Goals

 

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.

Examples of the tools you’ll need: a small screen, measuring cup, plastic cup for mixing, underglaze and brushes.

Examples of the tools you’ll need: a small screen, measuring cup, plastic cup for mixing, underglaze and brushes.

Download a printer-friendly version of this ceramic art lesson plan here:
4 Steps to Easy Color
Materials

  • At least 14 test tiles made from your clay body
  • Small plastic cups
  • Measuring spoons and cups
  • Small test sieve
  • Gram scale (optional)
  • Small bottle of black underglaze
  • Pre-mixed dry clear and matt glazes. These become your base glazes for a series of tests.
  • Coloring oxides – only small quantities needed, choose 6 from this list: red iron oxide, copper carbonate, cobalt carbonate, manganese dioxide, rutile, chrome oxide, titanium dioxide, commercial stains (colors of your choice)

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.

 

 

Mixed glaze tests from 2 base glazes, the measuring devices used and the glazed tiles, ready to be fired.

Glaze Calculation

 

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.

 

  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.
  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.
  3. Dip the top half of a test tile into the glaze for approximately 5 seconds. Repeat this with the other cups of glaze. You should then have 7 test tiles, 6 with colorants and 1 with just clear base glaze. Using a small brush dipped in either an 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.
  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 colored glazes 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 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:
Heating: 250°F/hr to cone 6
Cooling: 150°F/hr to 1500°F.


Jonathan Kaplan has more than 40 years experience in ceramics and is a regular contributor to PMI. For more information, visit www.jonathankaplanceramics.com.

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