Frits Dave Finkelnburg

Instructions

Frits are wonderful glaze ingredients. Getting familiar with frits can help solve problems and improve glazes. The challenge of frits lies in discovering which ones to use in a given situation, and why.

Defining the Terms

What are Frits 

What is a frit? Most simply, it’s a ground glass. However, it’s a glass of a special composition—typically high in flux elements, low in alumina, usually with enough silica to make a stable glass, and sometimes containing boron.

Fluxes that dissolve in a glaze slurry can cause problems during glaze firing. That’s why glaze recipes seldom use soluble fluxes. Frits can be wonderful glaze ingredients because they incorporate soluble elements in a glass matrix, thus rendering those elements insoluble.

At high temperatures (around cone 10) a useful source of insoluble fluxes that melts rapidly is feldspar. However, feldspars are slow to melt at low- and mid-range temperatures. Frits were originally developed to provide an insoluble source of fluxes for low and mid-range glazes and/or to permit formulation of faster melting glaze recipes.

Every frit has a particular ratio of flux elements to each other and, when used, to alumina and also to boron. Enough silica is included to make the frit stable and insoluble in the glaze slurry.

The remainder of the glaze recipe depends on the firing method, peak firing temperature, and the clay body the glaze is applied to. However, all fritted glaze recipes require enough clay or other viscosity modifier to suspend the glaze ingredients. Typically at least 10% of a white-burning ball clay or kaolin is used in the recipe. When kaolin is used, the dry glaze surface will be softer and glaze dusting may be a problem. Additional silica may be required in the recipe, since increasing proportions of silica are required as the firing temperature is increased.

Which Frit to Use

A great way to quickly gain familiarity with a variety of frits is by firing a small amount of each frit side by side in the same firing and then comparing the fired samples to each other. It is best to weigh the same amount of each frit and put each sample in a separate small, unglazed, deep tray or bowl.

Compare the visual characteristics such as fusibility, melt, opacity, transparency, fluidity, and stiffness of these fired samples to each other and to their published chemical compositions. Now you have information about the chemistry of each frit and the physical attributes that chemistry produces, information that will help you pick a frit for your glaze.

Developing a Fritted Glaze

The easiest way to incorporate or substitute a frit into a glaze recipe is to use glaze calculation software—this software is readily available and has dramatically simplified this task.

However, if you want to develop a glaze simply by experimentation, start with the basic materials.

All glazes require silica. It’s what forms the glass network. By itself, silica will not melt at ordinary kiln temperatures (silica melts at 3110°F (1710°C)). However, a frit provides necessary flux elements which, when mixed with silica, will make a combination that does melt readily in the kiln. Boron can also be used to speed up silica’s melting so the glaze melting temperature falls into the range suitable for your clay body.

When choosing a frit, decide whether you want a glaze that is transparent or opaque, what firing range you will use, and whether you want to brighten or mute any added colorants, then refer to your button tests and the frit chemistry to choose which frits to test.

Next, you’ll want to add some alumina to the mix, and alumina is usually sourced from clay. Alumina stiffens the melted glaze, stabilizes and hardens the fired glaze, and increases durability of the fired surface.

Additional raw materials can also be added depending on what you want the glaze to look like. Adding an excess of calcium can produce an opaque satin to matte glaze. This is because calcium compounds crystallize from the melted glaze as it cools. Alkali fluxes—sodium, potassium and lithium—all produce brighter colors.

Finally, test how well the glaze stays in suspension. If enough clay was added to provide required alumina, the glaze may suspend well on its own. If no more alumina is desired, a high-quality bentonite (used sparingly) can be a good alternative. Bentonites have extremely small particles, thus hold a great deal of water, and in excessive amounts may cause an applied glaze to dry too slowly.

Glaze suspension can be improved, and the glaze surface hardened simultaneously, by adding a cellulose gum. A common gum is CMC (carboxymethyl cellulose) VEEGUM® CER is a commercial blend of bentonite and CMC. Ball clay or kaolin can also be used, in small amounts, in combination with CMC, to suspend a glaze.

A useful rule of thumb is to use between 0.25–2% of bentonite and around 0.5% CMC. Begin testing at the low end of the addition rate. These ingredients will have a noticeable effect, so test small amounts of glaze carefully to avoid ruining an entire batch.

Note: When adding bentonite, either mix the dry ingredients very well with all other dry glaze ingredients or hydrate it thoroughly with a known amount of water for 24 hours, mix it well, then add it to the slurry as a weight percent of the glaze recipe.

This article was written by Dave Finkelnburg and excerpted from the  January 2014 issue of Ceramics Monthly.