| Answers from the CM Technical Staff
Q: I have been curious as to why I am getting varying results in my
glaze firings, specifically with the matt finishes I am supposed to be
receiving. I have been using a variety of commercial glazes that are
labeled as matt finishes. Some have been successful, while others are coming out glossy. I am finding this to be
inconsistent between the test kiln and the larger kiln I use.
Both are electric, fired to cone 5 in oxidation.
I have also experienced the reverse effects as well. I have seen pieces
with a glossy glaze applied, placed below kiln shelves, fire to a
matt/satin finish. The same glaze fires to the expected glossy finish
above the shelf in the same firing. I can only assume all sorts of
factors may cause these types of differences; temperature, soak time,
atmospheric conditions, size of firing load, cool down rates, etc.
Unfortunately, I have not been able to discover any consistency for the
reasons why this happens.
Lastly, I have heard it is possible to help reduce the glossiness of a
glaze by adding a dry white clay body to it. My tests have had little
noticeable effectiveness with this technique thus far. If there is any
validity to this technique, or, for that matter, any information on
other techniques to help accomplish this, I would appreciate it. Since
chemistry of the glaze, application and firing all play a part in the
finished piece, I'd like to hear what might cause these variations to
the surface luster.-J.D. |
a matter
of size
Test kilns can be heated very rapidly, but
they
also tend to cool rapidly as well. Neither of these is true of most
full-size kilns. To make a test-kiln firing predict what will come out
of a full-size kiln, it is necessary to fire the test kiln at
approximately the same rate, both during heating and while cooling
down---at least as far as about 700°C (1300°F). Below
that temperature,
the glaze is essentially a solid and no further change in glaze
appearance is going to take place. So the bottom line is, watch the
cooling rate as well as the heating rate in both types of kilns. To get
the same results from each, make those rates the
same. |
|
A: The gloss/matt problems you mention are unfortunately all too common. Here's what you need to understand to deal with them:
Glazes
that are glossy are fully melted into glass. These glazes have a
balance between fluxes (melters) and glass formers (silica and/or boron
compounds). However, there are two reasons glazes are matt. Glazes can
be matt because they just don't melt. Glazes can also appear matt
because crystals form from the chemicals (fluxes) in the molten glass.
The challenge for us as ceramic artists is to figure out which one is
happening with our glazes, in our studio---an unmelted matt or a
flux-rich matt?
If a glaze fires glossier as you increase firing
temperature, then the glaze is clearly unmelted. If you want to
increase gloss, you need to either add more flux (to lower the melting
temperature of the glaze) or increase the firing temperature and/or
soak time at peak firing temperature.
However, if slower cooling
makes a glaze more matt, then the glaze is rich in fluxes. Glazes that
are more matt in your large kiln, but appear glossy if fired in a test
kiln, are often of this type. Dense stacking, which slows cooling, can
also cause a matt glaze to fully develop. It is possible that a shelf directly above a piece can radiate heat down onto the piece, and trap heat below it, long enough to allow crystalline formations in the glaze.
Virtually all clay
bodies are refractory, so they don't melt very well. Most clay bodies
have at least five to ten times as much silica and alumina (compared to
fluxes) as a glaze will. So adding clay (or a clay body) to a glaze
will make the glaze drier. The alumina and silica in the clay body will
overwhelm the fluxes in the glaze and make the glaze more refractory.
This approach is temperature sensitive. Higher temperature equals
glossier glaze, so you need to balance firing temperature and the
amount of clay you add.
The most efficient way to test how much
clay you need to add is to make a simple line blend with one end being
your current glaze and the other end being that glaze plus more clay
body or silica than you think you will need. If you prepare a blend of
five to ten test tiles using increments of the two ends of the blend,
you are more likely to find the right mix than if you just pick an
amount and see what happens.
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Getting two glazes to look the same when one is fired in a test kiln and the other is fired in a full-size production kiln requires use of both witness cones and a good temperature indicator (pyrometer). Whether one uses a programmable kiln controller or controls the heating rate manually by adjusting the kiln switches, it is possible to heat two different kilns at the same rate to the same peak temperature simply by tracking time and firing temperature. The witness cones will indicate whether similar heating has occurred. However, the witness cone does not tell all. Glaze melting depends on actual peak firing temperature. The kiln must reach the same peak temperature AND the witness cone must be deformed by the same amount to consider two oxidation firings to be the same.
But wait, that's not all! There is something more, and in this case it is the cooling of the kiln. The important thing to remember about cooling and glaze surfaces is to control the speed. Slow cooling will cause matt glazes to develop more-and larger-crystals, because the glaze remains fluid longer, giving the flux materials time to re-form crystalline bonds, thus appearing more matt. Rapid cooling has the reverse effect, because all of those materials in suspension in the glass matrix don't have time to reorganize into their crystalline structures. Glazes that are very sensitive to cooling rate, in fact, will turn out matt with slow cooling but glossy when cooled very rapidly.
Dave Finkelnburg, CM Technical Editor
e-mail technical questions to editorial@ceramicsmonthly.org
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