Robert Wilt on sat 25 oct 97
First let me mention that there is an article by Jim Robinson
about Shino glazes in "Studio Potter" Vol. 21 #1 (December 1991).
The article includes a chart with several dozen recipes, sorted into
six major categories. This issue is available as a back issue for
$6.00 + $1.00 postage from Studio Potter, Box 70, Goffstown, NH
03045. Phone 603-774-3582.
Ian Currie also includes a chapter about Shinos in his book
"Stoneware Glazes".
Now the question(s). What is the mechanism of carbon trapping? Is the
gray/black color actually caused by smoke particles embedded in the
glaze? If so, how do they get there and why do they stay there? I know
that iron slips under a shino can also lead to a gray color, but it
doesn't seem to be quite the same. What kind of firing schedule do you
need to follow to maximize carbon trapping - what are the critical
points in the firing?
trying to understand the mysteries...
bob
Louis Katz on sun 26 oct 97
Hi Rob,
IMHO , the typical critical time for carbon trapping in shino glazes is
very early. We found that we had better results (at Kansas City Art
Institute in the late 70's) if we started our reduction at cone 012 than if
we started at 08. If you have seen Ken Ferguson's Movie, "Ferguson" you
will see that lots of smoke was an important part of the process. Kansas
City Art institute students developed a reputation for lots of smoke.
Late in my time at Kansas City, some things started to not add up. We found
kilns without much early reduction with a lot of carbon trapping. Sometimes
these kilns had had other problems. Somewhere along the line someone
discovered, or found out that if you threw some oil in the kiln (this will
create lots of smoke and flame and could create a danger of explosion from
unburnt gases coming out of the kiln)very late(5 minutes before closing the
kiln) in the firing that you could also trap carbon.
My soda kiln has one burner without a flame retention ring on it and it
leaves a trail of coarbon traping throughout the kioln unless I fire in
strict oxidation.
To get his wild Shinos Ken would spray his pieces with wood ash, or soda
ash or sometimes a thin coat of Albany slip or other iron bearing
ingredients. Too much soda ash causes some real glaze fit problems. You
need something that will really carry the mists away from you and others.
Louis
Craig Martell on sun 26 oct 97
Hi Robert:
Carbon Trap occurs when the ware is exposed to pure Carbon or soot during
the early stages of firing. I think this starts happening before any color
is seen in the kiln and continues until combustion accelerates and becomes
more efficient. The glazes are formulated to fuse early and quickly,
probably around 2000 degrees F. or shortly thereafter. Following low temp.
or body reduction period, the kiln is placed in a neutral to slightly
oxidizing mode to give a fast temperature rise to fuse the shinos quickly
and trap the carbon inclusions before they have time to burn away. If
reduction is continued past the body reduction stage, the temp rise is
sometimes (not always) too slow and the carbon is burned away. Once the
glazes have fused and the carbon is "trapped", you can reduce all you wish
and temp rise is no longer a factor.
I think that Ian Currie has included some firing schedules for shinos in his
book, Stoneware Glazes.
regards, Craig Martell-Oregon
Grimmer on sun 26 oct 97
Bob,
Carbon trap glazes typically have a source of soluble soda, usually soda
ash, in the recipe. On drying, this soda migrates to the surface of the raw
glaze with the H2O, and left there when the water evaporates. This soda
forms a very fine skin of what is essentially a ^010 (or lower) glaze. If
the kiln is reduced before that point, the soda will melt into a glaze and
trap any carbon in the raw shino. Fire a shino-type glaze in a bisque
kiln. Many will have a sheen at ^06.
A smokey reduction doesn't seem to be necessary to get carbon trapping.
I guess the CO somehow precipitates out particulate C in the glaze. Can
anyone add to this?
The same effect can be seen on porcelain in a salt kiln if you reduce
_real_ early. Some pieces will come out with a smokey gray glaze.
I always get the best carbon trapping when reduction starts at ^014 or
^012. Not always the best for ware strength, especially with stoneware,
but shino-types never fit well anyway.
steve grimmer
marion illinois
Robert Wilt wrote:
>
> ----------------------------Original message----------------------------
>
> Now the question(s). What is the mechanism of carbon trapping? Is the
> gray/black color actually caused by smoke particles embedded in the
> glaze? If so, how do they get there and why do they stay there? I know
> that iron slips under a shino can also lead to a gray color, but it
> doesn't seem to be quite the same. What kind of firing schedule do you
> need to follow to maximize carbon trapping - what are the critical
> points in the firing?
>
> trying to understand the mysteries...
>
> bob
June Perry on mon 27 oct 97
In my notes I have from a Ken Ferguson workshop years ago he stated that he
begins reduction as Cone 014 and never goes out of reduction. He reduces
hard form 08 to 02, hard at cone 3 and hard again at cone 8 till nine is over
and ten bending and ease up to cone 11.
Regards,
June
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