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4
Glazes
Glaze Surfaces
Light Reflection – Shiny, Matte, Dry Surfaces
Potters use the terms shiny, smooth, satin, satin matte,
matte, dry matte, and dry to indicate various types of
glaze surfaces. Each surface type has its own character.
Shiny glazes convey a feeling of elegance and
sophistication. They are used best on simple forms with
smooth unbroken surfaces. Shiny glazes strongly reflect
light. Complex, highly worked forms and surfaces can
reflect the light in confusing patterns. This is accentuated
when the glaze is shiny. Smooth simple clay surfaces, on
the other hand, are enriched by the depth and luxurious
character of shiny glazes.
Satin matte and matte opaque glazes have rich soft
surfaces. They always remind me of the soft surfaces of
clay while it is being formed. Their clay-like character
makes them valuable in both sculpture and pottery.
Matte glazes don’t reflect much light. They are
appropriate for highly manipulated forms where shiny
glazes would be confusing. They tend to convey a rugged
rock-like feeling. Their pleasing character is not marked
by the multiple reflections of shiny glazes; they are
therefore suitable for use on complex forms and highly
worked surfaces.
Glazes are a kind of glass, in many ways similar to
common window glass. Glazes, however, are not meant to
fit in a frame, they are meant to fit over a frame, which is
the form of the clay object. Glazes are the most commonly
used surface finishes in ceramics; they offer the choice of
a wide variety of visual effects, many of great beauty,
along with the virtues of durability and usefulness.
Glaze Makeup
Like clay bodies, all glazes contain silica, alumina, and
flux. The difference between the two is merely that glazes
contain more flux and more powerfully melting fluxes
than clay bodies. This similarity ensures that the bond
between the clay and glaze is very strong.
Glazes are formulated so they become glass-like
when fired to a desired cone or temperature range. They
are carefully devised to bond well with clay and to
color, decorate, and protect the form. Materials may be
added to the glaze to modify the color (colorants),
render the glaze opaque (opacifiers), or encourage
strong visual textures.
Glaze Character
Light Transmission – Transparency,
Translucency, Opacity
Glazes vary in the way they transmit light. A
transparent glaze is clear and very efficient in transmitting
light. Translucent glazes are cloudy and transmit light less
efficiently. Opaque glazes do not transmit light at all.
Transparent glazes may be colorless or stained with
color. Because well-formulated transparent glazes are
among the strongest and most practical glaze types, they
are often used on pieces intended for table use. In
oxidation-fired ceramics, transparent glazes are especially
luxurious when applied to white and porcelain bodies
because they dramatize the striking white color of the body.
Glazes take on many guises and vary a great deal in
character. Because of these wide variations, it is helpful to
group them into broad types or categories, determined by
the following criteria:
• Light reflection. Glazes range from glassy and highly
reflecting to satin matte, matte, dry, and stony.
• Light transmission. Glazes can be transparent,
translucent, or opaque.
• Visual texture. Glazes may have no visual texture or may
be marked with visual textures to one degree or another.
• Durability. Some glazes are soft and easily scratched,
others are hard and durable.
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Character of Glazes Fired
in Electric Kilns
Translucent glazes transmit light incompletely and
partially reveal the character of the clay body. The
cloudy surfaces of translucent glazes can be especially
appealing, as they can be softly modulated and can hint
at the character of the clay body under their surface.
Furthermore, they can pool and darken (especially if
they are colored) where they are thick. They are
especially rich over light-colored bodies. Translucent
glazes are useful for both pottery and sculpture. They
can be very smooth and durable, making them suitable
for table pieces.
Opaque glazes transmit little or no light. While they do
not reveal the character of the clay body, opaque glazes
can be quite rich and strongly marked with color or visual
texture.
If you look for it, you will see a special kind of
emotional tone in work fired in the electric kiln. The
difference is one of outlook. This work is not fire centered
– the fire is truly neutral. Instead it is likely to be centered
on imagery and process.
Color
After a period of time working with the electric kiln you
will become very familiar with the way the kiln affects
glaze color. Glaze color can be strong and very varied.
