Potters today are so fortunate to have a huge range of colors premixed and at their disposal in the form of commercial underglazes. But if you have ever wanted a slightly different color or finish to create the look you desire, modifying commercial underglazes is an option!
In this excerpt from the November/December 2020 issue of Pottery Making Illustrated, Paul Andrew Wandless explains how you can modify commercial underglazes with low fire commercial glazes to create your own unique surfaces. —Jennifer Poellot Harnetty, editor.
While there’s a wide range of underglaze colors available, achieving the specific color and surface you want can still be elusive. What if you want a little more color saturation from your underglaze and also want the surface to be semi-matte or satin? This would require adding more pigment for better color saturation and more flux for stronger melting to change surface qualities. A simple solution is to modify the underglaze with something that will address both of these characteristics at the same time. That something can be a glaze.
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Adding a commercial low-fire glaze to an underglaze will change the color saturation and melting point of the underglaze, but still keep you in the same firing range. This is a great way to use commercial ceramic finishes to expand your surface options. Evaporating a low-fire glaze and grinding it into a powder is the best way to mix it into the underglaze as a modifier. It’s more concentrated as an evaporated powdered glaze and will yield a stronger result.
Modification = Recipe Change
Modifying an underglaze means changing the chemistry of the underglaze through the introduction of new materials. The addition of these new materials will alter the base recipe of the underglaze. This means you now have a new recipe with new characteristics and qualities. In this case, with the addition of more flux from the low-fire glaze, the underglaze will start to resemble the recipe of an engobe.
With this in mind, the shrinkage rate of the modified underglaze may no longer be compatible with greenware. The additional flux causes a stronger melt, meaning the clay body and modified underglaze no longer shrink at the same rates. This will create a fit problem if the disparity is too great. When I modify an underglaze in this manner, I only use it on bisqueware to avoid any potential fit problems on greenware or with other slips and underglazes that would be applied at the greenware stage.
I also keep my firing temperature within the suggested range of the modifying commercial glaze. Low-fire glazes are typically fired to cone 06–04, but check the label for the recommended cone. I fire to cone 04 and haven’t had any problems at this temperature when modifying underglazes with commercial low-fire glazes.
Gather the following items for evaporating then grinding the glaze into a powder: A sheet of glass (frosted if possible; if it’s frosted, the dried material will release easier), a mortar and pestle, and a razor-blade scraper. The rest of the items are standard testing supplies: measuring spoons, small containers, mixing spatulas, an underglaze pencil, brushes, test tiles, and a notebook to record your experiments. Since we’re working with dry materials, you should also use a dust mask or a properly fitted respirator to protect from airborne particles.
Making Glaze Into Powdered glaze
Pour the low-fire glaze you chose as the modifier onto the plate of glass. Let it spread naturally across the surface, like pancake batter on a skillet. To begin with, pour just what you want for this experiment, so you don’t waste materials. Let it sit overnight or until all the physical water has evaporated away (1). Once fully dried, use a razor-blade scraper to remove it from the surface of the glass (2). The evaporated glaze will break into small shards as you scrape it up, which is perfect for the next step. Using the mortar and pestle, start grinding these shards into powder (3). Don’t put all the shards in the mortar at once. Start with a small amount, start grinding, then continue to add in small increments until done. If your mortar is small, grind the dried-glaze shards in several small batches.
Once all the shards are ground into a smooth and consistent powder, store it in an appropriately sized container (4) labeled with the name and cone of the glaze used. Now you can use it as a dry material to modify underglazes.
I use individual tiles (2-inch square) to make visual samples of the glazes and underglazes being used and test strips (2×5 inches) for the modified underglaze progression tests. The strips will ultimately show the underglaze being modified and a four-step progression with the modifier (5). Always make tiles from the clay body you use. This will give you an accurate sample of what the colors will look like on your work.
Liquid Underglaze + Powdered Glaze
Use the parts method for mixing the evaporated powdered glaze into the liquid underglaze because it’s simple, consistent, and repeatable. Each test mixture is made in its own cup to keep proportions consistent and repeatable.
Mixing note: If you choose an underglaze as a base and keep adding more powdered glaze to the same container of the base incrementally, the test will not be as reliable. Every time you add the evaporated powdered glaze to the base and then brush some of it onto the test tile, you’re removing material and changing the proportions of the mixture, which can give you misleading results.
Color Progression Test
This is a great way to see how the incremental addition of a powdered glaze will affect an underglaze. I use a five-step test strip for this color progression. The top section is the underglaze by itself and the next four sections incrementally add one part evaporated powdered glaze.
I treat the evaporated powdered glaze as if it was an additive for a base mix, to keep things simple for me. I use ¼ tsp as the part measurement and 10 parts (2½ tsp) of underglaze is my base of 100%. When I add 1 part evaporated powdered glaze to this base (6, 7), it makes my parts ratio 10:1. You can use any method you want to add the modifier incrementally, but make sure your base is always the same amount to keep that variable fixed. My test strips for this test are: 10:0 (base), 10:1, 10:2, 10:3, 10:4 (8).
Note: I don’t sieve my initial tests, because I’m just interested in the basic color and surface changes that may happen. I know there may be some small inconsistencies or slight speckling in some of the tests, which may be seen as flaws. These flaws tend to be seen in the 10:1 mix. In the higher mixing ratios, the color evens out due to the higher concentration of flux present.
Be sure to label your test tiles. I use a black underglaze pencil to label and number the bisque test tiles. Whatever numbering or labeling system you choose is fine, but have something that can be easily matched to your notes. I use a combination of roman numerals and letters for my tests.
Having the underglaze as the first section of the test strip is important, because you really get to see how the powdered glaze alters the color and surface as more is added. Generally, the 10:1 and 10:2 mixes have color shifts, but the surface is still matte with no sheen. The 10:3 mix has a color change and now has enough flux present to affect the surface as well. It’s a little smoother and may have a light sheen. The 10:4 mix has a strong color change and a smooth surface with a satin sheen.
In general you can expect the surfaces to go from matte to semi-matte to satin with the incremental addition of flux from the glaze modifier. Your color will progressively darken if the underglaze and modifier are of the same hue. When the modifier is a different hue than the underglaze, a new color will be created.
Paul Andrew Wandless is an artist, author, educator, and curator currently living and working in Chicago, Illinois. He has authored the books Image Transfer On Clay, 500 Prints on Clay and co-authored Alternative Kilns & Firing Techniques. He’s also featured in Fundamentals of Screen Printing On Clay with Paul Andrew Wandless available from Ceramic Arts Network. He currently serves on the Board of Trustees for Penland School of Crafts. You can learn more at www.studio3artcompany.com.