Perhaps one of the most enjoyable things about working with clay is the endless variety of techniques that can be tried and experimented with to yield new and exciting ways to decorate one's pots. Combination glazing, in which two or more glazes are combined to achieve various effects, can be performed in many different ways to achieve countless possibilities. These techniques are especially desirable for oxidation firing, as they put more life into the glazes.
You'll need at least two different glazes here, preferably ones which you've previously test-fired together to make sure they give pleasing results when overlapped. Start off by dipping your pot in the lighter of the two glazes, then let dry. Dip again, this time in the darker glaze. Once it has dried, take a mini-ribbon tool (made by Kemper: it has a triangular, open-ended wire on both ends of a wooden handle) and gently carve a pattern in the top layer of the glaze, exposing the lighter glaze underneath. Depending on how careful you are as you carve out the design, very intricate patterns may be achieved. Once fired, the carved patterns will appear in the color of the first glaze applied, surrounded by a "background" of the combined layered glaze color.
First, dip your pot into a layer of the glaze you'd like to use for the background. Next, use a natural sea sponge dipped in another color of glaze to achieve your desired pattern. Commercial sponges from a craft supplier, cut into various shapes of your liking, can also be used to obtain interesting effects. If you have time, experiment by mixing and matching various shapes and colors to see what sort of combined effects are most appealing.
Obtain some florist's foam (the stuff used to hold professional flower arrangements in place) from your local craft supplier. Then draw a pattern or design onto the surface of the foam. Carve out the design with your mini-ribbon tool, then dip the block into slip or glaze and use it as a stamp to decorate wide, flat forms which have already been dipped into a contrasting glaze and allowed to dry.
Dunk your whole pot into a light glaze; paint on a pattern with a wash like cobalt oxide (blue) or iron oxide (brown); then carefully paint wax resist over the wash pattern you just painted. When the wax is dry, dunk your pot into a second color. If you go over the line so you're waxing on top of the white, you'll have three glazes (white, cobalt, plus final). Extra detail can be achieved by carving through the second glaze.
For crisp, fine lines with sharp edges and angles, use thin masking tape instead of wax to achieve your resist pattern. This is done by glazing the entire pot, letting dry, and applying the masking tape in the pattern you desire. Dip the pot again, let dry, and remove the masking tape to expose the glaze underneath.
This technique works best on bowls or platters. Note that you must work fast to achieve optimum results.
Keep a dump bucket nearby, into which you'll be pouring your contaminated glazes. Select three or four different glazes which you would like to "marbleize," and pour various quantities into measuring cups, placing them all within easy reach. Start glazing by quickly pouring each color into your pot, then carefully swirl around the glazes to achieve a marble look without too much mixing. Quickly again, dump all of the glazes out of the pot and into your "dump" bucket.
This technique is reserved for application to the inside of the pot, as the glaze would run too much if applied to the pot's exterior. Once the inside of the pot has dried, pour on your exterior glaze (perhaps one of the four colors you just used).
Experiment with different quantities of glazes to get predominantly one color, for example. As you play with this technique, you will learn to control the flow of the glazes. Also try running your fingers through the glaze (like fingerpainting) to make it thick in some areas, thin in other areas.
Grace Lewis, CT technical editor, suggests sprinkling dry, powdered ingredients such as rutile or copper over wet glaze. Rutile, for example, can be used to add a sort of sparkling effect to glazes high in tin oxide. Copper, cobalt, manganese dioxide, or any fritted ceramic stains (i.e. Mason stains) can be used to achieve speckled colors on top of or between glaze layers.
Again, you must work quickly. Be sure to have a sieve, your powdered glaze chemical, and a bowl to catch the overflow nearby. Wear a dust mask or respirator. Start off by glazing your pot. While the glaze is still wet, sieve on your powdered chemical to the surface of the pot, and allow to dry as usual.
As with all of your glazing and firing techniques, remember to record each step in a notebook so you'll know how to repeat your favorite effects.
