Vitrification Nation

We’ve had a lot of questions lately about clay bodies and vitrification. We’ve noticed some manufacturers offer clays with a wide firing range – Cone 6 to 10 for example. Click the “READ MORE” button below to learn more about clay vitrification and why you should understand more on the topic.

Clay bodies are generally classified as:

earthenware or low fire

stoneware/porcelain or mid fire


stoneware/porcelain or high fire

Sometimes, clay manufacturers describe a clay body with a range that starts at Cone 04 (earthenware) and extends to Cone 10 (high fire.) While it is true that a clay may be used for a variety of purposes at a variety of temperatures, by definition a clay does not vitrify at earthenware temperatures, nor can a clay vitrify at both Cone 5 and Cone 10.

Two common household examples of clay that is vitrified are the brick used to build houses as well as clay sewage pipes.

So.. vitrification? What is it anyway? Clay vitrification is when clay becomes glassy and watertight but stops short of melting and slumping. Vitrification is dependent upon the clay being fired hot enough to become vitreous.

There may be projects or reasons to use a clay at it’s non-specified firing temperature. In essence, “under cooking” the clay. For instance, when sculpting a Cone 10 clay body might have *exactly* the look an artist is striving for, when that Cone 10 body is fired to only Cone 5.

A clay body must be fired hot enough to become mature, meaning it has become hard and durable but not yet necessarily vitrified.

Just because a clay and glaze are developed for the same temperature does not mean that they will fit will together and result in a pleasing pot. Every Cone 5 clay + every Cone 5 glaze will not result in a water tight pot, and so on. The outcome of each firing depends on numerous factors including glaze application, clay body, the expansion, contraction, and shrinking rates of the clay and glaze, and firing.

Within each of these factors, there are many components. How the glaze is applied and how thickly will be one portion of considering the glaze equation. Another will be clay body chemistry and the glaze pairing; the look of the glaze will absolutely be influenced by the color and ingredients of the clay. During the firing, hot or cool spots in the kiln, placement of the piece, ramping temperature, temperature holds, oxidation or reduction will play large parts in how the piece will look coming out of the kiln.

Kiln firings are always subject to the whims of the kiln gods as well. If a firing is re-created and doesn’t go as well as the previous firing, sacrifices to the kiln gods may be in order.

At Stone Leaf Pottery, we suggest you try the products that interest you. Pay attention and take notes to what you did and how you did it. Read the information the manufacturer prints on it’s product and ask the staff at Stone Leaf. We may have tried what you are attempting to do or chatted with another customer about a similar process.