Techno File: Ingredients’ Roles Steve Loucks

Appears in the November 2021 issue of Ceramics Monthly.

Standardizing the order for listing the ingredients in a glaze recipe—according to the ingredient’s dominant role in a glaze as a glass former, flux, or clay contributing ingredient—would greatly help users to better understand where a glaze gets its particular qualities.

Define the Terms 

Flux: A flux material, upon heating, reacts with silica and the other ingredients to lower their
melting temperatures.

Glass Former: A material that upon heating and followed by a relatively rapid cooling (relative from a geological perspective), forms a transparent amorphous solid, which we refer to as a glass. The main glass-forming material used by ceramic artists is silica, which gives glazes their fundamental look and quality. Boric oxide, which is also a glass former, is only available in combination with other chemicals and these compounds behave as flux ingredients.

Melting Temperature: This is both the temperature at which glaze ingredients melt to form a glaze, as well as the temperatures ceramic kilns readily achieve: generally, high-fire equates to cones 9 to 10 (2336–2381°F (1280–1305°C)); mid-range equates to cones 5 to 6 (2185–2232°F (1196–1222°C)); and low fire equates to cones 06 to 04 (1852–1958°F (1011–1070°C)).

Essential Glaze Materials and Component Parts

In theory, a glaze is made up of three essential materials: glass former, flux, and clay (which contributes refractories). Glass is the substance that makes a glaze surface at the most fundamental level. Silica is the main glass-forming material used by ceramic artists. Since silica melts at a very high temperature, flux materials are added to lower the melting point to temperatures our kilns can readily achieve. Clay is an aluminosilicate, contributing alumina and silica into the glaze. Its main role is contributing alumina, which stiffens the molten glaze to help control the flow of the glaze. The clay component in a bucket of liquid glaze helps to keep the ingredients in suspension.

In reality, a glaze is made using material ingredients that one buys from a ceramic supplier or digs from the ground. These ingredients fall into one or more of the following component parts or categories that make up the glaze in reality: feldspars, fluxes, clays, and glass formers. Collectively, these ingredients deliver the three essential glaze-making materials to make a base glaze. 

Most of our understanding of glazes comes from ceramic engineers. They think in terms of the molecules of certain chemicals such as silica, sodium, boron, calcium, and others that are in a glaze. The chemical analysis of a glaze is called the unity molecular formula or UMF, which is also sometimes called a Seger Formula. 

I like to think of glazes from the perspective of what the ingredients do and how they influence the glaze’s fired results. Every glaze contains the chemical silica, which is contributed from various ingredients. All feldspars contain some silica. Wollastonite, a flux ingredient, contains some silica. EPK kaolin and other clay-contributing ingredients contain some silica. So, what does silica do in a glaze? It can be confusing to figure this out because when the chemical silica is delivered into the glaze from many ingredients, its effects can’t be visually isolated from the effects caused by other chemicals contained in each ingredient. By contrast, when added as the ingredient silica (by itself), the influences on a glaze can be isolated and observed. In combination with the right ingredients at the right ratios, silica makes a glaze become glossy. If all glazes have silica in them, then what makes a glaze go matte? It’s complicated—but variables such as other materials present in the glaze, the firing temperature, and even the cooling rate are contributing factors. 

Component Parts Listed in a Glaze Recipe

To improve one’s understanding of glaze materials based on each ingredient’s dominant role (glass former, flux, or clay-contributing ingredient), I recommend that the ingredients in glaze recipes be listed in a standardized order of six component parts. The first four (feldspars, fluxes, clays, glass formers) make up the base glaze, followed by the addition of colorants and additives. 

1. Feldspars: List the feldspars in the recipe from highest to lowest amounts. Feldspar is a special ingredient that contains all three of the essential glaze-making materials. Feldspars can make a glaze by themselves, but the glazes are not perfect and need to be modified. Feldspars have silica for the glass-component part; various fluxing oxides (calcium, lithium, potassium, and sodium) for the flux-component part; and alumina for the clay-component part. These chemicals have been melted and combined together through geological processes to form feldspar. Feldspars are the foundation ingredient in a glaze that the other ingredients build upon and modify. That is why feldspars are almost always the highest amounts of all the ingredients in a glaze recipe. To emphasize the logic that feldspars should be listed first, remember that while their dominant role is to provide all three of the essential materials, the characteristics of the flux that’s in a particular feldspar lead to one feldspar being chosen over another. Some common feldspars are nepheline syenite, Minspar 200, Custer, Cornwall stone, petalite, and spodumene.