Modification of a base glaze recipe can be as simple as making changes to a bread recipe in your home kitchen. Much like you might substitute molasses for honey to make your bread appear dark brown rather than light-colored, you can add different colorants to your base glaze to achieve a wide variety of colors.
To begin experimenting with glaze colorants, it's best to start out with a reliable, basic base glaze recipe. Grace's Cone 6 Shiny Clear glaze will be used here to illustrate the procedure. (The recipe is Gerstley Borate, 50%; EPK, 17.5%; and Flint (Silica), 32.5%.)
Before doing the actual colorant testing, you need to make test tiles. This is easily done by rolling and cutting 3-in. square, 1/4-in. thick slabs of the clay you intend to glaze. Make impressions in, or texturize, half of each test tile's glazing surface so that your tests will show how the glaze reacts on both smooth and textured surfaces. Bisque the tiles at cone 06 or your usual bisque temperature.
Next, weigh out and mix together the dry ingredients of the base glaze (wearing a respirator and gloves, we hope). Once thoroughly dry mixed, weigh out the dry base glaze mix into 100-gram test batches.
This Cone 6 Glossy glaze is transparent. If you'd like your colored glazes to reveal the surface underneath, this glaze is ready for you to add colorants. But if you want the glaze to completely block out anything beneath it, make it opaque by adding various percentages of tin oxide to the 100-gram test batches, then fire to see which percentage you prefer. In general, anywhere from 2% to 5% should achieve a solid, white effect. Adjust your base glaze with the appropriate percentage of tin oxide.
Once your base glaze has the desired opacity, refer to the chart at right to select desired glaze colors and the colorants you'll need to achieve them. Then experiment with suggested percentages of the corresponding colorants by weighing out and adding them to your to your glaze test batches. Be sure to record the name of each colorant and percentage on the back of every test tile (you can paint this info on with red iron oxide) so that you'll be able to determine which combination you've used on the finished, fired test tiles.
Taken from Glaze Chemistry, a workshop by Grace Lewis, technical editor of Clay Times.
Many potters rely on glaze recipes available to them from pottery periodicals, books, college courses, online resources, friends, or colleagues...leaving little incentive for experimentation. It's true that formulation of an original, new glaze might seem intimidating at first, but a simple method called "line blending" can yield very rewarding, unique results.
A line blend is made by mixing together equal dry measures of two basic materials, or two separate mixes of materials, to slip consistency, then blending together these two thick liquids in varying proportions to yield a variety of different "test" recipes.
Usually, the two slips (made of clay and potash feldspar, for example, as they'll each offer silica, alumina, and fluxes) are blended together in six different proportions, beginning with 6 parts clay, 0 parts feldspar; then 5 parts clay, 1 part feldspar; 4 parts clay, 2 parts feldspar; 3 parts clay, 3 parts feldspar; 2 parts clay, 4 parts feldspar; 1 part clay, 5 parts feldspar; 0 parts clay, 6 feldspar.
Now apply each mixture to a test tile making sure to mark the back of the tile with the name of the combination (i.e., 4C/2F for 4 parts clay, 2 parts feldspar) and fire at your desired glaze temperature to test.
To obtain successful recipes, be sure to blend materials which together supply the three basic glaze ingredients: silica, alumina, and fluxes. (A detailed article on these materials will appear in next month's Claytime Companion.) Once you've developed a successful recipe, record it in the form of percentages (i.e., the 4/2 blend would be 66.66 clay, 33.33 feldspar).
Many a pottery student becomes intimidated when faced with the procedure of glaze formulation. Although some potters understand enough basic chemistry to perform complex glaze calculations without difficulty, those of us with non-scientific brains often struggle through the majority of glazing information. Too often, it's presented in the form of technical jargon too hard to follow.
If you're one of those people who has trouble with glaze chemistry, this article is for you! Following are basic concepts of glaze composition which will hopefully enlighten your understanding to the point where you can feel free to move on and experiment with the more advanced concepts on your own.
To start, think of a glaze simply as a thin coating of glass. The components which make up this glassy coating are applied in raw, liquid form, and turn into the glassy coating when fired at a very high temperature.
