Glazes are all glasses. There are essentially three categories of materials in a glaze: glass-formers, fluxes, and stabilizers.
Silica is the glass-former, but it melts at too high of a temperature to use in a glaze alone, so you need to add materials to decrease its melting point. This is where the second category of materials, fluxes, or melters comes in to play. These materials have many different names but really are all really just oxides, or combinations of oxygen atoms with atoms of another element. For instance, whiting supplies calcium oxide to a glaze. Dolomite supplies both calcium oxide and magnesium oxide, while talc only supplies magnesium oxide, lithium carbonate supplies lithium oxide.
More complex materials, like Custer feldspar and Kona F-4 feldspar, nepheline syenite and spodumene contain fluxes along with silica and alumina. They specifically supply potassium oxide, sodium oxide and lithium oxide, respectively, as well as some alumina and silica. All oxides also affect color development in a glaze.
The third group of materials needed in a glaze are called stabilizers. These materials control the flow of the molten glass on the surface of your pottery. The most important stabilizer is alumina, and it comes primarily from feldspar and clay.
So there it is, simply put, in very general terms! All the materials in the many buckets and bags in the studio supply these three categories of oxides that make glass. Mixed together with water and applied to your ware, they all interact with each other when subjected to heat, and form a glass coating called a glaze. We add metallic oxides or prepared ceramic stains to give the glaze color. Firing atmosphere plays an important part in the development of color and surface as well.
Glaze calculation provides a methodology of working with all these materials on a molecular level. This relationship allows us to balance the correct amounts of materials, provide the proper amounts of each oxide that will yield a glass of the correct expansion and melt at a given temperature. Looking at glazes this way provides a common way to analyze what occurs on an oxide-based level comparing equal parts to equal parts. Even the results derived from glaze calculation need to be tested.
4 Steps to Test
You can learn to experiment with both powdered and liquid commercial glazes in a simple, uncomplicated volumetric way without learning glaze calculation. In searching for different colors to use on your work, you can prepare simple color tests using commercially prepared dry mixed glazes. Not only is this a very easy way to see how glazes work, but you don’t even need to use a scale since you can use measuring spoons and cups.
Before you begin, you’ll need to make some test tiles from your clay body and gather a few rudimentary tools: small plastic cups, measuring spoons and cups, a small test sieve and a gram scale (optional).
Next, go to your ceramic supplier and purchase a few pounds of pre-mixed dry clear and matt glazes. These become your base glazes for a series of tests. You’ll also need a small quantity of the following coloring oxides: red iron oxide, copper carbonate, cobalt carbonate, manganese dioxide, rutile and chrome oxide. Also purchase a small bottle of black underglaze.
Step 1: Measure out ¹/2 cup of the dry clear glaze into a plastic cup. This is very close to 100 grams. Take 6 other cups and add the same amount of glaze, plus ¹/4 teaspoon of each coloring oxide per cup, which is roughly equal to 1 gram or 1% of the total amount of glaze material. You will have 6 test cups of colored glaze, and 7 cups total including the one with the base glaze and no colorants.
Step 2: Add ¹/2 cup of water to each cup. Mix the ingredients in each cup together with a small spatula, then pour through the test sieve into a clean cup. Label each cup with a marker in a manner of your choosing, and rinse the sieve before moving on.
Step 3: Dip the top half of a test tile into the glaze for approximately 5 seconds. You should then have 7 test tiles, 6 with colorants and 1 clear. Using a small brush dipped in iron oxide and water solution, or black underglaze, label each tile in an unglazed area. It’s best not to label the tiles on the bottom side that will come into contact with your kiln shelves as the underglaze will leave a ghost image. The label system should correspond to each cup.
Step 4: Using a clean brush, dip the brush into each colored glaze and paint a line of that color on the tile with just the clear glaze. This provides an example of how the color glaze will work over the clear glaze. You can also paint a line of color over the other glazes, just remember to use a systematic method and write it down so you’ll remember what you did after firing the tiles. For instance, the glaze with iron oxide can be painted over the tile with manganese dioxide as the colorant. There are many combinations using the 6 colored glazes.
Repeat the same test procedures with the matte glaze. You will then have 14 tests in all, 2 with each base glaze, and then 6 with each base and color. Fire to cone 6. If you have a kiln with a computer, program the following cycle:
Heating: 250°F/hr to cone 6
Cooling: 150°F/hr to 1500°F
This simple exercise provides a way to get started to see what a base glaze will do with the addition of colorant materials.
This article was excerpted from the January/February 2009 issue of Pottery Making Illustrated.
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