Steve Slatin on thu 22 sep 05
John --
It takes a while to learn to search the archives.
It's good that you tried (and it may protect you from
another exciting round of trash-the-newbie).
1. Clays and glazes are both generally somewhat
tolerant of slight under/overfiring. The effects will
vary somewhat, but the result will often be
acceptable. More on this later.
2. The 'rated' cone for clay is the cone at which you
should be confident it will be fully matured (if it's
well formulated). Most clays can be fired a cone or
two above their rated cone, and while they may become
somewhat more brittle, they generally do not melt into
a puddle.
3. The difference between cone 5 and cone 6 is rather
small. If you pack your kiln a bit too tightly,
you'll have that much range on a single shelf. (With
a traditional low-medium-high control kiln, even a
correctly packed kiln will often have that much
range.)
4. For clay, firing under the rated cone is often
more problematic (in my experience) than firing over.
At mid and high fire temps, you'll vitrify at *or*
above target cone; you will often not properly seal
surfaces at *under* target temp. Your ware may absorb
much more moisture if fired under target (which also
promotes crazing).
5. Glazes tend to be smoother-surfaced and more runny
at above target temp. Under they may not be
completely vitrified (even if they look OK). If you
take a ^8 glaze, for example, fire it at ^6 and it
looks OK, you still should test for glaze stability.
The fact that it's stable on your clay at ^8 doesn't
mean it will also be stable at ^6.
6. Color effects will often differ at even a half
cone difference, or with the same cone acheived with a
different ramp. Colorants here make a difference, the
colorants like Zircopax, with way high melting temps
will not vary significantly with firing differences of
one or two cones. A cobalt-bearing glaze, though,
fired a cone too high, may be quite unexpectedly
runny. Or a chrome-based glaze may fume off. Etc.
7. Many manufacturers give a range through which
their clay is usable. Bailey, for example, 'rates'
their mid-range clay at cone 4-7.
Hope some of this helps -- Steve Slatin
--- John Connolly wrote:
> Hola from Baja California,
>
> I've been a lurker here for only a short while, but
> have spent many
> hours in the archives leaping from thread to thread.
> I've only been
Steve Slatin --
Drove downtown in the rain
9:30 on a Tuesday night
Just to check out the
Late night record shop
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John Connolly on thu 22 sep 05
Hola from Baja California,
I've been a lurker here for only a short while, but have spent many
hours in the archives leaping from thread to thread. I've only been
potting for a little over three years and use the facilities of the
university here to learn and to fire. All of our work is at ^5 using
Laguna B-Mix and others. I have become most fond of laguna's Speckled
Buff. We fire in a Skutt Electric kiln and also in a locally built
raku kiln.
I plan to build my own kiln this winter. Most probably a catenary
kiln from Frank Colson's plans, fired by propane. It should easily be
capable of ^10.
My question(s) regard the seeming fact that most references in the
threads are to ^6 and ^10. Most glaze formulas on the web are for ^6
and ^10. I really have to search diligently to find any ^5 glazes.
This leads me to conclude that there is some inherent advantage in ^6
& 10. I am aware that there is better color response from some
colorants at ^10, but what are the other advantages or perchance,
disadvantages.
I'll be gratefull for any help I can get.
Incidently, I attended the high school where Mel Jacobsen taught, but I am sure I predated him by some time. I don't think they had a ceramics program in the late 50s, but did have a highly regarded art department. At the time, I was too much a confused teenager to believe that art had any application in my life. Oh well....
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