| Classroom Ceramics: A Guide to Clay and Glaze Selection |
Clays
When choosing a clay for a functional ceramic project, there are numerous
factors to consider: color, plasticity, porosity, vitrification, firing
temperature and glaze fit, just to name a few. Information on the
characteristics of each of Laguna's clays can be found in the Prepared
Clays section and will assist you in making the best possible
product choices for your needs. The Clay Application Charts shown
on this page are based on lab testing, input from our technical staff
and feedback from our customers.
Low Fire
Clays
Characteristics:
Low fire clay tends to be either reddish or white in color and because
of its porosity, glazing is required to create a waterproof surface.
When fired, these clays are non-vitreous and have low shrinkage. Low
fire dinnerware does not take extremely hot temperatures well, and
repeated exposure to such temperatures can result in cracking and
crazing. Laguna's low fire, high talc clays are used commercially,
by hobbyists and in educational programs.
Traditional
Applications:
- Low Fire Clays provide a good medium for small
throwing, hand building and modeling projects. Recommended clays
include: EM-207,
EM-210, EM-330,
EM-338, #10,
#20. The
following basic low fire clays contain sand to facilitate their
use on somewhat larger forms: EM-212,
EM-214, #10-G,
#20-G.
- Sculpture Clays are designed for larger projects
where varying thickness becomes a factor. Recommended Cone 06
clays: EM-215,
EM-234, #10-G,
#26.
Other low fire clays have specific uses which are usually described
in their title, i.e. 06
#5 Pressing and 06
#8 Pressing.
High Fire Stoneware Clays
Characteristics:
Cone 10 Earth-colored Stoneware Clays produce durable ware and most
will react favorably to the manipulation of air, gas and smoke in
reduction firing. These clays have good plasticity and will perform
well in both wheel throwing and hand building methods.
White Stoneware Clays also have good plasticity and contain sand to
increase their strength and workability, making them slightly open
when fired. These clays have become very popular because of the clean
canvas they provide for a myriad of glaze colors. Some iron specking
can occur but for the most part, glaze colors are not affected.
Cone 5 Stoneware Clays are similar to Cone 10 clays in their workability
and represent potential savings in lower firing costs. The lower firing
temperature and oxidation atmosphere allow for the use of a large
palette of ceramic stains and cause less warping of the ware.
Traditional Applications:
- Tableware may be made with stoneware clays
with smooth to slightly coarse fired surface textures and low
water absorption. Recommended clays include Cone 10: WC-370,
WC-841, WC-379,
WC-382, #510,
#590, #900;
Cone 5: WC-400,
WC-401, WC-402,
WC-877, #50,
#55, #65.
- Ovenware may be made from most stoneware clays
provided the ware is well designed and the user is aware of potential
thermal shock and how to avoid it. Cone 5 stonewares tend to be
less susceptible to thermal shock but all ovenware made of stoneware
should be heated and cooled gradually. Recommended clays include
Cone 10: WC-376,
WC-389, #750,
#910; Cone
5: WC-851, WC-877,
#30, #60,
#70.
- Wall Tile clay selection is completely dependent
on one’s individual design considerations. Recommended clays
include Cone 10: WC-376,
WC-389, #310,
#950; Cone
5: WC-394, WC-397,
#30, #45,
#66.
- Floor Tile, on the other hand, should be made
from a strong, durable clay to withstand wear and abuse. Commercial
producers employ special production techniques to increase the
durability and provide lower shrinkage. Recommended clays for
floor tile include Cone 10: WC-371,
WC-372, WC-381,
WC-885, #901;
Cone 5: WC-365,
WC-397, #30,
#45, #60.
Sculpture Clays contain significant amounts of sand
and grog in various mesh sizes and are formulated for a low rate of
shrinkage. These clays are designed to accommodate large handbuilt,
wheel thrown or sculpted projects. Recommended clays include WC-371,
WC-372, WC-381,
#901, #950
which are listed as Cone 10 but may be fired to Cone 5 or lower for
sculpture work; WC-390,
WC-392, #30,
#45, #66,
#75 are Cone
5 sculpture bodies, again often fired lower for sculpture.
Porcelains
Characteristics:
Porcelains are most commonly known for their whiteness and for having
some degree of translucency. They are pure and vitreous which gives
them their inherent glaze fit quality and unparalleled hardness and
durability. Laguna offers a variety of prepared moist porcelain clays
in firing temperatures ranging from Cone 10 to Cone 3.
Dave's Porcelain (WC-384)
and #570
are outstanding Cone 10 throwing bodies and are also excellent for
slab projects when a hard clay is desired. Cone 10 Windsor Porcelain
(WC-879)
is made with a highly plastic, imported kaolin, and translucent when
construction is thin. A good Cone 5 porcelain is #15.
Traditional Applications:
- Sanitaryware is usually made from porcelain
or porcelain-like clays. Hardness and durability as well as flawless
glaze fit are key factors in their function.
- Wind chimes create a nice sound when made of
thin porcelain. If other less vitreous clays are used, consider
glazing them to heighten their tone.
- Tableware, Tile, and Sculpture functions are
greatly enhanced when made with porcelain. The smooth, white,
hard qualities provide an ideal surface for glaze colors to achieve
depth and clarity. Porcelain is waterproof and very durable, making
its use for dinnerware especially popular.
