Understanding Ceramic Glaze Ingredients: A Simple Guide to What’s in a Ceramic Glaze

A ceramic glaze can seem complicated because each ingredient, whilst often having a primary purpose, often contributes in more than one way to the success of the glaze.

To explore this, I will use a traditional green celadon glaze. The Green Celadon recipe used as an example in this article is from John Britt's The Complete Guide to High-Fire Glazes: Glazing & Firing at Cone 10 (2007, p. 69).

Green Celadon

• Potash feldspar — 30%

• Silica — 30%

• Whiting — 20%

• Kaolin — 20%

• Red iron oxide — 1%

This is a simple-looking recipe, but it contains several different types of glaze ingredients. Some provide the basic glass, some help it melt, some control its behaviour, and one produces the characteristic colour.

It is worth remembering that a single raw material can perform more than one job.

For simplification, I have listed ingredients in their most common category,

The categories describe the main job of an ingredient, not everything that ingredient does.

Glass Formers — The Material That Becomes Glass

What are they?

Glass formers are the ingredients that provide the basic structure of the glassy glaze surface.

The most common glass formers in traditional ceramic glaze recipes are:

• Silica

• Flint

• Quartz

When a glaze is fired, the silica/flint/quartz combines with other ingredients to give a glaze its characteristic hard, smooth surface.

How are they represented in our Green Celadon?

In our Green Celadon recipe, the glass former is particularly easy to identify - 30% Silica

This means that almost a third of the recipe is a glass former.

Fluxes — Helping the Glaze Melt

What are they?

Silica melts at a much higher temperature than ordinary pottery kilns reach. A glaze cannot be simply made from silica alone, it needs other ingredients to help it to melt.

Primary fluxes are ingredients that help the glaze melt at a practical ceramic firing temperature. They are the ingredients that help turn the mixture of raw materials into a molten glaze during firing. Secondary fluxes are added to make the fired glaze insoluable and help with the strength and stability of the glaze. Common fluxes in ceramics are:

• Potassium and Soda Feldspars (Primary)

• Cornish Stone (Primary)

• Nepheline Syenite (Primary)

• Whiting (Secondary)

• Dolomite (Secondary)

• Talc (Secondary)

• Wood Ash (Secondary)

• Wollastonite (Secondary)

• Zinc Oxide (Secondary)

• Lithium Carbonate (Secondary)

• Barium Carbonate (Secondary)

• Strontium Carbonate (Secondary)

• Calcium Frit (Secondary)

• Borax Frit (Secondary)

• Gerstley Borate (Secondary)

How are they represented in our Green Celadon?

The primary flux-bearing material here is - 30% Potash Feldspar

Potash feldspar supplies potassium along with other glaze-forming oxides. It acts as an important source of flux in the glaze.

The secondary flux ingredient is - 20% Whiting

Whiting is calcium carbonate. During firing it decomposes and ultimately supplies calcium oxide to the glaze. Calcium is an important flux in many high-temperature glazes.

Stabilisers and Suspension Aids

What are they?

Stabilisers help control the glaze when it melts in the kiln, while suspension aids help the glaze behave properly before firing. Alumina is the principal stabilising oxide in a ceramic glaze. Clay materials such as kaolin/china clay and ball clay are particularly useful because they perform both jobs. Their alumina helps control the molten glaze, while their fine particles help keep the glaze suspended in water and help it adhere to the pot.

Common alumina sources for stabilising are:

• Kaolin/China Clay (both stabiliser and suspension aid)

• Ball Clay (both stabiliser and suspension aid)

A popular suspension aid is:

• Bentonite

How are they represented in our Green Celadon?

Our Green Celadon recipe contains - 20% Kaolin

The kaolin provides alumina, which helps control the molten glaze and reduces excessive running.

It is also contributing silica, so it isn't doing only one job. This is an important point when reading glaze recipes: 20% kaolin does not mean “20% stabiliser.” The raw material is supplying several different chemical components, and those components perform different functions once the glaze is fired.

Colourants and Opacifiers

What are they?

Colourants

Ingredients that produce or alter colour in the fired glaze.

Common colourant ingredients are:

• Iron oxide (red, yellow, black)

• Copper oxide/carbonate

• Cobalt oxide/carbonate

• Manganese

• Nickel oxide

• Chromium oxide

• Rutile

  
  

Opacifiers

Ingredients that reduce transparency and make a glaze more opaque.

Common example:

• Tin oxide

How are they represented in our Green Celadon?

Our Green Celadon recipe contains - 1% Red Iron Oxide

The iron provides the colour response. In reduction it can produce the characteristic green/blue-green celadon; in oxidation it can move towards yellow and brown.

Conclusion

The main components of a glaze can be summarised as follows, always remembering that ingredients do not fit neatly into one category, but can often be contributing in a variety of ways to the ultimate success of the glaze recipe

• Glass Formers — create the glass

• Primary Fluxes — help the glaze melt

• Secondary Fluxes — modify the melt

• Stabilisers & Suspension Aids — control the melt and glaze behaviour

• Colourants & Opacifiers — create colour and control transparency