Techno File: Subbing Fluxes

Many different glazes can easily be formulated from one base recipe by changing the source material for the flux, substituting the full amount of the flux for an entirely different one, or trying a combination of fluxes. The possibilities are virtually limitless.


Flux Power

A flux’s primary function is to lower the melting temperature of glass formers. Fluxes also influence color, strength, and durability of glazes. Flux oxides are sourced from various raw materials. First, you select a flux, then you decide which raw material you will use to supply that fluxing oxide. Depending on that source, you may be adding extra oxides to your recipe, which will affect the result of your glaze.

Let us say a glaze requires calcium oxide. At temperatures over cone 03, calcium oxide is a reliable flux that adds hardness and durability to a glaze, moderately affects color and is widely available. If the calcium is sourced from whiting, you will be adding calcium oxide alone to the glaze. If you source the calcium oxide from wollastonite (CaSiO2), you will be dragging along some silica, a glass former. This small addition of silica will not influence the color of the glaze. However, if you use dolomite CaMg(CO3)2 as the source for calcium oxide, magnesium comes along for the ride with the calcium oxide. Magnesium’s typical influence on a glaze is to create a matte surface and coloring oxides in a magnesium-dominated base become pastel.

Defining the Terms

Flashing: In these tests, when volatiles—such as sodium, boron, barium, zinc, and potassium—flux the unglazed clay areas, resulting in a shiny and toasty surface.

Technical Melter: Fluxing oxide in a glaze mixture.

Volatiles: Elements that vaporize from liquid to gas during firing.

Worm Tracks: Small rivulets forming long thin tracks in a glaze. In cone 6 oxidation firings, this can be caused by under firing glazes that contain a high amount of feldspathic materials.

Flux in Combinations

Most glaze recipes use a combination of fluxes for a few reasons. Grouping fluxes will help you achieve specific surfaces. By increasing or reducing the amounts of fluxes in combinations, you can increase or reduce the melting point of your glaze. This is somewhat like reducing the amount of sugar in a cake recipe—the decreased amount of sugar will not only change the taste, but also the consistency of the cake. You may have to add more liquid or more flour to get closer to duplicating the original recipe with reduced sugar. Similarly, the balance of fluxes determines the character of a glaze, so if you adjust the fluxes for a cone 9 glaze to melt at cone 6, you will likely have to adjust the other materials in your recipe in order make a glaze that resembles the original cone 9 glaze. Diverse fluxes in a glaze also usually yield a more stable and durable glaze than a single fluxed glaze.

It is not a good idea to formulate a glaze with a top-heavy amount of any single flux because if you run short or can no longer obtain the material (which is all too common these days), it will be difficult to reproduce your glaze. It will be easier to reproduce the original glaze if only a reasonable percent of a single ingredient in the glaze needs to be substituted by a material with similar properties to the one you ran out of.

If you need calcium oxide in your glaze, your material supplier may have four different sources for calcium oxide. Which one will you choose? Make tests using those different source materials and examine the results (1, 2). Make a test with the base, and a couple more with a controlled amount of cobalt and copper oxides or carbonates (0.5 to 3%). The base will show you the nature of the glaze surface. Is it melted, over or under fired, matte, satin, or gloss? Does it contain flaws such as crazing, pinholes, or blisters? The added coloring oxides will show how the different fluxes can influence color response. Is it muddy or bright, pastel or jewel toned? Does it give a turquoise when you expected celadon green or lilac when you were thinking cobalt blue? You can make endless adjustments by testing different fluxes in your glaze recipes.

As well as trying different source materials for the flux in your recipe, you can also substitute one flux for an entirely different one. Examine how the properties of the particular flux that was tested activate the surface for texture and color, and if the melting point is altered (under or over fired). In a base where a large amount of one particular flux is being substituted for another flux, the different characteristics of each flux used should be noticeable. Although I caution against the reliance of one flux in a recipe, using a large amount of one type in comparison has its purpose for observing a flux’s characteristics in test glazes.

Flux Sources for Cone 6 Oxidation Firing



The test tiles in these trials show various shades of blue and green, depending on the base, as well as different surfaces from gloss to matte, to textures including worm tracks, bubbles, pinholes, crazing, and tiny crystals. In several of the tests, adding the coloring oxides lowered the base glaze’s coefficient of thermal expansion enough to fix the crazing that occurred. Your perspective determines whether the surface and color is interesting, appealing, annoying, a special effect, or a fault.

You can speed up your test preparation time by mixing up one big batch of the base recipe then weighing out smaller batches for your tests and adding a fixed amount of one material to each test. This is faster than making 8 individual 100 gram batches of the base first, where you need to concentrate on many more measurements.

1 Three sources of calcium oxide: whiting, wollastonite, dolomite, with 2% cobalt oxide added to 30 Flux Base. 2 Three sources of calcium oxide: whiting, wollastonite, dolomite, with 2% copper carbonate added to 30 Flux Base.


The following examples use a 100-gram base recipe for all calculations.

Example 1:  8 fluxes to be tested in EZ Clear 34 Base: The base minus the variable flux = 66 grams. To mix enough for 8 tests, multiply following ingredients (in grams) by 8: 12 wollastonite, 24 kaolin, 30 silica. You will end up with 528 grams of the base recipe. Weigh 66 grams into 8 individual cups. Add 34 grams of an individual flux (8 selected) into each cup to yield 8 separate 100-gram test cups.

Example 2: 8 fluxes to be tested in 30 Flux Base: The base minus the variable flux = 70 grams. To mix enough for 8 tests multiply following ingredients (in grams) by 8: 40 nepheline syenite, 20 silica, 10 kaolin. You will end up with 560 grams of the base recipe. Weigh 70 grams into 8 individual cups. Add 30 grams of an individual flux (8 selected) into each cup to yield 8 separate 100-gram test cups.

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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.


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