Paul Borian on sun 2 jul 06
I have been working with slip glazes for a long time but only recently
started mixing large batches so as to integrated them into a consistent
product line. A smaller batch mixed a while back started coming out with a
nice variegated color to it, very similar to how Ohata red comes out in my
kiln, with various shades of browns and reds. The slip glaze never did this
until i started adding bone ash and feldspar (both g-200 and neph sy) along
with whiting (and also some agricultural-grade limestone called "Plum Run"
lime which is also called "dolomite lime" at the farmer's co-op where i
bought 100 lbs of it a for a few dollars.
I make notes off the specific quantites of the materials i add each time
but the other day when i mixed a new batch, trying to replicate what i did
last time, i noticed that the fired glaze did not seem to have the
variegated color response. So it seems that there is a fairly delicate
balance in materials needed to get this and it would help if i knew
specifically which of the materials i am using are the ones that cause this
to happen. Does anyone know?
thanks for any help,
Paul
Ivor and Olive Lewis on mon 3 jul 06
Dear Paul Borian,
Difficult to answer your questions since you give no indication if your =
modifications are systematic or random.
However, Bone Ash may be a clue. This can contribute to a second non =
miscible Phosphate glass phase. The second factor is the quality of the =
materials you are using. If they are not homogenous, and soil additives =
supplied for agricultural purposes are of doubtful grading or purity, =
then your "Lime Dolomite" may be another source of variation.
All I can suggest is that you test, observe, record, reason and =
evaluate.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on wed 5 jul 06
HI Paul,
You may find those variations are linked to cooling rates as well as what
the glazes are made up of.
If you calculate your glazes you will probably begin to notice certain
commonalties between glazes that variegate. One of the strengths of
calculation - if you notice something particular with the molecular formula
of a glaze - doing something you like - it is easy to push the glaze
further in that direction.
If you do not have a calculation program - let me know - perhaps I can
distill out some reasons to explain why those glazes are doing what they
do.
RR
>I have been working with slip glazes for a long time but only recently
>started mixing large batches so as to integrated them into a consistent
>product line. A smaller batch mixed a while back started coming out with a
>nice variegated color to it, very similar to how Ohata red comes out in my
>kiln, with various shades of browns and reds. The slip glaze never did this
>until i started adding bone ash and feldspar (both g-200 and neph sy) along
>with whiting (and also some agricultural-grade limestone called "Plum Run"
>lime which is also called "dolomite lime" at the farmer's co-op where i
>bought 100 lbs of it a for a few dollars.
>I make notes off the specific quantites of the materials i add each time
>but the other day when i mixed a new batch, trying to replicate what i did
>last time, i noticed that the fired glaze did not seem to have the
>variegated color response. So it seems that there is a fairly delicate
>balance in materials needed to get this and it would help if i knew
>specifically which of the materials i am using are the ones that cause this
>to happen. Does anyone know?
>thanks for any help,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
John Britt on wed 5 jul 06
Paul,
Don't know exactly what is happening, but bone ash is the distinguishing
factor in "Tomato Reds=94, (a.k.a. Ohata Kaki, Persimmon, etc) with 9 =96 15=
%
red iron oxide. Using real bone ash vs. synthetic bones will change
things. Using various forms of iron oxide will change things.
Also, the firing cycle is extremely important. It may be just a small
variation in firing cycle and atmosphere and/or cooling. Reduction will
turn it brown while late oxidation will turn it orange. (Also, try
refiring the brown Ohata Kaki to cone 07 in your electric. It will often
change the reduced iron on the surface to oxidized iron or from brown to
orange/red.) When and how much you reduce or oxidize the kiln, also has a
great effect on the color and variegation of the glaze. How it cools, in
what atmosphere, how long at each temperature. It is surprising but even
varying the amount of work in the kiln will change the cooling cycle
(having lots of kiln shelves and work in a kiln is a lot of mass, which
holds heat and slows cooling).
That =93Dolomitic Limestone=92 is not really whiting but more like dolomite.=
And the composition is varied. It adds both MgO and CaO to the glaze.
Magnesium oxide is a factor in creating oil spot glazes (variegated).
Hope it helps,
John Britt
www.johnbrittpottery.com
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