You will see glaze colors that are clean and bright and
seem to have a weightless character. You will see blue and
green colors that are cool in tone. Even warm colors such
as tan and orange will be bright.
Visual Texture
Though glazes can be completely free of visual texture
(especially when fired in oxidation), the surfaces of
many glazes exhibit a rich variety of these textures,
including crystal patterns, light and dark modulations,
pooling, and random flow. Glazes marked with visual
textures are much valued because of their rich, varied,
and often surprising imagery. Many of these textures
have a way of emphasizing manipulations on the surface
of the clay body, an especially welcome characteristic.
The addition of materials containing elements such as
lithium, titanium, and zirconium encourages visual
textures where they might otherwise not appear in glazes
fired in an oxidation atmosphere. A number of recipes
on pages 277 to 284 have been formulated to encourage
the formation of visual textures.
Creating Glaze Imagery
The ceramist who works in the electric kiln can create
glaze imagery in a great variety of ways. Visual texture is
the primary imagery tool for ceramists working in fuelburning kilns.
Visual texture can be pleasing in the electric kiln but it
is usually quite subtle and sober in character. It is
important to consider a great variety of image creation
methods when working with the electric kiln.
Combinations of engraved or relief imagery with glazes
can be very rich. Another effective work strategy is
combining application methods such as splashed, poured,
and sprayed imagery. These too can help the ceramist
create a rich surface.
Painted imagery works well in the electric kiln. It is
likely to have a highly graphic character and it allows the
creation of complex imagery, even highly delineated
imagery, if the ceramist wishes.
Utility and Durability
Glazes vary greatly in durability. If you are glazing
utilitarian pieces, look for good durability and easy
cleaning. Look for glazes that resist scratches,
abrasions, and are free from crazing, bubbling, and
flaking. You may want to use shiny or satin shiny
glazes because they are more easily cleaned than
glazes with matte or dry surfaces.
Many glaze materials that encourage rich character
and visual textures also encourage soft, easily worn
surfaces. The wish for a beautiful surface must be
balanced against the need for durability. Ceramists
who create nonutilitarian pieces tend to emphasize the
look of the glaze while those who create utilitarian
pieces or work meant for outdoor sites favor durable
surfaces.
Glaze Color in Electric
Kilns
Many contemporary ceramic artists want as wide a
variety of color choice as possible. One of the most
persuasive reasons for working with glazes fired in the
electric kiln is the availability of a broad spectrum of rich
color.
We bring color to glazes using minerals that have color
as an attribute. Only minerals can survive the heat of the
kiln. And since not every colored mineral can survive this
Left: David Furman. “The Troublemaker.” 18" x 7" x 14". Strikingly rich color is very important to Furman. He works on
slipcast midrange porcelain bisque fired to cone 5 because the white body enhances color. He applies layers of color,
oxides, glazes, and underglaze stains, each one applied and fired separately so as not to allow the color to become
muddy. He aims for a surface with strong color and a sense of depth. The work is fired up to 12 times at 2000° F. He then
applies a clear glaze fired to the same temperature to finish the piece.
Photo: Michael Honer
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Bryan Hiveley. “Next Step.” 49" x 46" x 18". Glazes, slips, and underglazes fired to cone 04. The glazes in this piece are
fine examples of what can be done in the low fire. It is not that they are brilliant and flamboyant, but rather that they are
high key and represent a nontraditional way of thinking about ceramic color. Hiveley states that it is his goal “to suggest
universal biomorphic forms so that viewers will reexamine the natural world around them, reconsidering the importance
of the most natural wonders as well as what would seem most insignificant.”
Photo: Bryan Hiveley
heat, only a select group of colored minerals are useful to
the ceramist. Some of these minerals come as colored
clays, some as minerals found in nature, and some are
naturally occurring minerals that have been modified to
suit our needs. We call the minerals found in nature
“colorants” and the minerals that have been modified to
suit our needs “prepared stains.”
The naturally occurring colorant rutile is useful for
creating warm-toned colors with some of the character of
bronze. Additions of the naturally occurring colorants
copper and cobalt encourage cool colors such as green,
aqua, blue, and purple.
