PPP: A Guide to Clay and Glaze Selection for the Classroom
With both the increasing popularity of the ceramic arts and the technological advances being made in the ceramic field, the selection of ceramic materials has expanded and become more complex. To simplify the selection process, the following guide describes the most appropriate clays and glazes for a variety of classroom projects.
ClaysWhen choosing a clay for a functional ceramic project, a number of factors should be considered, including color, plasticity, porosity, vitrification, firing temperature and glaze fit-to name just a few.
High-Fire Stoneware Clays
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.
- 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, #900, #590 and #510; and cone 5: WC-877, WC-400, WC-401, WC-402, #65, #55 and #50.
- 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 stoneware clays 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 and #910; and cone 5: WC-851, WC-877, #70, #60 and #30.
- Wall tile clay selection is completely dependent on one's individual design considerations. Recommended clays include cone 10: WC-389, WC-376, #950 and #310; and cone 5: WC-397, WC-394, #66, #45 and #30.
- 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 durability and provide lower shrinkage. Recommended clays for floor tile include cone 10: WC-371, WC-372, WC-381, WC 885 and #901; and cone 5: WC-397, WC-365, #30, #60 and #45.
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 hand-built, wheel-thrown or sculpted projects. Recommended clays include WC-381, WC-371, WC-372, #901 and #950, which are listed as cone 10 but may be fired to cone 5 or lower for sculpture work. WC-390, WC-392, #30, #66 and #75 are cone 5 sculpture bodies, and they are also often fired lower for sculpture.
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. A variety of prepared moist porcelain clays are available 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 it is translucent when construction is thin. A good cone 5 porcelain is #15, and a pure white translucent cone 6 porcelain is S-3126 (FROST).
- 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.
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. Low-fire, high-talc clays are used commercially by hobbyists and in educational programs.
- 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 and #20. The following basic low-fire clays contain sand to facilitate their use on somewhat larger forms: EM-212, EM-214, #10-G and #20-G.
- Low-fire sculpture clays are designed for larger projects where varying thickness becomes a factor. Recommended cone 06 clays: EM-215, EM-234, EM-337, #10-G, #26.
- Other low-fire clays have specific uses that are usually described in their title, i.e., 06 #5 Pressing and 06 #8 Pressing.
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.
OvenCraft and Dry-Hard
OvenCraft, OvenCraft II and Dry-Hard clays are 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 and OvenCraft II will harden after baking for an hour or less in your kitchen oven at 350°F, 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 OvenCraft II and Dry-Hard are available in buff, brown, white and terra cotta. These clays may be painted with any acrylic enamel, tempera or watercolor paint. They are more suited for decorative than functional ware, and are perfect for aspiring young artists.
GlazesThe wide variety of lead-free, high-fire glazes represent formulas developed during the renaissance of high-fire pottery over the past 40 years, and new formulas are constantly in development.
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 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 unique quality in the ware. Recommended glazes include WC-527 Chun, V-18 #41 White, WC-556 Iron Red and 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.
Three base glazes are available in the stock cone 10 dry glaze line: SG-91 White Gloss, SG-82 Brilliant Black and SG-93 Clear. These glazes are 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-558 Jon's Clear and WC-520 #16 Transparent.
Cone 5 Glazes
Most of Laguna's lead-free cone 5 dry glazes 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-157 Clear and SG-138 White 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.
Moroccan Sand cone 5 liquid glazes are for brushing applications. 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 are available in pints and gallons (most are also available in dry form) and 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 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. An extensive line of lead-free, low-fire glazes and underglazes are available for use with low-firing modeling clays, such as EM-207 and EM-210. Lead-free should be specified when ordering these glazes.
The use of lead-free clear glazes over underglaze colors allows for precise color and design considerations. Some tips for 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.
EM-2001 is a 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.
Editor's note: This information has been reprinted with permission from Laguna Clay Co. and is not intended to be representative of all clay or glaze products. For additional details regarding the materials included in this article, visit http://www.lagunaclay.com .
SIDEBAR: GlossaryGlaze 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-vitrified
Vitrification: 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