Adventures in Glaze Firing

By Janet (imapigeon) on April 5, 2010

Unloading a glaze kiln is always an adventure! One of the things I hear most from students is, "Why didn't my pot turn out the color I expected?". Here are some variables to consider:

The basis of all glaze is silica, clay, and other mineral elements that will affect the melting temperature, opacity, texture and other characteristics. Glaze colorants are mineral oxides (iron, cobalt, tin, etc.) When a glaze is fired, all of the ingredients combine, react and bond chemically with each other and the available oxygen in the kiln. The amount of oxygen available during firing significantly impacts how both the clay and the glaze will turn out.

At various temperatures in a firing cycle, clay and glaze molecules become unstable.  They start looking around for other molecules they can bond with to re-stabilize.  In an electric kiln, the firing atmosphere is typically "oxidation", which means that there are lots of oxygen molecules available during the firing. In a gas kiln it is easier to do either oxidation or "reduction" firing, which means at specific temperatures, gas burners and dampers are partially closed to reduce the amount of available oxygen. (I've read that it's possible to do reduction in an electric kiln, but that it's hard on the elements and may reduce their life cycle---and I haven’t actually tried this myself.)

The kiln’s firing atmosphere has a significant impact on how a piece will turn out.  For example, an iron-rich glaze will typically be greenish in an oxidation firing, while the same glaze in a reduction firing will typically be brown.  Commercial glazes are formulated so they react in SOMEWHAT predictable ways, depending on what kind of clay body they’re used on, how they are applied, and what cone/temperature they reach.  High-fired red glazes are particularly challenging---they're tempramental and difficult to get.

However, these are just a few of the contributing variables to all the chemical changes that occur inside a kiln during the firing cycle.  Other factors are the shapes and sizes of the pieces and how closely spaced they are.  Each piece may be affected by where it sits----its proximity to elements or burners, spyholes, and the top & bottom of the kiln.  How close it is to other pieces, and even the glazes on nearby pieces will likely have an impact.  In an outdoor gas kiln, the flow volume, chimney diameter and height, and even weather will also affect each firing.

So the next time you pick up a glazed ceramic piece, think of all the variables that contributed to how it turned out!!

The many faces of Popsicle Slide Glaze:


Related articles:
clay, firing, glaze, glazing, kiln, oxidation, pots, pottery, reduction

About Janet
I've been involved with clay off and on since 1972----I love getting my hands in the mud! I've taken classes at several colleges, managed ceramic shops, and taught many students. After retiring in 2009 from environmental, health and safety, I began dedicating several hours a week to clay projects in my little studio. I especially like throwing planters, bowls and lidded jars, and I've made hundreds of handpainted tiles for my house. I do much of my firing at home, where I have both electric and gas kilns.

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Comments and discussion:
Subject Thread Starter Last Reply Replies
Great ceramiccolors Jan 10, 2014 9:34 PM 0
Great! Sharon Apr 18, 2010 6:30 PM 14

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