All glazes are made up of a minimum of three basic ingredients. They include: the glass former (silica), the melter (flux), and the sticker (refractory element).
The "glass former" is silica. Silica is the dioxide form of silicon (SiO2) occuring as quartz, sand flint, and agate. When fired by itself, silica will not melt until it reaches an extremely high temperature: 3100 degrees F or 1705 degrees C. However, since most types of clay mature at temperatures ranging from 1085-2471 degrees F (585-1355 degrees C), the use of silica alone will not work as a glaze. Another ingredient is therefore necessary in the glaze mix to lower the melting temperature of the silica.
This "melter" ingredient is also known as a "flux." In glazes which are to be fired at lower temperatures, lead or sodium is often used as the flux. In high-fired glazes, feldspar and calcium compounds can serve as the flux.
The third main component of a glaze is the "sticker," also known as the "refractory element." The reason for this ingredient is to keep the glaze from running off the pot and to make it stronger and more durable for practical use. Alumina is usually used as the sticker or "refractory element" because it combines well with silica to produce a tough, shock-resistant, scratch-resistant coating of glass. It bonds well to the clay because it is a main ingredient in clay.
When combined in various proportions, these three basic ingredients will yield a wide variety of "base" glazes. The melting points and ratios of the three basic ingredients to each other determines the firing temperature of each glaze: either "low-fire" (cone 4 and below), "mid-range" (cone 5-8), or "high-fire" (cone 9-12).
These base glazes will each produce one of three basic types of glaze finishes: glossy, semi-gloss, or matte. Generally, the more "mature" or "overfired" glazes yield glossy results; glazes which are somewhat "underfired" will result in matte or non-glare effects.
Now that you're familiar with the three basic components of glazing, consider how ingredients are added to the glaze mix to provide special effects. The most basic of these additives include opacifiers and colorants.
Opacifiers are chemicals added to the glaze to make it less translucent or more opaque. In other words, you would not be able to see through a very opaque glaze (sort of like adding a lot of the color white to a clear glaze). Tin oxide is considered an optimum (although expensive) opacifier because it's completely opaque and lends a pleasing, buttery quality to the glaze.
Colorants are just what you might expect: ingredients added to give color to your base glaze. These can be added in the form of raw oxides & carbonates like cobalt oxide or copper carbonate to achieve blue or green results, for example. Most ceramic suppliers also offer pre-blended versions of colorants to simplify the process for you (fritted ceramic oxides such as mason stains, for example).
Another addition to the basic glaze is the "suspender." This is an additive (usually bentonite, a form of volcanic clay) mixed with all the others (2% ratio, preferably in dry form) to keep those chemicals from settling to the bottom of a liquid glaze batch. In this way, you're not forced to spend 15 minutes stirring up the glaze before you apply it!
The following recipe for a reliable, clear, glossy cone 6 oxidation glaze is very basic and inexpensive. It is a good glaze for use over underglaze brushwork and oxides because it won't react with underglazes to change colors.
Beginners' note: This glaze may be used in an electric kiln that can fire up to cone 6 or higher. Base glaze ingredients are listed in percentages to total 100; the suspender percentage (and any colorants or opacifiers) may then be added. For convenience, amounts are usually measured in dry grams so each measurement equals one gram (i.e., 50 grams gerstley borate, etc.) when mixing a dry test batch of glaze. If you multiply each ingredient by 400, you'll end up with a 4000-gram glaze batch. After mixing with water to a consistency of table cream, this is enough glaze to store in a 5-gallon bucket so you can dip the entire pot when glazing. Always wear a respirator when mixing glazes; wash up thoroughly afterward.
(flux) Gerstley Borate........50.0 %
(alumina) EPK/Kaolin............ 17.5
This glaze should be applied thinly. If thick, it could appear cloudy or milky.
If you wish to experiment with this base glaze, try adding 2-5% tin oxide to make the glaze opaque. For color, add 1/2% to 4% coloring oxides: 1-2% cobalt for blue, 6-15% iron oxide for brown to reds, or 1-2% max. copper for green results.
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