Glaze stains encourage the broadest color range
(especially in the low fire) and include most of the colors
available to artists who work in any art media, including
the brightest varieties of orange, yellow, burgundy, violet,
and pink in ceramics.
Using Clay to Color Glaze
Almost all glaze recipes contain clay. Clay is a source
of alumina and silica in glazes. It also serves to make the
glaze easier to use by keeping it in suspension. Another
very important aspect of glaze clays is their contribution
to glaze color. Often we add clays that are white or cream
in color. We use these to make colorless transparent glazes
and white or cream-colored glazes.
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Dee Schaad. “Plate.” 8 1/2" diameter. Schaad bisque fired this piece to cone 03, then applied commercially prepared glazes
and lusters (all from Amaco). He applied these using brushed and splashed applications and fired the piece to cone 05.
Here the brilliant colors of the low-fire work, in tandem with a classic plate form, produce a persuasive and distinctive piece.
Photo: Dee Schaad
Barnard is widely available in North America but similar
clays are found all over the globe. They too can encourage
very interesting results in the red-brown, dark brown, and
umber range.
The contribution to glaze color from colored clays is
particularly significant to ceramists who fire in the electric
kiln. Colored clays are especially useful when used as glaze
colorants because their rich warm tones are otherwise
difficult to obtain in an electric kiln firing. They contribute
warm ivory, cream, earth orange, red, and brown colors.
It is surprising that the results from high impurity clays
are so different from the results of using the colorant iron
oxide. Iron oxide would seem to be a perfect colorant. In
the electric kiln, however, iron oxide seems to encourage
drab neutral tones (greenish ocher and soft grass green
colors are common).
In most places it is easy to find an iron-rich clay for use
in glazes. In North America we can use Cedar Heights
Redart, a widely distributed red clay. While it is best
known as an ingredient in clay bodies, Redart is a fine
glaze clay. Glazes that contain Redart are reliable,
durable, and visually appealing. You will find many glaze
recipes that call for red clays in the recipe section (pages
277 to 284).
A number of recipes call for Barnard, an alternative
clay that is very dark-colored and high in impurities.
Glaze Colorants
Glaze colorants are important sources of ceramic color.
They are refined from minerals that are found in nature.
The ceramist adds them to the glaze in varying amounts,
usually 1% to 5% of the total recipe. Their unfired color
does not reflect the color they will have when they are
fired. This color change introduces unpredictability in
the glazing process. When you use colorants you never
know exactly how the piece will look until it is fired. In
time you can become familiar with the color changes that
take place in the firing and learn to expect them, but even
an experienced ceramist will encounter the occasional
surprise from a glaze firing.
Some colorants are quite toxic. No recipe in this book
calls for the use of toxic colorants. Stains can be used as
a substitute for some of the toxic colorants and in other
cases, no appropriate colorant or stain is available.
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Frank Martin. “Platter.” 3" x 11" x 9". Martin works with a studio-made white body, and with glazes fired to cone 4. He says
of his work, “My goal is to challenge the viewer’s notion of an object’s use while assuring that it performs flawlessly as a
functional piece and is, at the same time, beautiful.” It is the combination of images here (Martin calls this “the
reassembling of disparate elements”) that gives this piece its strong character and its special identity.
Photo: Frank Martin
Glaze Colorant Chart
Colorant
Unfired Color
Fired Color
chrome oxide
dark-colored clay
copper carbonate
copper oxide
red iron oxide
strong grass green
cream, brown, rust, burnt orange
soft gray-green
black
iron red, brown
black iron oxide
manganese
rutile
vanadium
black
black
tan
bright yellow
same as unfired
toxic – replace with chrome green stain
same as unfired
strong clear green
strong clear green
earth yellow or soft yellow (with calcium), otherwise gray-brown
dark-colored clays are often superior to red iron oxide in the
electric fire
gray, brown
soft brown, pink
toxic
tan, brown
same as unfired
toxic – safely replaced with titanium stains
Notes
Right: Yoshiro Ikeda. “Passion.” 22" x 12" x 6". The artist has combined a strong sculptural form and a surface treatment
marked by a crawling glaze and painted imagery to make a piece with a rich and complex character.
Photo: Yoshiro Ikeda
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