 | Final temperature (Even if fired to the same Cone, the temperature may
vary from one part of the kiln to another. Or two people might interpret
the bending of the Cones differently.) |
| Rate of change in temperature, particularly cooling rate. This is a
main reason why glazes look different when fired on a whole piece than
when fired in a test kiln, because the smaller test kilns usually cool
faster. |
| Clay body (Color as well as the materials in the clay body. For
example, iron in a clay will often cause spotting through the glaze, or
will act as a flux causing glazes to melt earlier.) |
| The thickness or thinness of the application or the method of
application (sprayed, dipped, or brushed.) |
| Variations in materials (Glaze materials are taken from the earth, and
their compositions are not pure. There is always some variation from
batch to batch, and sometimes this variation is enough to affect the
glaze in a substantial way.) |
| Atmosphere: The amount of oxygen present or not present (reduction) |
| The presence of other glazes nearby. |
| Particle size differences. If a material is available in 200 mesh and
325 mesh, these will melt differently and give different effects. |
| Mixing and screening. How well the materials are mixed and to what
mesh they are screened will affect the final result. |
| Venting. Whether and how much a kiln is vented can affect the final
firing. |
As you can see, there are many factors, so it is not surprising that it
is hard to replicate a glaze.
Commercial glazes have to be formulated to work over as wide of a range
as possible, or they would not be marketable. Not that they are perfect.
As you can see, there is such a variety of factors that can affect them,
that sometimes they will vary too. But they are usually chosen for their
more stable properties.
In fact, that’s why some people think
commercial glazes tend to be “boring”. You are not likely to get a
beautiful commercial multi-colored breaking glaze, because to achieve
such a glaze requires a very specific range of conditions. (Breaking
means that it changes colors or textures around the piece, such as where
there is variation in thickness or surface texture.)
Commercial low fire glazes are the most stable. Because of the way they
have been formulated and the lower temperatures they are fired at, they
usually are very consistent. They can still be affected by the above
factors, but are less susceptible to minor changes. This is why people
who work with low fire glazes are usually those who are looking for
consistent, repeatable colors, rather than unexpected, breaking colors.
Generally, the higher the temperature, the more interaction there is
between the clay and the glaze. So clay body will play an increasingly
important role as you go up in temperature. At Cone 6 there is some
interaction. At Cone 10 there is a lot of interaction.
Aside from looks, there are other problems with traveling glazes. For
example, a glaze that is stable for one person might run or spit all
over your shelves, causing damage. Or it may be toxic.
To understand how materials can affect a glaze, let’s look at an
ingredient used in some glazes: Cornwall Stone. Cornwall Stone is a low
iron feldspar material used primarily as a flux in clay bodies and
glazes. It melts at 1150-1300C. Remember, Cornwall Stone is a material
that is mined from the earth. But let’s look at its typical chemical
composition.
|
CaO |
1.81% |
|
K2O |
4.31% |
|
MgO |
0.14% |
|
Na2O |
3.30% |
|
TiO2 |
0.15% |
|
Al203 |
16.31% |
|
SiO2 |
73.80% |
|
Fe2O3 |
0.20% |
|
Total |
100% |
Now you don’t have to know any chemistry at this point, other than to
see that there are many chemical compounds in this single ingredient.
Calcium Oxide, Magnesium Oxide, Titanium Dioxide, etc. Each of these
ingredients contributes something to a glaze.... (What, you will learn in
a later tip, and it’s nowhere near as difficult as it might seem now!)
But one batch of Cornwall Stone may differ from the next in the
proportions of the ingredients shown. This can obviously affect how your
glaze will perform.
It is for this reason that professional potters
often buy their raw materials in very large batches, so they are ensured
the same composition once they get their recipe working. This is not
feasible for most part time (non-production) potters who only mix a
small amount of glaze at a time.
Now that I’ve warned you about the dangers of using glaze recipes,
some of you are still wondering where you can find such recipes. We
maintain a listing of recipe sources at this page. Glaze
Recipes (note, this is a link off the main page)
Copyright 2001 Cindi Anderson
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