Ever wonder what happens to a typical cone-5 clay body during a kiln firing to turn it from a soft, relatively weak piece of bone-dry ware into a hard, dense piece of ceramic art? Here’s what happens.

Define the Terms 

Fluxing Ingredients: Minerals or frits containing elements such as sodium and potassium that lower the temperature at which a liquid glass phase forms in a clay body. 

Iron Pyrite: A metallic, crystalline mineral composed of iron and sulfur. Technically it is iron disulfide, but most commonly it’s known as fool’s gold. Its chemical formula is FeS².

Refractory: A material that is resistant to being changed by heat.

Sintering: Fusion of the contact points of particles of a dry powder to each other to form a porous solid.

Vitrification: Melting of all or a portion of dry ceramic materials into a glass. In ceramic art, the glass formed frequently contains unmelted or only partially melted particles of quartz.

Firing Science

Free water is evaporated during candling. At about 451°F (233°C) any organic materials contaminating the clay begin to burn off. Clays usually contain 0.1 to 3.5% organic matter by weight. 

Next, beginning around 1022°F (550°C), the clay body loses water that is chemically bound in the crystal lattice of the clay particles. Frits, as well as quartz, feldspar, and other minerals do not contain a significant amount of water.

At about 1652°F (900°C), minerals containing sulfur, such as iron pyrite, also burn off. The weight of the bound water, organics, and sulfur typically driven off in a firing is often listed by clay vendors as a percent of the weight of the dry clay and is listed as the clay’s loss on ignition, or LOI.

By the time the pyrite begins to burn, crystals of clay have begun to sinter, and the body takes on a very soft bisque or biscuit consistency. Continued firing produces a harder, but still porous piece.

The sintering process changes around 1922°F (1050°C) to vitrification. That is, part of the body begins to melt into liquid glass. The hotter the kiln gets, the further this process goes and more the body shrinks. That’s the case until the kiln and clay reach cone 5, about 2170°F (1188°C). If the firing continues and the kiln becomes even hotter, the clay will eventually begin to swell again. That’s because so much glass phase will form that bubbles of kiln vapor will become trapped in the liquid gas and expand.

At cone 5, most of the clay body will be either the glass phase or the remainders of the quartz particles that have not melted into the glass phase and crystals of mullite. The mullite is what’s left of the clay particles as the heating causes them to give up part of their silica. Typically, that silica becomes part of the glass phase. However, if there’s not enough alumina in the body, the silica from the clay forms crystals of cristobalite.

Right: Feldspar, quartz, and clay particles are a jumbled mass in the formed, but unfired clay body. As sintering proceeds during the firing, clay particles begin to fuse to other particles in the piece including other bits of clay.

Testing and Fixing Vitrification

If a glazed piece remains porous after the glaze firing, you now understand what has happened. Not enough glass has formed in the body. In other words, vitrification of the body has been insufficient.

How do you tell if the work is not vitrified enough? The best way is to weigh an unglazed test tile, then place it in gently boiling distilled water for 5 hours, then transfer it to cold water or allow it to cool in the water for 24 hours, and finally pat it dry with a towel and immediately reweigh it. Its weight gain should typically be between 1 and 3%.

In the worst case, if the form is a vessel, perhaps a vase, you will notice that it will not hold water. Sometimes the leakage is subtle. The vase will leave a damp spot where it’s left to sit. Firing test tiles is a much better way to discover under-vitrification than ruining good furniture.

There are several ways to deal with insufficient vitrification. The simplest, and possibly the best, is to switch to a clay body that becomes more vitrified at the firing temperature being used.

Another solution commonly used is to fire the work to a higher temperature. Testing is necessary to determine how much hotter the piece needs to be fired. A hotter firing temperature will increase glaze flow, however. Whether the change is problematic will depend on how much hotter the firing has to be to vitrify the body. Only testing will reveal that.

Less commonly done but sometimes necessary is to modify the clay body recipe by changing the ratio of fluxing ingredients to refractory ingredients. Either removing refractory ingredients or adding flux materials lowers the vitrification temperature of the body.

the author Dave Finkelnburg is a studio potter, practicing engineer, and a regular contributor to Ceramics Monthly. He earned his master’s degree in ceramic engineering from Alfred University.