Understanding the Role of Fluxes and Oxides in Pottery Glazes

Pottery glazes blend chemistry with artistry, where fluxes like soda feldspar and oxides such as lanthanides are pivotal. They lower silica’s melting point and enhance color vibrancy. This article explores their roles, chemical interactions, and practical applications, offering insights for potters to create enduring glazes that blend artistry with scientific precision.

Fluxes

The chemistry of pottery glazes involves many different materials. The most important material is silica, which acts as the glass-forming agent. Since the melting point of silica is very high (1700 Centigrade), a flux must be used to lower its temperature. Different fluxes have varying abilities to do this and each affects the melt and character of the glaze.

An example of a common pottery flux is soda feldspar, also known as nepheline syenite or Kona F-4. Another is a calcium oxide source such as whiting or dolomite. In addition, there are non-alkaline earth metal oxides that act as fluxes such as barium carbonate or strontium carbonate.

To compare the effects of these and other possible materials, it is necessary to convert a percentage glaze recipe into a molecular formula (unity formula). This is done by using the unique molecular weights of each oxide and adding them together. This allows apples-to-apples comparisons between recipes, even when they are not calculated with the same software.

Oxides

The lanthanide oxides offer a way to achieve clean color without the muddying effect of stains. They tend to be denser than stain colors and offer a wide range of glaze effects in the right firing temperature range. These oxides are often mixed with a small amount of Gerstley borate or Gillespie borate to ensure that they will dissolve in the molten glass and stay suspended as they color.

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ceramic clay body will also act as a source of silica, alkaline earth metals, and other oxides that help make a glazed plastic. It will also contribute its unique properties such as a low rate of expansion and a high melting point. A glaze can be made without the clay component but it will not function as well, and the fired results will be less desirable. Clay is important for the suspension of the rest of the glaze slurry. It also imparts a certain character to the glaze that is impossible to duplicate with kaolin or silica alone.

Characteristics

There are several reasons why potters choose to use a flux or combination of fluxes in their glaze recipes. The most obvious reason is to lower the melting temperature of the glaze. This is the main function of the fluxes, and it is important to understand this aspect.

Another reason is to create a glaze with specific characteristics. This could be a high gloss or a low viscosity. For example, a glaze recipe containing iron oxide can be difficult to control if the glaze is not saturating in its melt because it tends to precipitate out during cooling. This can be avoided by using a flux such as calcium carbonate to keep the iron in solution.

Some of the most common fluxes include whiting, dolomite (calcium and magnesium carbonates), talc, wollastonite (calcium silicate), and frits. Wood ash is also a good source of calcium and acts as a flux in its own right. It can be used to help stabilize glazes and reduce drying shrinkage.

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Applications

Assuming you have the correct mix of glass formers and fluxes, the next challenge is keeping the resulting molten glaze from flowing right off the pot. To do this, a plasticizer must be added to the recipe.

The simplest plasticizer is calcium carbonate (CaCO), which is also a natural flux in some clays. Talc is another common choice, especially for middle-temperature glazes.

Lithium is an active flux at lower temperatures and can reduce crazing. It has a low rate of expansion and can be traded against higher-expansion fluxes to lower the melting point of a glaze without significant color effects.

Generally, the percentage of fluxing oxides in a glaze should not exceed 18%. This number is based on the Unity Molecular Formula, or UMF for short. The UMF calculates the relative molecular weights of oxides so that apples-to-apples comparisons can be made. It is a useful tool for comparing recipes, evaluating glazes, and revising melting points.

Achieving Vibrant Pottery Glazes

In mastering pottery glazes, understanding the intricate roles of fluxes and oxides is crucial. Fluxes like soda feldspar and oxides such as lanthanides play pivotal roles in lowering melting points and enhancing color vibrancy. By carefully selecting and balancing these materials, potters can create enduring glazes that blend artistry with scientific precision. Whether you’re a novice exploring a pottery supply store, looking online for a “pottery supply store near me” or an experienced ceramic artist refining your craft, the chemistry behind glazes offers endless possibilities for creative expression and technical mastery.