Imagine a glaze that fires perfectly at both cone 10 and cone 6, in reduction and oxidation, and in a soda firing, yet still produces a variety of exciting, stable colors. A glaze that fits this description is Turner’s White, which consists of common, inexpensive ingredients.
Turner’s White has great viscosity but is not runny; applies very well on bisque whether you spray, dip or pour; and doesn’t settle out in the bucket over time so remixing is fairly easy.
The goal: take this one stable glaze and learn about its properties, as well as discover new variations. In this way, you can increase the variety of surfaces possible using a limited number of materials.
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Testing the Base Glaze
Take two directions with the glaze—first explore Turner’s White (or your chosen glaze) by changing the ingredients within the recipe. For the second round of tests, explore color development using a variety of coloring oxides and commercial stains.
1. Changing ingredients
Make test batches where you increase one ingredient by 100 grams and another test where you omit the ingredient altogether. You could also choose to increase the ingredient by 100% then decrease it to 0% rather than use a specific gram amount so your test batches can be larger or smaller than the examples here. Mix each test glaze in a tall disposable cup, and label both the cup and the test tile accordingly. Do these two tests for each ingredient in the recipe, one for reduction firing, and the other for oxidation firing. This means that for the Turner’s White recipe, you will have a total of 18 cups, two of the base glaze, and 16 for the materials tests.
These tests do not require any glaze re-calculation but will give you a better understanding of what certain chemicals do in a glaze. Glazes are composed of glass formers, fluxes and stabilizers. Each of the chemicals included in a recipe fits into one (sometimes more) of these categories, depending on its chemical composition. As you work, you’ll start to see patterns, with some materials causing glazes to run, while others cause it to look dry or stiff.
As a general guideline for what you might expect:
Turner’s White’s original recipe produces a nice matt white surface fired to cone 6 electric.
Adding silica, Turner’s White fluxes more, and at the same temperature gives a more glossy, white surface, but is still very stable.
Adding zircopax and firing to cone 6 electric results in a superb white semigloss surface, and omitting zircopax, produces a nice, light beige.
Adding dolomite or talc also makes Turner’s White flux when fired to cone 6 electric.
Adding EPK yields a more textured, rough surface, like a slip or engobe.
In the cone 10 reduction tests, eliminating feldspars from the recipe gave a creamy matt surface.
Eliminating silica from the recipe gave a stone white matt surface.
Omitting zircopax and firing to cone 10 reduction gave an interesting, celadon-like surface.
Tests increasing either talc or dolomite at cone 10 reduction seemed to form a crystalline texture on the surface but were runny as well.
Note: The brown specks on the cone 10 reduction tests were produced by iron in the stoneware clay body.
2. Color Development
Using Turner’s White (or your chosen glaze) as a base, and leaving out the zircopax (or any opacifier), add a variety of colorants—copper carbonate, cobalt carbonate, rutile, red iron oxide, chrome oxide, Mason stains, encapsulated pigment stains and others.
Fire the tests to cone 6 (or the maturation temperature for your glaze) in both electric and gas reduction. The test results will be both interesting and disappointing.
For your tests, be sure to try a variety of percentages as well as different colorant oxides. What happens when you oversaturate a glaze with colorant? What are the other properties of the glaze that this affects? What observations can you make about the relative strength of different colorants?
As a general guideline here are some results from color tests with Turner’s White:
- 2% copper carbonate gives light turquoise colors
- 4% copper carbonate produced dark green in electric firings.
- A combination of red iron oxide and rutile produce a buckwheat color when fired in electric.
- Adding 2% cobalt oxide and 4% manganese dioxide give a nice purple both in oxidation and reduction.
- Nickel at 4% in oxidation produces a mustard colored matt surface but produced a chartreuse color and rough surface in reduction.
I encourage those of you who have one favorite glaze to try out one of the two directions and see what happens, maybe you’ll discover some remarkable surface or color. In any case, you’ll better understand the glaze, and what each ingredient contributes to the mix.
Kristina Bogdanov teaches ceramics at Ohio Wesleyan University in Delaware, Ohio.