Glazes and Glaze Types
Cones are used to measure temperatures in ceramics
firing. You buy a certain number of cone, and when if falls
over you have reached the appropriate temperature. 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.
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.
Low fire glazes are typically fired at cone
06-04. At these lower temperatures you can get very vivid,
bright colors which are difficult or impossible to get at higher
temperatures.
High fire glazes are typically fired at cone
9-10.
Mid fire glazes fall in between, and are fired at
cone 4-6.
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 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.
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!
