Alisa Liskin Clausen on mon 30 dec 02
Dear John and Clayart,
I have been reading this recent discussion of results with interest.
Thinking over the past two years of firing with two different kilns, this is
what I can generally say I have observed in my firings:
When I first started firing electric, I did not slow cool, but soaked at the
top temp.
of 1220c for 15 minutes. From there, I shut off the kiln. The kiln was very
old, very
big and very thick walled. It took 48 hours to cool down naturally. At
that time, I was using
pretty much prefab glazes, either glosses or mattes, nothing that should
have been semi to
either side. They all fired as they should, the mattes being matte. I was
just beginning to make glaze tests. Some of the mattes were all right, but
there were consistent problems with some of the recipes for mattes, firing
glossy. Once in a while I had some tiny blisters in the glosses and some
craters in the mattes. Besides from breaking down kiln problems, I had no
consistent glaze problems due to the firing schedule. Tony's Crystal glaze
had crystals
fired by this schedule. However, tests for Charcoal Matte and Tony Hansens
Matte among others, were considerably glossy. At this time, I think it is
because of my frit sub. and not the firing schedule. Key to me here, is
that the kiln took so long to cool down naturally.
Later, I began to mix only glazes from raw materials. I combined a
recommended schedule from a friend and one I read from Cindy Strnad (hope
she comes back on the list), where I soaked 15 minutes at the top and then
fast cooling to 800c, holding it there for one hour, and then cooling down
by shutting off the kiln, 48 hours. Some of the gloss glazes were all
right, but some were overfired. They ran or had craters, and the clear
glaze over oxides, dragged almost all of the brushwork downward. The mattes
were good and I could not see a marked change in the mattes going more
matte.
I went back to holding at the top for only 5 minutes and holding at 800c for
one hour.
Last year I bought a new kiln. A round Skutt. Half the size, half the
weight, less than half the time to cool down naturally than my old Dagny.
I first began firing with a hold at the top for 15 minutes and then shut off
the kiln.
The kiln cools down in about 12 hours by itself. The mattes were not great.
The
glosses were all right, but again, the mattes lacked that topcoat sheen they
had in my old kiln.
If readers are confused by what I mean by this sheen, I mean simply that the
mattes I like have a
waxy finish rather than a very dry feel. There are many types of mattes,
dry, semi, waxy, crater, etc. But for my purposes, I was trying to get the
right surface for more or less waxy mattes. It was not happening in my new
kiln with no slowing down on the way down.
Instead of going back to the schedule of 5 minutes at the top and an hour
hold at 800c, I started using the following firing schedule 100c p/h to
600c (212f - 1112f)
150c p/h to 1100c (330f - 2012f)
100c p/h to 1220c (212f - 2228f)
cool down max. per hour to 1100c
cool 80c per hour to 800c
shut off kiln
suggested by John and Ron.
The mattes were good once again. I credit this to the crystal chains that
were allowed to form by the slow cooling, as I learned from John Hesselberth
when we discussed Charcoal Grey glaze. I am wondering if my old kiln was so
well insulated that it held the heat long enough on the way down, that the
crystals chains could form without any slowing down of the cool down?
To this day however, I have never been able to get Xavier Green or Charcoal
Grey to look anything but glossy with my frit.
In my new kiln, I have used J and R's schedules for many firings with good
results. But it
was taking many hours, I cannot remember, but about 15 or so. It was too
long for me to run my kiln with the turnovers I needed starting around
October. I began firing again by holding at the top for 5 minutes, fast
down to 800c and shutting down.
I do not notice a distinct difference in my mattes between holding at 800c
for one hour and J and R's schedule. In all of the firings there were
mattes, semi mattes, semiglosses and glosses stacked in the same kiln
firing. Cyrstal glazes as well.
The intended surface for most glazes was successful, as well as the
formation of crystals in the crystal glazes. The crystals were small and
clustered, a few larger stray ones, but distinct nonetheless. (I was only
testing crystal glaze test tiles in my firing ramp). My J and R glazes all
fire well with this schedule, similar to the book's examples. So, for now,
I am satisfied with holding at the top for 5 minutes and holding for one
hour at 800c.
One firing last month was all Tony Hansen's 5 x 20, plus some small saucers
with crystal glazes. I fired the kiln with just a 5 minute hold at the top,
because I forgot about the saucers.
There were still distinct crystals in the crystal glazes. But unlike a
matte glaze, with a tight cover of tiny crystals, the ground for these
glazes is glossy and there are some small crystals floating through it.
However, I was surprised to see the crystals with this fast ramp and cool
down.
So, I think that we can make a lot of ramps and a lot of tests and compare
differences, minute to major, depending on the glaze's temperament as well
as the degree of ramp differences from tile to tile.
Almost like making a line blend for firing ramps, keeping the hold at the
top consistent, and varying the cool down rates. Or otherwise from now
until we get tired of it or until we are personally satisfied.
John, I think this is a very interesting topic and I hope that you will keep
us posted of your testing with ramps.
regards from Alisa in Denmark
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