???" All I can say is that hope springs eternal.......
I count myself as a beginner in the glaze analysis game, so any feedback
from the glaze gurus is appreciated.
Ruth Ballou
rballou@mnsinc.com
-----------Original message----------------------------
>>I have been having problems with a relatively new, to me, glaze. I got it on
>>Clayart months ago and another version from Claytimes.
>>IE 27 A/O
>>Nep Sy 67
>>Dolomite 10
>>Kaolin 8
>>Silica 9
>>Zinc oxide 6
>>Plus copper carb 1.5
>> cobalt ox .25
>>SHINY SOFT GREEN w pink flecks
>>above recipe with Copper carb 3%
>>When I tested these glazes several months ago they were shiny and lucious. I
>>bisque at ^06 and fire at^6 in an electric kiln.
>>I use a brown stoneware that has lots of Redart and some ball clay and
>>recently recieved a new shipment. The supplier says the ingredients have not
>>changed. [ I do have a few pounds of old clay that I have just made a few
>>pots of to see if it is the clay.]
>>The glazes, instead of consistent shine, have dull, matt black or brown
>>patches (mostly on the insides of bowls, plates etc. Sometimes there are
>>yellow crystal patches. In doing research, I found in Hammer that dolomite
>>can be adversly affected by sulphur, which is present in Redart. I have found
>>no other reference to any other ingredient that may cause this problem.
>>So, my question to you all, is:Instead of bisquing to ^06 to drive out the
>>sulphur, would ^05 do it? I don't want to go much higher because I am afraid
>>it will affect my glaze applications.
>>Thanks in advance,
>>Louise, in Cincinnati
>
>Ron Roy
>Toronto, Canada
>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
| |
|
