Welcome to Part 3 of our exploration into the primary components of ceramic glazes! In our previous articles we covered the role of glass-formers and fluxes, and today we’re going to examine another fundamental aspect of glaze chemistry: stabilizers. Also known as refractories, we’ll explore the importance, types, and impact these materials have on the development of your glazes. 

The Role of Stabilizers

Refractories or stabilizers are perhaps best thought of as the counterpart to fluxes. Where the flux lowers the melting point of the glass-former, the stabilizer helps moderate that melt so that your glaze does not simply run off your pot. It’s a stiffener, controlling the fluidity of your melting glaze. 

Stabilizers also play a vital role in increasing the strength of our glazes, being the primary source of durability in glaze. They reduce thermal expansion, and increase the glaze’s resistance to acid and other chemicals.

As a general rule, the more stabilizer in your recipe, the higher the melting point, and therefore the higher the firing temperature.

Alumina in its raw state

Sourcing Refractories in Glaze

Just like with our glass-former, our stabilizer is primarily sourced by one oxide. In this case we’re talking about alumina, or Al₂O₃. Let’s have a closer look:

Alumina (Aluminum Oxide), Al₂O₃

This material is considered an intermediate oxide because it helps build strong chemical links between fluxes and SiO2. It also helps to prevent crystallization in glazes, providing the levels of certain fluxes are not too high. 

Interestingly, knowing the ratio of alumina to silica in your glaze can help you predict the surface quality of your glaze. A ratio of 1:5 will give a matte glaze and a ratio of 1:8 gives a glossy glaze, with satin glazes lying in the middle. 

So what materials do we use for alumina? You may have noticed alumina or alumina hydrate available at ceramic suppliers, but we don’t actually tend to use these raw forms in our glazes very often. This is because in this form it doesn’t melt well enough to yield its oxides. These materials are more commonly used for making kiln wash. 

For the purpose of our glazes, we source our aluminum oxide from several of the same materials that provide our silica or our flux. These include clays such as kaolin, feldspars such as nepheline syenite, and frits.

These materials are a better source of Al₂O₃ because the oxide is already bound to silica particles, allowing it to better melt into the glaze. 

Considering their vital role, becoming familiar with stabilizers is much easier than understanding your fluxes, because there’s only one oxide you need to recognize. The only real challenge here is learning how much each source material is providing. To help with this, we recommend having a look at material resources such as Digital Fire or Glazy, that have searchable material databases that list the chemical makeup of ceramic materials. As a mini challenge, why not look up the following materials: Kaolin, Nepheline Syenite, and Custer Feldspar. Which has the highest amount of alumina, and which has the lowest? And as a bonus, what other components are they contributing?

Just as glass-formers and fluxes lay the groundwork for glaze chemistry, stabilizers play an equally vital role in shaping the final outcome of our recipes. With today’s article we’ve uncovered the significance of aluminum oxide in providing stability, durability, and unique aesthetic qualities to ceramic glazes. And while this article rounds out our exploration of the three primary components of glaze formation, we’re not quite finished with this series just yet. Be sure to check back soon for Part 4, where we’ll look at an essential component in the aesthetic quality of our glazes: colorants! 

We have a wide variety of workshops available covering all aspects of glazing and surface decoration. If you would like to elevate your skills in this area, be sure to check them out today!