Techno File: Bully Material Testing


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Parts By Volume Mixing
When added as a relatively large percent to the overall balance of a glaze base, a glaze material’s specific characteristic will have a dominating effect on the recipe. The higher the proportion of the single material in your test, the more you will see of how this material behaves when fired. It is a good show-and-tell exercise to mix up an individual glaze material with water, apply it thick and thin to a test tile, then fire it; however, it is more useful to fire an excessive proportion of a material in a simple base recipe so you are already one step closer to a working glaze when you examine the fired results. With the addition of a clay, a glass former, and a flux(es), the individual materials are helped to melt, fuse to the clay body, and also develop a color response.

Brian Gartside, a New Zealand potter (www.gartside.info), uses a parts-by-volume glaze-recipe mixing approach and throws all kinds of materials together, fires the tests, and observes the results. His curiosity has prompted me to explore an alternative method to glaze calculation, and simply raid the glaze material shelves to find interesting combinations. Gartside’s relaxed attitude toward glaze mixing and application depends on parts measurements and simple 50/50 mixes of glaze ingredients.

It will always be essential to know your glaze oxides in order to fully understand why your results look the way they do, but it is not essential for mixing materials, firing them, studying the results, and applying this evidence-based knowledge to further testing or using it in your glaze formulation. Whether your approach is seriously technical, haphazard, or something in between, you will get test results you can learn from with the parts-by-volume method.

50/50 Testing
To test this method, I mixed a simple glaze base with a balance of common glaze materials: silica (glass former), nepheline syenite (a feldspathoid that is virtually a glaze in itself containing K2O.3Na2O.4Al2O3.8SiO2), and clay to stiffen the glaze. For the purpose of allowing each bully ingredient to flex its muscles, the glaze balance is 50% of the base glaze recipe (the control base) and the additional 50% is a single material (the bully material). This large percentage will show how these singular ingredients behave when fired in a simple glaze base. These tests can be made with any material from your glaze pantry or anything else you are curious to test.

Now you have a 50/50% glaze test by volume. Parts volume is a good measuring method for this experiment because it is quick and you can make a lot of tests in record time. You also do not need to be so concentrated, because you are not working with precise gram weights.


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However, when you see results you like and want to develop or refine the glaze with glaze calculation programs, you can easily recalculate the recipe to percentages.

Converting Parts to Percent
To convert a parts recipe to a percent recipe, start with Gartside’s Basic Parts Glaze: Add up the parts. In this case, the parts total is 14. Use the percentage formula: % of material = (parts of material divided by parts total) x 100 (rounded to the closest whole decimal).

(4 divided by 14) x 100 = 29
(2 divided by 14) x 100 = 14
(1 divided by 14) x 100 = 7
29 + 29 + 14 + 14 + 7 + 7 = 100%

Individual materials take up the same volume in a fixed unit of measurement, however, if you pour them out and weigh them, they will not have the same weights because some are light or heavy, dense or powdery.

Conversely, take 100 grams of nepheline syenite, kaolin, and silica and they will fill a different amount of volume in a fixed unit of measure, because of their different consistencies. The amount of volume a material takes up in the unit of measure (i.e cup or teaspoon) may be the same, but each material will have a different gram weight. Despite this, the conversion from parts to % normally yields the expected results.

Glaze Boats
For these tests, I made glaze boats that are extruded slabs, cut to about 4×2 inches. The edges on the top and bottom (held vertically) are sliced about ½ inch from the edge and inward to form a tab.2 These tabs are then folded over each other to form a boat (1). Each boat can handle about 50 grams of mixed glaze. The floor of the boat, where there is a stamped impression, allows for a thick and thin application of the glaze. This is where you can see if the glaze breaks or pools. The sides of the boat are high enough that you can also get an impression of how stiff or fluid your glaze is. By adding a small brush detail of a coloring oxide such as cobalt, you can easily see if there is movement in the glaze based on how much the brush stroke bleeds or falls, as well as get an indication of color response. When firing, if you avoid glazing the boat’s top edges, you can stack them in the kiln to maximize room.


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Raid the Pantry
The materials I used as my 50% bully ingredient came from raiding my own glaze pantry. I sourced information on each material from a basic description found in Frank and Janet Hamer’s The Potter’s Dictionary of Materials and Techniques. Virtually all ceramic materials are worth testing. Materials of more unusual nature, even raw materials from your backyard or woods can be tested in this or a similar base and the results can be inspiring. I have even tested crushed mussel shells and ochre from the beach and brook by my home.


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Reading the Results
I made a palette key of the tests for each group so that I could refer to them quickly. (The boats give the true nature of glaze, as it provides different applications and surfaces to study the nature of the glaze tested.) The palette keys are also useful to see one material next to another.

Looking at the 50/50 boat tests (2), notice the flow or stiffness of the glazes—some completely covered the stamped design while others were very thick, but still fluid enough to reveal the stamp. Some glazes turned into crazed glass and some into dry riverbeds. Whatever the result, it is a step forward to a glaze effect you will develop further, with or without a sensible glaze balance, depending on your individual goal. Glaze experiments require as little as two teaspoons of whatever you have in your glaze pantry.

The palette key gives an indication of what the bully material looks like in the glaze base, but the boats give so much more information due to their surface area and ability to show any activation or movement in the glaze. Adding a colored oxide helps to show how fluid or stiff the glaze is. You can now compare the fired tests to the information sourced from the dictionary of materials. For example, the test with zinc shows clearly that it is a flux, but it is also refractory (with a high melting point of 3587°F (1975°C)), adds opacity, and helps with crazing, but adversely causes crawling. In this test, zinc shows its characteristics quite well.

Some of the bullies are strong fluxes. They made the glaze base so fluid that the individual oxide brush markings were indistinguishable due to so much movement on the glaze surface. Another test with spodumene (a source for lithium) stressed the clay body so much, it dunted. Without using a glaze calculation program, we can assume that this test’s low COE caused the dunting.

the author Alisa Liskin Clausen is an American-born ceramic artist with a BFA from Syracuse University. She has lived and worked as a potter in Denmark for the past 20 years, focusing her work on glaze development from local materials.

1 Ian Currie Revealing Glazes, page 151
2 www.gartside.info, Technical, Obnoxious Glazing
3 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, Fourth Edition. Page 366, Zinc
4 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, Fourth Edition. Page 110, Dunting
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