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OvenCraft and Dry-Hard
Clays
OvenCraft and Dry-Hard Clays are remarkable ceramic clays that can
be thrown on a Potter's Wheel or hand modeled like any high quality
sculpting clay, and all three will harden without having to be kiln
fired. OvenCraft
will harden after baking for an hour or less in your kitchen oven
at 350 degrees, while Dry-Hard
clays will actually reach an incredible hardness by naturally air
drying for only a couple of days (each of these clays comes with detailed
instructions). OvenCraft is available in a warm speckled brown, and
Dry-Hard is available in buff, brown, white and terra cotta. These
clays may be painted with any acrylic enamel, tempera or water color
paint. They are more suited for decorative than functional ware, and
are perfect for aspiring young artists.
Note: Forming methods, drying techniques and firing schedules will
all affect your results with any clay. Always test an unfamiliar product
before ordering it in quantity.
Glazes
The wide variety of lead-free, high fire glazes Laguna Clay offers
represent formulas developed during the renaissance of high fire pottery
over the past forty years. We continue to concentrate on developing
lead-free formulas for our broad palette of glaze colors. Again, you
will find detailed information describing the key characteristics
of each glaze in our catalog as well as detailed instructions for
brushing, dipping, or spraying.
Cone 10
Reduction
Cone 10 stoneware glazes are unique in that the firing plays such
an important role in the process. Most of the Cone 10 glazes we offer
are intended to be enhanced by reduction firing. Impurities and trace
metals in the clay and glaze will react with carbon in the kiln atmosphere
to create spotting. Reduction firing also causes glazes to attain
a warm color and a smooth, durable surface quality.
Oxides react in various and sometimes spectacular ways depending on
the glaze compositions. Each firing creates a certain unique quality
in the ware. Recommended glazes include WC-527 Chun, V-18 #41 White,
WC-556 Iron Red, WC-553 K9.
Cone 10 Oxidation
Most stoneware glazes can be fired in oxidation. Cone 10 oxidation
firings come into play when more controlled, stable and reliable results
are needed. Many commercial manufacturers employ high fire oxidation
to produce durable ware with reliable results. Items like fine dinnerware,
jewelry and sanitaryware are often fired in this range and atmosphere.
Laguna offers three base glazes in our stock Cone 10 dry glaze line
formulated specifically for this application: SG-82 Brilliant Black,
SG-91 White Gloss, and SG-93 Clear, formulated specifically for color
development with or without the use of opacifiers. Also available
are two clear base glazes with a Cone 5 to 10 firing range: WC-520
#16 Transparent and WC-558 Jon's Clear.
Cone
5 Glazes
Most of the lead-free Cone 5 dry glazes we offer are formulated to
be fired in oxidation. Each has its own distinct surface quality,
and reaction to colorants will vary from one to the next. Two glazes,
SG-98 Light Blue and SG-99 Brown, are formulated to be used without
additional color. SG-138 White and SG-157 Clear are both excellent
base glazes for decorating with stains or color additions. Both glazes
contain no zinc and allow the use of chrome tin stains.
Laguna's Moroccan Sand Cone 5 liquid glazes are for brushing application.
This series features over 80 glazes in a wide variety of colors and
surfaces. All of these glazes are lead-free and safe for food and
beverage containers except for the four metallic glazes in the 900
series.
These liquid glazes in pints and gallons (most also available in dry
form) provide an excellent vehicle for beginners to learn the ins
and outs of glaze application. Those more adept in ceramics can utilize
brush application techniques to create detailed and ornate designs.
Low Fire Glazes
Low fire talc bodies provide an excellent medium for ceramic art and
a base for a large selection of commercially available textures, satins,
satin mattes and vivid gloss colors. Laguna offers an extensive line
of lead-free, low fire glazes and underglazes perfect for use with
our low firing modeling clays such as EM-207 and EM-210. Specify lead-free
when you order.
The use of lead-free clear glazes over underglaze colors allows for
precise color and design considerations. Laguna's EM-2001 is a spectacular
crystal clear transparent, perfect for use over colorful underglazes.
Also recommended are the lead-free EM-1000 Series Glazes and the EM-8000
Series Underglazes.
Some hints when using underglaze decorations:
- Bisque at Cone 05 - 04 and glaze fire to Cone 06. The higher
bisque temperature will burn out material that might otherwise
cause problems in the glaze firing.
- Leadless clear glazes are best when applied very thin. If the
glaze surface feels dry, apply a thin coat of glaze and refire.
- White underglaze may be used to mute underglaze colors.
Glossary
- Glaze fit - The lack of tension and amount
of fusion between the glaze and the clay.
- Opacifier - A material used to whiten a glaze
or make it opaque.
- Plasticity - The ability of a damp clay body
to yield under pressure without cracking and to retain the formed
shape after the pressure is released.
- Porosity - The state of being porous, open
or semi-vitrefied.
- Vitrefication - The fusion of clay particles
in fired clay.
- Warping - The deformation of ceramic forms
occurring during drying or firing.
- Workability - A clay's usefulness in various
hand forming methods.
Reference Materials
- School Texts
- Technical Data & General Information
- Forming Process
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