2. Fluxes: List the flux ingredients from highest to lowest amounts. Flux ingredients generally lower the melting temperature of the feldspar and/or the glass former in the glaze and give the glaze its main characteristics and qualities. Common flux ingredients are barium carbonate, bone ash, borax, dolomite, frits, Gerstley borate, lithium carbonate, magnesium carbonate, soda ash, strontium carbonate, talc, whiting, wollastonite, and zinc.

3. Clays: List the clay ingredients from highest to lowest amounts. Clay controls the melt and flow of the glaze. In high amounts, clay promotes matte glaze surfaces, particularly if the recipe calls for a low amount of silica. EPK kaolin is the ingredient that is most often used for the clay component part. Other common clay ingredients are 6 tile kaolin, Grolleg kaolin, OM 4 ball clay, and Cedar Heights Redart. 

4. Glass Formers: Finally, list the glass-forming ingredient, which is almost always silica. It makes the glaze go glossy, slightly raises the melting temperature, and makes the glaze harder and more durable. 

These four component parts that comprise the base glaze ingredients should be listed in the order outlined above, and the amounts should total to 100%. Note: If a recipe does not equal 100, you should recalculate percentages using the actual total. (Note: In Ceramics Monthly, the ingredients are listed by component part, but organized alphabetically within the subgroups). 

Additional Components

Below the base glaze ingredient list are two more component parts: colorants and additives. Common colorants are chrome oxide; cobalt carbonate; copper carbonate; iron oxide; manganese dioxide; nickel oxide; rutile; commercial stains; and the opacifiers tin oxide, titanium dioxide, and Zircopax. The most common additive is bentonite, which is used to help keep the glaze ingredients in suspension. The colorants and additives should be listed in percentages based on the 100% base glaze. 

Materials Contributing Boron

As we saw above, the three essential glaze-making materials and four component parts that supply them are not always the same as the ingredients that are purchased at a ceramic-supply store. For example, some glazes may have boron in them. But, boron by itself is not available at a ceramic supply store. It is available in ingredients that contain boron in combination with other chemicals. These ingredients are frits, Gerstley borate, and Gerstley borate substitutes. To explain, Gerstley borate is an ingredient made up of a combination of calcium carbonate and boric oxide. When the ingredient Gerstley borate is added into a bucket of glaze, it is adding boric oxide, or boron as it is commonly known, which is a secondary glass former, along with calcium carbonate, which is a flux. Gerstley borate melts in the low-fire temperature range and behaves as a strong, low-melting flux. Therefore, it is listed as a flux ingredient, which is its dominant role in a glaze.

Testing Ingredient Roles and Effects

Nepheline Syenite Line Blend with Gerstley Borate (1): This test tile shows the feldspar nepheline syenite at 100% on the left, with the colorant copper carbonate at 2%, and then incremental 10% additions of the flux Gerstley borate. Note how the Gerstley borate lowers the melting temperature of the nepheline syenite and eventually causes it to flow uncontrollably. Therefore, as visually demonstrated, Gerstley borate’s dominant role in the glaze is as a flux ingredient. 

Listing the glaze ingredients according to their role and component part is helpful when observing and comparing fired glazes and their respective ingredients. One can see general characteristics, influences, or responses among glazes that have relatively high amounts of similar ingredients in them. For example, fired glazes that have relatively high amounts of silica in them are glossy. Glazes with relatively low amounts or no clay in them are runny after firing, as exemplified in the first glaze test tile, which contains only feldspar and colorant with additions of the flux Gerstley borate and no clay (see 1). Glazes containing relatively high amounts of Gerstley borate are usually glossy after firing. Glazes that have relatively high amounts of the flux strontium carbonate may look runny after firing, yet they remain matte (see Component Part Test A images 2–5). The list goes on and on.

To definitively show the role of a particular ingredient or component part and its influence or effect on the glaze, it is best to make a series of test tiles. The tiles will be used to test the effects of eliminating the particular ingredient from the glaze and then adding it back into the glaze in incremental amounts. 

For great results, use my L-shaped test-tile design, which is shown in this article and also explained in the October 2018 issue of Ceramics Monthly. This test-tile design directly shows how additions of an ingredient influence the glaze, how the glaze responds to vertical flow and horizontal pooling, and how it breaks over texture. And very importantly, all of the testing information is written on the back of the tile with an underglaze pencil, so the information is always right there and cannot be lost in misplaced notebooks. 

Two component-part tests (A and B) are included here to explain my approach to understanding the role of an ingredient and visually show how it affects a glaze. I chose two simple glaze recipes, where each has only one ingredient per component part. There are four test tiles per glaze (one test tile showing the results of removing and then incrementally adding in each of the component parts: feldspar, flux, clay, and glass former). 

