Glazes and Glaze Types
Cones are used in ceramics instead of
temperature. A cone is a special piece of clay shaped like a
triangle that you put in your kiln. As it heats up it
softens, and it is calibrated bend over when your clay and glaze
have received the right amount of heat. So for example, if
you use a glaze that says "fire to Cone 04", your glaze
will be ready when a Cone 04 bends. The Orton
Cone Chart gives the approximate temperature equivalents for
the various cone, however, the cone actually falls based on the
total amount of heat it absorbed, not the maximum temperature it
reached, so the temperatures can vary.
Low fire glazes are typically fired at cone
06-04. High fire glazes are typically fired at cone
9-10. Mid fire glazes fall in between, and are fired at
cone 4-6.
Low fire was historically used to achieve very vivid,
bright colors. Mid and high fire were historically used to
achieve more muted, earthy colors. But glazes have continued
to improve and now many different glaze characteristics can be
achieved at each of the firing temperatures. High fire is
not often used in electric kilns because it is harder on the kiln
and takes so much more power. Therefore, few commercial
glazes are available in High fire.
Low fire takes less power, but mid-high fire which
is fired using stoneware and porcelain clay, is stronger and more
durable.
Glazes come in WET
form (pre-mixed in pints or gallons), and DRY
form which you mix with water and put through a sieve. Wet
is more popular in most cases and almost all low fire glazes, but
mid-high fire glazes are sometimes available in dry.
Typically WET glazes are designed for
brushing and DRY glazes are designed
for dipping or pouring. Dry glazes can be made brushable
with the addition of a brushing medium.
Underglazes are used to get consistent colors that
stay where you put them (regular glazes have a tendency to move
around during firing). Sometimes they are left as is, and
sometimes are covered with a clear or translucent glaze and
refired.
Overglazes are primarily accent products applied over pieces that have
previously been glaze fired. They typically are lustrous or have
special pearlescent qualities. The piece is refired with the
overglaze at very low temperatures, such as cone 016-018.
When using overglazes, apply them to surfaces that are free of
dust, skin oils, and lotions to ensure proper adhesion. Wipe
surfaces with rubbing alcohol prior to application (except when
used over crackle glazes.)
Stains and Oxides can be wedged into clay to color it, added to clear glazes, or made into a
thick liquid and used for drawing on or coloring clay, similar to
a slip.
Slips are basically watered down clay, usually with color
added. Engobes are similar.
Acrylic Paints are
used to paint bisque (clay that has already been fired
once). Paints are not re-fired in a kiln.
There is oxidation firing, and reduction
firing. Reduction keeps the oxygen from reaching your
glazes. Oxidation is typically done in an electrical kiln,
basically because it is hard on the electric kiln elements to do
reduction. Temperatures range from low fire to high
fire. Reduction is typically done in gas firings, where it
is usually high fire, and is used in other firings types such as
Raku which is low fire.
Most of the prepared glazes you can buy are for
oxidation. However, some can be used in reduction.
Most of the prepared glazes you can buy are
formulated for brushing on. Chemicals are added to keep the
particles in suspension and the glaze easy to brush on.
Usually 3 coats of glaze are required.
Many commercial glazes are also available dry, to
be mixed with water and sometimes a suspension agent. A 25
lb bag of dry glaze will typically make about 3 gallons of glaze.
Although some commercial high fire glazes are
available, most people use hand-made glazes for high
fire work. These glazes are based on recipes and use glaze
chemicals such as those sold at BigCeramicStore.com. These
glazes are typically mixed to be used for dipping or
spraying. The Excellent
Ceramic Links page lists many sources for glaze recipes.
Making your own glazes can be an intense scientific exploration,
as there are many factors that affect how well they perform:
firing type, firing speeds, clay body used, etc. To make
your own glazes you need a good mask (many of the chemicals
are poisonous), a variety of sieves and large buckets, an accurate
scale, and a great deal of patience!
Please note that there are many factors that
affect how well any glaze performs. Getting the right fit of
the glaze to the clay body is extremely complex and fills many
books. If your glaze and your clay don't "fit":
bond together, expand at the same rate, etc, your glaze will craze
or crack or the piece can even crack. The benefit of
commercially prepared glazes is that they have been formulated and
tested to work in a variety of conditions, they are very stable
(don't run all over your kiln shelves), and they are easy to get
and use.
We definitely recommend that you get some basic
books on ceramics as you get started!
