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Glaze Exploration

Cones are used in ceramics instead of temperature. A cone is a special piece of clay shaped like a triangle that you put in your kiln. As it heats up it softens, and it is calibrated bend over when your clay and glaze have received the right amount of heat. So for example, if you use a glaze that says "fire to Cone 04", your glaze will be ready when a Cone 04 bends.

Cone Chart
Cone Number	Tempurature	Color of Fire	What Happens?	Types of Ware and Glaze
022-014	1094 -1540	Dull Red	Dehydration begins and organic matter burns out	Overglaze colors enamels and chrome red glazes
013 – 011	1582 – 1641	Cherry Red		Lustre glazes
010 – 07	1679 – 1809	Orange		Low fire earthenware and lead
06 - 5	1855 – 2205	Yellow	Red clays mature and then melt. Buff clays mature	China and earthenware
6 - 15	2269 - 2615	White	Stoneware clays mature	Stoneware

Low & High Fire

Low fire was historically used to achieve very vivid, bright colors. Mid and high fire were used to achieve more muted, earthy colors. But glazes have continued to improve and now many different glaze characteristics can be achieved at each of the firing temperatures. High fire is not often used in electric kilns because it is harder on the kiln and takes so much more power. Therefore, few commercial glazes are available in High fire.

Low fire takes less power, but mid-high fire which is fired using stoneware and porcelain clay, is stronger and more durable.

Different Types of Glazes

Underglazes are used to get consistent colors that stay where you put them (regular glazes have a tendency to move around during firing). Sometimes they are left as is, and sometimes are covered with a clear or translucent glaze and refired.

Overglazes are primarily accent products applied over pieces that have previously been glaze fired. They typically are lustrous or have special pearlescent qualities. The piece is refired with the overglaze at very low temperatures, such as cone 016-018. When using Overglazes, apply them to surfaces that are free of dust, skin oils, and lotions to ensure proper adhesion. Wipe surfaces with rubbing alcohol prior to application (except when used over crackle glazes.)

Stains and Oxides can be wedged into clay to color it, added to clear glazes, or made into a thick liquid and used for drawing on or coloring clay, similar to a slip. The image to the right has a combination of black and red iron oxides used to give it a weather leathered look.

Slips are basically watered down clay, usually with color added. Engobes are similar.

Most of the prepared glazes you can buy are formulated for brushing on. Chemicals are added to keep the particles in suspension and the glaze easy to brush on. Usually 3 coats of glaze are required.

Many commercial glazes are also available DRY, to be mixed with water and sometimes a suspension agent. A 25 lb bag of DRY glaze will typically make about 3 gallons of glaze.

Although some commercial High fire glazes are available, most people use hand-made glazes for high fire work. These glazes are typically mixed to be used for dipping or spraying. The Excellent Ceramic Links page lists many sources for glaze recipes. Making your own glazes can be an intense scientific exploration, as there are many factors that affect how well they perform: firing type, firing speeds, clay body used, etc. To make your own glazes you need a good mask (many of the chemicals are poisonous), a variety of sieves and large buckets, an accurate scale, and a great deal of patience!

There are many factors that affect how well any glaze performs. Getting the right fit of the glaze to the clay body is extremely complex and fills many books. If your glaze and your clay don't "fit" (meaning bond together or expand at the same rate) your glaze will craze or crack or the piece can even crack. The benefit of commercially prepared glazes is that they have been formulated and tested to work in a variety of conditions, they are very stable (don't run all over your kiln shelves), and they are easy to get and use.