Learn more about glazes through empirical testing. Glazes are all glasses, formed of different materials. There are essentially three categories of materials in a glaze: glass-formers, fluxes and stabilizers.
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4 Steps to Easy Color
The 3 Glaze Components
Silica is the glass-former, but it melts at too high a temperature to use in a glaze, 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 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 the fluxes 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.
Four Steps to Testing Color
Before diving in to glaze calcuation, experiment with both powdered and liquid commercial glazes in a simple, uncomplicated volumetric way to learn how different glazes and colorants work (or don’t work) together.
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.
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 the appropriate cone based on the glaze bases you chose to test (low, medium or high fire).
If you have a kiln with a computer, program the following cycle:
Jonathan Kaplan has more than 40 years experience in ceramics and is a regular contributor to PMI. For more information, visit www.jonathankaplanceramics.com.