The test series consists of six stripes per tile. The first stripe on each tile is the original glaze recipe for comparison purposes. The second stripe has none of the component-part test ingredient in it. The following stripes have additions of the component-part test ingredient that has been added back into the glaze mixture in the particular percentages as outlined below. 

Component-Part Test A

The component part for each test tile was eliminated and then added back into the glaze base in increasing increments.

Feldspar Test: CPT-A Feldspar Line Blend with Nepheline Syenite (2). The additions of the ingredient nepheline syenite mainly gave the glaze more body with each addition taking it from a 45-gram glaze mixture that had no feldspar in it to a 125-gram base glaze. The nepheline syenite did not change the glaze quality significantly once it was added into the glaze, which is one of the reasons why a feldspar makes a great foundation ingredient. Only the color response changed dramatically due to the decreasing percentages of the colorant relative to the overall amount of base-glaze ingredients. 

Flux Test: CPT-A Flux Line Blend with Strontium Carbonate (3). Additions of the ingredient strontium carbonate gave this glaze three major qualities. First, it has a turquoise color response instead of green. Second, it gave the glaze a matte surface quality. Third, it lowered the melting temperature of the glaze and eventually made it flow uncontrollably, yet it remained matte.

Clay Test: CPT-A Clay Line Blend with EPK Kaolin (4). Additions of the ingredient EPK kaolin had three main effects upon the glaze. First, it controlled the flow of the glaze. Second, adding EPK kaolin raised the melting temperature of the glaze, which made it go matte and eventually underfired. A third but minor effect was that high percentages of EPK kaolin washed out the color.

Glass Former Test: CPT-A Glass Former Line Blend with Silica (5). Additions of the ingredient silica had two main effects on the glaze. First, as the amount of silica increased, it made the matte glaze eventually turn glossy. Second, adding silica raised the melting temperature of the glaze, which helped control the flow of the glaze and eventually made it slightly underfired. In the tests with high amounts of silica that resulted in a glossy glaze, the color response shifted from turquoise to green.

Component-Part Test B

The component part for each test tile was eliminated and then added back into the glaze base in increasing increments.

Feldspar Test: CPT-B Feldspar Line Blend with Nepheline Syenite (6). The additions of the ingredient nepheline syenite gave the glaze more volume, going from a 60-gram base glaze with no feldspar in it to a 120-gram base glaze. Additions of nepheline syenite did not change the glaze quality significantly—it made the glaze shift from a semi-gloss to a glossy fired surface. It also had a minimal effect on the color response. 

Flux Test: CPT-B Flux Line Blend with Whiting (7). Additions of the ingredient whiting gave this glaze two major qualities. First, it has a green color response with copper carbonate due to the effects of calcium carbonate on the colorant. Second, it made the glaze turn glossy with medium amounts of whiting added, and matte with very high amounts added. Whiting is considered to be a mid- to high-fire flux, which is why it did not make the glaze flow uncontrollably.

Clay Test: CPT-B Clay Line Blend with EPK Kaolin (8). Additions of the ingredient EPK kaolin had three main effects upon the glaze. One, it controlled the flow of the glaze. Two, it raised the melting temperature of the glaze, which made it eventually go matte and underfired. Note: In low amounts, it initially made the glaze go glossy. Remember that EPK kaolin also contains silica. Three, additions of high amounts of EPK kaolin diluted the color response.

Glass Former Test: CPT-B Glass Former Line Blend with Silica (9). Additions of the ingredient silica had two main effects on the glaze. First, it made the glaze go glossy. Second, in high amounts, it raised the melting temperature of the glaze, which helped to control the flow of the glaze. Note how it minimally affected the color response.

Developing New Glazes and Know-How

Often, more than just one stripe can result in an interesting glaze. After the test tiles are fired, choose which stripe(s) is most desirable, and recalculate the amounts of all ingredients to equal 100% for your new glaze recipe(s).

I highly recommend arranging the ingredients in your glaze recipes according to their dominate role in the glaze as a feldspar, flux, clay, and glass former (for the base glaze), then the colorant, and additive ingredients. As you fire the glazes, observe and take notes. You will see patterns or commonalities among the glazes for the component parts, which will help you to better understand glazes, how particular ingredients affect a glaze, and possible ways to adjust the glaze’s ingredients to improve them. Enjoy great results! 

the author Steve Loucks earned an MFA from the New York State College of Ceramics at Alfred University and is a Professor Emeritus from Jacksonville State University, Jacksonville, Alabama, where he taught for 26 years. Steve wrote and self-published the book Glazes From a Potter’s Perspective: a Simple, Kitchen-Method Approach to Understanding Glazes. To learn more, visit www.stevelouckspottery.com.