JOHN NEELY on mon 13 oct 97
Having taken one solo on this tune, I guess I'm obligated to take another
turn.
Louis Katz wrote:
> But although glaze is relatively impermeable after it has melted, it
> is not impermeable and reduction in the kiln after the glaze is
> melted does have an effect. This effect can be seen by firing a Raku
> glaze such as Copper Penny or most other glazes of that ilk firing it
> in clean oxidation until it is melted and then reducing the kiln
> while holding the temperature or allowing it to drop slightly. During
> this process if your peice is close enough to the spy you can watch
> the reduced copper "floc" together in little clumps (probably some
> sort of crystalization rather than flocculation). If the pots are
> pulled and quenched the coloration stays.
Copper Penny is a perfect example of what I meant when I said that some clays
and glazes are quite susceptible to *superfical* oxidation and reduction. That
copper luster is on the surface. Ever wonder why the copper disappears when
those pots sit around for a while (there was a thread on this a while back.)
Did you ever put those pots in a swimming pool to cool them? That chlorinated
water will eat through the luster in short order. When you say that "cooled
normally in the kiln it disappears, " what is "normal?" Oxidised cooling? The
luster oxidises. I haven't done much of this, but if you do a reduced cooling
like that used for hard lusters ala Alan Caiger Smith or Alan Peascod, I am
quite certain that you can maintain the luster. Copper red is a different
situation. I think that the flocs that are involved in copper red are of
colloidal scale - not easily viewed in the hot kiln.
> However, It used to be where I went to school that
> stoneware kilns would be fired in oxidation between cone 1 and cone 7
> or so. Once we started firing in light reduction all the way from
> cone 08 we had much better reduction of our glazes and more
> consistent results. We assumed and I still beleive that this was
> because the reduction was eeasier to acheive before the glaze melted.
I'm not sure I've got the whole picture here. What was the atmosphere before
cone 1 and after cone 7? Was there an initial period of reduction at a lower
temperature in the first instance? Reduction reactions do happen faster as the
temperature goes up, so if an initial attempt at reduction at a low
temperature has been unsuccessful, continued reduction as the temperature goes
up IS more likely to achieve the desired results. If the firing was oxidised
until cone 7, then you have a different situation. Most stoneware clays are
pretty vitreous by cone 7, which means that the body colour will be little
affected by the atmosphere (meaning oxidised color). Glazes that do not melt
until late in the firing will still be dramatically affected by the reducing
atmosphere after cone 7. Northern Sung celadons would be an example of this
scenario. High calcium glazes are dry until very close to the end of the
firing and then , whoosh, they're fluid. Conversely things that melt earlier
need to be reduced earlier.
and to Paul Wilmoth who wrote:
> To put it simply: no post fire reduction = no black core(for that
> particular situation)
>
> Point in fact is that the clay can be reduced further (to the point
> of black core) after the body has reached vitrification. This black
> color of the body happened after the kiln had reached cone 10, cooled
> for 5 hours then was reduced heavily for 30 minutes at dull red color
> It did also make the wares weak and brittle.
and then:
> I am not spouting off theory - I am reporting results
I don't mean to be contentious, nor do I doubt the veracity of your report. I
am inclined to wonder about your explanation (theory.) Like Vince Pitelka, who
pulled me into this, I am at a loss to explain the situation as you have
presented it. And like Ron Roy, who questioned the possibility of "over
reduction" to begin with, I don't understand physically what is meant by "clay
can be reduced further." What is the mechanism? The body that you are using
would be vitrified, and with about 2 percent iron, assuming it was indeed
reduced on the way up in both cases, should be pretty grey (the color that it
would be under clear glaze.) Further assuming that you can make carbon
penetrate vitrified reduced clay, what is that carbon going to react with? Are
you suggesting that the black iron oxide already incorpoated in the glassy
melt responsible for vitrification would be reduced to metallic iron? I can
only believe that there is another explanation - that there is some factor
here that has not been accounted for. Anybody else want to take a stab at
this?
In any event, I'll give your body a try and see what I can come up with in the
next few weeks.
John Neely
Utah State University
| |
|
